(en)The present invention provides methods and compositions for improving the drought resistance of plants. More specifically, the present invention utilizes overexpression of a molybdenum cofactor sulfurase in plants and plant cells.

1.ApplicationNumber: US-23103502-A
1.PublishNumber: US-7038111-B2
2.Date Publish: 20060502
3.Inventor: ZHU JIAN-KANG
XIONG LIMING

4.Inventor Harmonized: ZHU JIAN-KANG(US)
XIONG LIMING(US)

5.Country: US
6.Claims:
(en)The present invention provides methods and compositions for improving the drought resistance of plants. More specifically, the present invention utilizes overexpression of a molybdenum cofactor sulfurase in plants and plant cells.

7.Description:
(en)CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims benefit to U.S. Provisional Application Ser. No. 60/317,724, filed on Sep. 6, 2001, incorporated herein by reference in its entirety.


STATEMENT OF GOVERNMENTAL SPONSORSHIP
The invention described herein was supported by NSF grants IBN-9808398 and DBI-9813360. The government may have certain rights in this invention.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and compositions for increasing stress tolerance in plants. More specifically, the present invention provides methods and compositions for improving drought resistance, salt tolerance, and resistance to freezing.
2. Background of the Invention
Plants respond to environmental challenges in part by altering their gene expression profile that ultimately leads to various adaptive responses at cellular and whole plant levels (Bray, 1993; Thomashow, 1999; Hasegawa et al., 2000; Zhu et al., 1997; Ingram and Bartel, 1996). One important regulator of plant responses to abiotic stress environments is the phytohormone abscisic acid (ABA). ABA is involved in plant responses to abiotic stress such as low temperature, drought, and salinity, as well as the regulation of plant growth and development including embryogenesis, seed dormancy, shoot and root growth and leaf transpiration (Koornneef et al., 1998; McCourt, 1999; Leung and Giraudat, 1998; Rock, 2000). Evidence for a role of ABA in stress-responsive gene regulation in plants has been two-fold. Firstly, under cold, drought, or salt stress conditions, plants accumulate increased amount of ABA, with drought stress having the most prominent effect on ABA accumulation. Secondly, the expression of many stress-responsive genes is induced by exogenous ABA, and their stress-inducibility is decreased in mutant plants defective in ABA biosynthesis or responsiveness.
Genetic analysis based on the inhibitory effect of ABA on seed germination has yielded mutants with reduced ABA biosynthesis or altered ABA responsiveness (Koornneef et al., 1998; McCourt, 1999; Leung and Giraudat, 1998; Rock, 2000). The former group of mutants in Arabidopsis includes aba1, aba2, and aba3. The ABA1 gene encodes a zeaxanthin epoxidase that functions in an early step of ABA biosynthesis by converting zeaxanthin to violaxathin. Molecular cloning of ABA2 or ABA3 has not been reported thus far. Common phenotypes of these aba mutants include loss in seed dormancy, germination resistance to NaCl stress, and withering when transferred from high humidity to low humidity conditions. The utilization of ABA deficient mutants along with ABA response mutants in stress gene regulation studies led to the notion that stress-responsive gene expression in plants is mediated by both ABA-dependent and ABA-independent pathways (Shinozaki and Yamaguchi-Shinozaki, 1997; Leung and Giraudat, 1998; Rock, 2000; Thomashow, 1999). Although the molecular mechanisms underlying the differences between ABA-dependent and ABA-independent gene regulation is unclear, analysis of the promoters of stress-responsive genes and the isolation of transcription factors that activate these genes support that there are distinct regulatory mechanisms for the different pathways. The ABRE (ABA-responsive element) complex in these promoters mediates gene induction by ABA (Guiltinan et al., 1990; Yamaguchi-Shinazaki and Shinozaki, 1994; Shen and Ho, 1995; Vasil et al., 1995), whereas the DRE/CRT (dehydration-responsive element) mediates cold and osmotic stress responsiveness independently of ABA (Yamaguchi-Shinozaki and Shinozaki, 1994; Stockinger et al., 1997). Despite these differences in transcriptional activation, genetic analysis has indicated that the ABA-dependent and ABA-independent pathways have extensive interactions or crosstalk in controlling gene expression under abiotic stresses (Ishitani et al., 1997; Xiong et al., 1999a).
As is well-appreciated in the field, there remains a need for methods of improving the resistance of plants to drought.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods and compositions for increasing stress tolerance in plants.
It is another object of the present invention to provide plants and plant cells which have increased stress resistance.
The objects of the present invention, and others, may be accomplished with a method of increasing stress resistance in a plant, comprising overexpressing a molybdenum cofactor sulfurase in the plant.
The objects of the present invention may also be accomplished with a method of increasing stress resistance in a plant cell, comprising overexpressing a molybdenum cofactor sulfurase in the plant cell.
The objects of the present invention may also be accomplished with a plant or a plant cell transformed with a nucleic acid which encodes a molybdenum cofactor sulfurase.
Thus, the present invention also provides a method of producing such a plant or plant cell, by transforming a plant or plant cell with the nucleic acid which encodes the molybdenum cofactor sulfurase.
The present invention also provides an isolated and purified molybdenum cofactor sulfurase having the amino acid sequence of SEQ ID NO: 2.
The present invention also provides a method of producing the molybdenum cofactor sulfurase described above, comprising culturing host cells which have been transformed with a nucleic acid encoding the molybdenum cofactor sulfurase under conditions in which the molybdenum cofactor sulfurase is expressed, and isolating the molybdenum cofactor sulfurase.
In another embodiment, the present invention provides an isolated and purified enzyme having molybdenum cofactor sulfurase activity, wherein the amino acid sequence of the enzyme has a homology of from 70% to less than 100% to SEQ ID NO: 2
The present invention also provides a method of producing the enzyme described above, comprising culturing host cells which have been transformed with a nucleic acid encoding the enzyme under conditions in which the enzyme is expressed, and isolating the enzyme.
The present invention has been accomplished using a reporter gene approach to genetically dissect ABA and stress signal transduction in Arabidopsis. ABA and stress responsive bioluminescent plants were constructed by introducing the firefly luciferase reporter gene under control of the RD29A promoter (containing both ABRE and DRE/CRT elements; Yamaguchi-Shinozaki and Shinozaki, 1994) into Arabidopsis. The RD29A-LUC plants were mutagenized, and mutants with abnormal bioluminescence in response to cold, drought, salt or ABA were isolated (Ishitani et al., 1997). One group of mutants exhibit reduced luminescence responses to NaCl stress. Here, we present the characterization and cloning of two allelic mutants from this group. These two mutants, designated los5-1 and los5-2 (low expression of osmotically responsive genes), show reduced expression of stress-responsive genes under both cold and osmotic stress conditions. While the role of LOS5 in osmotic stress regulation of gene expression is mediated by ABA, the regulation of cold responsiveness by LOS5 is not dependent on ABA. The function of LOS5 in cold and osmotic stress responsive gene expression is independent of CBF/DREB1 or DREB2A transcription factors. los5 mutant plants are more susceptible to damage by freezing, salt and drought stresses, suggesting that LOS5 is critical for plant stress tolerance. The mutant plants also show enhanced transpirational water loss and accumulate less ABA in response to drought stress. Allelic tests show that los5 is allelic to the aba3 mutation. Map-based cloning of LOS5/ABA3 reveals that it encodes a putative molybdenum cofactor (MoCo) sulfurase that catalyzes the sulfuration of the desulfo form of MoCo, which is consistent with previous findings that the aba3 lesion is in the introduction of S into MoCo (Schwartz et al., 1997a). Sulfurylated MoCo is a cofactor of ABA-aldhyde oxidase that functions in the last step of ABA biosynthesis. Expression of the LOS5/ABA3 gene is up-regulated by ABA, salt and drought stresses. These data provide important insights into ABA biosynthesis and significantly further an understanding of stress gene regulation and stress tolerance.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following Figures in conjunction with the detailed description below.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 : Luminescence phenotypes of los5 mutant plants.
(A) Morphology of wild-type (left) and los5-1 (right) seedlings on an agar plate.
(B) Luminescence of (A) after low-temperature treatment at 0° C. for 48 hr.
(C) Morphology of wild-type (left) and los5-1 (right) seedlings on an agar plate.
(D) Luminescence after treatment with 100 μM ABA for 4 hr.
(E) Morphology of wild-type (left) and los5-1 (right) seedlings on filter paper saturated with 300 mM NaCl.
(F) Luminescence of (E) after 5 hr of 300 mM NaCl treatment.
(G) Quantitation of the luminescence intensities of wild type and los5-1 plants in response to cold (0° C. for 48 hr), ABA (100 μM for 4 hr), or NaCl (300 mM for 5 hr) treatment as shown in FIGS. 1B , 1 D, and 1 F. Also shown are those of untreated plants (control).
(H) Low temperature dosage-response curve. Treatments at −5° C. or −10° C. lasted for 3 hr following by incubating at room temperature for 2 hr. Treatment at other temperatures lasted for 48 hr before imaging.
(I) NaCl dosage-response curve. Treatment time was 3 hr.
The color scale at right shows the luminescence intensity from dark blue (lowest) to white (highest). Data in (G) to (I) represent means and standard errors (n=20). Open symbols, wild type; black symbols, los5-1.
FIG. 2 : Transcript levels of stress-responsive genes in los5-1 and wild-type plants. Seedlings were either not treated (control, C), or treated with low temperature (Cold, 0° C. for 24 hr), 100 μM ABA for 2 hr, 300 mM NaCl for 3 hr, or 30% PEG for 5 hr. Actin was used as a loading control. WT, wild type.
FIG. 3 : Freezing sensitivity of los5-1 plants.
(A) Plants before freezing treatment.
(B) Plants after freezing treatment (−7° C. for 5 hr). The picture was taken 7 days after freezing treatment.
FIG. 4 : Proline accumulation, and osmotic stress sensitivity of los5-1 mutant plants.
(A) Proline accumulation in los5-1 and wild type plants that were untreated (control), or 150 mM NaCl treated, or 50 μM ABA-treated. Data represent means and standard error (n=3).
(B) Drought sensitivity as measured by electrolyte leakage in wild type and los5-1 plants treated with 30% PEG. Data represent means and standard error (n=4).
(C) los5-1 plants are more sensitive to NaCl stress. One-week-old los5-1, los6/aba1, and wild type seedlings were transferred from MS nutrient agar medium to MS agar plate without NaCl (0 mM NaCl) or with 100 mM NaCl. Note that los5-1 mutant leaves were bleached due to NaCl stress. The pictures were taken 3 weeks after the seedlings were transferred to the treatment plates.
