Characterization of a gene family encoding abscisic acid- and environmental stress-inducible proteins of alfalfa

Molecular Mechanism of Exogenous ABA to Enhance UV-B Resistance in Rhododendron chrysanthum Pall. by Modulating Flavonoid Accumulation

With the depletion of the ozone layer, the intensity of ultraviolet B (UV-B) radiation reaching the Earth's surface increases, which in turn causes significant stress to plants and affects all aspects of plant growth and development. The aim of this study was to investigate the mechanism of response to UV-B radiation in the endemic species of Rhododendron chrysanthum Pall. (R. chrysanthum) in the Changbai Mountains and to study how exogenous ABA regulates the response of R. chrysanthum to UV-B stress. The results of chlorophyll fluorescence images and OJIP kinetic curves showed that UV-B radiation damaged the PSII photosystem of R. chrysanthum, and exogenous ABA could alleviate this damage to some extent. A total of 2148 metabolites were detected by metabolomics, of which flavonoids accounted for the highest number (487, or 22.67%). KEGG enrichment analysis of flavonoids that showed differential accumulation by UV-B radiation and exogenous ABA revealed that flavonoid biosynthesis and flavone and flavonol biosynthesis were significantly altered. GO analysis showed that most of the DEGs produced after UV-B radiation and exogenous ABA were distributed in the cellular process, cellular anatomical entity, and catalytic activity. Network analysis of key DFs and DEGs associated with flavonoid synthesis identified key flavonoids (isorhamnetin-3-O-gallate and dihydromyricetin) and genes (TRINITY_DN2213_c0_g1_i4-A1) that promote the resistance of R. chrysanthum to UV-B stress. In addition, multiple transcription factor families were found to be involved in the regulation of the flavonoid synthesis pathway under UV-B stress. Overall, R. chrysanthum actively responded to UV-B stress by regulating changes in flavonoids, especially flavones and flavonols, while exogenous ABA further enhanced its resistance to UV-B stress. The experimental results not only provide a new perspective for understanding the molecular mechanism of the response to UV-B stress in the R. chrysanthum, but also provide a valuable theoretical basis for future research and application in improving plant adversity tolerance.

Abscisic Acid Affects Phenolic Acid Content to Increase Tolerance to UV-B Stress in Rhododendron chrysanthum Pall

The presence of the ozone hole increases the amount of UV radiation reaching a plant's surface, and UV-B radiation is an abiotic stress capable of affecting plant growth. Rhododendron chrysanthum Pall. (R. chrysanthum) grows in alpine regions, where strong UV-B radiation is present, and has been able to adapt to strong UV-B radiation over a long period of evolution. We investigated the response of R. chrysanthum leaves to UV-B radiation using widely targeted metabolomics and transcriptomics. Although phytohormones have been studied for many years in plant growth and development and adaptation to environmental stresses, this paper is innovative in terms of the species studied and the methods used. Using unique species and the latest research methods, this paper was able to add information to this topic for the species R. chrysanthum. We treated R. chrysanthum grown in a simulated alpine environment, with group M receiving no UV-B radiation and groups N and Q (externally applied abscisic acid treatment) receiving UV-B radiation for 2 days (8 h per day). The results of the MN group showed significant changes in phenolic acid accumulation and differential expression of genes related to phenolic acid synthesis in leaves of R. chrysanthum after UV-B radiation. We combined transcriptomics and metabolomics data to map the metabolic regulatory network of phenolic acids under UV-B stress in order to investigate the response of such secondary metabolites to stress. L-phenylalanine, L-tyrosine and phenylpyruvic acid contents in R. chrysanthum were significantly increased after UV-B radiation. Simultaneously, the levels of 3-hydroxyphenylacetic acid, 2-phenylethanol, anthranilate, 2-hydroxycinnamic acid, 3-hydroxycinnamic acid, α-hydroxycinnamic acid and 2-hydroxy-3-phenylpropanoic acid in this pathway were elevated in response to UV-B stress. In contrast, the study in the NQ group found that externally applied abscisic acid (ABA) in R. chrysanthum had greater tolerance to UV-B radiation, and phenolic acid accumulation under the influence of ABA also showed greater differences. The contents of 2-phenylethanol, 1-o-p-coumaroyl-β-d-glucose, 2-hydroxy-3-phenylpropanoic acid, 3-(4-hydroxyphenyl)-propionic acid and 3-o-feruloylquinic ac-id-o-glucoside were significantly elevated in R. chrysanthum after external application of ABA to protect against UV-B stress. Taken together, these studies of the three groups indicated that ABA can influence phenolic acid production to promote the response of R. chrysanthum to UV-B stress, which provided a theoretical reference for the study of its complex molecular regulatory mechanism.

