The present invention relates to a gene implicated in abiotic stress tolerance and growth promotion and a composition for improving abiotic stress tolerance and promoting growing of transformed plants with the same.

Due to their sessile nature, higher plants are constantly faced with various adverse environmental factors, including drought, high salt, heavy metals, cold, heat shock, and ozone, during their whole life span. These abiotic stresses are a limiting factor for the growth and development of crop plants. Water deficiency causes dramatic reduction of crop production globally, and the decreasing availability of fresh water may pose a future threat to humans and higher plants. Plants have cellular and genetic defense mechanisms to enhance their tolerance to transient and long-term water shortages by triggering signaling network pathways and inducing stress-responsive genes (Shinozaki and Yamaguchi-Shinozaki, 2007). However, for stress tolerance or sensitivity, our knowledge concerning the biological functions of stress-related genes in higher plants is still rudimentary. Therefore, it is important to study the functions of stress responsive genes to increase the productivity of crop plants.

Plants have diverse defense strategies to reinforce their tolerance against unfavorable conditions by triggering signaling network pathways and inducing the stress-responsive genes. The present inventors have found that three AtSRP (Arabidopsis thaliana stress related protein) genes which encode homologs to the small rubber particle protein (SRPP) in rubber trees (Hevea brasiliensis) were rapidly induced by dehydration in Arabidopsis thaliana. Natural rubber is cis-1,4-polyisoprene produced through mevalonate pathway in cytosolic fractions (latex) of latex vessel tissues (Oh et al. 1999; Sookmark et al. 2002 ; Kim et al. 2004; Chow et al. 2007). Since Arabidopsis thaliana does not produce natural rubber, it was unexpected that the Arabidopsis thaliana water stress-induced genes shared sequence homology with rubber biosynthetic genes.

SRPP, originally known as a latex allergin, is a protein tightly bound on a small rubber particle in the latex of rubber trees (Hevea brasiliensis).

Bark tissue of rubber trees is constantly stripped, which is known as a tapping process, to collect rubber latex. Therefore, plugging of latex vessels is essential for rubber trees to prevent the loss of their cytoplasmic components, such as primary metabolites, and to prevent pathogen infection of the latex vessel tissues (Wititsuwannakul et al., 2008b). In the process of latex vessel plugging, hevein or Hevea latex lectin (HLL) interacts with rubber particle (RP) protein to form a rubber latex coagulum (Gidrol et al., 1994; Wititsuwannakul et al., 2008a). Recently, SRPP was purified as an RP glycoprotein that bound HLL and was, therefore, termed HLL-binding protein (HLLBP) (Wititsuwannakul et al., 2008c). Interaction between an N-acetylglucosamine moiety in SRPP and HLL may modulate the degree of latex coagulation in response to tapping and mechanical wounding.

However, roles of the SRPP associated with abiotic stresses such as drought, high salt and cold have been not known yet.

Throughout this application, various publications and patents are referred and citations are provided in parentheses. The disclosures of these publications and patents in their entities are hereby incorporated by references into this application in order to fully describe this invention and the state of the art to which this invention pertains.

The present inventors have made intensive studies to increase the productivity of crop plants by developing genes for improving a tolerance to abiotic stresses including drought, high salt and cold of the plants. As results, they have discovered that the improved tolerance to abiotic stresses may be obtained when expressions of three AtSRP (Arabidopsis thaliana stress related protein) genes which encode homologs to the small rubber particle protein (SRPP) in rubber trees (Hevea brasiliensis) were increased in plants.

Accordingly, it is an object of this invention to provide a composition for improving the tolerance of a plant to an abiotic stress.

It is another object of this invention to provide a composition for promoting growth of a plant.

It is still another object of this invention to provide a method for improving the tolerance of a plant to an abiotic stress.

It is further object of this invention to provide a method for promoting growth of a plant.

Other objects and advantages of the present invention will become apparent from the following detailed description together with the appended claims and drawings.