FIG. 5 : Leaf morphology and wilty phenotypes of los5-1 mutant plants.
(A) Wild type plants in soil.
(B) los5-1 mutant plants in soil.
(C) Wild type and los5-1 rosette plants are turgid immediately after root detachment.
(D) los5-1 plants are wilty 10 min after root detachment. Arrow points to a wilty leaf in los5-1.
(E) Morphology of wild type (left) and los5-1 inflorescence 10 min after being moved from 90% relative humidity to ˜30% relative humidity. Note los5-1 plants are wilty.
(F) Accumulative transpirational water loss in detached los5-1 and wild type shoots with or without 100 μM ABA treatment. Data are means and standard error (n=4).
FIG. 6 : Cold- or salt stress-regulation of RD29A-LUC expression in wild type, los5-1 and los6/aba1 seedlings as affected by exogenous ABA.
(A) Salt stress responsiveness in los5-1 and los6-1 mutants is rescued by ABA application.
(B) Cold responsiveness in los5-1 mutant is not rescued by ABA application. Data are means and standard errors (n—20). Cold, 0° C. for 48 hr; ABA, 100 μM for 4 hr; and Na, 300 mM NaCl for 4 hr.
FIG. 7 : Positional cloning of LOS5 and the organization of LOS5 gene and gene product.
(A) LOS5 was mapped to the upper arm of chromosome I and located on the BAC clone F19K19.
(B) Structure of the LOS5 gene and position of los5/aba3 mutations. Positions are relative to the translation initiation codon. Filled boxes indicate the open reading frame and lines between boxes indicate introns.
(C) Overall structure of LOS5 protein.
FIG. 8 : Sequence of the cDNA sequence encoding the LOS5 protein (SEQ ID NO: 1).
FIG. 9 : Sequence alignment of LOS5 and its homologs from other organisms. Residues in black shade indicate identity and gray shade indicates similarity. Dotted lines indicate gaps that introduced to maximize alignment. The putative pyridoxal phosphate-binding motif is solid-underlined and the conserved cysteine motif is dash-underlined. The conserved critical lysine residue in the PLP domain is indicated with an asterisk, and the conserved cysteine residue is indicated with a square. Also shown are the positions of los5/aba3 mutations (filled circles indicate introduced stop codon). Sequence accession numbers for LOS5/ABA3 and its homologs are as follows: LOS5/ABA3, AY034895 (SEQ ID NO: 2); Human, BAA91354 (SEQ ID NO: 3); Fruitfly (Mal protein of Drosophila melanogaster; SEQ ID NO: 5), AAF50901; cattle (MCSU of Bos Taurus; SEQ ID NO: 4), BAA98133; and Aspergillus (HxB of Aspergillus nidulans; SEQ ID NO: 6), AAF22564.
FIG. 10 : LOS5/ABA3 gene expression and stress gene regulation in ABA deficient or insensitive mutants.
(A) LOS5 expression in different parts of plants.
(B) Up-regulation of LOS5 expression by drought in wild type seedlings.
(C) LOS5 expression under different stress treatments in wild type and los5-1 plants.
(D) Expression of LOS5 and selected other stress-responsive genes in los5-1, aba1-1, and abi1-1 plants.
(E) CBF2 and DREB2A expression in ABA-deficient mutants.
Control, untreated; cold, 4° C. for 12 hr; ABA, 100 μM ABA for 4 hr; NaCl, 300 mM NaCl for 5 hr; PEG, 30% PEG for 5 hr. Tubulin and actin are shown as loading controls.
FIG. 11 : Reaction catalyzed by LOS5/ABA3.
Desulfo/dioxyo form of molybdenum cofactor (MoCo) (left) needs to be sulfurylated at one of the two terminal oxyo groups by LOS5/ABA3 molybdenum cofactor sulfurase to generate the sulfide form of MoCo (right). The sulfide form MoCo is a cofactor of aldehyde oxidase (AO) and xanthine dehydrogenase (XDH), whereas the dioxyo form of MoCo is the cofactor for nitrate reductase (NR) and sulfite oxidase (SOX). AO catalyzes the last step of ABA biosynthesis. The immediate sulfur donor could be a cysteine residue, originated from LOS5/ABA3 or other sources. The structure of pterin and its numbering scheme are according to Rajagoplant (1991).
FIG. 12 : Reduced water loss rate from leaves of Arabidopsis plants overexpressing the LOS5 gene. Leaves were detached from rosette stage soil-grown seedlings and placed under room conditions under light. Pictures were taken at the indicated time after detachment.



DETAILED DESCRIPTION OF THE INVENTION
Description of the Preferred Embodiments
As described in detail in the Examples below, overexpression the molybdenum cofactor sulfurase described herein provides improved drought resistance in plants. Such plants are also expected to be also more resistant to salt stress and freezing stress. This is because drought tolerance is intimately connected with salt and freezing tolerance, especially with regard to ABA. The loss of function los5 mutants are more sensitive to salt or freezing stress, also suggesting that the overexpression plants are likely more tolerant to salt and freezing stresses. Therefore, in the context of the present invention, the expression “increasing stress tolerance in plants” refers one or any combination the following: increased drought tolerance, increased resistance to salt (i.e., soil salinity), and increased freezing tolerance. As will be readily appreciated by those skilled in the art, the increased tolerance to stress in the plants overexpressing the molybdenum cofactor sulfurase refers to the corresponding plants which do not overexpress this enzyme.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art of molecular biology. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be limiting.
Reference is made to standard textbooks of molecular biology that contain definitions and methods and means for carrying out basic techniques, encompassed by the present invention. See, for example, Maniatis et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York (1982) and Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York (1989); Methods in Plant Molecular Biology, Maliga et al, Eds., Cold Spring Harbor Laboratory Press, New York (1995); Arabidopsis, Meyerowitz et al, Eds., Cold Spring Harbor Laboratory Press, New York (1994) and the various references cited therein.
The term “plant” includes whole plants, plant organs (e.g., leaves, stems, roots, etc.), seeds and plant cells and progeny of same. The class of plants which can be used in the methods of the invention is generally as broad as the class of higher plants amenable to transformation techniques, including both monocotyledonous and dicotyledonous plants. Preferred plants include rice, corn, wheat, cotton, peanut, and soybean. Thus, in one embodiment of the present invention, the salt tolerance of a plant can be enhanced or increased by increasing the amount of protein available in the plant, preferably by the enhancement of the molybdenum cofactor sulfurase gene in the plant.
Thus, one embodiment of the present invention are plant cells carrying the polynucleotides of the present invention, and preferably transgenic plants carrying the isolated polynucleotides of the present invention.
As used herein, the term “enhancement” means increasing the intracellular activity of one or more enzymes in a plant cell and/or plant which are encoded by the corresponding DNA. Enhancement can be achieved with the aid of various manipulations of the bacterial cell. In order to achieve enhancement, particularly over-expression, the number of copies of the corresponding gene can be increased, a strong promoter can be used, or the promoter- and regulation region or the ribosome binding site which is situated upstream of the structural gene can be mutated. Expression cassettes which are incorporated upstream of the structural gene act in the same manner. In addition, it is possible to increase expression by employing inducible promoters. A gene can also be used which encodes a corresponding enzyme with a high activity. Expression can also be improved by measures for extending the life of the mRNA. Furthermore, enzyme activity as a whole is increased by preventing the degradation of the enzyme. Moreover, these measures can optionally be combined in any desired manner. These and other methods for altering gene activity in a plant are known as described, for example, in Methods in Plant Molecular Biology, Maliga et al, Eds., Cold Spring Harbor Laboratory Press, New York (1995).
A gene can also be used which encodes a corresponding or variant enzyme with a high activity. Preferably the corresponding enzyme has a greater activity than the native form of the enzyme, more preferably at least in the range of 5, 10, 25% or 50% more activity, most preferably more than twice the activity of the native enzyme.
In the context of the present application, a polynucleotide sequence is “homologous” with he sequence according to the invention if at least 70%, preferably at least 80%, most preferably at least 90% of its base composition and base sequence corresponds to the sequence according to the invention. According to the invention, a “homologous protein” is to be understood to comprise proteins which contain an amino acid sequence at least 70% of which, preferably at least 80% of which, most preferably at least 90% of which, corresponds to the amino acid sequence which is shown in SEQ ID NO: 2; wherein corresponds is to be understood to mean that the corresponding amino acids are either identical or are mutually homologous amino acids. The expression “homologous amino acids” denotes those which have corresponding properties, particularly with regard to their charge, hydrophobic character, steric properties, etc. Thus, the protein may be from 70% up to less than 100% homologous to SEQ ID NO: 2.
Homology, sequence similarity or sequence identity of nucleotide or amino acid sequences may be determined conventionally by using known software or computer programs such as the BestFit or Gap pairwise comparison programs (GCG Wisconsin Package, Genetics Computer Group, 575 Science Drive, Madison, Wis. 53711). BestFit uses the local homology algorithm of Smith and Waterman, Advances in Applied Mathematics 2: 482–489 (1981), to find the best segment of identity or similarity between two sequences. Gap performs global alignments: all of one sequence with all of another similar sequence using the method of Needleman and Wunsch, J. Mol. Biol. 48:443–453 (1970). When using a sequence alignment program such as BestFit, to determine the degree of sequence homology, similarity or identity, the default setting may be used, or an appropriate scoring matrix may be selected to optimize identity, similarity or homology scores. Similarly, when using a program such as BestFit to determine sequence identity, similarity or homology between two different amino acid sequences, the default settings may be used, or an appropriate scoring matrix, such as blosum45 or blosum80, may be selected to optimize identity, similarity or homology scores.
The present invention also relates to polynucleotides which contain the complete gene with the polynucleotide sequence corresponding to SEQ ID NO. 1 or fragments thereof, and which can be obtained by screening by means of the hybridization of a corresponding gene bank with a probe which contains the sequence of said polynucleotide corresponding to SEQ ID NO. 1 or a fragment thereof, and isolation of the DNA sequence.
Polynucleotide sequences according to the invention are suitable as hybridization probes for RNA, cDNA and DNA, in order to isolate those cDNAs or genes which exhibit a high degree of similarity to the sequence of SEQ ID NO: 1.
Polynucleotide sequences according to the invention are also suitable as primers for polymerase chain reaction (PCR) for the production of DNA which encodes an enzyme having molybdenum cofactor sulfurase activity.