OsABT Is Involved in Abscisic Acid Signaling Pathway and Salt Tolerance of Roots at the Rice Seedling Stage

Rice is a staple cereal crop worldwide, and increasing its yields is vital to ensuring global food security. Salinity is a major factor that affects rice yield. Therefore, it is necessary to investigate salt tolerance mechanisms in rice. Proteins containing WD40 repeats play important roles in eukaryotic development and environmental adaptation. Here, we showed that overexpression of OsABT, a gene encoding a WD40-repeat protein, enhanced salt tolerance in rice seedlings by regulating root activity, relative conductivity, malondialdehyde and H₂O₂ content, and O₂^•− production rate. Root ion concentrations indicated that OsABT overexpression lines could maintain lower Na⁺ and higher K⁺/Na⁺ ratios and upregulated expression of salt-related genes OsSOS1 and OsHAK5 compared with the wild-type (WT) Nipponbare plants. Furthermore, Overexpression of OsABT decreased the abscisic acid (ABA) content, while downregulating the ABA synthesis genes OsNCED3 and OsNCED4 and upregulating the ABA catabolic gene OsABA8ox2. The yeast two-hybrid and bimolecular fluorescence complementation analyses showed that OsABT interacted with the ABA receptor proteins OsPYL4, OsPYL10, and PP2C phosphatase OsABIL2. A transcriptome analysis revealed that the differentially expressed genes between OsABT overexpression lines and WT plants were enriched in plant hormone signal transduction, including ABA signaling pathway under salt stress. Thus, OsABT can improve the salt tolerance in rice seedling roots by inhibiting reactive oxygen species accumulation, thereby regulating the intracellular Na⁺/K⁺ balance, ABA content, and ABA signaling pathway.

Role of Glycine max ABSCISIC ACID INSENSITIVE 3 (GmABI3) in lipid biosynthesis and stress tolerance in soybean

Soybean is an important oilseed crop and primary dietary protein resource. The limited understanding of soybean oil biosynthesis has become a significant obstacle for the improvement of soybean oil production. A transcription factor ABSCISIC ACID INSENSITIVE 3 (ABI3) is known for its role in plant development and seed dormancy in many crops. The current study was aimed to functionally characterise ABI3 homologue in Glycine max L. For this purpose, the GmABI3 gene was cloned and ectopically expressed in wildtype and abi3 mutant Arabidopsis. The GmABI3 expression in the atabi3 mutant enhanced the triacylglycerol (TAG) content (7.3%) in addition to modified fatty acid composition. The GmABI3 increased eicosenoic acid (20:1) up to 6.5% in genetically complemented Arabidopsis mutant seeds, which is essential for long-chain fatty acid synthesis. The transgenic GmABI3/wildtype seeds contain 34.9% more TAG content compared with wildtype seeds. The results showed that GmABI3 is responsible for seed-specific TAG and long-chain fatty acid biosynthesis in soybean. The exposure to cold and heat stress and exogenous supply of abscisic acid and jasmonic acid altered the level of GmABI3 in treated seeds and leaves. It also concluded that GmABI3 could regulate stress tolerance in soybean, which applies to a wide variety of crops to deal with biological stresses.

Biotechnological Perspectives of Omics and Genetic Engineering Methods in Alfalfa

For several decades, researchers are working to develop improved major crops with better adaptability and tolerance to environmental stresses. Forage legumes have been widely spread in the world due to their great ecological and economic values. Abiotic and biotic stresses are main factors limiting legume production, however, alfalfa (Medicago sativa L.) shows relatively high level of tolerance to drought and salt stress. Efforts focused on alfalfa improvements have led to the release of cultivars with new traits of agronomic importance such as high yield, better stress tolerance or forage quality. Alfalfa has very high nutritional value due to its efficient symbiotic association with nitrogen-fixing bacteria, while deep root system can help to prevent soil water loss in dry lands. The use of modern biotechnology tools is challenging in alfalfa since full genome, unlike to its close relative barrel medic (Medicago truncatula Gaertn.), was not released yet. Identification, isolation, and improvement of genes involved in abiotic or biotic stress response significantly contributed to the progress of our understanding how crop plants cope with these environmental challenges. In this review, we provide an overview of the progress that has been made in high-throughput sequencing, characterization of genes for abiotic or biotic stress tolerance, gene editing, as well as proteomic and metabolomics techniques bearing biotechnological potential for alfalfa improvement.