• Fig. 1 represents that AtSRPs are expressed in various organs of Arabidopsis. Fig. 1a represents results of analyzing AtSRPs expression by RT-PCR at 7 developmental stages. 18s rRNA was used as a loading control.
  WS; whole seedling, RT; root, RL; rosette leaf, ST; stem, F; Flower, SQ; sillique, SD; seed.
  Fig. 1b represents results of analyzing AtSRPs expression by real-time RT-PCR in various organs of Arabidopsis. Results are means ± S.D. from four repeated experiments.
• Fig. 2 represents that AtSRPs are up-regulated by ABA and abiotic stresses. Total RNA was prepared from light-grown, 7-dold Arabidopsis seedlings, which had been treated with 100 mM ABA (1.5-3 hrs), drought (1.5-3 hrs), cold (4°C for 4.5-24 hrs), or high salinity (300 mM NaCl for 3 hrs). Induction patterns of AtREFPs were investigated by RT-PCR (Fig. 2a) or real-time qRT-PCR (Fig. 2b). The RAB18 and RD29A genes were used as positive controls for ABA and abiotic stress, respectively. AtACT8 transcript levels were used as loading controls. Data represent means ± SD from four independent experiments.
• Fig. 3 represents that AtSRP1, membrane associated protein, possessed lectin binding property. Fig. 3a represents results of the biochemical analysis using affinity chromatography with concanavalin A. Fig. 3b represents results of the membrane association test. AtSRPs were transiently over-expressed using transient assay. Then, total membrane pellets from Arabidopsis seedlings were resuspended in 1% (v/v) Triton X-100, 2 M urea, 1 M NaCl, or 0.1 M Na₂CO₃. Suspensions were recentrifuged at 125,000g to separate solubilized fractions (S) and insoluble fractions (P), which were analyzed by immunoblotting using Flag antibody.
• Fig. 4 represents phenotypic characterizations of T3 35S:AtSRPs transgenic Arabidopsis plants and knock-out mutants. AtSRPs-overexpresors grew more rapidly than wild type plants. In contrast, knock-out mutants showed opposite phenotypes. Fig. 4a represents morphological comparisons of light-grown 3-day-old wild type and 35S:AtSRPs transgenic seedlings and knock-out mutants. Fig. 4b represents differences in growth patterns of early roots in wild type, AtSRPPs-overexpressing, and knock-out seedlings. Error bars indicate mean ±SD (n = 70). Fig. 4c represents determination of the dimensional parameters of second leaves 2 weeks after germination. The second leaves were detached from wild type, transgenic, and knock-out mutant lines, scanned for image analysis using SCIONIMAGE program, and their blade widths and lengths, petiole lengths, and blade areas were calculated. Error bars indicate mean ±SD (n = 35). Fig. 4d represents growth morphology of wild type, 35S:AtSRPs transgenic Arabidopsis, and knock-out mutant plants grown under normal growth conditions 28 days after germination (left panel). Right panel represents differences in growth patterns of inflorescence stems of wild type and transgenic Arabidopsis plants 20 to 28 days after germination. Error bars indicate mean ±SD (n = 80).
• Fig. 5 represents faster-growing phenotypes of 35S:AtSRPs transgenic plants correlated with enhanced cell cycle progression. Fig. 5a represents longitudinal views of root tips from 3-day-old wild type and AtSRPs-overexpressing transgenic and knock-out mutant seedlings. Root sections were stained with propidium iodide and analyzed by confocal microscopy. Fig. 5b represents results of confocal microscope demonstrating the epidermal and palisade mesophyll cells from the second leaves of wild type, AtSRPs-overexpressors, and knock-out mutants. Scale bars = 20 µm.
• Fig. 6 represents that loss of function of AtSRPs showed susceptible phenotype against salt stress. It shows morphologies of 12-day-old wild-type, AtSRPs-overexpressing transgenic, and knock-out mutant seedlings in 100 mM NaCl media.
• Fig. 7 represents that AtSRPs play a role as a positive factor for drought defensive responses. Fig. 7a represents survival rates of wild type and 35S:AtSRPs transgenic plants after drought stress. Light-grown 25-day-old wild type, 3-week-old transgenic plants, and 26-day-old knock-out mutants were further grown for 8 days without watering. Plants were then re-watered and their survival rates were determined after 3 days. The survival rate was considered as the ability of plants to continue to grow after returned to normal water conditions following eight-day drought stress. The survival rates are following: wild type, 26.97% (24 out of 89 plants); 35S:AtSRP1 transgenic line, 64.84% (59 out of 91 plants); 35S.AtSRP2 transgenic line, 66.29% (59 out of 89 plants); 35S:AtSRP3 transgenic line, 58.02% (47 out of 81 plants); atsrp1 mutant line, 3.90% (3 out of 77 plants) and atsrp3 mutant line, 0.00% (0 out of 101 plants). Fig. 7b represents results of measurement of cut rosette water loss (CRWL) rates. The rosette leaves were detached from light-grown 3-week-old wild type, 35S:AtSRPs transgenic plants, and knock-out mutants, placed on open-lid Petri dishes, and incubated for 0-5 hrs at room temperature. Decreases in fresh weights were determined. Water loss is expressed as the percentage of initial fresh weight of the detached leaves. Error bars indicate mean ± SD (n=20). Fig. 7c represents amount (mg/g DW) of chlorophyll a + chlorophyll b in leaves of wild, 35S:AtSRPs-overexpressing plants, and knock-out mutants. Error bars indicate mean ± SD (n=3).
• Fig. 8 represents analysis results of proline content of leaf discs before and after drought treatments. Data were obtained from three independent experiments. Bars represent means ± SD (n = 3). 35S:AtSRPs transgenic leaves retained their leaf water more effectively than did wild-type leaves under drought conditions. In contrast, knockout mutants showed opposite phenotypes.
• Fig. 8 schematically represents AtSRP1, AtSRP2 and AtSRP3 cDNA clones.