Oligonucleotides such as these, which serve as probes or primers, can contain more than 30, preferably up to 30, more preferably up to 20, most preferably at least 15 successive nucleotides. Oligonucleotides with a length of at least 40 or 50 nucleotides are also suitable.
The term “isolated” means separated from its natural environment.
The term “polynucleotide” refers in general to polyribonucleotides and polydeoxyribonucleotides, and can denote an unmodified RNA or DNA or a modified RNA or DNA.
The term “polypeptides” is to be understood to mean peptides or proteins which contain two or more amino acids which are bound via peptide bonds.
The polypeptides according to invention include polypeptides corresponding to SEQ ID NO. 2, and also includes those, at least 70% of which, preferably at least 80% of which, are homologous with the polypeptide corresponding to SEQ ID No. 2, and most preferably those which exhibit a homology of least 90% to 95% with the polypeptide corresponding to SEQ ID NO. 2 and which have molybdenum cofactor sulfurase activity. Thus, the polypeptides may have a homology of from 70% to up to 100% with respect to SEQ ID NO: 2.
The invention also relates to coding DNA sequences which result from SEQ ID NO. 1 by degeneration of the genetic code. In the same manner, the invention further relates to DNA sequences which hybridize with SEQ ID NO. 1 or with parts of SEQ ID NO. 1. Moreover, one skilled in the art is also aware of conservative amino acid replacements such as the replacement of glycine by alanine or of aspartic acid by glutamic acid in proteins as “sense mutations” which do not result in any fundamental change in the activity of the protein, i.e. which are functionally neutral. It is also known that changes at the N- and/or C-terminus of a protein do not substantially impair the function thereof, and may even stabilize the function.
In the same manner, the present invention also relates to DNA sequences which hybridize with SEQ ID NO. 1 or with parts of SEQ ID NO. 1. Finally, the present invention relates to DNA sequences which are produced by polymerase chain reaction (PCR) using oligonucleotide primers which result from SEQ ID NO. 1. Oligonucleotides of this type typically have a length of at least 15 nucleotides.
The terms “stringent conditions” or “stringent hybridization conditions” includes reference to conditions under which a polynucleotide will hybridize to its target sequence, to a detectably greater degree than other sequences (e.g., at least 2-fold over background). Stringent conditions are sequence-dependent and will be different in different circumstances. By controlling the stringency of the hybridization and/or washing conditions, target sequences can be identified which are 100% complementary to the probe (homologous probing). Alternatively, stringency conditions can be adjusted to allow some mismatching in sequences so that lower degrees of similarity are detected (heterologous probing).
Typically, stringent conditions will be those in which the salt concentration is less than about 1.5 M Na ion, typically about 0.01 to 1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. for short probes (e.g., 10 to 50 nucleotides) and at least about 60° C. for long probes (e.g., greater than 50 nucleotides). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. Exemplary low stringency conditions include hybridization with a buffer solution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecyl sulphate) at 37° C., and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 M trisodium citrate) at 50 to 55° C. Exemplary moderate stringency conditions include hybridization in 40 to 45% formamide, 1 M NaCl, 1% SDS at 37° C., and a wash in 0.5× to 1×SSC at 55 to 60° C. Exemplary high stringency conditions include hybridization in 50% formamide, 1 M NaCl, 1% SDS at 37° C., and a wash in 0.1×SSC at 60 to 65° C.
Specificity is typically the function of post-hybridization washes, the critical factors being the ionic strength and temperature of the final wash solution. For DNA—DNA hybrids, the Tm can be approximated from the equation of Meinkoth and Wahl, Anal. Biochem., 138:267–284 (1984): Tm=81.5oC.+16.6 (log M)+0.41 (% GC)−0.61 (% form)−500/L; where M is the molarity of monovalent cations, % GC is the percentage of guanosine and cytosine nucleotides in the DNA, % form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. The Tm is the temperature (under defined ionic strength and pH) at which 50% of a complementary target sequence hybridizes to a perfectly matched probe. Tm is reduced by about 1° C. for each 1% of mismatching; thus, Tm, hybridization and/or wash conditions can be adjusted to hybridize to sequences of the desired identity. For example, if sequences with approximately 90% identity are sought, the Tm can be decreased 10° C. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (Tm) for the specific sequence and its complement at a defined ionic strength and pH. However, severely stringent conditions can utilize a hybridization and/or wash at 1, 2, 3, or 4° C. lower than the thermal melting point (Tm); moderately stringent conditions can utilize a hybridization and/or wash at 6, 7, 8, 9, or 10° C. lower than the thermal melting point (Tm); low stringency conditions can utilize a hybridization and/or wash at 11, 12, 13, 14, 15, or 20° C. lower than the thermal melting point (Tm). Using the equation, hybridization and wash compositions, and desired Tm, those of ordinary skill will understand that variations in the stringency of hybridization and/or wash solutions are inherently described. If the desired degree of mismatching results in a Tm of less than 45° C. (aqueous solution) or 32° C. (formamide solution) it is preferred to increase the SSC concentration so that a higher temperature can be used. An extensive guide to the hybridization of nucleic acids is found in Current Protocols in Molecular Biology, Chapter 2, Ausubel, et al., Eds., Greene Publishing and Wiley-Interscience, New York (2000).
Thus, with the foregoing information, the skilled artisan can identify and isolated polynucleotides which are substantially similar to the present polynucleotides. In so isolating such a polynucleotide, the polynucleotide can be used as the present polynucleotide in, for example, increasing the salt tolerance of a plant.
One embodiment of the present invention is methods of screening for polynucleotides which have substantial homology to the polynucleotides of the present invention, preferably those polynucleotides encode a protein having molybdenum cofactor sulfurase activity.
The polynucleotide sequences of the present invention can be carried on one or more suitable plasmid vectors, as known in the art for plants or the like.
In one embodiment, it may be advantageous for propagating the polynucleotide to carry it in a bacterial or fungal strain with the appropriate vector suitable for the cell type. Common methods of propagating polynucleotides and producing proteins in these cell types are known in the art and are described, for example, in Maniatis et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York (1982) and Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, New York (1989).
In another preferred embodiment the polynucleotide comprises SEQ ID NO: 1, polynucleotides which are complimentary to SEQ ID NO: 1, polynucleotides which are at least 70%, 80% and 90% identical to SEQ ID NO: 1; or those sequence which hybridize under stringent conditions to SEQ ID NO: 1, the stringent conditions comprise washing in 5×SSC at a temperature from 50 to 68° C. Thus, the polynucleotide may be from 70% up to less than 100% identical to SEQ ID NO: 1.
In another preferred embodiment the polynucleotides of the present invention are in a vector and/or a host cell. Preferably, the polynucleotides are in a plant cell or transgenic plant. Preferably, the plant is Arabidopsis thaliania or selected from the group consisting of wheat, corn, peanut cotton, oat, and soybean plant. In a preferred embodiment, the polynucleotides are operably linked to a promoter, preferably an inducible promoter.
In another preferred embodiment the present invention provides, a process for screening for polynucleotides which encode a protein having molybdenum cofactor sulfurase activity comprising hybridizing the polynucleotide of the invention to the polynucleotide to be screened; expressing the polynucleotide to produce a protein; and detecting the presence or absence of molybdenum cofactor sulfurase activity in the protein.
In another preferred embodiment, the present invention provides a method for detecting a nucleic acid with at least 70% homology to nucleotide SEQ ID NO: 1, sequences which are complimentary to SEQ ID NO: 1 and/or which encode a protein having the amino acid sequence in SEQ ID NO: 2 comprising contacting a nucleic acid sample with a probe or primer comprising at least 15 consecutive nucleotides of the nucleotide sequence of SEQ ID NO: 1, or at least 15 consecutive nucleotides of the complement thereof.
In another preferred embodiment, the present invention provides a method for producing a nucleic acid with at least 70% homology to the polynucleotides of the present invention comprising contacting a nucleic acid sample with a primer comprising at least 15 consecutive nucleotides of the nucleotide sequence of SEQ ID NO: 1, or at least 15 consecutive nucleotides of the complement thereof.
In another preferred embodiment, the present invention provides a method for making molybdenum cofactor sulfurase protein, comprising culturing the host cell carrying the polynucleotides of the invention for a time and under conditions suitable for expression of molybdenum cofactor sulfurase, and collecting the molybdenum cofactor sulfurase protein.
In another preferred embodiment, the present invention provides a method of making a transgenic plant comprising introducing the polynucleotides of the invention into the plant.
In another preferred embodiment, the present invention provides method of increasing the salt tolerance of a plant in need thereof, comprising introducing the polynucleotides of the invention into said plant.
Methods, vectors, and compositions for transforming plants and plant cells in accordance with the invention are well-known to those skilled in the art, and are not particularly limited. For a descriptive example see Karimi et al., TRENDS in Plant Science, Vol. 7, No. 5, May 2002, pp. 193–195, incorporated herein by reference.
In another preferred embodiment, the present invention provides an isolated polypeptide comprising the amino acid sequence in SEQ ID NO: 2 or those proteins that are at least 70%, preferably 80%, preferably 90% and preferably 95% identity to SEQ ID NO: 2, where the polypeptides have molybdenum cofactor sulfurase activity. Thus, the enzyme has a homology of from 70% to less than 100% homology to SEQ ID NO: 2.
To understand low temperature and osmotic stress signaling in plants, we isolated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2, which are impaired in gene induction by cold and osmotic stresses. Expression of RD29A-LUC (firefly luciferase reporter gene under control of the stress responsive RD29A promoter) in response to cold and salt/drought is reduced in the los5 mutants but the response to ABA remains unaltered. RNA blot analysis indicates that the los5 mutation reduces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt or drought stress. los5 plants are ABA-deficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with another ABA deficient mutant aba1 reveals that the impaired low temperature gene regulation is specific to the los5 mutation. Genetic tests suggest that los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encodes a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the generation of sulfurylated form of MoCo, a cofactor required by aldehyde oxidase that functions in the last step of ABA biosynthesis in plants. The LOS5/ABA3 gene is expressed ubiquitously in different plant parts and the expression level increases in response to drought, salt, or ABA treatment. Our results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stress-responsive gene expression and stress tolerance.