Identification of Single-Nucleotide Polymorphic Loci Associated with Biomass Yield under Water Deficit in Alfalfa (Medicago sativa L.) Using Genome-Wide Sequencing and Association Mapping

Alfalfa is a worldwide grown forage crop and is important due to its high biomass production and nutritional value. However, the production of alfalfa is challenged by adverse environmental factors such as drought and other stresses. Developing drought resistance alfalfa is an important breeding target for enhancing alfalfa productivity in arid and semi-arid regions. In the present study, we used genotyping-by-sequencing and genome-wide association to identify marker loci associated with biomass yield under drought in the field in a panel of diverse germplasm of alfalfa. A total of 28 markers at 22 genetic loci were associated with yield under water deficit, whereas only four markers associated with the same trait under well-watered condition. Comparisons of marker-trait associations between water deficit and well-watered conditions showed non-similarity except one. Most of the markers were identical across harvest periods within the treatment, although different levels of significance were found among the three harvests. The loci associated with biomass yield under water deficit located throughout all chromosomes in the alfalfa genome agreed with previous reports. Our results suggest that biomass yield under drought is a complex quantitative trait with polygenic inheritance and may involve a different mechanism compared to that of non-stress. BLAST searches of the flanking sequences of the associated loci against DNA databases revealed several stress-responsive genes linked to the drought resistance loci, including leucine-rich repeat receptor-like kinase, B3 DNA-binding domain protein, translation initiation factor IF2, and phospholipase-like protein. With further investigation, those markers closely linked to drought resistance can be used for MAS to accelerate the development of new alfalfa cultivars with improved resistance to drought and other abiotic stresses.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

MsZEP, a novel zeaxanthin epoxidase gene from alfalfa (Medicago sativa), confers drought and salt tolerance in transgenic tobacco

KEY MESSAGE

The zeaxanthin epoxidase gene ( MsZEP ) was cloned and characterized from alfalfa and validated for its function of tolerance toward drought and salt stresses by heterologous expression in Nicotiana tabacum. Zeaxanthin epoxidase (ZEP) plays important roles in plant response to various environment stresses due to its functions in ABA biosynthetic and the xanthophyll cycle. To understand the expression characteristics and the biological functions of ZEP in alfalfa (Medicago sativa), a novel gene, designated as MsZEP (KM044311), was cloned, characterized and overexpressed in Nicotiana tabacum. The open reading frame of MsZEP contains 1992 bp nucleotides and encodes a 663-amino acid polypeptide. Amino acid sequence alignment indicated that deduced MsZEP protein was highly homologous to other plant ZEP sequences. Phylogenetic analysis showed that MsZEP was grouped into a branch with other legume plants. Real-time quantitative PCR revealed that MsZEP gene expression was clearly tissue-specific, and the expression levels were higher in green tissues (leaves and stems) than in roots. MsZEP expression decreased in shoots under drought, cold, heat and ABA treatment, while the expression levels in roots showed different trends. Besides, the results showed that nodules could up-regulate the MsZEP expression under non-stressful conditions and in the earlier stage of different abiotic stress. Heterologous expression of the MsZEP gene in N. tabacum could confer tolerance to drought and salt stress by affecting various physiological pathways, ABA levels and stress-responsive genes expression. Taken together, these results suggested that the MsZEP gene may be involved in alfalfa responses to different abiotic stresses and nodules, and could enhance drought and salt tolerance of transgenic tobacco by heterologous expression.

Identification of Loci Associated with Drought Resistance Traits in Heterozygous Autotetraploid Alfalfa (Medicago sativa L.) Using Genome-Wide Association Studies with Genotyping by Sequencing

Drought resistance is an important breeding target for enhancing alfalfa productivity in arid and semi-arid regions. Identification of genes involved in drought tolerance will facilitate breeding for improving drought resistance and water use efficiency in alfalfa. Our objective was to use a diversity panel of alfalfa accessions comprised of 198 cultivars and landraces to identify genes involved in drought tolerance. The panel was selected from the USDA-ARS National Plant Germplasm System alfalfa collection and genotyped using genotyping by sequencing. A greenhouse procedure was used for phenotyping two important traits associated with drought tolerance: drought resistance index (DRI) and relative leaf water content (RWC). Marker-trait association identified nineteen and fifteen loci associated with DRI and RWC, respectively. Alignments of target sequences flanking to the resistance loci against the reference genome of M. truncatula revealed multiple chromosomal locations. Markers associated with DRI are located on all chromosomes while markers associated with RWC are located on chromosomes 1, 2, 3, 4, 5, 6 and 7. Co-localizations of significant markers between DRI and RWC were found on chromosomes 3, 5 and 7. Most loci associated with DRI in this work overlap with the reported QTLs associated with biomass under drought in alfalfa. Additional significant markers were targeted to several contigs with unknown chromosomal locations. BLAST search using their flanking sequences revealed homology to several annotated genes with functions in stress tolerance. With further validation, these markers may be used for marker-assisted breeding new alfalfa varieties with drought resistance and enhanced water use efficiency.