In one aspect of this invention, there is provided a composition comprising composition comprising a nucleotide sequence encoding the amino acid sequence selected from the group consisting of SEQ ID NOs:4 to 6 for use in the improvement of the tolerance of a plant to an abiotic stress.

The present inventors have made intensive studies to increase the productivity of crop plants by developing genes for improving a tolerance to abiotic stresses including drought, high salt and cold of the plants. As results, they have discovered that the improved tolerance to abiotic stresses may be obtained when expressions of three AtSRP (Arabidopsis thaliana stress related protein) genes which encode homologs to the small rubber particle protein (SRPP) in rubber trees (Hevea brasiliensis) were increased in plants.

According to a preferred embodiment, the present nucleotide sequence encoding the amino acid sequence of SEQ ID NOs:4, 5 and 6 comprises the nucleotide sequence as set forth in SEQ ID NO: 1, 2 and 3, respectively.

According to a preferred embodiment, the amino acid sequence of SEQ ID NOs:4, 5 and 6 is the sequence of SRPP-like proteins present in Arabidopsis. These proteins are named as AtSRP (Arabidopsis thaliana stress related protein) 1, AtSRP 2 and AtSRP 3, respectively. As shown in Examples, the present inventors have found that expressions of these proteins or their coding genes were increased by various abiotic stresses, and the improved tolerance to abiotic stresses may be obtained when these genes were over-expressed in plants.

It would be obvious to the skilled artisan that the nucleotide sequences used in this invention are not limited to those listed in the appended Sequence Listings. The nucleotide sequences described herein are illustrative and their biological equivalents may be also used in this invention for enhancement of tolerance to a drought stress and promotion of flowering or growing in plants. In this regard, the sequence variations should be construed to be covered by the present invention.

For nucleotides, the variations may be purely genetic, i.e., ones that do not result in changes in the protein product. This includes nucleic acids that contain functionally equivalent codons, or codons that encode the same amino acid, such as six codons for arginine or serine, or codons that encode biologically equivalent amino acids.

Considering biologically equivalent variations described hereinabove, the nucleic acid molecule of this invention may encompass sequences having substantial identity to them. Sequences having the substantial identity show at least 80%, more preferably at least 90%, most preferably at least 95% similarity to the nucleic acid molecule of this invention, as measured using one of the sequence comparison algorithms. Methods of alignment of sequences for comparison are well-known in the art. Various programs and alignment algorithms are described in: Smith and Waterman, Adv. Appl. Math. 2:482(1981); Needleman and Wunsch, J. Mol. Bio. 48:443(1970); Pearson and Lipman, Methods in Mol. Biol. 24: 307-31(1988); Higgins and Sharp, Gene 73:237-44(1988); Higgins and Sharp, CABIOS 5:151-3(1989) Corpet et al., Nuc. Acids Res. 16:10881-90(1988) Huang et al., Comp. Appl. BioSci. 8:155-65(1992) and Pearson et al., Meth. Mol. Biol. 24:307-31(1994). The NCBI Basic Local Alignment Search Tool (BLAST) [Altschul et al., J. Mol. Biol. 215:403-10(1990)] is available from several sources, including the National Center for Biological Information (NBCI, Bethesda, Md.) and on the Internet, for use in connection with the sequence analysis programs blastp, blasm, blastx, tblastn and tblastx. It can be accessed at http://www.ncbi.nlm.nih.gov/BLAST/. A description of how to determine sequence identity using this program is available at http://www.ncbi.nlm.nih.gov/BLAST/blasChelp.html.