Results
Isolation of Arabidopsis Mutants with Reduced RD29A-LUC Induction by Salt Stress
Seeds from Arabidopsis plants expressing the RD29A-LUC transgene (referred to as wild type) were mutagenized with ethyl methanesulfonate (EMS) and seedlings of the M 2 generation were screened for mutants with altered regulation of the transgene (Ishitani et al., 1997). One group of mutants was isolated that exhibit a clear reduction in NaCl-induced luminescence. Two allelic mutants, designated as los5 (los1 to los4 are mutants with specific defects in low temperature signaling, J.-K. Zhu, unpublished data), that show reduced luminescence induction in response to both NaCl and cold, were chosen for detailed characterization.
As shown in FIG. 1 , the luminescence intensities in los5-1 mutant seedlings were considerably lower than those in the wild type when treated with cold (0° C.) for 48 hr ( FIG. 1B ) or 300 mM NaCl for 5 hr ( FIG. 1F ). In contrast, the luminescence expression in response to ABA (100 μM for 5 hr) is not lower than that in the wild type ( FIG. 1D ). Without stress treatment there was virtually no luminescence expression in either the wild type or los5 mutant. Quantification of the luminescence intensities in FIG. 1 indicates that the levels of RD29A-LUC expression in los5-1 seedlings are only 8% and 2% of the wild-type levels for cold and NaCl treatments, respectively, whereas the expression levels under ABA treatment are virtually the same for the mutant and wild type ( FIG. 1G ).
To determine whether the los5 mutant has an altered threshold response to either cold or salt stress, different low temperatures or NaCl dosages were applied and the luminescence expression in los5 and wild type plants was quantified. Results show that los5 mutant plants exhibited a consistently lower luminescence expression under the temperatures tested ( FIG. 1H ), and increased NaCl concentrations also did not result in a recovery of the expression to the wild type levels ( FIG. 1I ). This indicates that the reduced responsiveness to cold or osmotic stress in los5 is not due to altered induction thresholds of these stresses.
los5-1 mutant plants were backcrossed to the wild type plants. Analysis of luminescence expression of the F 1 seedlings and the selfed F 2 population indicated that los5 is a recessive mutation in a nuclear gene (Table 1). Pairwise crosses with other mutants that also show reduced luminescence responses to NaCl treatment identified a second allele, los5-2 (Table 1). The los5-2 mutant has identical phenotypes as los5-1 (data not shown).
Reduced Gene Regulation by Salt, Drought, and Cold in los5 Mutants
To assess whether the los5 mutation has a similar effect on expression of the endogenous RD29A as it has on the RD29A-LUC transgene, RNA blot analysis was carried out with total RNA isolated from los5-1 and wild type seedlings that were not stress-treated (control), or treated with either cold (0° C.) for 24 hr, 100 μM ABA for 2 hr, 300 mM NaCl for 3 hr, or 30% PEG for 5 hr. The results show that whereas ABA induction of the endogenous RD29A was not substantially affected, NaCl induction was almost completely blocked by the los5 mutation ( FIG. 2 ). The mutation also clearly reduced RD29A expression in response to cold treatment. To distinguish whether the effect of NaCl is due to ionic or osmotic stress, polyethylene glycerol (PEG, average molecular weight 6,000) was used. The steady-state transcript level of RD29A was also greatly reduced in los5 in response to PEG treatment ( FIG. 2 ). This indicates that the los5 mutation reduces gene expression under osmotic stress.
The los5 mutation also has dramatic effects on the expression of other stress-responsive genes under osmotic or cold stress treatment. Several stress-responsive genes including COR15A (Lin and Thomashow, 1992), KIN1 (Kurkela and Franck, 1990), COR47 (Gilmour et al., 1992), RD22 (Yamaguchi-Shinozaki et al., 1992), P5CS were analyzed. The los5 mutation almost completely blocks the osmotic stress induction of COR15A, KIN1, and P5CS ( FIG. 2 ). It also substantially reduces the induction of RD22 and COR47 by NaCl and PEG. Interestingly, whereas ABA-induction of COR47 is enhanced by the los5 mutation, the induction of COR15A and P5CS by ABA is reduced by the mutation ( FIG. 2 ). As a control, the transcript level of an actin gene was determined and the result shows that its expression is not changed by stress treatments and not substantially different between the mutant and the wild type under the respective treatments ( FIG. 2 ).
los5 Mutant Plants are More Sensitive to Freezing Stress
The reduced expression of RD29A and other stress-responsive genes in los5 might have an impact on the stress tolerance of the mutant plants. To test the sensitivity of los5 mutant plants to low temperature, los5-1 and wild type plants were incubated at 4° C. for up to 4 weeks. No significant difference in growth was found between the mutant and wild type, indicating that LOS5 is not critical for chilling resistance. To test whether los5 mutant plants are defective in cold acclimation, wild type and los5-1 rosette plants growing in soil ( FIG. 3A ) was cold-acclimated (4° C. under light) for 48 hr. The plants were then treated at −7° C. for 5 hr. After incubation in the growth chamber for one day, clear difference was observed: whereas 97% of the wild type plants survived −7° C. freezing, all los5-1 mutant plants were killed ( FIG. 3B and data not shown). The result shows that los5 mutant plants have reduced freezing tolerance.
los5 Mutant Plants are More Sensitive to Drought and Salt Stress Damage
Steady-state RNA levels as shown in FIG. 2 have revealed a remarkable reduction in los5-1 mutant plants in the expression of stress-responsive genes in response to salt or drought (i.e., PEG) treatments. Except for the P5CS (Δ 1 -pyrroline-5-carboxylate synthase) gene, the function of most the genes examined is not clear. P5CS catalyzes the rate-limiting step in the biosynthesis of proline, a major osmolyte important for plant tolerance to freezing, drought, and salt stresses (e.g., Xin and Browse, 1998; Roosens et al., 1999; Hong et al., 2000). Proline contents in wild type and los5-1 plants treated with ABA or salt stress were measured. In the absence of stress treatment (i.e. control), los5-1 mutant plants had a slightly higher proline content than wild type plants. The proline content in both los5-1 and the wild-type plants increased in response to 150 mM NaCl treatment. However, the increase in los5-1 plants is less than that in wild-type plants. los5-1 mutant and the wild-type plants had similar proline contents when treated with 50 μM ABA ( FIG. 4A ).
To determine the drought sensitivity in los5 mutant plants, los5-1 and wild type seedlings were treated with 30% PEG and electrolyte leakage was measured as an indicator of drought-induced cellular damage. While los5-1 seedlings had higher electrolyte leakage than the wild type even without stress treatment, PEG treatment resulted in an electrolyte leakage in the mutant that is twice as much as in wild type plants ( FIG. 4B ), indicating that los5 mutant plants are more sensitive to drought stress.
Although los5-1 mutant seeds were more tolerant to NaCl stress at germination, mutant root growth did not differ from that of the wild type in their NaCl sensitivity (data not shown). Despite similar levels of inhibition of root growth by NaCl stress, los5-1 mutant plants did show increased sensitivity to NaCl in the shoot. At NaCl concentrations of 75 mM or higher, los5-1 mutant seedlings became yellowish and were killed by prolonged exposure to the stress, whereas wild type plants survived ( FIG. 4C , and data not shown).
LOS5 Mutant Plants are Deficient in Stress-induced ABA Accumulation
In addition to changes in stress sensitivity, los5 mutant plants are also altered in development. Under our long day growth conditions, los5-1 plants flowered approximately 5 days earlier than wild type plants. Besides having a more dark green color, los5-1 mutant leaves are narrower and slightly serrated in the edge, as compared to the more round leaves of wild type plants ( FIGS. 5A and 5B ). In fact, los5 mutant plants can be distinguished from the wild type based on these leaf characteristics. These visible phenotypes are shared by the los5-2 mutant plants, co-segregate with the los5 luminescence phenotypes, and are present in los5-1 plants that had been backcrossed with the wild type for 4 times.
When the aboveground parts are detached from the roots at the rosette stage ( FIG. 5C ), the younger leaves of los5-1 plants withered within 10 min under our room conditions (22±2° C., ˜30% relative humidity). In contrast, wild type leaves remained turgid under the same conditions ( FIG. 5D ). The inflorescence of adult los5 plants also readily became wilty when the plants were removed from growth chambers (22° C.±2° C., 90% relative humidity) to our room conditions, whereas the wild type inflorescence remained turgid ( FIG. 5E ). These observations suggest that los5 mutant plants may have a higher transpiration rate. Measurement of transpirational water loss shows that los5 mutant plants indeed lost water much faster than wild type plants ( FIG. 5F ), indicating potential defects in stomatal regulation, which is a typical phenotype of ABA deficient or insensitive mutants.
To determine whether los5 mutant plants are deficient in ABA or are insensitive to ABA, wild type and los5-1 plants at the rosette stage were sprayed with 100 μM ABA 3 hr before detaching the aerial parts for water loss measurement. FIG. 5F shows whereas the ABA treatment did not significantly affect transpirational water loss from wild type plants, the treatment reduced substantially the rate of water loss from los5-1 plants. This observation is consistent with ABA deficiency in los5-1 mutant plants and suggests that los5-1 is not ABA-insensitive.
ABA contents in los5-1 and wild type plants were measured by using an immunoassay. In the absence of stress treatment, ABA contents in the wild type and los5-1 leaves are essentially the same (Table 2). When detached leaves were allowed to lose 30% of their fresh weight, ABA contents in both the wild type and los5-1 mutant increased. However, the magnitudes of the increases are quite different. Whereas ABA content in the wild type increased by more than 300% in response to the water stress, the increase in los5 leaves was only 80%, with the wild type having nearly 250% as much ABA as in the mutant (Table 2).
These data clearly show that los5-1 is an ABA deficient mutant. To test whether los5 is allelic to known ABA deficient mutants, los5-1 was initially crossed with aba1 and aba2. Analysis of the F 1 plants indicated that los5-1 is not allelic to either one (Table 1 and data not shown). When the aba3-1 mutant became available later, we crossed it to los5-1 and analyzed the phenotypes of F 1 and F 2 seedlings by luciferase imaging and water loss measurement. Luminescence images showed that the resulting F 1 seedlings had a low expression of luminescence when treated with 300 mM NaCl, indicating los5 is likely allelic to aba3. However, measurement of transpirational water loss in the F 1 seedlings was less conclusive in part because when crossed to the wild type RD29A-LUC plants (C24 background), the aba3/ABA3 heterozygous plants showed an incomplete recessive phenotype, i.e., the F 1 plants lost water slower than aba3 but faster than the RD29A-LUC wild type. Although the F 1 plants (los5/aba3) from a cross between los5-1 and aba3-1 lost water faster than the F 1 plants from a cross between RD29A-LUC wild type and aba3-1, the water loss rate of los5-1/aba3-1 heterozygous plants was still lower than either los5-1 or aba3-1 mutant (data not shown). The intermediate phenotypes probably have to do with the genetic differences between the two ecotypes (C24 vs. Columbia).