System responses to long-term drought and re-watering of two contrasting alfalfa varieties

Systems analysis of two alfalfa varieties, Wisfal (Medicago sativa ssp. falcata var. Wisfal) and Chilean (M. sativa ssp. sativa var. Chilean), with contrasting tolerance/sensitivity to drought revealed common and divergent responses to drought stress. At a qualitative level, molecular, biochemical, and physiological responses to drought stress were similar in the two varieties, indicating that they employ the same strategies to cope with drought. However, quantitative differences in responses at all levels were revealed that may contribute to greater drought tolerance in Wisfal. These included lower stomatal density and conductance in Wisfal; delayed leaf senescence compared with Chilean; greater root growth following a drought episode, and greater accumulation of osmolytes, including raffinose and galactinol, and flavonoid antioxidants in roots and/or shoots of Wisfal. Genes encoding transcription factors and other regulatory proteins, and genes involved in the biosynthesis of osmolytes and (iso)flavonoids were differentially regulated between the two varieties and represent potential targets for improving drought tolerance in alfalfa in the future.

Nitrogen metabolism responses to water deficit act through both abscisic acid (ABA)-dependent and independent pathways in Medicago truncatula during post-germination

The modulation of primary nitrogen metabolism by water deficit through ABA-dependent and ABA-independent pathways was investigated in the model legume Medicago truncatula. Growth and glutamate metabolism were followed in young seedlings growing for short periods in darkness and submitted to a moderate water deficit (simulated by polyethylene glycol; PEG) or treated with ABA. Water deficit induced an ABA accumulation, a reduction of axis length in an ABA-dependent manner, and an inhibition of water uptake/retention in an ABA-independent manner. The PEG-induced accumulation of free amino acids (AA), principally asparagine and proline, was mimicked by exogenous ABA treatment. This suggests that AA accumulation under water deficit may be an ABA-induced osmolyte accumulation contributing to osmotic adjustment. Alternatively, this accumulation could be just a consequence of a decreased nitrogen demand caused by reduced extension, which was triggered by water deficit and exogenous ABA treatment. Several enzyme activities involved in glutamate metabolism and genes encoding cytosolic glutamine synthetase (GS1b; EC 6.3.1.2.), glutamate dehydrogenase (GDH3; EC 1.4.1.1.), and asparagine synthetase (AS; EC 6.3.1.1.) were up-regulated by water deficit but not by ABA, except for a gene encoding Δ(1)-pyrroline-5-carboxylate synthetase (P5CS; EC not assigned). Thus, ABA-dependent and ABA-independent regulatory systems would seem to exist, differentially controlling development, water content, and nitrogen metabolism under water deficit.

Molecular cloning and characterization of a novel dehydrin gene from Ginkgo biloba

A full-length cDNA encoding a dehydrin was cloned from the living fossil plant Ginkgo biloba by rapid amplification of cDNA ends (RACE). The cDNA, designated as GbDHN, was 813 bp long containing an open reading frame of 489 bp. The deduced GbDHN protein had 163 amino acid residues, which formed a 17 kDa polypeptide with a predicted isoelectric point (pI) of 5.75. GbDHN had an S-segment and a K-segment, indicative of dehydrins, but no Y-segments. Homology analysis indicated that the S-segment and K-segment of GbDHN shared identity with those of other reported dehydrins, indicating that GbDHN belonged to dehydrin superfamily. Genomic sequence of GbDHN was also cloned using genomic walker technology. By comparing genomic DNA with the cDNA, it was found that there was a 257-bp intron in this gene. Promoter analysis indicated that it contained six CAAT boxes, one TATA box, one ABRE box and one GC-motif in the 5'-flanking region. Southern blot analysis revealed that GbDHN belonged to a single copy gene family. RT-PCR analysis revealed that GbDHN constitutively expressed in stems and roots. The increased expression of GbDHN was detected when G. biloba seedlings were treated with exogenous abscisic acid (ABA), salt stress and drought stress. These results indicate that the GbDHN has the potential to play a role in response to ABA and environmental stresses that can cause plant dehydration.

Identification and expression analysis of cold-regulated genes from the cold-hardy Citrus relative Poncirus trifoliata (L.) Raf

Citrus is a cold-sensitive genus and most commercially important varieties of citrus are susceptible to freezes. On the other hand, Poncirus trifoliata (L.) Raf. is an interfertile Citrus relative that can tolerate temperatures as low as -26 degrees C when fully cold acclimated. Therefore, it has been used for improving cold tolerance in cold-sensitive commercial citrus rootstock varieties and in attempts to improve scion varieties. In this study, cDNA libraries were constructed from both 2-day cold-acclimated and from non-acclimated Poncirus seedlings using a subtractive hybridization method with the objective of identifying cold-regulated genes. A total of 192 randomly picked clones, 136 from the cold-induced library and 56 from the cold-repressed library, were sequenced. The majority of these clones showed sequence homology to previously identified cold-induced and/or environmental stress-regulated genes in Arabidopsis. In addition, some of them shared homology with cold and/or environmental stress-induced genes previously identified in other herbaceous and woody perennial plants and some showed no homology with sequences in GenBank. When these 192 cDNAs were analyzed by reverse northern blot with cold-acclimated and non-acclimated probes, 92 of the cDNAs displayed significantly increased expression, ranging from 2 to 49-fold, during cold acclimation; all 92 were from the cold-induced library. Surprisingly no clones displayed significantly repressed expression in response to cold. Analysis of a number of selected genes individually in northern blots of mRNA from cold-acclimated and non-acclimated plants largely confirmed the reverse northern analysis, verifying induction of expression of selected cDNAs in response to cold. The results showed that subtractive hybridization is an efficient method for identification of cold-induced genes in plants with limited sequence information available. This study also revealed that genes induced during cold acclimation of the cold-hardy citrus relative P. trifoliata are similar to those in Arabidopsis, indicating that similar pathways may be present and activated during cold acclimation in woody perennial plants.