According to a preferred embodiment, the abiotic stress is selected from the group consisting of a drought stress, a low-temperature stress and a salt stress.

To introduce a foreign nucleotide sequence into plant cells or plants may be performed by the methods (Methods of Enzymology, Vol. 153, 1987) known to those skilled in the art. The plant may be transformed using the foreign nucleotide inserted into a carrier (e.g., vectors such as plasmid or virus) or Agrobacterium tumefaciens as a mediator (Chilton et al., Cell, 11: 263-271 (1977)) and by directly inserting the foreign nucleotide into plant cells (Lorz et al., Mol. Genet., 199: 178-182 (1985); the disclosure is herein incorporated by reference). For example, electroporation, microparticle bombardment, polyethylene glycol-mediated uptake may be used in the vector containing no T-DNA region.

Generally, Agrobacterium-mediated transformation is the most preferable (U.S. Pat. Nos. 5,004,863, 5,349,124 and 5,416,011), and the skilled artisan can incubate or culture the transformed cells or seeds to mature plants in appropriate conditions.

The term "plant(s)" as used herein, is understood by a meaning including a plant cell, a plant tissue and a plant seed as well as a mature plant.

The plants applicable of the present invention include, but not limited to, food crops such as rice plant, wheat, barley, corn, bean, potato, Indian bean, oat and Indian millet; vegetable crops such as Arabidopsis sp., Chinese cabbage, radish, red pepper, strawberry, tomato, watermelon, cucumber, cabbage, melon, pumpkin, welsh onion, onion and carrot; crops for special use such as ginseng, tobacco plant, cotton plant, sesame, sugar cane, sugar beet, Perilla sp., peanut and rape; fruit trees such as apple tree, pear tree, jujube tree, peach tree, kiwi fruit tree, grape tree, citrus fruit tree, persimmon tree, plum tree, apricot tree and banana tree; flowering crops such as rose, gladiolus, gerbera, carnation, chrysanthemum, lily and tulip; and fodder crops such as ryegrass, red clover, orchardgrass, alfalfa, tallfescue and perennial ryograss.

According to a preferred embodiment, the nucleotide sequence is contained a recombinant plant expression vector; and the recombinant plant expression vector comprises (i) the nucleotide sequence; (ii) a promoter which is operatively linked to the nucleotide sequence of (i) and generates RNA molecules in plant cells; and (iii) a poly A signal sequence inducing polyadenylation at the 3'-end of the RNA molecules.

The term "operatively linked" as used herein refers to functional linkage between a nucleic acid expression control sequence (such as a promoter, signal sequence, or array of transcription factor binding sites) and a second nucleotide sequence, wherein the expression control sequence affects transcription and/or translation of the nucleic acid corresponding to the second sequence.

The vector system of this invention may be constructed in accordance with conventional techniques described in Sambrook et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press(2001), teachings of which are incorporated herein by reference.

The suitable promoter in the present invention includes any one commonly used in the art, for example SP6 promoter, T7 promoter, T3 promoter, PM promoter, maize-ubiquitin promoter, Cauliflower mosaic virus (CaMV)-35S promoter, Nopalin synthase (nos) promoter, Figwort mosaic virus 35S promoter, Sugarcane bacilliform virus promoter, commelina yellow mottle virus promoter, photo-inducible promoter of small subunit of Ribulose-1,5-bis-phosphate carboxylase (ssRUBISCO), cytosolic triosphosphate isomerase (TPI) promoter in rice, adenine phosphoribosyltransferase (APRT) or octopine synthase promoter in Arabidopsis. Preferably, the promoter used in this invention is CaMV 35S.

