Molecular and physiological responses to water deficit in drought-tolerant and drought-sensitive lines of sunflower. Accumulation of dehydrin transcripts correlates with tolerance

Plant dehydrins and dehydrin-like proteins: characterization and participation in abiotic stress response

Abiotic stress has a significant impact on plant growth and development. It causes changes in the subcellular organelles, which, due to their stress sensitivity, can be affected. Cellular components involved in the abiotic stress response include dehydrins, widely distributed proteins forming a class II of late embryogenesis abundant protein family with characteristic properties including the presence of evolutionarily conserved sequence motifs (including lysine-rich K-segment, N-terminal Y-segment, and often phosphorylated S motif) and high hydrophilicity and disordered structure in the unbound state. Selected dehydrins and few poorly characterized dehydrin-like proteins participate in cellular stress acclimation and are also shown to interact with organelles. Through their functioning in stabilizing biological membranes and binding reactive oxygen species, dehydrins and dehydrin-like proteins contribute to the protection of fragile organellar structures under adverse conditions. Our review characterizes the participation of plant dehydrins and dehydrin-like proteins (including some organellar proteins) in plant acclimation to diverse abiotic stress conditions and summarizes recent updates on their structure (the identification of dehydrin less conserved motifs), classification (new proposed subclasses), tissue- and developmentally specific accumulation, and key cellular activities (including organellar protection under stress acclimation). Recent findings on the subcellular localization (with emphasis on the mitochondria and plastids) and prospective applications of dehydrins and dehydrin-like proteins in functional studies to alleviate the harmful stress consequences by means of plant genetic engineering and a genome editing strategy are also discussed.

Involvement of dehydrin proteins in mitigating the negative effects of drought stress in plants

Drought stress-induced crop loss has been considerably increased in recent years because of global warming and changing rainfall pattern. Natural drought-tolerant plants entail the recruitment of a variety of metabolites and low molecular weight proteins to negate the detrimental effects of drought stress. Dehydrin (DHN) proteins are one such class of proteins that accumulate in plants during drought and associated stress conditions. These proteins are highly hydrophilic and perform multifaceted roles in the protection of plant cells during drought stress conditions. Evidence gathered over the years suggests that DHN proteins impart drought stress tolerance by enhancing the water retention capacity, elevating chlorophyll content, maintaining photosynthetic machinery, activating ROS detoxification, and promoting the accumulation of compatible solutes, among others. Overexpression studies have indicated that these proteins can be effectively targeted to mitigate the negative effects of drought stress and for the development of drought stress-tolerant crops to feed the ever-growing population in the near future. In this review, we describe the mechanism of DHNs mediated drought stress tolerance in plants and their interaction with several phytohormones to provide an in-depth understanding of DHNs function.

Plant Group II LEA Proteins: Intrinsically Disordered Structure for Multiple Functions in Response to Environmental Stresses

In response to various environmental stresses, plants have evolved a wide range of defense mechanisms, resulting in the overexpression of a series of stress-responsive genes. Among them, there is certain set of genes that encode for intrinsically disordered proteins (IDPs) that repair and protect the plants from damage caused by environmental stresses. Group II LEA (late embryogenesis abundant) proteins compose the most abundant and characterized group of IDPs; they accumulate in the late stages of seed development and are expressed in response to dehydration, salinity, low temperature, or abscisic acid (ABA) treatment. The physiological and biochemical characterization of group II LEA proteins has been carried out in a number of investigations because of their vital roles in protecting the integrity of biomolecules by preventing the crystallization of cellular components prior to multiple stresses. This review describes the distribution, structural architecture, and genomic diversification of group II LEA proteins, with some recent investigations on their regulation and molecular expression under various abiotic stresses. Novel aspects of group II LEA proteins in Phoenix dactylifera and in orthodox seeds are also presented. Genome-wide association studies (GWAS) indicated a ubiquitous distribution and expression of group II LEA genes in different plant cells. In vitro experimental evidence from biochemical assays has suggested that group II LEA proteins perform heterogenous functions in response to extreme stresses. Various investigations have indicated the participation of group II LEA proteins in the plant stress tolerance mechanism, spotlighting the molecular aspects of group II LEA genes and their potential role in biotechnological strategies to increase plants' survival in adverse environments.

CaDHN4, a Salt and Cold Stress-Responsive Dehydrin Gene from Pepper Decreases Abscisic Acid Sensitivity in Arabidopsis

Dehydrins play an important role in improving plant resistance to abiotic stresses. In this study, we isolated a dehydrin gene from pepper (Capsicum annuum L.) leaves, designated as CaDHN4. Sub-cellular localization of CaDHN4 was to be found in the nucleus and membrane. To authenticate the function of CaDHN4 in cold- and salt-stress responses and abscisic acid (ABA) sensitivity, we reduced the CaDHN4 expression using virus-induced gene silencing (VIGS), and overexpressed the CaDHN4 in Arabidopsis. We found that silencing of CaDHN4 reduced the growth of pepper seedlings and CaDHN4-silenced plants exhibited more serious wilting, higher electrolyte leakage, and more accumulation of ROS in the leaves compared to pTRV2:00 plants after cold stress, and lower chlorophyll contents and higher electrolyte leakage compared to pTRV2:00 plants under salt stress. However, CaDHN4-overexpressing Arabidopsis plants had higher seed germination rates and post-germination primary root growth, compared to WT plants under salt stress. In response to cold and salt stresses, the CaDHN4-overexpressed Arabidopsis exhibited lower MDA content, and lower relative electrolyte leakage compared to the WT plants. Under ABA treatments, the fresh weight and germination rates of transgenic plants were higher than WT plants. The transgenic Arabidopsis expressing a CaDHN4 promoter displayed a more intense GUS staining than the normal growth conditions under treatment with hormones including ABA, methyl jasmonate (MeJA), and salicylic acid (SA). Our results suggest that CaDHN4 can protect against cold and salt stresses and decrease ABA sensitivity in Arabidopsis.

Integration of transcriptomic and metabolic data reveals hub transcription factors involved in drought stress response in sunflower (Helianthus annuus L.)

By integration of transcriptional and metabolic profiles we identified pathways and hubs transcription factors regulated during drought conditions in sunflower, useful for applications in molecular and/or biotechnological breeding. Drought is one of the most important environmental stresses that effects crop productivity in many agricultural regions. Sunflower is tolerant to drought conditions but the mechanisms involved in this tolerance remain unclear at the molecular level. The aim of this study was to characterize and integrate transcriptional and metabolic pathways related to drought stress in sunflower plants, by using a system biology approach. Our results showed a delay in plant senescence with an increase in the expression level of photosynthesis related genes as well as higher levels of sugars, osmoprotectant amino acids and ionic nutrients under drought conditions. In addition, we identified transcription factors that were upregulated during drought conditions and that may act as hubs in the transcriptional network. Many of these transcription factors belong to families implicated in the drought response in model species. The integration of transcriptomic and metabolomic data in this study, together with physiological measurements, has improved our understanding of the biological responses during droughts and contributes to elucidate the molecular mechanisms involved under this environmental condition. These findings will provide useful biotechnological tools to improve stress tolerance while maintaining crop yield under restricted water availability.