Expression of PsGRP1, a novel glycine rich protein gene of Pisum sativum, is induced in developing fruit and seed and by ABA in pistil and root

A novel glycine-rich protein gene, PsGRP1, has been identified in Pisum sativum L. Accumulation of PsGRP1 transcripts was observed in reproductive organs and vegetative tissues. They were localized in endocarp sclerenchyma during fruit development in cells that will lignify. PsGRP1 expression was also detected in senescent pistils and developing seeds and induced by ABA treatment in presenescent pistils. A raise in the expression was also observed in roots after treatment with ABA or mannitol but not under cold stress. A mannitol treatment induced a rise in ABA levels and fluridone treatment counteracted the mannitol induction of PsGRP1 expression. The results suggest a possible role for PsGRP1 in differentiation of the endocarp sclerenchyma and during seed development, pistil senescence and osmotic stress under ABA control.

Proteomic profiling unravels insights into the molecular background underlying increased Aphanomyces euteiches-tolerance of Medicago truncatula

To investigate the molecular mechanisms underlying susceptibility of legumes to the root pathogen Aphanomyces euteiches (oomycota), comparative proteomic studies have been carried out. In a first approach, we have analysed two Medicago truncatula lines of the French CORE collection (F83.005-5 (R2002) and F83.005-9 (R2002)), which showed either increased or decreased susceptibility to A. euteiches as compared to the widely adopted line A17. Several proteins were identified to be differentially induced after pathogen challenge in the two M. truncatula accessions with altered disease susceptibility, whereof proteins with increased abundances in the more resistant line F83.005-9 could be involved in mechanisms that lead to an improved disease resistance. Among these proteins, we identified two proteasome alpha subunits, which might be involved in defense response. To broaden our studies on A. euteiches-tolerance of M. truncatula, we investigated two other phenomena that lead to an either increased A. euteiches-resistance or to an enhanced susceptibility. The topic of an enhanced plant resistance to A. euteiches was studied in plants showing a bioprotective effect of a pre-established arbuscular mycorrhiza (AM) symbiosis. Evaluation of root fresh weights and pathogen spreading in the root system clearly indicate that mycorrhizal plants show increased A. euteiches-resistance as compared to non-mycorrhizal plants. Proteome analyses revealed the induction of similar protein patterns as in the M. truncatula accessions with comparatively high resistance level to A. euteiches. In a third approach, increased A. euteiches susceptibility was effected by exogenous abscisic acid (ABA) application prior to root infection. Evaluation of the abundance levels of a group of pathogenesis related class 10 (PR10)-like proteins, which were previously identified to be regulated after A. euteiches infection, revealed a correlation between the abundance levels of these proteins and the A. euteiches infection level or severity.

Proteomic approach: identification of Medicago truncatula proteins induced in roots after infection with the pathogenic oomycete Aphanomyces euteiches

The legume root rot disease caused by the oomycete pathogen Aphanomyces euteiches is one major yield reducing factor in legume crop production. A comparative proteomic approach was carried out in order to identify proteins of the model legume Medicago truncatula which are regulated after an infection with A. euteiches . Several proteins were identified by two dimensional gel electrophoresis to be differentially expressed after pathogen challenge. Densitometric evaluation of expression values showed different regulation during the time-course analysed. Proteins regulated during the infection were identified by matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). Among the differentially expressed proteins, two encoded putative cell wall proteins and two were designated as small heat shock proteins. Furthermore, an isoform of the chalcone-O-methyltransferase was found to be increased in infected roots. The majority of induced proteins belonged to the family of class 10 of pathogenesis related proteins (PR10). Previously, various PR10-like proteins have been shown to be regulated by general stress or abscisic acid (ABA). Therefore, these proteins were further investigated concerning their regulation in response to drought stress and exogenous ABA-application. Complex regulation patterns were identified: three of the A. euteiches -induced PR10-like proteins were also induced by exogenous ABA- but none of them is induced after drought stress. In contrast, three of these proteins are down-regulated by drought stress. Hence, the strong expression of different PR10-family members and their regulation profiles indicates that this set of proteins plays a major role during root adaptations to various stress conditions.