LOS5 Regulates Cold- and Osmotic Stress-responsive Gene Expression Through Distinct Mechanisms
Gene expression analysis in los5-1 mutant plants suggests a critical role of LOS5 in the regulation of stress-responsive genes by salt and drought and to a less extent, by cold ( FIG. 1 and FIG. 2 ). In our mutant screen, a second genetic locus, LOS6, was defined by los6-1 mutant plants that show reduced gene induction by salt/drought treatments (J.-K. Zhu, unpublished observation). Interestingly, genetic analysis showed that los6 is allelic to aba1. This provides an excellent opportunity to study the role of ABA in cold and osmotic stress regulation of gene expression by using the very sensitive and reliable RD29A-LUC reporter and comparing two different ABA deficient mutants, los5 and los6. RD29A-LUC induction by salt stress is significantly reduced in los5 and los6 ( FIG. 6A ). Interestingly, when ABA was simultaneously administered with salt stress, RD29A-LUC expression in both los5 and los6/aba1 were restored to the wild type level or higher, indicating that exogenous ABA complements the reduced salt induction phenotype. This suggests that reduced gene induction by salt and drought in both los5-1 and los6/aba1 mutants can be solely accounted for by ABA deficiency.
Gene induction by cold was also analyzed using these two mutants. Without stress treatment, los6, like los5 and the wild type, did not show any luminescence expression (data not shown). To our surprise, whereas cold induction of RD29A-LUC in los5-1 is significantly reduced, the expression in los6/aba1 is significantly increased ( FIG. 6B ). This increased luminescence expression in los6 was consistently observed in numerous independent experiments. To test the role of ABA in cold gene regulation, ABA was administered together with cold treatment. Measurement of RD29A-LUC expression shows that ABA treatment restored the luminescence expression in los6/aba1 close to the wild type level ( FIG. 6B ), indicating that the elevated cold induction in los6/aba1 may be an indirect consequence of ABA deficiency. In contrast, application of ABA to los5 mutant seedlings failed to restore RD29A-LUC expression to the wild type level. In fact, los5 plants did not appear to respond to this ABA treatment as compared to cold treatment alone ( FIG. 6B ), suggesting that the reduced gene induction by cold in los5 mutants is not a result of ABA deficiency.
Map-based Cloning of the LOS5 Gene
To clone the LOS5 gene, homozygous los5-1 mutant plants in the C24 ecotype were crossed with wild type plants in the Columbia ecotype. The resulting F 1 plants were allowed to self. Initially, the F 2 seeds were plated on MS agar plates and seedlings were analyzed for luminescence expression. Putative mutants were selected by their reduced luminescence expression under cold treatment as well as salt treatment. Selected seedlings were then transferred to soil and adult plants were tested for the wilty phenotype under reduced humidity conditions. Simple sequence length polymorphism (SSLP) markers distributed throughout the five Arabidopsis chromosomes that exhibit a size polymorphism between C24 and Columbia ecotypes were used for genetic mapping. The genetic mapping placed the LOS5 locus to the upper arm of chromosome I, between the SSLP markers AtEAT1 and nga248 ( FIG. 7A ). This region corresponds roughly to where the ABA3 locus was mapped (Léon-Kloosterzie et al., 1996). As most of the mutant seedlings selected after the NaCl treatment failed to survive, we later used the salt tolerance phenotype of los5 mutant seeds during germination to select mutants for mapping. Specifically, the segregating F 2 seeds were plated on agar medium supplemented with 200 mM NaCl, a salt concentration that inhibits the germination of wild type but not los5 mutant seeds. Putative los5 seedlings were then transferred to soil and later confirmed by examination of their wilty phenotype.
While the fine mapping of LOS5 was in progress, genomic DNA sequence corresponding to the LOS5 region was released. Given the ABA deficiency phenotype, the availability of genomic sequence makes it possible to select candidate genes to find the los5 mutation. A detailed examination of the genes on BAC clones in this region identified several candidates that could potentially function in ABA biosynthesis. Among them, the F19K19.13 gene on the BAC clone F19K19 appears to be a good candidate ( FIG. 7A ). BLAST searches suggest that the predicated gene product has high similarity to molybdenum cofactor sulfurase from other organisms. Previous studies have indicated that the genetic lesion in aba3 is in the introduction of S into the molybdenum cofactor (MoCo) (Schwartz et al., 1997a) and sulfurylated MoCo is required by aldehyde oxidase (Schwartz et al., 1997a; Akaba et al., 1998; Sagi et al., 1999) which functions in the last step of ABA biosynthesis (Schwartz et al., 1997a).
Genomic DNA corresponding to F19K19.13 was amplified by PCR from wild type and los5-1 mutant plants and sequenced. Comparison between the sequences revealed a G to A change 1083 bp downstream of the predicted translation initiation codon in los5-1. The F19K19.13 gene was then amplified from los5-2 and sequenced. Sequence analysis identified a G to A change in los5-2, 1040 bp downstream of the predicted translation initiation codon. These results strongly suggest that the F19K19.13 gene is LOS5.
The F19K19.13 gene product is predicted to function in ABA biosynthesis at a step corresponding to ABA3. Our genetic analysis also suggested that los5-1 is likely allelic to aba3-1 (Table 2). Taken together, these results strongly suggested that F19K19.13 is the ABA3/LOS5 gene. To test this hypothesis, we sequenced the F19K19.13 gene from aba3-1 and aba3-2 alleles, and the DNA sequences were compared with those from Columbia (aba3-1 background) and Landsberg (aba3-2 background) wild type plants, respectively. Results showed that in aba3-1, a G to A change occurred at position 3707 while in the aba3-2 allele, there are three mutations in a row with a single non-mutated nucleotide spacing them: a G to A change at position 3176, a T to A change at position 3178, and a deletion of T at position 3180. The nature of the mutations is consistent with the type of mutagens used: aba3-1 was caused by EMS, while aba3-2 was caused by γ-ray irradiation (Léon-Kloosterzie et al., 1996).
All the changes in F19K19.13 DNA sequence in the four los5/aba3 mutant alleles are predicted to cause changes in the predicted open reading frame. Together, the above data unequivocally demonstrate that the LOS5 locus is identical to ABA3, and the LOS5/ABA3 gene is F19K19.13. After we have identified the los5 mutation, a third mutant allele of ABA3, frs1/aba3-3, was recently reported (Llorente et al., 2000). The freezing sensitive phenotype and the degree of ABA deficiency in frs1/aba3-3 (Llorente et al., 2000) are very similar to those in los5. For consistency, we propose to rename los5-1 as aba3-4, and los5-2 as aba3-5.
LOS5/ABA3 Encodes a Molybdopterin Cofactor Sulfurase
To obtain the cDNA sequence of LOS5, reverse transcriptase-PCR was carried out with mRNA extracted from wild type Columbia plants. The RT-PCR product was cloned and sequenced (accession number AY034895). After the LOS5 cDNA was cloned, an identical sequence (accession number AF325457, submitted by Bittner F. and Mendel R. R.) was released in the Genebank. Comparison with genomic DNA sequence revealed that the LOS5 gene consists of 21 exons and 20 introns ( FIG. 7B ). The open reading frame consists of 2457 nucleotides and is predicted to encode a protein of 819 amino acids with an estimated molecular weight of 91.8 kDa. The nucleic acid and amino acid sequences are shown in FIG. 8 (SEQ ID NO: 1 and 2).
The los5-1 mutation occurred at the 4 th exon and changes a tryptophan residue at amino acid position 120 to a stop codon and thereby truncates the protein. The los5-2 mutation also occurred at the 4th exon and changes a small glycine residue at position 106 to a larger, negatively charged glutamic acid residue. The aba3-1 mutation occurred at the 13 th exon and changes a glycine at position 469 to a glutamic acid residue. The aba3-2 mutations occurred at the junction between 10 th and 11 th exons. The aba3-2 mutation changes the leucine residue at position 387 to a stop codon and thereby truncates the protein from the 11 th exon ( FIGS. 7B and 8 ).
Database searches showed that LOS5/ABA3 has high sequence homology to the molybdopterin cofactor sulfurase (MCSU) recently identified from cattle (Watanabe et al., 2000), which belongs to a highly conserved protein family found from bacteria to human ( FIG. 9 ). Overall, the LOS5 protein has 35% amino acid sequence identity and 53% similarity to the human homolog, 34% identity and 49% similarity to the Mal protein of Drosophila melanogaster, 35% identity and 51% similarity to MCSU of cattle (Watanabe et al., 2000), and 31% identity and 48% similarity to the HxB protein of Aspergillus nidulans (Amrani et al., 1999). The entire sequence of LOS5/ABA3 appears to consist of three domains. The N-terminal domain shows high sequence homology to the Class V pyridoxal 5′-phosphate-dependent aminotransferases of type I fold, represented by the isopenicillin N epimerase, phosphoserine aminotransferase, aspartate decarboxylase, the small subunit of cyanobacterial soluble hydrogenase and the NifS proteins from Azotobacter vinelandii. Recently, the structures of several NifS-like proteins were solved (Fujii et al., 2000; Kaiser et al., 2000), thus making it possible to identify conserved motifs in the NifS-like domain of LOS5/ABA3 ( FIG. 7C ). This includes a putative pyridoxal phosphate binding motif and a conserved cysteine motif ( FIG. 7C ). The respective key residues in these motifs are marked in FIG. 10 . It is noteworthy that several putative proteins in the Arabidopsis genome show significant sequence similarity to this NifS-like domain (data not shown). The second domain, the junction that connects the NifS-like domain to the C-terminal domain shows little sequence homology to other proteins except among members of this MCSU family. The C-terminal domain shows significant sequence similarity with a group of unknown proteins found both in Arabidopsis and in other organisms. However, the Arabidopsis genome does not contain any other protein with significant overall sequence homology to the full-length LOS5/ABA3 protein, implying that LOS5/ABA3 is a single copy gene in the genome.