An analysis of the development of cauliflower seed as a model to improve the molecular mechanism of abiotic stress tolerance in cauliflower artificial seeds

The development stages of conventional cauliflower seeds were studied and the accumulation of dehydrin proteins through the maturation stages was investigated with the aim of identifying methods to improve the viability of artificial seeds of cauliflower. While carbohydrate, ash and lipids increased throughout the development of cauliflower traditional seeds, proteins increased with the development of seed and reached the maximum level after 75 days of pollination, however, the level of protein started to decrease after that. A significant increase in the accumulation of small size dehydrin proteins (12, 17, 26 KDa) was observed during the development of cauliflower seeds. Several experiments were conducted in order to increase the accumulation of important dehydrin proteins in cauliflower microshoots (artificial seeds). Mannitol and ABA (Absisic acid) increased the accumulation of dehydrins in cauliflower microshoots while cold acclimation did not have a significant impact on the accumulation of these proteins. Molybdenum treatments had a negative impact on dehydrin accumulation. Dehydrins have an important role in the drought tolerance of seeds and, therefore, the current research helps to improve the accumulation of these proteins in cauliflower artificial seeds. This in turns improves the quality of these artificial seeds. The current results suggest that dehydrins do not play an important role in cold tolerance of cauliflower artificial seeds. This study could have an important role in improving the understanding of the molecular mechanism of abiotic stress tolerance in plants.

Faba bean drought responsive gene identification and validation

This study was carried out to identify drought-responsive genes in a drought tolerant faba bean variety (Hassawi 2) using a suppressive subtraction hybridization approach (SSH). A total of 913 differentially expressed clones were sequenced from a differential cDNA library that resulted in a total of 225 differentially expressed ESTs. The genes of mitochondrial and chloroplast origin were removed, and the remaining 137 EST sequences were submitted to the gene bank EST database (LIBEST_028448). A sequence analysis identified 35 potentially drought stress-related ESTs that regulate ion channels, kinases, and energy production and utilization and transcription factors. Quantitative PCR on Hassawi 2 genotype confirmed that more than 65% of selected drought-responsive genes were drought-related. Among these induced genes, the expression levels of eight highly up-regulated unigenes were further analyzed across 38 selected faba bean genotypes that differ in their drought tolerance levels. These unigenes included ribulose 1,5-bisphosphate carboxylase (rbcL) gene, non-LTR retroelement reverse related, probable cyclic nucleotide-gated ion channel, polyubiquitin, potassium channel, calcium-dependent protein kinase and putative respiratory burst oxidase-like protein C and a novel unigene. The expression patterns of these unigenes were variable across 38 genotypes however, it was found to be very high in tolerant genotype. The up-regulation of these unigenes in majority of tolerant genotypes suggests their possible role in drought tolerance. The identification of possible drought responsive candidate genes in Vicia faba reported here is an important step toward the development of drought-tolerant genotypes that can cope with arid environments.

Stress Sensitivity Is Associated with Differential Accumulation of Reactive Oxygen and Nitrogen Species in Maize Genotypes with Contrasting Levels of Drought Tolerance

Drought stress decreases crop growth, yield, and can further exacerbate pre-harvest aflatoxin contamination. Tolerance and adaptation to drought stress is an important trait of agricultural crops like maize. However, maize genotypes with contrasting drought tolerances have been shown to possess both common and genotype-specific adaptations to cope with drought stress. In this research, the physiological and metabolic response patterns in the leaves of maize seedlings subjected to drought stress were investigated using six maize genotypes including: A638, B73, Grace-E5, Lo964, Lo1016, and Va35. During drought treatments, drought-sensitive maize seedlings displayed more severe symptoms such as chlorosis and wilting, exhibited significant decreases in photosynthetic parameters, and accumulated significantly more reactive oxygen species (ROS) and reactive nitrogen species (RNS) than tolerant genotypes. Sensitive genotypes also showed rapid increases in enzyme activities involved in ROS and RNS metabolism. However, the measured antioxidant enzyme activities were higher in the tolerant genotypes than in the sensitive genotypes in which increased rapidly following drought stress. The results suggest that drought stress causes differential responses to oxidative and nitrosative stress in maize genotypes with tolerant genotypes with slower reaction and less ROS and RNS production than sensitive ones. These differential patterns may be utilized as potential biological markers for use in marker assisted breeding.

Response of sunflower (Helianthus annuus L) genotypes to PEG-mediated water stress

Drought tolerance of two sunflower (Helianthus annuus L.) genotypes, cultivated cultivar 1114 and interspecific line H. annuus × H. mollis, was studied under laboratory conditions using PEG-6000. Four levels of osmotic stress (−0.4, −0.6, −0.8 and −1.0 MPa) were created and performances were monitored against a control. Physiological and biochemical stress determining parameters such as malondialdechyde (MDA), proline content, and hydrogen peroxide (H2O2) were compared between seedlings of both genotypes. The results indicated that both genotypes have similar responses at four osmotic potentials for all traits studied. All seedling growth parameters such as germination percentage, root length, shoot length, root and shoot dry weight decreased with increasing osmotic stress. MDA, proline, and H2O2 were found to be increased at different osmotic gradients in comparison to control. Cultivar 1114 was less affected than the interspecific line under these stress conditions. The data observed in the experiments revealed that perennial wild H. mollis can hardly be considered to be an excellent candidate of drought tolerance genes.

Identification and functional roles of CaDIN1 in abscisic acid signaling and drought sensitivity

Plants frequently face challenges caused by various abiotic stresses, including drought, and have evolved defense mechanisms to counteract the deleterious effects of these stresses. The phytohormone abscisic acid (ABA) is involved in signal transduction pathways that mediate defense responses of plants to abiotic stress. Here, we report a new function of the CaDIN1 protein in defense responses to abiotic stress. The CaDIN1 gene was strongly induced in pepper leaves exposed to ABA, NaCl, and drought stresses. CaDIN1 proteins share high sequence homology with other known DIN1 proteins and are localized in chloroplasts. We generated CaDIN1-silenced peppers and overexpressing transgenic Arabidopsis plants and evaluated their response to ABA and drought stress. Virus-induced gene silencing of CaDIN1 in pepper plants conferred enhanced tolerance to drought stress, which was accompanied by low levels of lipid peroxidation in dehydrated leaves. CaDIN1-overexpressing transgenic plants exhibited reduced sensitivity to ABA during seed germination and seedling stages. Transgenic plants were more vulnerable to drought than that by the wild-type plants because of decreased expression of ABA responsive stress-related genes and reduced stomatal closure in response to ABA. Together, these results suggest that CaDIN1 modulates drought sensitivity through ABA-mediated cell signaling.