According to a preferred embodiment, the 3'-non-translated region causing polyadenylation includes that from the nopaline synthase gene of Agrobacterium tumefaciens (NOS 3' end) (Bevan et al., Nucleic Acids Research, 11(2):369-385(1983)), that from the octopine synthase gene of Agrobacterium tumefaciens, the 3'-end of the protease inhibitor I or II genes from potato or tomato, the CaMV 35S terminator, and OCS (octopine synthase) terminator. Most preferably, the 3'-non-translated region causing polyadenylation in this invention is NOS.

Optionally, the present vector for plants may further carry a reporter molecule (e.g., genes for luciferase and β-glucuronidase). In addition, the vector may contain antibiotic resistant genes as selective markers (e.g., neomycin phosphotransferase gene (nptII) and hygromycin phosphotransferase gene (hpt)).

According to a preferred embodiment, the plant expression vector of this invention is Agrobacterium binary vectors.

The term "binary vector" as used herein, refers to a cloning vector containing two separate vector systems harboring one plasmid responsible for migration consisting of left border (LB) and right border (RB), and another plasmid for target gene-transferring. Any Agrobacterium suitable for expressing the nucleotide of this invention may be used, and most preferably, the transformation is carried out using Agrobacterium tumefaciens GV3101.

Introduction of the recombinant vector of this invention into Agrobacterium can be carried out by a large number of methods known to one skilled in the art. For example, particle bombardment, electroporation, transfection, lithium acetate method and heat shock method may be used. Preferably, the electroporation is used.

According to the present invention, the present AtSRPs-overexpressing transgenic plants grew more rapidly with longer roots, larger leaves, and bolted earlier with prolonged inflorescences. The growth rates of the transgenic plants are enhanced using properties of the AtSRPs genes, whereby they may effectively applied for cultivating the plants with novel function of rapid growing, and biomass.

In another aspect of this invention, there is provided a method for improving the tolerance of a plant to an abiotic stress, comprising:

1. (a) introducing a nucleotide sequence encoding the amino acid sequence selected from the group consisting of SEQ ID NOs:4 to 6 to a cell of the plant; and
2. (b) obtaining a transgenic plant exhibiting improved tolerance to an abiotic stress from the cell of the plant.

In still another aspect of this invention, there is provided a composition comprising a nucleotide sequence encoding the amino acid sequence selected from the group consisting of SEQ ID NOs:4 to 6 for use in the improvement of the growth of a plant.

In further aspect of this invention, there is provided a method for promoting the growth of a plant, comprising:

1. (a) introducing a nucleotide sequence encoding the amino acid sequence selected from the group consisting of SEQ ID NOs:4 to 6 to a cell of the plant; and
2. (b) obtaining a transgenic plant exhibiting improved tolerance to an abiotic stress from the cell of the plant.

As the present amino acid sequence, the present nucleotide sequence and the plant expression vector comprising thereof are mentioned above, they are omitted herein to avoid excessive overlaps.

The features and advantages of the present invention will be summarized as follows:

1. (a) The present invention provides a method for improving the tolerance of a plant to an abiotic stress and a method for promoting growing of a plant.
2. (b) The present nucleotide sequence is involved in abiotic stress tolerance such as drought, low-temperature and salt stresses of plants. Therefore, the overexpressing transgenic plants have excellent tolerances to these abiotic stresses, whereby they may be useful as novel functional crops which are affected by climates and environments of the cultivated areas.
3. (c) In addition, where the plants are transformed with the present nucleotide sequence, the growth abilities of the transgenic plants are remarkably enhanced,

The present invention will now be described in further detail by examples. It would be obvious to those skilled in the art that these examples are intended to be more concretely illustrative and the scope of the present invention as set forth in the appended claims is not limited to or by the examples.

Total mRNA was extracted from 5-day-old Arabidopsis thaliana seedlings to perform RT-PCT (Seo et al., 2008). cDNA was synthesized by reverse transcription. To amplify specifically AtSRP1, AtSRP2 and AtSRP3 genes, primer pairs used in PCR were prepared by searching cDNA sequence to each of AtSRP1, AtSRP2 and AtSRP3 genes from a database of the Arabidopsis Information Resource (TAIR, http://www.arabidopsis.org), and subsequently, tagging BamHI at CDS 5' and SacI at CDS 3'. Each of the amplified cDNA of AtSRP1, AtSRP2 and AtSRP3 gene was ligated to pGEM-T Easy vector (Promega, http://www.promega.com) to cloning, and the sequence was analyzed by sequencing reaction (Table 1). The analyzed sequence was exactly corresponded with that of TAIR (http://www.arabidopsis.org). In addition, it was found that At1g67360 (AtSRP1) included CDS consisting of 723 bp, At2g47780 (AtSRP2) included CDS consisting of 708 bp, and At3g05500 (AtSRP3) included CDS consisting of 741 bp.