Purification and characterization of a barley aleurone abscisic acid-binding protein

A protein designated ABAP1 and encoded by a novel gene (GenBank accession number AF127388) was purified and shown to specifically bind abscisic acid (ABA). ABAP1 protein is a 472-amino acid polypeptide containing a WW protein interaction domain and is induced by ABA in barley aleurone layers. Polyclonal antiidiotypic antibodies (AB2) cross-reacted with purified ABAP1 and with a corresponding 52-kDa protein associated with membrane fractions of ABA-treated barley aleurones. ABAP1 genes were detected in diverse monocot and dicot species, including wheat, tobacco, alfalfa, garden pea, and oilseed rape. The recombinant ABAP1 protein optimally bound (3)H-(+)-ABA at neutral pH. Denatured ABAP1 protein did not bind (3)H-(+)-ABA, nor did bovine serum albumin. The maximum specific binding as shown by Scatchard plot analysis was 0.8 mol of ABA mol(-1) protein with a linear function of r(2) = 0.94, an indication of one ABA-binding site with a dissociation constant (K(d)) of 28 x 10(-9) m. ABA binding in aleurone plasma membranes showed a maximum binding capacity of 330 nmol of ABA g(-1) protein with a K(d) of 26.5 x 10(-9) m. The similarities in the dissociation constants for ABA binding of the recombinant protein and that of the plasma membranes suggest that the protein within the plasma membrane fraction is the native form of ABAP1. The stereospecificity of ABAP1 was established by the incapability of ABA analogs and metabolites, including (-)-ABA, trans-ABA, phaseic acid, dihydrophaseic acid, and (+)-abscisic acid-glucose ester, to displace (3)H-(+)-ABA bound to ABAP1. However, two ABA precursors, (+)-ABA aldehyde and (+)-ABA alcohol, were able to displace (3)H-(+)-ABA, an indication that the structural requirement of ABAP1 at the C-1 position is not strict. Our data show that ABAP1 exerts high binding affinity for ABA. The interaction is reversible, follows saturation kinetics, and has stereospecificity, thus meeting the criteria for an ABA-binding protein.

Linear amplification coupled with controlled extension as a means of probe amplification in a cDNA array and gene expression analysis during cold acclimation in alfalfa (Medicago sativa L.)

This study describes a rapid and simple way to amplify limited amounts of probes used for cDNA array hybridization while maintaining the original representation of transcripts in the samples. The approach is based on linear amplification of cDNA-coupled controlled extension of amplified products and yielded a 50-75-fold increases in hybridization signal intensity. Controlled extension of products is achieved either by adjusting the amplification conditions or by using a digested template. Linear amplification with controlled extension generates a population of fragments consisting mainly of 3'-end portions of original transcripts and ranging in length from 200 to 800 nucleotides. cDNA array analysis revealed that amplified and non-amplified probes generate expression profiles with correlations ranging from r=0.857 to 0.895. Up to 90% of cDNA clones, differentially expressed during cold acclimation in alfalfa, could be detected with both types of probes. This amplification method should increase the utility of cDNA arrays for identifying novel differentially expressed genes as well as expression profiling in specialized tissues or cells when the amount of analysed material is limited. The possibility of diminishing cross-hybridization of long genes sharing high sequence homology and improving the hybridization kinetics of complex probes after amplification is also discussed.

Plant glycine-rich proteins: a family or just proteins with a common motif?

Twelve years ago a set of glycine-rich proteins (GRP) of plants were characterized and since then a wealth of new GRPs have been identified. The highly specific but diverse expression pattern of grp genes, taken together with the distinct sub-cellular localisation of some GRP groups, clearly indicate that these proteins are implicated in several independent physiological processes. Notwithstanding the absence of a clear definition of the role of GRPs in plant cells, studies conducted with these proteins have provided new and interesting insights on the molecular and cell biology of plants. Complex regulated promoters and distinct mechanisms of gene expression regulation have been demonstrated. New protein targeting pathways, as well as the exportation of GRPs from different cell types have been discovered. These data show that GRPs can be useful as markers and/or models to understand distinct aspects of plant biology. In this review, the structural and functional features of this family of plant proteins will be summarised. Special emphasis will be given to the gene expression regulation of GRPs isolated from different plant species, as it can help to unravel their possible biological functions.

PLANT COLD ACCLIMATION: Freezing Tolerance Genes and Regulatory Mechanisms

Many plants increase in freezing tolerance upon exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. In this review, recent advances in determining the nature and function of genes with roles in freezing tolerance and the mechanisms involved in low temperature gene regulation and signal transduction are described. One of the important conclusions to emerge from these studies is that cold acclimation includes the expression of certain cold-induced genes that function to stabilize membranes against freeze-induced injury. In addition, a family of Arabidopsis transcription factors, the CBF/DREB1 proteins, have been identified that control the expression of a regulon of cold-induced genes that increase plant freezing tolerance. These results along with many of the others summarized here further our understanding of the basic mechanisms that plants have evolved to survive freezing temperatures. In addition, the findings have potential practical applications as freezing temperatures are a major factor limiting the geographical locations suitable for growing crop and horticultural plants and periodically account for significant losses in plant productivity.