Physiological and gene expression responses of sunflower (Helianthus annuus L.) plants differ according to irrigation placement

To investigate effects of soil moisture heterogeneity on plant physiology and gene expression in roots and leaves, three treatments were implemented in sunflower plants growing with roots split between two compartments: a control (C) treatment supplying 100% of plant evapotranspiration, and two treatments receiving 50% of plant evapotranspiration, either evenly distributed to both compartments (deficit irrigation - DI) or unevenly distributed to ensure distinct wet and dry compartments (partial rootzone drying - PRD). Plants receiving the same amount of water responded differently under the two irrigation systems. After 3 days, evapotranspiration was similar in C and DI, but 20% less in PRD, concomitant with decreased leaf water potential (Ψleaf) and increased leaf xylem ABA concentration. Six water-stress responsive genes were highly induced in roots growing in the drying soil compartment of PRD plants, and their expression was best correlated with local soil water content. On the other hand, foliar gene expression differed significantly from that of the root and correlated better with xylem ABA concentration and Ψleaf. While the PRD irrigation strategy triggered stronger physiological and molecular responses, suggesting a more intense and systemic stress reaction due to local dehydration of the dry compartment of PRD plants, the DI strategy resulted in similar water savings without strongly inducing these responses. Correlating physiological and molecular responses in PRD/DI plants may provide insights into the severity and location of water deficits and may enable a better understanding of long-distance signalling mechanisms.

Genome-wide analysis of drought induced gene expression changes in flax (Linum usitatissimum)

A robust phenotypic plasticity to ward off adverse environmental conditions determines performance and productivity in crop plants. Flax (linseed), is an important cash crop produced for natural textile fiber (linen) or oilseed with many health promoting products. This crop is prone to drought stress and yield losses in many parts of the world. Despite recent advances in drought research in a number of important crops, related progress in flax is very limited. Since, response of this plant to drought stress has not been addressed at the molecular level; we conducted microarray analysis to capture transcriptome associated with induced drought in flax. This study identified 183 differentially expressed genes (DEGs) associated with diverse cellular, biophysical and metabolic programs in flax. The analysis also revealed especially the altered regulation of cellular and metabolic pathways governing photosynthesis. Additionally, comparative transcriptome analysis identified a plethora of genes that displayed differential regulation both spatially and temporally. These results revealed co-regulated expression of 26 genes in both shoot and root tissues with implications for drought stress response. Furthermore, the data also showed that more genes are upregulated in roots compared to shoots, suggesting that roots may play important and additional roles in response to drought in flax. With prolonged drought treatment, the number of DEGs increased in both tissue types. Differential expression of selected genes was confirmed by qRT-PCR, thus supporting the suggested functional association of these intrinsic genes in maintaining growth and homeostasis in response to imminent drought stress in flax. Together the present study has developed foundational and new transcriptome data sets for drought stress in flax.

Disorder and function: a review of the dehydrin protein family

Dehydration proteins (dehydrins) are group 2 members of the late embryogenesis abundant (LEA) protein family. The protein architecture of dehydrins can be described by the presence of three types of conserved sequence motifs that have been named the K-, Y-, and S-segments. By definition, a dehydrin must contain at least one copy of the lysine-rich K-segment. Abiotic stresses such as drought, cold, and salinity cause the upregulation of dehydrin mRNA and protein levels. Despite the large body of genetic and protein evidence of the importance of these proteins in stress response, the in vivo protective mechanism is not fully known. In vitro experimental evidence from biochemical assays and localization experiments suggests multiple roles for dehydrins, including membrane protection, cryoprotection of enzymes, and protection from reactive oxygen species. Membrane binding by dehydrins is likely to be as a peripheral membrane protein, since the protein sequences are highly hydrophilic and contain many charged amino acids. Because of this, dehydrins in solution are intrinsically disordered proteins, that is, they have no well-defined secondary or tertiary structure. Despite their disorder, dehydrins have been shown to gain structure when bound to ligands such as membranes, and to possibly change their oligomeric state when bound to ions. We review what is currently known about dehydrin sequences and their structures, and examine the various ligands that have been shown to bind to this family of proteins.

Significant relationships among frost tolerance and net photosynthetic rate, water use efficiency and dehydrin accumulation in cold-treated winter oilseed rapes

Five winter oilseed rape cultivars (Benefit, Californium, Cortes, Ladoga, Navajo) were subjected to 30 days of cold treatment (4 °C) to examine the effect of cold on acquired frost tolerance (FT), dehydrin (DHN) content, and photosynthesis-related parameters. The main aim of this study was to determine whether there are relationships between FT (expressed as LT50 values) and the other parameters measured in the cultivars. While the cultivar Benefit accumulated two types of DHNs (D45 and D35), the other cultivars accumulated three additional DHNs (D97, D47, and D37). The similar-sized DHNs (D45 and D47) were the most abundant; the others exhibited significantly lower accumulations. The highest correlations were detected between LT50 and DHN accumulation (r=-0.815), intrinsic water use efficiency (WUEi; r=-0.643), net photosynthetic rate (r=-0.628), stomatal conductance (r=0.511), and intracellular/intercellular CO2 concentration (r=0.505). Those cultivars that exhibited higher Pn rate in cold (and further a significant increase in WUEi) had higher levels of DHNs and also higher FT. No significant correlation was observed between LT50 and E, PRI, or NDVI. Overall, we have shown the selected physiological parameters to be able to distinguish different FT cultivars of winter oilseed rape.

Drought effect on growth, gas exchange and yield, in two strains of local barley Ardhaoui, under water deficit conditions in southern Tunisia

Two local barley strains cv. Ardhaoui originated from Tlalit and Switir, sourthern Tunisia were grown in pots in a glasshouse assay, under well-watered conditions for a month. Plants were then either subjected to water deficit (treatment) or continually well-watered (control). Control pots were irrigated several times each week to maintain soil moisture near field capacity (FC), while stress pots experienced soil drying by withholding irrigation until they reached 50% of FC. Variation in relative water content, leaf area, leaf appearance rate and leaf gas exchange (i.e. net CO(2) assimilation rate (A), transpiration (E), and stomatal conductance (gs)) in response to water deficit was investigated. High leaf relative water content (RWC) was maintained in Tlalit by stomatal closure and a reduction of leaf area. Reduction in leaf area was due to decline in leaf gas exchange during water deficit. Tlalit was found to be drought tolerant and able to maintain higher leaf RWC under drought conditions. Water deficit treatment reduced stomatal conductance by 43% at anthesis. High net CO(2) assimilation rate under water deficit was associated with high RWC (r = 0.998; P < 0.01). Decline in net CO(2) assimilation rate was due mainly to stomatal closure. Significant differences between studied strains in leaf gas exchange parameters were found, which can give some indications on the degree of drought tolerance. Thus, the ability of the low leaf area plants to maintain higher RWC could explain the differences in drought tolerance in studied barley strains. Results showed that Tlalit showed to be more efficient and more productive than Switir.

Cloning and characterization of dehydrin gene from Ammopiptanthus mongolicus

Based on the sequence of an expressed sequence tag, the full-length cDNA of 1,008 nucleotides was cloned from Ammopiptanthus mongolicus by rapid amplification of cDNA ends. It was designated as AmDHN, encoding a protein of 183 amino acids. The calculated molecular weight of the AmDHN protein is 18.4 k Da, and theoretical isoelectric point is 5.78. The AmDHN localized in nucleus. Under normal growth conditions, differential expression of AmDHN exhibited that the expression was the highest in seeds and the lowest in flowers. AmDHN could be induced by NaCl, PEG6000, ABA and drought treatments. Salt and drought resistances of transgenic plants with overexpression of AmDHN are improved. Taken together, these results demonstrated that AmDHN could regulate the expression of abiotic-responsive genes and plays important roles in modulating the tolerance of plants to abiotic stresses.