LOS5/ABA3 is Ubiquitously Expressed and its Expression is Enhanced by ABA and Drought Stress
To analyze the expression pattern of the LOS5/ABA3 gene, full-length LOS5 cDNA was used as a probe in RNA blot analysis, using total RNA extracted from different parts of unstressed wild type plants. The result indicates that LO5 is constitutively expressed at a relatively low level in all plant parts examined, including roots, stems, leaves, flowers and siliques ( FIG. 10A ). Interestingly, the transcript level of LOS5 increases significantly in response to drought ( FIG. 10B ), ABA, NaCl or PEG treatments ( FIG. 10C ). Cold treatment has no significant effect on LOS5 expression ( FIGS. 10C and 10D ). The steady-state LOS5 transcript levels in los5-1 seedlings are considerably lower than those in the wild type under the treatment conditions ( FIG. 10D ), suggesting that the mutant transcript with a pre-mature stop codon may trigger RNA surveillance mechanisms that remove abnormal transcripts (Hilleren and Parker, 1999).
The expression of LOS5 was also analyzed in another ABA deficient mutant, aba1-1 (Koornneef et al., 1982), and in an ABA-insensitive mutant, abi1-1 (Koornneef et al., 1984). The results indicate that the induction of LOS5 by osmotic stress in aba1 is not substantially different from that in the wild type. In the abi1-1 mutant, the induction of LOS5 appears slightly lower compared to that in the wild type ( FIG. 10D ).
Comparison between the Effects of los5/aba3, aba1 and abi1 Mutations on Stress Gene Regulation
In contrast to los5-1, the aba1-1 mutation does not decrease the induction of the RD29A transcript by cold or ABA ( FIG. 10D ). In fact, cold induction of RD29A appears higher in aba1-1 compared to that in the wild type. The aba1-1 mutation does decrease the induction of RD29A by NaCl or PEG stress, although the effect is not as dramatic as that of los5-1. The different effects on RD29A transcript induction by los5-1 and aba1-1 are in general agreement with the findings on the effects of los5-1 and los6-1 (a different aba1 allele) on RD29A-LUC expression ( FIG. 6 ). aba1-1 enhances ADH induction by cold and ABA but decreases the induction by NaCl or PEG stress ( FIG. 10D ). This is again in sharp contrast with the los5-1 mutation, which decreases the ADH induction by either cold, ABA, NaCl or PEG ( FIG. 10D ). Both aba1-1 and los5-1 nearly completely blocks RAB18 induction by osmotic stresses ( FIG. 10D ). However, los5-1 but not aba1-1, also blocks RAB18 induction by ABA ( FIG. 10D ). Compared with aba1-1 or los5-1, the abi1-1 mutation has little effect on RD29A or ADH induction by the stresses ( FIG. 10D ). Nevertheless, the induction of RAB18 by ABA, NaCl or PEG is reduced in the abi1-1 mutant ( FIG. 10D ).
Because CBF (Stockinger et al., 1997) and DREB2 (Liu et al., 1998) family of transcription factors are known to bind to the DRE element present in the promoters of RD29A and several other stress responsive genes, we were interested in determining whether stress induction of these transcription factors is affected by the ABA deficient or insensitive mutations. The results show that the expression of the cold-specific CBF2/DREB1C is not significantly affected by los5-1/aba3-4 or aba3-1, but is enhanced in the aba2-1 mutant ( FIG. 10E ). It was reported that DREB2A is induced specifically by osmotic stress (Liu et al., 1998). Under our treatment conditions, DREB2A expression is also up-regulated by cold stress. Interestingly, whereas none of the ABA-deficient mutations significantly affects osmotic stress-induction of DREB2A, los5 shows increased cold-induction of DREB2A compared to the wild type ( FIG. 10E ).
Discussion
The phytohormone ABA plays many significant roles in plant growth and development, and in plant responses to environmental stresses. Understanding ABA biosynthesis pathways in plants is thus of critical importance. ABA biosynthesis mutants serve as excellent tools to for understanding ABA biosynthesis and for studying gene regulation in response to stressful environments. In Arabidopsis and other plants such as maize, tobacco, and tomato, genetic analysis based on ABA promotion of seed dormancy has yielded a series of mutants that are defective in ABA biosynthesis (for recent review, see Koornneef et al., 1998; Cutler and Krochko, 1999; Loitenberg et al., 1999). Characterization of these mutants along with biochemical studies has revealed that in plants ABA is synthesized from an ‘indirect’ pathway via the cleavage of a carotenoid precursor. The Arabidopsis aba1 mutant (and tobacco aba2) is defective in the epoxidation of zeaxanthin and antheraxanthin to violaxanthin (Rock and Zeevaart, 1991) and the affected gene encodes a zeaxanthin epoxidase (Marin et al., 1996). Oxidative cleavage of the 9-cis-neoxanthin by the VP14 protein yields xanthoxin. The VP14 gene was isolated by using the maize vp14 mutant (Tan et al., 1997) and encodes a 9-cis-epoxycarotenoid dioxygenase (NCED) (Schwartz et al., 1997b). It is thought that xanthoxin is converted to ABA by a two-step reaction via AB-aldehyde. The Arabidopsis aba2 mutant is impaired in the first step of this reaction, thus unable to convert xanthoxin into AB-aldehyde. The Arabidopsis aba3 mutant is defective in the last step of ABA biosynthesis, i.e. the conversion of AB-aldehyde to ABA (Schwartz et al., 1997a), which is catalyzed by AB-aldehyde oxidase. Mutations in either the aldehyde oxidase apoprotein (e.g., Seo et al., 2000b) or molybdenum cofactor (MoCo) synthase impair ABA biosynthesis and lead to ABA deficiency in plants.
Molybdenum cofactor consists of a single Mo atom coordinated to the sulfur atoms of an organic moiety, molybdopterin. MoCo is highly conserved in cellular organisms and is used for the transfer of an oxygen atom in redox reactions involved in the metabolism of nitrogen, sulfur, and carbon (for review, see Kisher et al., 1997). Defects in MoCo biosynthesis have been reported to be associated with many diseases in humans and other animals (for review, see Reiss, 2000). In plants, three groups of MoCo-containing enzymes have been described (for review, see Mendel and Schwarz, 1999): nitrate reductase (NR), xanthine dehydrogenase (XDH) and aldehyde oxidase (AO). A fourth group, sulfite oxidase, also exists in the completely sequenced Arabidopsis genome. Different from nitrate reductase and sulfite oxidase that use dioxyo form of MoCo, however, both xanthine dehydrogenase and aldehyde oxidase require that the MoCo be modified in the last step of biosynthesis with the insertion of a sulfur atom to replace one of the two terminal oxygen atoms ( FIG. 11 ). This sulfuration reaction is catalyzed by molybdopterin cofactor sulfurase (MCSU). Mutants defective in this step have been identified from Drosophila melanogaster (maroon-like, mal) (Wahl et al., 1982), Aspergillus nidulans (hxB) (Scazzocchio, 1973; Amrani et al., 1999) and cattle (Watanabe et al., 2000). Plant mutants defective in this sulfuration step have also been identified, i.e., tobacco aba1 (Leydecker et al., 1995), tomato flacca (Marin and Marion-Poll, 1997), and Arabidopsis aba3 (Schwartz et al., 1997a). Biochemical characterizations suggest that these plant mutants are all defective in the last step of ABA biosynthesis. As expected, the defects are specific to AO and XDH but not to NR (Leydecker et al., 1995; Marin and Marion-Poll, 1997; Schwartz et al., 1997a). Like in the Drosophila mal mutant, resulfuration of the plant mutant extracts with Na 2 S restores XDH and AO activities in Arabidopsis aba3 (Schwartz et al., 1997a), tobacco aba1 (Akaba et al., 1998), and tomato flacca (Sagi et al., 1999). These previous studies suggest that the wild type genes corresponding to the respective mutations likely encode molybdenum cofactor sulfurases that function in the last step of MoCo modification, which is specifically required by aldehyde oxidases and xanthine dehydrogenases for their catalytic activities.
The identification of los5 mutants as impaired in ABA biosynthesis and the cloning of the LOS5/ABA3 gene demonstrate that indeed LOS5/ABA3 encodes a putative molybdenum cofactor sulfurase (MCSU). As free molybdenum cofactor is extremely unstable, to date there has been no report on sulfuration enzyme activity of MCSU using either MoCo or other substrates. Nonetheless, abundant evidence both from plant and animal studies that resulfuration of mutant extracts restores AO and XDH or other enzyme activities strongly suggests that MCSU have in vivo sulfuration activity against the desulfo form of MoCo. Additionally, sequence comparisons with other proteins also support the potential catalytic property of MCSU.
Like other MCSU proteins, the LOS5/ABA3 gene product has extensive sequence similarity to the NifS-like protein in its N-terminal region ( FIG. 7C ). The NifS protein of Azotobacter vinelandii is required for the activity of nitrogenase, the only Mo-containing enzyme that does not use molybdopterin cofactor but instead requires an iron-molybdenum-sulfur cluster for electron transfer. Although the exact function of NifS protein in nitrogen fixation is not known, NifS in vitro was shown to be able to use L-cysteine as a substrate to form alanine and sulfide. Thus, NifS appears to act as a cysteine desulfurase in the biogenesis of the metallocluster by mobilizing an inorganic sulfide originated from the substrate L-cysteine (Zheng et al., 1993). This reaction is quite similar to the chemical reactions expected to be catalyzed by LOS5/ABA3 in the sulfuration of the desulfo-MoCo. In fact, both the pyridoxal phosphate binding motif and the conserved cysteine motif required for NifS are all well conserved in LOS5/ABA3 and its homologs in other organisms ( FIGS. 7C and 8 ). Both by analogy with members of the class V aminotransferase-like proteins and by experimental analysis (Zheng et al., 1993), pyridoxal phosphate (PLP) was found to be a cofactor for NifS protein. Given the high similarity with NifS and other PLP-dependent proteins in this class, it is likely that PLP is also a cofactor for LOS5/ABA3. By comparing LOS5/ABA3 with other NifS proteins whose structures were recently solved (Fujii et al., 2000; Kaiser et al., 2000), the conserved lysine at position 271 ( FIG. 9 ) is probably the residue where PLP is covalently attached to make an Schiff base (Fujii et al., 2000). Likewise, the conserved cysteine residue at position 430 ( FIG. 10 ) is a likely S donor for the trans-sulfuration reaction ( FIG. 11 ). Both conserved motifs are found in all LOS5/ABA3 homologs ( FIG. 10 ). It is expected that both the PLP and the Cysteine motifs are required for the catalytic activity of LOS5/ABA3. Interestingly, the aba3-1 mutation occurs just outside of the NifS-like domain, but is still in a highly conserved region, suggesting that this region is also required for the enzyme function.