Mode of action of the COR15a gene on the freezing tolerance of Arabidopsis thaliana

Constitutive expression of the cold-regulated COR15a gene of Arabidopsis thaliana results in a significant increase in the survival of isolated protoplasts frozen over the range of -4.5 to -7 degreesC. The increased freezing tolerance is the result of a decreased incidence of freeze-induced lamellar-to-hexagonal II phase transitions that occur in regions where the plasma membrane is brought into close apposition with the chloroplast envelope as a result of freeze-induced dehydration. Moreover, the mature polypeptide encoded by this gene, COR15am, increases the lamellar-to-hexagonal II phase transition temperature of dioleoylphosphatidylethanolamine and promotes formation of the lamellar phase in a lipid mixture composed of the major lipid species that comprise the chloroplast envelope. We propose that COR15am, which is located in the chloroplast stroma, defers freeze-induced formation of the hexagonal II phase to lower temperatures (lower hydrations) by altering the intrinsic curvature of the inner membrane of the chloroplast envelope.

THE MOLECULAR BASIS OF DEHYDRATION TOLERANCE IN PLANTS

Molecular studies of drought stress in plants use a variety of strategies and include different species subjected to a wide range of water deficits. Initial research has by necessity been largely descriptive, and relevant genes have been identified either by reference to physiological evidence or by differential screening. A large number of genes with a potential role in drought tolerance have been described, and major themes in the molecular response have been established. Particular areas of importance are sugar metabolism and late-embryogenesis-abundant (LEA) proteins. Studies have begun to examine mechanisms that control the gene expression, and putative regulatory pathways have been established. Recent attempts to understand gene function have utilized transgenic plants. These efforts are of clear agronomic importance.

Post-transcriptional regulation of bifunctional alpha-amylase/subtilisin inhibitor expression in barley embryos by abscisic acid

Changes in bifunctional alpha-amylase/subtilisin inhibitor (BASI) expression induced by abscisic acid (ABA) were studied using in vitro cultured barley (Hordeum vulgare cv. Bonanza) embryos. The steady-state levels of BASI mRNA and BASI protein were increased by exogenously applied ABA. Accumulation of BASI protein was preceded by an increase in message level. The results suggest that ABA does not affect BASI mRNA translation. Nuclear run-on assays demonstrated that ABA had no effect on transcriptional activity. BASI mRNA was not detectable in the embryos treated with a protein synthesis inhibitor, cycloheximide, which had no inhibitory effect on BASI transcription rate. We propose that ABA increases the stability of BASI mRNA through synthesis of a short-lived protein that protects the message.

Induction of chilling resistance by water stress, and cDNA sequence analysis and expression of water stress-regulated genes in rice

Exposure of seedlings of a chilling-sensitive variety of rice (Oryza sativa L. cv. Wasetoittu) to water stress (0.5 M mannitol, 30 min) at room temperature induced a degree of chilling resistance. No such resistance was induced by exogenous abscisic acid (ABA) application (10 microM, 60 min). Upon short-term water stress, new transcripts were expressed in both seedlings and suspension-cultured cells. We suggest that the genes induced by short-term water stress, and not those induced by ABA, are related to acquired chilling resistance in this chilling-sensitive rice variety. A total of nine different cDNA clones, specifically induced by short-term water stress, were isolated by differential hybridization and partial sequencing. Northern hybridization analysis using RNAs from the seedlings subjected to chilling after water stress treatment reveal three distinct groups of above mentioned nine cDNA clones: wsi (water stress-induced) 18, 76, and 724, representative of genes whose expression increases, decreases, and remains almost fixed during chilling, respectively. The nucleotide and deduced amino acid sequences of the three representative clones were determined. Characteristic features of wsi18 are the presence of one set of amino acid sequence repeats, a conserved amino acid sequence common to LEA-group genes in the N-terminal region, and an alanine- and lysine-rich tract in the C-terminal region.

A cold-induced gene from Medicago sativa encodes a bimodular protein similar to developmentally regulated proteins

A new cold-regulated (COR) gene, msa CIC, was isolated by differential screening of a cDNA library from cold-acclimated crowns of alfalfa (Medicago sativa L. cv. Apica). Transcripts of msa CIC were not detectable in unacclimated alfalfa and accumulated to higher levels in cold-acclimated plants of the cold-tolerant cv. Apica than in those of the cold-sensitive cv. CUF-101. The DNA sequence analysis of a full-length cDNA clone revealed that msa CIC encodes for a putative protein (MSACIC) of 166 amino acids with distinct proline-rich and hydrophobic domains. Protein sequence comparisons indicated that MSACIC is similar to a group of bimodular proteins that are developmentally regulated in other plant species.