The histone H1 variant accumulates in response to water stress in the drought tolerant genotype of Gossypium herbaceum L

We have optimized and improved the protocol for extraction of histone proteins from Gossypium herbaceum. Histone proteins were isolated by acid extraction method and fractionation of histone proteins were performed using RP-HPLC (reverse-phase high performance liquid chromatography). Analysis of histones from drought tolerant (Vagad) and drought sensitive genotype (RAHS-14) indicated that the tolerant genotype Vagad encodes a 29 kDa protein. Protein sequencing on MALDI TOF/TOF revealed that the 29 kDA protein shared sequence similarity with another drought-inducible linker histone-H1.S reported in tomato. This H1.S like linker histone was not found in RAHS-14 in our study. We further examined the expression of H1 variant at the transcript and protein levels and found that it was induced specifically in the tolerant genotype Vagad.

The physiology and proteomics of drought tolerance in maize: early stomatal closure as a cause of lower tolerance to short-term dehydration?

Understanding the response of a crop to drought is the first step in the breeding of tolerant genotypes. In our study, two maize (Zea mays L.) genotypes with contrasting sensitivity to dehydration were subjected to moderate drought conditions. The subsequent analysis of their physiological parameters revealed a decreased stomatal conductance accompanied by a slighter decrease in the relative water content in the sensitive genotype. In contrast, the tolerant genotype maintained open stomata and active photosynthesis, even under dehydration conditions. Drought-induced changes in the leaf proteome were analyzed by two independent approaches, 2D gel electrophoresis and iTRAQ analysis, which provided compatible but only partially overlapping results. Drought caused the up-regulation of protective and stress-related proteins (mainly chaperones and dehydrins) in both genotypes. The differences in the levels of various detoxification proteins corresponded well with the observed changes in the activities of antioxidant enzymes. The number and levels of up-regulated protective proteins were generally lower in the sensitive genotype, implying a reduced level of proteosynthesis, which was also indicated by specific changes in the components of the translation machinery. Based on these results, we propose that the hypersensitive early stomatal closure in the sensitive genotype leads to the inhibition of photosynthesis and, subsequently, to a less efficient synthesis of the protective/detoxification proteins that are associated with drought tolerance.

Physiology and proteomics of drought stress acclimation in sunflower (Helianthus annuus L.)

An easy and manageable in vitro screening system for drought tolerance of sunflower seedlings based on MS media supplemented with polyethylene glycol 6000 was evaluated. Morphological and physiological parameters were compared between control (-0.05 MPa) and drought-stressed (-0.6 MPa) seedlings of Helianthus annuus L. cv. Peredovick. There was a significant growth deficit in drought-stressed plants compared to control plants in terms of hypocotyl length, and shoot and root fresh mass. Shoot growth was more restricted than root growth, resulting in an increased root/shoot ratio of drought-stressed plants. Accumulation of osmolytes such as inositol (65-fold), glucose (58-fold), proline (55-fold), fructose (11-fold) and sucrose (eightfold), in leaves of drought-stressed plants could be demonstrated by gas-liquid chromatography. Soluble protein patterns of leaves were analysed with two-dimensional gel electrophoresis (2D-PAGE) and MALDI-TOF mass spectrometry. A set of 46 protein spots allowed identification of 19 marker proteins. Quantitative changes in protein expression of drought-stressed versus control plants were detected. In leaves of drought-stressed sunflower seedlings six proteins were significantly up-regulated more than twofold: a putative caffeoyl-CoA 3-O-methyltransferase (4.5-fold), a fructokinase 3 (3.3-fold), a vegetative storage protein (2.5-fold), a glycine-rich RNA binding protein (2.2-fold), a CuZn-superoxide dismutase (2.1-fold) and an unknown low molecular weight protein (2.3-fold). These proteins represent general stress proteins induced under drought conditions or proteins contributing to basic carbon metabolism. The up-regulated proteins are interesting candidates for further physiological and molecular investigations regarding drought tolerance in sunflower.

Biochemical and molecular changes in buckwheat leaves during exposure to salt stress

In spite of the great nutritive and pharmacological potentials of buckwheat, data about the abiotic stress tolerance of this plant species are very limited. The aim of this work was to analyze the biochemical and molecular response of buckwheat plants in the middle vegetative phase against short- and long-term salt stress. Changes in relative water content, level of lipid peroxidation, content and localization of H2O2 as well as changes in antioxidative enzyme activity and expression of ubiquitin and dehydrins, were investigated. Reasons for observed buckwheat salt stress sensitivity as well as possibilities for enhancing stress tolerance are discussed.

Terminal drought-tolerant pearl millet [Pennisetum glaucum (L.) R. Br.] have high leaf ABA and limit transpiration at high vapour pressure deficit

It was previously shown that pearl millet genotypes carrying a terminal drought tolerance quantitative trait locus (QTL) had a lower transpiration rate (Tr; g cm(-2) d(-1)) under well-watered conditions than sensitive lines. Here experiments were carried out to test whether this relates to leaf abscisic acid (ABA) and Tr concentration at high vapour pressure deficit (VPD), and whether that leads to transpiration efficiency (TE) differences. These traits were measured in tolerant/sensitive pearl millet genotypes, including near-isogenic lines introgressed with a terminal drought tolerance QTL (NIL-QTLs). Most genotypic differences were found under well-watered conditions. ABA levels under well-watered conditions were higher in tolerant genotypes, including NIL-QTLs, than in sensitive genotypes, and ABA did not increase under water stress. Well-watered Tr was lower in tolerant than in sensitive genotypes at all VPD levels. Except for one line, Tr slowed down in tolerant lines above a breakpoint at 1.40-1.90 kPa, with the slope decreasing >50%, whereas sensitive lines showed no change in that Tr response across the whole VPD range. It is concluded that two water-saving (avoidance) mechanisms may operate under well-watered conditions in tolerant pearl millet: (i) a low Tr even at low VPD conditions, which may relate to leaf ABA; and (ii) a sensitivity to higher VPD that further restricts Tr, which suggests the involvement of hydraulic signals. Both traits, which did not lead to TE differences, could contribute to absolute water saving seen in part due to dry weight increase differences. This water saved would become critical for grain filling and deserves consideration in the breeding of terminal drought-tolerant lines.

Analysis of gene expression in response to water deficit of chickpea (Cicer arietinum L.) varieties differing in drought tolerance

BACKGROUND

Chickpea (C. arietinum L.) ranks third in food legume crop production in the world. However, drought poses a serious threat to chickpea production, and development of drought-resistant varieties is a necessity. Unfortunately, cultivated chickpea has a high morphological but narrow genetic diversity, and understanding the genetic processes of this plant is hindered by the fact that the chickpea genome has not yet been sequenced and its EST resources are limited. In this study, two chickpea varieties having contrasting levels of drought-tolerance were analyzed for differences in transcript profiling during drought stress treatment by withdrawal of irrigation at different time points. Transcript profiles of ESTs derived from subtractive cDNA libraries constructed with RNA from whole seedlings of both varieties were analyzed at different stages of stress treatment.