The overall structure of LOS5/ABA3 is reminiscent of a chimeric protein that has evolved by fusing two separate proteins. The C-terminal domain does not have a known function yet, but is likely also important, considering the high similarity in this domain among the MCSU proteins from diverse organisms, and its high similarity to several unknown proteins in the Arabidopsis genome.
One interesting note related to the apparently multiple functional domain structure is that both the los5-1 and los5-2 mutants have a unique leaf morphology ( FIG. 5B ), whereas in aba3-1 and aba3-2, there is no such clear alteration in leaf form (data not shown). It is worth noting that both the los5-1 and los5-2 mutations occurred at the N-terminal part, whereas the aba3-1 and aba3-2 mutations occurred at the middle of the protein. It is therefore tempting to speculate that the N-terminal domain in the aba3-1 and aba3-2 mutant proteins may still have some activity that is required for maintenance of the wild type leaf morphology. On the other hand, the los5-1 and los5-2 mutant forms may have lost this activity, resulting in an altered leaf form.
Since LOS5/ABA3 is a single copy gene in the Arabidopsis genome, it is not surprising that it is ubiquitously expressed ( FIG. 10A ). This is in contrast with the aldehyde oxidase (AAO) gene family where each member has a different expression pattern (Seo et al., 2000b). However, it is interesting to note that the tomato flacca mutant was reported to lose AO and XDH activities in the shoot but retain measurable activities in the roots where a notable amount of ABA accumulates (Sagi et al., 1999). This raises the possibility that there may exist more than one LOS5/ABA3-like MCSU genes in tomato, and a root-specific isoform(s) may remain active in the flacca mutant. In addition to impaired ABA biosynthesis in flacca mutant shoot, it was shown that the mutation might reduce the transport of ABA from the root to shoot as well (Sagi et al., 1999).
In the present study, we showed that the expression of the LOS5/ABA3 gene is up-regulated when plants are treated with drought, salt or ABA ( FIG. 10 ). We note that the promoter region of LOS5/ABA3 contains putative ABREs (e.g. ACGTGG at −253 upstream from translation initiation codon) and DRE/CRT-like elements, suggesting that the LOS5/ABA3 gene may be regulated by ABA and drought/salt stress in a way similar to other stress-responsive genes. In the ABA biosynthesis pathway, it is generally thought that the rate-limiting step is in the oxidative cleavage of the 9-cis-neoxanthin catalyzed by the VP14 protein (Tan et al., 1997; Schwartz et al., 1997b; Loitenberg et al., 1999; Iuchi et al., 2000; Thompson et al., 2000; Taylor et al., 2000). Given the relative low abundance of LOS5/ABA3 transcript and the fact it is the only gene encoding MCSU in Arabidopsis, it is likely that LOS5/ABA3 may also regulate ABA biosynthesis. Low temperature appears to have little effect on the expression of LOS5/ABA3 ( FIGS. 9C and 9D ), consistent with its limited effect on endogenous ABA biosynthesis (Thomashow, 1999). Qin and Zeevaart (1999) also found that low temperature did not induce the expression of PvNECD1 (a VP14 homolog in bean). Drought treatments (20% fresh weight loss and incubated for 3 or 6 hr) significantly increased the expression of LOS5/ABA3 ( FIG. 10B ), yet the same treatments failed to up-regulate AAO3 gene expression (data not shown). This lack of AAO3 induction differs from the observation by Seo et al (2000a) that dehydration (i.e., in air flow hood for 3 hr) significantly induced AAO3 expression in Arabidopsis shoot. The reason for this discrepancy is probably because our stress condition (dehydration in still air for approximately 40 min to lose 20% fresh weight, followed by incubation at 100% humidity for 3 or 6 hr) was not as severe. Our result implies that LOS5/ABA3 may be the key regulator in this last step of ABA biosynthesis. Consistent with this speculation, Sagi et al. (1999) found that sulfuration with Na 2 S ‘superinduced’ the activity of AO and XDH in wild type tomato crude extracts, suggesting that MoCo sulfuration limits AB-aldehyde oxidase activity. ABA up-regulation of LOS5/ABA3 expression ( FIGS. 9C and 9D ) is very intriguing and may suggest a positive feedback regulation of ABA biosynthesis by ABA.
The availability of plant mutants defective in ABA biosynthesis has provided an excellent opportunity to study gene regulation by ABA under various abiotic stress conditions. In doing so, most researchers have used aba1 or abi1 and abi2 along with the respective wild types. The Arabidopsis ABA deficient mutant aba1 was the first mutant defective in ABA biosynthesis isolated in Arabidopsis (Koornneef et al., 1982). Recently, additional ABA deficient Arabidopsis mutants, aba2 and aba3, became available (Léon-Kloosterziel et al., 1996). However, stress gene regulation in these recently isolated mutants has not been reported.
Extensive studies with aba1 or abi1/2 mutants have yielded considerable, yet, sometimes conflicting information. For example, whereas Savoure et al. (1997) reported that the expression of P5CS genes is independent of ABA since they observed that the expression level is similar in wild type and in aba1 under cold or drought treatments. They suggested that ABA might affect proline biosynthesis posttranscriptionally (Savoure et al., 1997). On the other hand, Yoshiba et al. (1999) reported that AtP5CS1 induction by drought and salt stress is regulated both by ABA-dependent and ABA-independent pathways. Furthermore, Strizhov et al. (1997) found that stress-induced P5CS1 gene expression absolutely requires ABA, which is consistent with our findings here ( FIG. 2 ). To help clear the confusion, several reviews have been published and some consensuses have been reached (Leung and Giraudat, 1998; Shinozaki and Yamaguichi-Shinozaki, 1997; Thomashow, 1999; Rock, 2000). Although low temperature treatment can trigger a transient increase in ABA and application of ABA can induce the expression of cold-responsive genes at warm temperatures and increases plant freezing tolerance, a general consensus is that ABA does not have an important role in regulating the expression of the DRE/CRT class of genes (Thomashow, 1999). In the present study, we found that los5-1 seedlings show a dramatic reduction in the expression of the RD29A-LUC transgene under low temperature treatment ( FIGS. 1B and 1G ). RNA blot analysis showed that the induction of COR15, KIN1, COR47, RD29A, RAB18, and ADH by low temperature is also significantly reduced in los5-1 mutant plants ( FIGS. 2 and 9D ). The reduction in cold-regulated gene expression as seen in los5-1 was, however, not observed in aba1-1 ( FIG. 10D ). In fact, cold-induction of both ADH and RD29A was enhanced in aba1-1 ( FIG. 10D ). Similarly, cold induction of RD29A-LUC expression was enhanced in los6/aba1 mutant seedlings ( FIG. 6B ). Our results with los6/aba1 are consistent with previous studies on cold regulated gene expression carried out with aba1-1 (reviewed in Thomashow, 1999). The different effects of los5 and aba1 mutations raise the question whether the significant role played in cold-regulated gene expression by LOS5/ABA3 is a result of ABA-deficiency. This was partially addressed by our experiment as shown in FIG. 6B : whereas treatment with ABA complemented los6/aba1 defect in cold-regulated RD29A-LUC expression, the same treatment failed to rescue los5/aba3 ( FIG. 6B ). This suggests that in addition to its role in ABA biosynthesis, LOS5/ABA3 may have additional roles in cold regulation. Consistent with this notion is the finding that although exogenous ABA achieves similar or slightly higher expression of COR47, RD22, RD29A, and ADH ( FIGS. 2 , 6 , and 9 D) in los5 relative to that in the wild type, ABA fails to induce the expression of COR15 and P5CS, and has a reduced induction of KIN1 ( FIG. 2 ). These results strongly suggest that cold signaling requires a function of LOS5/ABA3 that is not directly related to ABA biosynthesis. Presently, it is unclear how LOS5/ABA3 is involved in cold or ABA regulation of some genes.
In contrast to low temperature, drought stress can dramatically stimulate de novo ABA biosynthesis and thus ABA is more closely involved in drought/salt stress responses (Ingram and Bartels, 1996; Bray, 1993). Gene regulation by drought has been thought to involve both ABA-dependent and ABA-independent pathways (Shinozaki and Yamaguchi-Shinozaki, 1997). Because genes such as RD29A, KIN1, and COR47 have both ABRE complex and the DRE/CRT elements, presumably, they can be activated by abiotic stress alone in the absence of ABA. Analysis using aba1 or abi mutants indeed showed that this is likely the case (Thomashow, 1999; Shinozaki and Yamaguchi-Shinozaki, 1997). However, our results with los5 mutants clearly present a quite different picture. Under osmotic stress, the expression of RD29A-LUC is almost abolished in los5 ( FIGS. 1F and 1G ). RNA blot analyses also found that the los5 mutation virtually blocks the induction of COR15, KIN1, RD22, P5CS, and RAB18 by salt and drought (PEG) stress ( FIGS. 2 and 9D ), and it severely impairs the induction of RD29A, COR47, and ADH by the stresses ( FIGS. 2 and 9D ). To ascertain whether the effect of los5 on osmotic stress-regulated gene expression is accounted for by ABA deficiency, we applied ABA together with salt stress and analyzed RD29A-LUC expression. The result indicates that ABA restores the RD29A-LUC expression to the wild type levels both in los5-1 and in los6/aba1 ( FIG. 6A ). Our previous RNA blot analysis with the wild type plants showed that under this treatment condition, the RD29A-LUC luminescence expression faithfully mirrors the pattern of endogenous RD29A expression (Xiong et al., 1999b). These results suggest that the observed defects in salt/drought-regulated gene expression in los5 mutant plants are most likely a consequence of ABA deficiency. Currently, an alternative possibility that LOS5 may have yet unknown roles that are not related to ABA biosynthesis in regulating ‘ABA-independent’ osmotic stress signaling cannot be completely ruled out. This is because the reduced magnitude of gene induction by drought or salt in aba1/los6 was not as dramatic as in los5 ( FIG. 6A ), although these mutants show a similar extent of ABA deficiency by bulk quantitative measurement.