Two ABA-responsive proteins from pea (Pisum sativum L.) are closely related to intracellular pathogenesis-related proteins

We report here the isolation of cDNAs encoding two abscisic acid-responsive pea (Pisum sativum L.) proteins, ABR17 and ABR18, which are synthesized during late seed development in vivo. Southern blot analyses suggest that ABR17 cDNA corresponds to a single-copy gene, but ABR18 is one member of a family of closely related sequences in the pea genome. The deduced amino acid sequences of ABR17 and ABR18 cDNAs showed similarity to those of the pea disease resistance response proteins, to pathogenesis-related and to stress-induced proteins in other species and to the major birch pollen allergen Betv1.

Two related, low-temperature-induced genes from Brassica napus are homologous to the human tumour bbc1 (breast basic conserved) gene

In order to identify genes involved in cold acclimation, we have constructed a cDNA library from Brassica napus (cv. Samouraï) cold-acclimated etiolated seedlings. By differential screening, a cDNA clone named pBnC24 (Brassica napus Cold), corresponding to a new cold-inducible plant gene, was isolated. Northern blot hybridizations using total RNA from acclimated and unacclimated seedlings confirmed that BnC24 represents a cold-regulated gene. In contrast with a number of cold-inducible plant genes, BnC24 does not seem to be responsive to abscisic acid (ABA). In addition, further screening of the 'cold-acclimated' cDNA library using pBnC24 cDNA as a probe, allowed the isolation of a second type of homologous cDNA. Sequence analysis showed that the two BnC24 genes encode basic 24 kDa proteins, which are highly hydrophilic and rich in alanine, lysine and arginine. The nucleotide and deduced amino acid sequences of these clones do not show any homology with other previously described cold-induced plants genes. However they have strong homology with a recently discovered human tumour gene, bbcl (breast basic conserved), which seems to be highly conserved in eukaryotes.

A novel blue light- and abscisic acid-inducible gene of Arabidopsis thaliana encoding an intrinsic membrane protein

Continuous irradiation with blue light (400-500 nm) induces flower formation in plantlets of Arabidopsis thaliana (C24) while red light (600-700 nm) is ineffective. This observation started a search for genes that are activated by blue light and initiate the morphogenic programme leading to flower formation. Several genes were identified via their cDNAs. From these clone AthH2, with an open reading frame for a hydrophobic 30.5 kDa polypeptide, was selected for further characterization of the corresponding gene. From a genomic library a DNA fragment of about 6.4 kb was isolated, comprising the coding region as well as 5'-upstream and 3'-downstream flanking segments. The coding region is composed of four exons, which specify a polypeptide of 286 amino acids. Several potential regulatory elements were found between position -670 and -1140 including GA and ABA sequence motifs. The latter could account for the observed induction of the AthH2 gene by ABA. Southern blot analysis of Arabidopsis genomic DNA suggests that the AthH2 gene is encoded by a single-copy gene. Hydropathy plots and secondary structure analysis of the putative polypeptide predict six membrane-spanning domains implicating a function as transmembrane channel protein. It displays significant homology with the proteins TR7a of pea (82%) and RD 28 of A. thaliana (68%).

A leaf-specific gene stimulated by light during wheat acclimation to low temperature

We report here the identification and characterization of a new leaf-specific light-stimulated gene induced during cold acclimation of wheat. Sequence analysis revealed that the gene encodes a protein of 19 kDa with a pI of 8.8. This is a novel protein with a particular charge distribution. The C-terminal half has a high propensity to form an alpha-helix and contains all the acidic amino acids with a net negative charge of -7. On the other hand, the N-terminal half is rich in proline, lysine and arginine with a net positive charge of +10. These properties are commonly found in several transcription factors. The protein is also rich in alanine (21%), is hydrophilic but not boiling soluble in contrast to other alanine-rich proteins. During low temperature exposure, the corresponding mRNA accumulates rapidly in the leaf and remains at a constant level in two tolerant cultivars used. However, in a less tolerant cultivar, the mRNA level declines despite maintaining the plants at 4 degrees C. Southern blot analysis indicates that the differential expression in the less tolerant genotype is not due to a different genomic organization or gene copy number. The mRNA was specifically localized in leaf tissues and increased several-fold during the greening at 4 degrees C. Furthermore, this gene is not induced in callus cultures acclimated in the absence or presence of light. This suggests that the full expression of this gene is dependent on organized leaf tissue. The expression of this gene was not affected by ABA, drought, heat shock, salinity, wounding or anaerobiosis, demonstrating that it is specifically induced by low temperature. The Wcs19 mRNA is preferentially expressed in tolerant Gramineae species.