RESULTS

A series of comparisons of transcript abundance between two varieties at different time points were made. 319 unique ESTs available from different libraries were categorized into eleven clusters according to their comparative expression profiles. Expression analysis revealed that 70% of the ESTs were more than two fold abundant in the tolerant cultivar at any point of the stress treatment of which expression of 33% ESTs were more than two fold high even under the control condition. 53 ESTs that displayed very high fold relative expression in the tolerant variety were screened for further analysis. These ESTs were clustered in four groups according to their expression patterns.

CONCLUSIONS

Annotation of the highly expressed ESTs in the tolerant cultivar predicted that most of them encoded proteins involved in cellular organization, protein metabolism, signal transduction, and transcription. Results from this study may help in targeting useful genes for improving drought tolerance in chickpea.

Differential accumulation of dehydrins in response to water stress for hybrid and common bermudagrass genotypes differing in drought tolerance

Expression of dehydrin proteins may be induced or enhanced by environmental stresses that lead to cell dehydration. The objective of the this study was to investigate genetic variation in dehydrin protein accumulation in response to drought stress of whole-plants or dehydration of detached leaves and to identify dehydrins differentially expressed in bermudagrass (Cynodon spp.) genotypes differing in drought tolerance. Plants of four hybrid bermudagrass (Cynodondactylon L. xCynodontransvaalensis L.) ('Tifway', 'Tifdwarf', 'Tifeagle', 'Kan1') and four common bermudagrass (Cynodon dactylon) ('C299', 'Sportbermuda', 'H10', and 'H19') genotypes were subjected to 14d of drought stress and detached leaves of two genotypes were exposed to dehydration in growth chambers. Turf quality and leaf relative water content (RWC) decreased while electrolyte leakage (EL) increased during whole-plant drought stress for all genotypes, with more pronounced changes in each parameter for 'C299' and 'Tifeagle' than those for other genotypes ('Tifway', 'Kan 1', 'Sportbermuda', 'H10', and H19'), suggesting that the former two genotypes were more sensitive to drought stress than the other genotypes. During dehydration of detached leaves, relative water loss rate (RWL) was significantly lower in drought-tolerant 'Tifway' than in drought-sensitive 'C299'. Immunoblotting analysis indicated that no dehydrin polypeptides were detected in all genotypes under well-watered conditions. A 24-kDa polypeptide was detected in 'C299' at 6 d of drought, but not in the other genotypes. The dehydrin polypeptides of about 14-74kDa accumulated at 10d of drought stress and in a range of RWL for detached leaves, and two dehydrins (31 and 40kDa) exhibited differential accumulation in the drought-sensitive 'C299' and tolerant 'Tifway', as demonstrated by the whole-plant drought responses. The 31-kDa dehydrin polypeptide was present only in 'Tifway' and 'H19' at 10d of drought stress, and accumulated with the increasing RWL in detached leaves of 'Tifway'. The expression level of 40-kDa dehydrin polypeptides was greater in 'Tifway'' than in 'C299' at the same level of water deficit (from 10% to 65% RWL). These results indicated that the accumulation of 31- and 40-kDa dehydrins may contribute to drought or dehydration tolerance in warm-season bermudagrass.

Identification of quantitative trait loci for ABA sensitivity at seed germination and seedling stages in rice

Abscisic acid (ABA) is one of the important plant hormones, which plays a critical role in seed development and adaptation to abiotic stresses. The sensitivity of rice (Oryza sativa L.) to exogenous ABA at seed germination and seedling stages was investigated in the recombinant inbred line (RIL) population derived from a cross between irrigated rice Zhenshan 97 and upland rice IRAT109, using relative germination vigor (RGV), relative germination rate (RGR) and leaf rolling scores of spraying (LRS) or culturing (LRC) with ABA as sensitivity indexes. The phenotypic correlation analysis revealed that only RGV at germination stage was positively correlated to ABA sensitivity at seedling stage. QTL detection using composite interval mapping (CIM) and mixed linear model was conducted to dissect the genetic basis of ABA sensitivity, and the single-locus QTLs detected by both methods are in good agreement with each other. Five single QTLs and six pairs of epistatic QTLs were detected for ABA sensitivity at germination stage. Eight single QTLs and five pairs of epistatic QTLs were detected for ABA sensitivity at seedling stage. Two QTLs were common between LRS and LRC; and one common QTL was detected for RGV, LRS and LRC simultaneously. These results indicated that both single and epistatic loci were involved in the ABA sensitivity in rice, and the genetic basis of ABA sensitivity at seed germination and seedling stage was largely different.

RcDhn5, a cold acclimation-responsive dehydrin from Rhododendron catawbiense rescues enzyme activity from dehydration effects in vitro and enhances freezing tolerance in RcDhn5-overexpressing Arabidopsis plants

Dehydrins (DHNs) are typically induced in response to abiotic stresses that impose cellular dehydration. As extracellular freezing results in cellular dehydration, accumulation of DHNs and development of desiccation tolerance are believed to be key components of the cold acclimation (CA) process. The present study shows that RcDhn5, one of the DHNs from Rhododendron catawbiense leaf tissues, encodes an acidic, SK(2) type DHN and is upregulated during seasonal CA and downregulated during spring deacclimation (DA). Data from in vitro partial water loss assays indicate that purified RcDhn5 protects enzyme activity against a dehydration treatment and that this protection is comparable with acidic SK(n) DHNs from other species. To investigate the contribution of RcDhn5 to freezing tolerance (FT), Arabidopsis plants overexpressing RcDhn5 under the control of 35S promoter were generated. Transgenic plants exhibited improved 'constitutive' FT compared with the control plants. Furthermore, a small but significant improvement in FT of RcDhn5-overexpressing plants was observed after 12 h of CA; however, this gained acclimation capacity was not sustained after a 6-day CA. Transcript profiles of cold-regulated native Arabidopsis DHNs (COR47, ERD10 and ERD14) during a CA time-course suggests that the apparent lack of improvement in cold-acclimated FT of RcDhn5-overexpressing plants over that of wild-type controls after a 6-day CA might have been because of the dilution of the effect of RcDhn5 overproduction by a strong CA-induced expression of native Arabidopsis DHNs. This study provides evidence that RcDhn5 contributes to freezing stress tolerance and that this could be, in part, because of its dehydration stress-protective ability.

Gene expression analysis in citrus reveals the role of gibberellins on photosynthesis and stress

The effect of gibberellins (GA) on internode transcriptome was investigated in transgenic Carrizo citrange (Citrus sinensis x Poncirus trifoliata) plants overexpressing endogenous CcGA20ox1 (encoding a GA biosynthetic gene), and in non-transformed explants treated with GA(3), using a citrus cDNA microarray. Substantial modulation of gene expression was found in sense CcGA20ox plants. Extensive up-regulation of genes involved in photosynthesis and carbon utilization, and down-regulation of those involved in protein synthesis and ribosome biogenesis were shown for the first time in plants with higher GA content. Importantly, increase of net photosynthesis in attached leaves was also demonstrated. Expression of other genes belonging to functional groups not reported previously to be regulated by GA (mainly abiotic and biotic stresses, and cuticle biosynthesis), and genes involved in cell division and cell wall architecture were also differentially expressed. Culture of citrus explants for 24 h in GA(3) solution produced much lower changes in the transcriptome compared with CcGA20ox plants (1.6% versus 16%, respectively, of total genes in the microarray), suggesting that most of the changes observed in CcGA20ox plants were a consequence of a long-standing GA effect. Interestingly, genes related to abiotic and biotic stresses were similarly modulated in transgenics and GA(3)-treated explants.