To analyze the AtSRPs expression in transcription level in various tissues, RT-PCR and real-time RT-PCR were performed (Seo et al., 2008). Primers used in the analysis are shown in Table 1. Primers for cloning, preparations of transgenic plants, and protein expressions Gene Forward primer Reverse primer Cloning of AtSRPs ORF, RT-PCR and preparation of transgenic plant AtSRP1 5'-gcggatccatggagacagagaagaaaaatag-3' 5'-gcgagctcctactccgaatcagacgatg-3' AtSRP2 5'-gcggatccatggctgaagatgaaatagtagtc-3' 5'-gcgagctctcaatcagctcgacactgatc-3' AtSRP3 5'-gcggatccatggctactcaaacggatctc-3' 5'-gcgagctctcaatcaagtggatggaactc-3' 18s rRNA 5'-gcatttgccaaggatgtttt-3' 5'-gtacaaagggcagggacgta-3' At4CT8 5'-tactgattacctcatgaagatccttac-3' 5'-aaacgatgtctctttagtttagaagc-3' Quantitative real time RT-PCR primer AtSRP1 5'-tgattctctcctggtctttc-3' 5'-ttctctgtcgccttgtagat-3' AtSRP2 5'-ttttcgtcgatcgtaagg-3' 5'-aatgctcttggtagtgtcca-3' AtSRP3 5'-ccctgttgaggtccttaaat-3' 5'-gccacaatgggtgctat-3'

AtSRPs were fused with 2xFlag epitope, expressed to BL21-CodonPlus (DE3) RIL (Stratagene), and purified. The proteins were quantified using BSA as a standard protein (Bradford, 1976).

To investigate binding properties to lectin of AtSRPs, two proteins were used as follows: 2xFlag-AtSRPs were expressed in E. coli, and purified to obtain protein; and 35S:2XFlag-AtSRPs were transiently over-expressed in tobacco to obtain protein.

The two proteins were subject to concanavalin A binding assay according to the manufacturer's method (Con A Sepharose 4B-GE-71707700AG; GEHealthcare Life Sciences). The resultants were subject to immunoblotting using Flag antibodies. As a result, the only band was elucidated in the lane of 35S:2XFlag-AtSRPs using tobacco system, indicating that the only AtSRP1 glycosylated in plant system binds to lectin. Accordingly, it would be understood that the AtSRP1 is a glycoprotein (Fig. 3a).

To investigate association properties to membrane of AtSRPs, 35S:2XFlag-AtSRPs were transiently over-expressed in tobacco to obtain protein. The protein was extracted to use. Then, the protein was subject to membrane association test according to Kim and Bassham (Kim and Bassham, 2011). The resultants were subject to immunoblotting using Flag antibodies. As a result, the band of the supernatant in the 0.1 M Na₂CO₃-treated membrane pellet was darker than that of the precipitate (Fig. 3b), addressing that the AtSRP1 protein binding to membrane was eluted into the supernatant by Na₂CO₃ that weakens binding of the AtSRP1 protein to membrane. Accordingly, it would be understood that AtSRPs proteins are membrane associated proteins.

The AtSRPs cDNA were introduced into the corresponding sites of the binary vector pBI121 to overexpress. The used primers are shown in Table 1. The resulting fusion gene was transferred to Agrobacterium tumefaciens. Agrobacterium cells were transformed into Arabidopsis plants by means of the floral-dip method (Joo et al., 2004). Seeds were germinated on 0.5 X Murashige and Skoog (MS) medium with 30 µg/ml kanamycin to select independent transgenic plants. atsrp1 and atsrp3 used T-DNA insertion lines (atsrp1: GABI_3095G05, atsrp3: WiscDSLOXHS192) which were purchased from ABRC (www.arabidopsis.org). atsrp2 used knock-down line, which is prepared by RNAi method, to select plants in which expression in transcription level is lower (Lee and Kim, 2010).