The los5 mutation seems to have little effect on salt stress-regulated DREB2A expression ( FIG. 10E ). This raises the possibility that DREB2A function may require ABA-dependent factor(s) to activate downstream gene expression. Previously, our genetic analysis using RD29A-LUC as a molecular marker has shown that ABA-dependent and ABA-independent signaling pathways may not function independently of each other. Rather, there exists extensive connections or ‘crosstalk’ between them (Ishitani et al., 1997; Xiong et al., 1999a). The present molecular characterization of los5 mutants further casts doubt on the ABA-independency of ‘ABA-independent’ stress signal transduction pathways, at least as far as the DRE/CRT genes are concerned. Furthermore, it has been shown that ectopic expression of DREB2A alone does not activate downstream gene expression (Liu et al., 1998). As has been suggested by Liu et al. (1998), DREB2A activity in activating stress-responsive genes may require posttranscriptional modifications. Thus, it is possible that phosphorylation/dephosphorylation of DREB2A or the functions of its co-factors may be dependent on ABA-regulated molecules such as ABI1, ABI2, CDPKs, or numerous other ABA-responsive regulatory factors (e.g., Leung et al., 1997; Leung and Giraudat, 1998; Finkelstein and Lynch, 2000; Rock, 2000; Merlot et al., 2001). This interdependency of ABA and stress signaling may underlie the mechanisms for synergistic effect of ABA and drought/salt stress on the regulation of stress-responsive genes as observed in the present study ( FIG. 6A ) and elsewhere (Bostock and Quatrano, 1992; Xiong et al., 1999b).
EXAMPLES
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.
Example 1
Characterization of the LOS5/ABA3 Locus
Isolation of los5 Mutants
Arabidopsis thaliana (C24 ecotype) expressing the RD29A-LUC transgene (referred to as the wild type) were obtained by Agrobacterium -mediated transformation (Ishitani et al., 1997). Seeds of wild type RD29A-LUC were mutagenized by ethyl methanesulfonate (Ishitani et al., 1997). M 2 were planted on 0.6% agar plates containing full strength Murashige and Skoog salt base (MS salt base; JRH Biosceicnes, Lenex, Kans.) and were germinated and grown at 22±2° C. under continuous white light. One-week-old seedlings were screened for mutants with altered RD29A-LUC (i.e. luminescence) expression in response to low temperature, ABA, or osmotic stress using a thermoelectrically cooled CCD camera. For luminescence imaging, plants were sprayed with 1 mM luciferin in 0.01% Triton X-100 and then kept in the dark for 5 min before imaging. All images were acquired with a 5 min exposure time. The luminescence intensity of each seedling was quantified with the WinView software.
Stress and ABA Treatment
For ABA treatment, 100 μM (±)-cis, trans-abscisic acid in H 2 O was sprayed uniformly on leaves of seedlings and the plants were incubated at room temperature under cool-white light for 4 hr (for luminescence imaging) or 3 hr (for RNA analysis). For NaCl or PEG treatment, seedlings on MS plates were transferred to filter paper saturated with MS solution supplemented with 300 mM NaCl or 30% of polyethylene glycol (molecular weight 6,000) and incubated for 5 hr. Unless otherwise stated, cold treatment for image analysis and RNA analysis were done by incubating seedlings growing in plates at 0° C. in the dark for 48 hr (for imaging) or 12 hr (for RNA analysis). Because longer treatment at −5 or −10° C. will result in freezing of the agar media, these freezing temperature treatments only lasted for 3 hr. After the treatment, the plates were placed at room temperature for 2 hr to thaw before luminescence images acquisition. For ABA plus NaCl treatment, seedlings were transferred onto a filter paper saturated with 300 mM NaCl in MS solution and sprayed immediately with 100 μM ABA. The plants were then incubated under cool-white light for 4 hr before luciferase imaging. For low temperature plus ABA treatment, seedlings on agar plates were briefly incubated at 0±1° C. for 10 min and then sprayed with 100 μM ABA and incubated in the dark at 0±1° C. for 48 hr before image analysis.
Genetic Analysis of los5 Mutants and Map-based Cloning of LOS5
For genetic analysis, the los5 mutants were crossed to the wild type and other mutants we isolated with similar luminescence phenotypes. The F 1 and F 2 seedlings were subjected to luminescence analysis and scored for los5 luminescence phenotypes. los5-1 was also crossed with aba1, aba2, and aba3 mutants (obtained from Arabidopsis Biological Resource Center, Columbus, Ohio). Part of the resulting F 1 and F 2 seedlings was treated with 300 mM NaCl for luminescence image analysis and part was directly planted in soil for scoring wilty phenotypes under reduced humidity. For genetic mapping of the los5 mutation, los5-1 mutant in the C24 ecotype was crossed with wild type plants of the Columbia ecotype. The resulting F 1 plants were allowed to self and homozygous los5-1 mutants in the segregating F 2 population were selected based on their reduced luminescence when cold-treated and a control treatment with ABA were used to rule out those that did not have the RD29A-LUC transgene. A part of the seedlings were also tested for their reduced luminescence under NaCl treatment. Mapping of LOS5 was carried out as described previously (Lee et al., 2001). SSLP markers were developed by surveying released genomic DNA sequences for simple repeats using the RepeatMasker program (http://ftp.genome.washington.edu/cgi-bin/RepeatMasker). Primer pairs flanking these simple repeats that generate PCR products with size polymorphisms on 4% agarose gels between the C24 and Columbia ecotypes were used as molecular markers for mapping.
RNA Analysis
Ten-day-old seedlings on MS agar plates were treated were treated with cold, ABA, NaCl, or PEG as described in the above section. For drought treatment, seedlings at rosette stage in potted soil was detached from soil surface and allowed to lose 20% of their fresh weight. The dehydrated materials were then incubated in a container with 100% relative humidity for 3 hr or 6 hr before being frozen in liquid N 2 for RNA extraction. Total RNA from control or treated plants was extracted and analyzed as described (Ishitani et al., 1998). Gene-specific probes were as described (Ishitani et al., 1998; Lee et al., 2001).
Stress Tolerance Assays
For freezing sensitivity assay, wild type and los5 seedlings growing in potted soil in a growth chamber (22±2° C., 16 hr light and 8 hr dark) for 3 weeks were first incubated at 4° C. in the light for 48 hr to cold-acclimate. After this cold acclimation, the plants were subjected to freezing at −7° C. for 5 hr. Upon finishing the treatment, plants were immediately transferred to 4° C. under white light and were incubated overnight. In the following morning the plants were placed into a growth chamber. Seedling damage was scored at times as indicated in the text.
For NaCl tolerance tests, 7-day-old seedlings of los5-1 and wild type growing on MS plates (solidified with 1.2% agar) were transferred to MS agar plates supplemented with different concentrations of NaCl. The plates were then placed vertically at 22±2° C. under white light and root elongation was measured daily for up to 10 days. To measure ion leakage in seedlings induced by PEG treatment, one-week-old wild type and los5-1 seedlings growing in MS agar plates were carefully removed from the plate, briefly rinsed in distilled water and placed in solutions containing 30% of polyethylene glycol (PEG) (mw 6,000) for 5 hr. After the treatment, seedlings were rinsed briefly in distilled water and immediately placed in a tube with 5 ml H 2 O. The tube was then gently agitated for 3 hr before the electrolyte content was measured. Four replicates of each treatment were conducted.
Water Loss Measurements
For water loss measurement, plants at the rosette stage were detached from soil surface and weighed immediately in a plastic weighing boat. The boat with the plants was then placed on laboratory bench (relative humidity ˜30%) and weighed at designated time intervals. There were four replicates for each line. Percent loss of fresh weight was calculated based on the initial weight of the plants.
Proline Assays
One-week-old seedlings of los5-1 and wild type grown in MS agar plate were sprayed with 50 μM ABA or transferred on to filter paper in a petri dish saturated with 150 mM NaCl and incubated at 22±2° C. under white light for 24 hr. After the treatment, the samples were frozen in liquid nitrogen and kept at −80° C. for proline assay. Proline concentration was determined as described by Bates et al. (1973).
ABA Measurement
Rosette leaves were excised from 3-week-old mutant and wild type plants grown in soil and placed on lab bench. After the leaves lost 30% of initial fresh weight (over a period of approximately 2 hrs), they were placed in a sealed plastic bag with wet paper towels for an additional 5 hrs. Unstressed control leaves were placed directly in a high humidity sealed plastic bag without losing fresh weight. The tissues were then frozen in liquid nitrogen and ground into powder. One gram of the tissues was suspended in 15 mL of extraction solution containing 80% methanol, 100 mg/L butylated hydroxytoluene and 0.5 g/L citric acid monohydrate. The suspension was stirred overnight at 4° C. and centrifuged at 1,000 g for 20 min. The supernatant was transferred to a new tube and dried under vacuum. The dry residue was dissolved with 100 μL of methanol plus 900 μL of tris-buffered saline (50 mM Tris, 0.1 mM MgCl 2 .6H 2 O, 0.15 M NaCl, pH 7.8). ABA concentration in the solution was then determined using the Phytodetek ABA immunoassay kit (Idetek, Inc., Sunnyvale, Calif.).
Example 2
Overexpression of the LOS5 Gene Increases Plant Tolerance to Drought Stress
Since loss of function mutations in the LOS5 gene resulted in de-regulated expression of stress-responsive genes and increased sensitivity to drought and salt stress, it was hypothesized that increased expression of LOS5 might affect gene expression pattern under stress and increase plant drought tolerance.
To test this possibility, LOS5 full length cDNA was transcriptionally fused to the CaMV 35S promoter and a transcriptional enhancer. The insert was cloned into the binary vector pCAMBIA1200. The resulting plasmid was transferred into Arabidopsis ecotypes Columbia and C24 (containing the RD29A-LUC reporter gene), respectively. Around 70 transformants were selected in each ecotype background for further tests. To examine the impact of overexpressing LOS5 gene on stress gene regulation and plant water relations, seedlings of transgenic plants were assayed for 1) their expression of the RD29A-LUC transgene under salt stress treatment; 2) their transpiration rates by measuring water loss rate of detached leaves, and 3) drought tolerance in soil growing plants.
The results with 7 lines in the C24 background indicated that upon treatment with 300 mM NaCl, the expression level for the RD29A-LUC reporter gene in LOS5-overexpressing line is 39 to 96 percent higher than in the wild type plants. During a 4-hr period, the water transpiration rate from detached leaves of LOS5-overexpressing plants is significantly lower than that in the wild type plants in three lines tested. This reduced transpiration can also be seen when detached leaves were let to dry at the room condition (see FIG. 12 ). The reduced transpiration will significantly increase drought tolerance of the plants.
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Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
The content of Xiong et al., The Plant Cell, Volume 13, September 2001, pp. 2063–2083, is incorporated herein by reference in its entirety.