Mycorrhizal and non-mycorrhizal Lactuca sativa plants exhibit contrasting responses to exogenous ABA during drought stress and recovery

The arbuscular mycorrhizal (AM) symbiosis enhances plant tolerance to water deficit through the alteration of plant physiology and the expression of plant genes. These changes have been postulated to be caused (among others) by different contents of abscisic acid (ABA) between AM and non-AM plants. However, there are no studies dealing with the effects of exogenous ABA on the expression of stress-related genes and on the physiology of AM plants. The aim of the present study was to evaluate the influence of AM symbiosis and exogenous ABA application on plant development, physiology, and expression of several stress-related genes after both drought and a recovery period. Results show that the application of exogenous ABA had contrasting effects on AM and non-AM plants. Only AM plants fed with exogenous ABA maintained shoot biomass production unaltered by drought stress. The addition of exogenous ABA enhanced considerably the ABA content in shoots of non-AM plants, concomitantly with the expression of the stress marker genes Lsp5cs and Lslea and the gene Lsnced. By contrast, the addition of exogenous ABA decreased the content of ABA in shoots of AM plants and did not produce any further enhancement of the expression of these three genes. AM plants always exhibited higher values of root hydraulic conductivity and reduced transpiration rate under drought stress. From plants subjected to drought, only the AM plants recovered their root hydraulic conductivity completely after the 3 d recovery period. As a whole, the results indicate that AM plants regulate their ABA levels better and faster than non-AM plants, allowing a more adequate balance between leaf transpiration and root water movement during drought and recovery.

Transcriptional profiles of primary metabolism and signal transduction-related genes in response to water stress in field-grown sunflower genotypes using a thematic cDNA microarray

A sunflower cDNA microarray containing about 800 clones covering major metabolic and signal transduction pathways was used to study gene expression profiles in leaves and embryos of drought-tolerant and -sensitive genotypes subjected to water-deficit stress under field conditions. Using two-step ANOVA normalization and analysis models, we identified 409 differentially expressed genes among genotypes, water treatment and organs. The majority of the cDNA clones differentially expressed under water stress was found to display opposite gene expression profiles in drought-tolerant genotype compared to drought-sensitive genotype. These dissimilarities suggest that the difference between tolerant and non-tolerant plants seems to be associated with changes in qualitative but not quantitative mRNA expression. Comparing leaves and embryos, 82 cDNA clones showing organ-specific variation in gene expression levels were identified in response to water stress across genotypes. Genes related to amino acids and carbohydrates metabolisms, and signal transduction were induced in embryos and repressed in leaves; suggesting that vegetative and reproductive organs respond differentially to water stress. Adaptive mechanisms controlling water deficit tolerance are proposed and discussed.

Overexpression of barley hva1 gene in creeping bentgrass for improving drought tolerance

The objectives of this study were to test the feasibility of introducing barley hva1 gene, a LEA3 member, into perennial grass species using the Agrobacterium-mediated transformation technique and to determine whether heterologous expression of hva1 would alleviate water-deficit injury in grass species. Creeping bentgrass (Agrostis stolonifera var. palustris), a drought-intolerant grass species, was transformed transiently or stably using three different promoters in conjunction with the downstream report/target genes. Two abscisic acid (ABA)-inducible promoters, ABA1 and ABA2 derived from ABA-response complex (ABRC3) were used to examine stress-responsive expression of the green fluorescent protein (GFP). Transient expression of GFP demonstrated the inducibility of ABA1 and ABA2 promoters in response to exogenous ABA application. The ABA2 promoter was further studied for stress-responsive expression of hva1 and a maize Ubi-1 promoter was tested for constitutive expression of the gene. In the T(0) generation, the Ubi-1::hva1 transformants displayed variable expression levels of HVA1 protein under normal growth conditions. The hva1 gene in the ABA2::hva1 transformants maintained low expression under well-watered conditions, but was upregulated under water-deficit conditions. The tolerance to water deficit of T(0) transgenic lines was assessed by measuring leaf relative water content and visually rating the severity of leaf wilting during to water stress. Under water-stressed conditions, some transgenic lines maintained high water content in leaves and showed significantly less extent of leaf wilting compared with non-transgenic control plants. These results indicated that the introduction of barley hva1 gene using constitutive or stress-inducible promoters lessened water-deficit injury in creeping bentgrass, suggesting that heterologous expression of LEA3 protein genes may enhance the survival ability of creeping bentgrass in water limiting environments.

Light induces expression of a dehydrin-encoding gene during seedling de-etiolation in sunflower (Helianthus annuus L.)

The effects of light quality on the expression of a sunflower dehydrin-encoding gene, HaDhn1, were studied during seedling de-etiolation. Seeds were germinated in the dark and, after 5 days, seedlings were maintained well watered and de-etiolated under different lights for 3, 6, 12, and 24h. Exposure to white light stimulated HaDhn1 transcript accumulation in the cotyledons of these seedlings, contrary to seedlings grown in the dark. HaDhn1 transcripts increased also treating plantlets with monochromatic lights, especially red light. The increase of HaDhn1 transcripts is provoked by the formation of the active form of phytochrome. Further experiments, performed saturating active phytochrome by yellow light, in combination or not with blue light, showed that also cryptochrome can increase HaDhn1 transcripts accumulation after exposure to light. In situ analysis of HaDhn1 expression domains in cotyledons of light-treated seedlings showed a hybridisation signal spread in all mesophyll cells, especially in the basal portion and in the vascular tissue. In the distal portion of the cotyledons, less intense signal was observed. Western blot analysis indicated that HaDhn1 transcription is not followed by dehydrin-protein accumulation. The isolated putative promoter sequence of the HaDhn1 gene showed that different putative cis-elements recognisable by transcription factors occur in the isolated sequence, including a putative light-responsive G-box. On the whole, our results indicate that HaDhn1 gene expression is induced by light during de-etiolation, in absence of water stress.

Genotypic variability in sequence and expression of HVA1 gene in Tibetan hulless barley, Hordeum vulgare ssp. vulgare, associated with resistance to water deficit

Late embryogenesis abundant (LEA) proteins are thought to protect against water stress in plants. Characteristics of sequence and expression of barley gene HVA1, a member of LEA group 3 protein, were investigated in hulless barley (Hordeum vulgare ssp. vulgare), associated with phenotypically diverse drought-tolerant genotypes. Sensitive and tolerant genotypes were identified from Tibetan populations of cultivated hulless barley, based on scores of water loss rate (WLR), maldondialdehyde (MDA), and proline content. The results indicated that lower MDA contents, lower scores of WLR, and higher proline contents were associated with drought-tolerant genotypes in hulless barley. Notably, differential trends of expression patterns were detected among the selected contrasting genotypes, depending on the duration of dehydration stress. The HVA1 gene tended to respond earlier in the tolerance (after 2 h) compared with sensitive genotypes (after 4 h). Results of quantitative real-time PCR indicated that the relative level of HVA1 expression was always higher in tolerant genotypes, rapidly increasing at the earlier stages (after 2–4 h of dehydration). However, HVA1 expressions of sensitive genotypes had a fast increase from 8 to 12 h of stress. Variable numbers of the 11-amino-acid-motif in LEA3 proteins were not consistent with the lines of drought resistance in hulless barley. Molecular characteristic of LEA3 protein in tolerant lines existed in the consistency of Gln32, Arg33, and Ala195 in Tibetan hulless barley. The present study may indicate that the differential HVA1 gene has a functional role in the dehydration tolerance in hulless barley. The authors suggested that the observed variability in sequence and expression of HVA1 could be related to the diverse drought-tolerant genotypes in crops.