Medium was prepared to contain 0.5 x MS (including vitamin B5), 1% sucrose and 0.8% agar (select agar; Life Technology, Rockvile, MD, USA) (pH 5.7). Surfaces of seeds of wild type plants and AtSRPs-overexpressing transgenic plants were sterilized, planted and grown under light condition at 22°C. After inhibition of radicle germination, they were daily observed for 5 days. In order to observe the root growth, seeds were observed until 5th day after germination. During the root growth, the root shoot-tip was marked in outside of culture plates, and the root length was measured using ScionImage software (Scion Corp., Frederic, MD, USA). In order to observe the leaf size, the seeds were planted in sterile soil. After 12 days, sizes of the leaves from a seed leaf to a third leaf in wild type plants and AtSRPs-overexpressing transgenic plants were compared. In addition, the second leaf was measured in leaf length, leaf width and petiole length using ScionImage software (Scion Corp., Frederic, MD, USA).

In order to observe the flower stalk rising time, seeds of wild type plants and overexpressing transgenic plants were planted in sterile soil, and grown under light condition at 22°C. At 19 days after germination, the flower stalk length was measured. As a result, when wild type plants and AtSRPs-overexpressing transgenic plants were compared each other, there was no difference in germination ability. However, AtSRPs-overexpressing transgenic plants grew more rapidly with longer roots, larger leaves, and longer height. Therefore, it could be demonstrated that AtSRPs act as a positive regulator in plant growth (Figs. 4a-d).

Leaves of 5-day-old wild-type and transgenic Arabidopsis thaliana were washed in a solution of chloral hydrate with the reported method (Kwon et al. 2009). Cell layer was observed by bright-field microscope (BX51 fluorescence microscope, Olympus, Japan). For longitudinal views of root tips, roots of 5-day-old wild-type and transgenic Arabidopsis thaliana were stained by propidium iodide, and the images were obtained by confocal microscope with the method of Seo et al. (2008) (Fig. 5).

5-day-old wild type, 35S:CaSRP1 transgenic and each of knock-out plants, which had been grown under normal growth conditions, were subjected to high salt stress by growing in high salt-media containing 100mM of NaCl for 7 days. Then, root lengths of each of plants were examined. As a result, each of the knock-out plants has a more sensitive phenotype to high salt stress compared with wild-type plants. Therefore, it could be demonstrated that AtSRPs act as a positive regulator in high salt stress response (Fig. 6).

3-week-old wild type, 35S:CaSRP1 transgenic and each of knock-out plants, which had been grown under normal growth conditions, were subjected to drought stress by withholding water for 8 days. The plants were then re-watered and their phenotypes were examined after 3 days. Survival was defined as the ability to resume growth when returned to normal conditions following water stress (Cho et al. 2008). Each of survival rates was as follows (Fig. 7a):

• wild type plants, 26.97% (24 out of 89 plants); 35S:AtSRP1 transgenic plants, 64.84% (59 out of 91 plants); 35S:AtSRP2 transgenic plants, 66.29% (59 out of 89 plants); 35S:AtSRP3 transgenic plants, 58.02% (47 out of 81 plants); atsrp1 mutant plants, 3.90% (3 out of 77 plants); atsrp3 mutant plants, 0.00% (0 out of 101 plants). In addition, transgenic plants showed significantly lower decreases of the moisture contents by the lapse of time, as compared to the wild-type and knock-out mutant plants (Fig. 7b).

Chlorophyll was extracted in leaves of wild-type and AtSRPs-overexpressing transgenic Arabidopsis thaliana plants after drought stress by the method of Bae et al. (2009) using 80% acetone. The remaining leaf extract was dried at 105°C for 16 hours, and weight of the dry extract was measured (Welti et al. 2002). Proline content of the leaves was measured by the method of Claussen (2005). As a result, transgenic plants exhibited significantly higher levels of proline contents, indicating that transgenic plants retained their leaf water more effectively than did wild-type leaves under drought conditions. In contrast, knoc-kout mutant plants showed opposite phenotypes (Fig. 8).

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present disclosure. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the disclosure as set forth in the appended claims.