Genetic variability for physiological traits under drought conditions and differential expression of water stress-associated genes in sunflower (Helianthus annuus L.)

Genotypic variation for water status and gas exchange parameters under different water treatments (well-watered and water-stressed plants before and after rehydration) were investigated in a population of recombinant inbred lines (RILs) of sunflower (Helianthus annuus L.). Afterwards, four RILs and parental lines presenting contrasting responses to dehydration and rehydration were selected to determine the differential expression of four water-stress associated genes: aquaporin, dehydrin, leafy cotyledon1-like protein and fructose-1,6 bisphosphatase. Water stress revealed a high genetic variability for water status and gas exchange parameters when compared with well-watered genotypes. Genetic gain when selected RILs were compared with the best parent was significant for most traits due to transgressive segregation. QTL mapping and graphical genotyping showed that RILs carrying different genomic regions for some QTLs presented also physiological different characteristics as well as gene expression patterns. The expression level of aquaporin genes in leaves of four RILs and their parents was down regulated by water stress and was associated with relative water content (RWC). Down-regulation was also associated with genomic regions having alleles with negative effects on plant water status. The level of dehydrin transcripts increased in leaves of all studied RILs in response to water stress. Transcript accumulations of dehydrin and leafy cotyledon1-like genes, likely involved in protective tolerance processes, were not correlated directly with plant water status or QTL effects. Down-regulation of fructose-1,6 bisphosphatase was observed under water stress. Net photosynthesis rate (P(n)) and the fructose-1,6 bisphosphatase gene expression levels were associated mainly after rehydration. This phenomenon indicates an association between physiological response to water stress and differential expression of water-stress related genes.

Drought stress response in wheat: physiological and molecular analysis of resistant and sensitive genotypes

Water deficit is a severe environmental stress and the major constraint on plant productivity with an evident effect on plant growth. The aim of this work was to study Triticum and Aegilops seedlings differing in their response to drought stress at the physiological and molecular levels. The identification of resistant and sensitive genotypes was firstly based on the relative water content (RWC) measurement. Further characterization of genotypes contrasting in their response to water stress was performed at the physiological level by determination of RWC, water loss rate (WLR) and free proline content after different hours of dehydration. Modification in the expression level of five dehydrin (DHN) genes was also analysed by reverse transcription-polymerase chain reaction (RT-PCR). Five cDNAs coding for different DHNs were identified and characterized. These genes are not expressed in the well-watered plants, but only in the stressed plants. Four of these cDNAs are related to novel DHN sequences. The results obtained clearly indicate a relation between the expression of these genes and tissue water content. In particular, in the resistant genotypes the expression of DHN genes is initiated even though tissue hydration levels are still high, indicating also in wheat the involvement of these proteins in water retention.

Plant dehydrins--tissue location, structure and function

Dehydrins (DHNs) are part of a large group of highly hydrophilic proteins known as LEA (Late Embryogenesis Abundant). They were originally identified as group II of the LEA proteins. The distinctive feature of all DHNs is a conserved, lysine-rich 15-amino acid domain, EKKGIMDKIKEKLPG, named the K-segment. It is usually present near the C-terminus. Other typical dehydrin features are: a track of Ser residues (the S-segment); a consensus motif, T/VDEYGNP (the Y-segment), located near the N-terminus; and less conserved regions, usually rich in polar amino acids (the Phi-segments). They do not display a well-defined secondary structure. The number and order of the Y-, S-and K-segments define different DHN sub-classes: Y(n)SK(n), Y(n)Kn, SK(n), K(n) and K(n)S. Dehydrins are distributed in a wide range of organisms including the higher plants, algae, yeast and cyanobacteria. They accumulate late in embryogenesis, and in nearly all the vegetative tissues during normal growth conditions and in response to stress leading to cellular dehydration (e.g. drought, low temperature and salinity). DHNs are localized in different cell compartments, such as the cytosol, nucleus, mitochondria, vacuole, and the vicinity of the plasma membrane; however, they are primarily localized to the cytoplasm and nucleus. The precise function of dehydrins has not been established yet, but in vitro experiments revealed that some DHNs (YSK(n)-type) bind to lipid vesicles that contain acidic phospholipids, and others (K(n)S) were shown to bind metals and have the ability to scavenge hydroxyl radicals [Asghar, R. et al. Protoplasma 177 (1994) 87-94], protect lipid membranes against peroxidation or display cryoprotective activity towards freezing-sensitive enzymes. The SK(n)-and K-type seem to be directly involved in cold acclimation processes. The main question arising from the in vitro findings is whether each DHN structural type could possess a specific function and tissue distribution. Much recent in vitro data clearly indicates that dehydrins belonging to different subclasses exhibit distinct functions.

A novel ABA-hypersensitive mutant in Arabidopsis defines a genetic locus that confers tolerance to xerothermic stress

Hot and dry air (harmattan or xerothermic climate) greatly inhibits plant growth, particularly flowering and seed setting of crops. Little is known about the mechanism of plant response to this extreme environmental stress due to the lack of valuable genetic resource. Here, we report the isolation and characteristics of a unique Arabidopsis mutant, hat1 (harmattan tolerant 1), which shows high tolerance to hot and dry air. Under normal growth conditions, the mutant does not differ in morphology and soil drought tolerance compared to the wild type. When subjected to high temperature (42 degrees C) and low humidity (10-15%), however, it could survive up to 6 days, while the wild type (Col-0) died after 24 h. The hat1 mutant also exhibits enhanced tolerance to soil drought, but only under xerothermic conditions. Mutant plants tightly close their stomata to retain water under xerothermic stress, and are more tolerant to high salinity at all developmental stages, accumulating less Na+ and more K+ than wild-type plants during NaCl treatment. Interestingly, hat1 plants are also ABA-hypersensitive. Genetic analysis revealed that the hat1 phenotype is caused by a dominant mutation at a single nuclear locus. Mapping studies indicate that Hat1 is located at an interval of 168 kb on chromosome 5 in which 21 genes are known to be regulated by diverse abiotic stresses. A mutant of this kind, to our knowledge, has not been previously reported. Thus, this report serves as a starting point in the genetic dissection of the plant response to xerothermic stress, and provides physiological and genetic evidence of the existence of a novel abiotic stress response pathway that is also ABA-dependent.

Expression of SK3-type dehydrin in transporting organs is associated with cold acclimation in Solanum species

The expression of a gene, encoding a dehydrin protein designated as DHN24 was analyzed at the protein level in two groups of Solanum species differing in cold acclimation ability. The DHN24 protein displays consensus amino acid sequences of dehydrins, termed K- and S-segments. The S-segment precedes three K-segments, classifying the protein into SK3-type dehydrins. A group of Solanum species able to cold acclimation constituted by S. sogarandinum and S. tuberosum, cv. Aster, and a second one composed of a S. sogarandinum line, that lost ability to cold acclimation, and of S. tuberosum, cv. Irga, displaying low ability to cold acclimation were studied. Under control conditions, noticeable levels of the DHN24 protein was observed in stems, tubers, and roots of Solanum species. No protein was detected in leaves. During low temperature treatment the DHN24 protein level substantially increased in tubers, in transporting organs and in apical parts, and only a small increase was observed in leaves. The increase in protein abundance was only observed in the plants able to cold acclimate and was found to parallel the acclimation capacity. Upon drought stress, the DHN24 level decreased in stems and in leaves, but increased in apical parts. These results suggest that Dhn24 expression is regulated by organ specific factors in the absence of stress and by factors related to cold acclimation processes during low temperature treatment in collaboration with organ-specific factors. A putative function of the SK3-type dehydrin proteins during plant growth and in the tolerance to low temperature is discussed.

Over-expression of tobacco NtHSP70-1 contributes to drought-stress tolerance in plants

HSP70, a heat shock protein, is a molecular chaperone responsive to various environmental stresses. Here, NtHSP70-1 was a drought-/ABA-inducible gene. We monitored the expression of CaERD15 (early responsive to dehydration) with exposing plants to progressive drought stress. Its activity was used as an indicator of water-deficit conditions. To analyze the protective role of HSP70, we obtained transgenic tobacco plants that constitutively expressed elevated levels of the tobacco HSP70, NtHSP70-1, as well as transgenic plants containing either the vector alone or else having NtHSP70-1 in the antisense orientation. Plants with enhanced levels of NtHSP70-1 in their transgenic sense lines exhibited tolerance to water stress. Under progressive drought, the amount of leaf NtHSP70-1 was correlated with maintenance of optimum water content, with contents being higher in the leaves of dehydrated transgenic sense plants than in those of either the control (vector-only) or the transgenic antisense plants. Moreover, the expression of CaERD15 was considerably reduced in tobacco plants that over-expressed NtHSP70-1. These results suggest that elevated levels of NtHSP70-1 can confer drought-stress tolerance.

Expression analysis of a stress-modulated transcript in drought tolerant and susceptible cultivars of sorghum (Sorghum bicolor)

The present study reports the cloning of a 581 bp sequence, designated as SbEST8, from the osmotically stressed germinated seeds of a drought tolerant cultivar of sorghum (Sorghum bicolor). The SbEST8, which shows no homology with the reported gene sequences, is present in multiple copies and lacks restriction fragment length polymorphism among different sorghum cultivars. The expression of SbEST8 in the germinating seeds of sorghum was modulated by different abiotic stresses. Kinetic studies revealed that imposition of osmotic stress after 8h resulted in maximum levels of SbEST8 mRNA in the germinating seeds of cv. ICSV-272, with further stress causing a decline to undetectable levels by 16 h. However, relieving the stress after 12h resulted in an enhancement of SbEST8 mRNA levels for at least another 4h following which it declined. The decrease in SbEST8 mRNA levels in the leaves at 30 DAS in response to drought stress was observed only in the drought susceptible cultivar (CSV-216), whereas its expression was either increased substantially or remained unaffected in the tolerant cultivars, thus suggesting its role in water stress tolerance.

Changes in gene expression in maize kernel in response to water and salt stress

Increasing pressure on limited water resources for agriculture, together with the global temperature increase, highlight the importance of breeding for drought-tolerant cultivars. A better understanding of the molecular nature of drought stress can be expected through the use of genomics approaches. Here, a macroarray of approximately 2500 maize cDNAs was used for determining transcript changes during water- and salt-stress treatments of developing kernels at 15 days after pollination. Normalization of relative transcript abundances was carried out using a human nebulin control sequence. The proportions of transcripts that changed significantly in abundance upon treatment (>2-fold compared to the control) were determined; 1.5% of the sequences examined were up-regulated by high salinity and 1% by water stress. Both stresses induced 0.8% of the sequences. These include genes involved in various stress responses: abiotic, wounding and pathogen attack (abscisic acid response binding factor, glycine and proline-rich proteins, pathogenesis-related proteins, etc.). The proportion of down-regulated genes was higher than that for up-regulated genes for water stress (3.2%) and lower for salt stress (0.7%), although only eight genes, predominantly involved in energy generation, were down-regulated in both stress conditions. Co-expression of genes of unknown function under defined conditions may help in elucidating their roles in coordinating stress responses.

Comparative expression of five Lea Genes during wheat seed development and in response to abiotic stresses by real-time quantitative RT-PCR

Gene expression profiles of group 2 (dehydrins) and group 4 Late embryogenesis abundant (Lea) genes in developing seeds of Triticum durum and T. aestivum and in coleoptiles and coleorhizae of T. durum seedlings were monitored by real-time quantitative RT-PCR. The five genes exhibited clear differences in their accumulation pattern in wheat seed and in response to dehydration, low temperature, salinity and ABA. Td29b, Td16 and Td27e gene transcripts accumulate late in embryogenesis as expected for Lea genes, Td11 gene transcripts were present throughout seed development whereas no Td25a gene transcripts were detected in seeds. Drastic changes in the relative levels of Td29b, Td16, Td27e and Td11 transcripts occurred at the shift between the cell expansion and desiccation phases. All genes except the Td11 gene are more highly induced by dehydration in coleorhizae than in coleoptiles. In contrast, response to low temperature, salinity or ABA is higher in coleoptiles than in coleorhizae. Depending on both the gene and on the type of stress, a wide range of induction levels (8- to 100,000-fold) was observed.

Evaluation of the role of genes encoding for dehydrin proteins (LEA D-11) during drought stress in arbuscular mycorrhizal Glycine max and Lactuca sativa plants

In this study, it has been determined whether the arbuscular mycorrhizal (AM) symbiosis is able to alter the pattern of dehydrin (LEA D-11 group) transcript accumulation under drought stress, and whether such a possible alteration functions in the protection of the host plants against drought. Two dehydrin-encoding genes have been cloned from Glycine max (gmlea 8 and gmlea 10) and one from Lactuca sativa (lslea 1) and they have been analysed for their contribution to the response against drought in mycorrhizal soybean and lettuce plants. Results with soybean plants showed that most of the treatments did not show LEA gene expression under well-watered conditions. The higher gene expression was found in non-inoculated plants subjected to drought. Only plants singly inoculated with Bradyrhizobium japonicum showed an important level of LEA gene expression under well-watered conditions and a reduced level under drought-stress conditions. The same results were confirmed in subsequent experiments and at the latest stage of a time-course experiment. In lettuce, the lslea 1 gene was also induced by drought stress in all treatments. However, the level of induction was clearly higher in roots from non-inoculated plants than in roots from the two AM treatments assayed. The overall results demonstrated that the levels of lea transcript accumulation in mycorrhizal treatments subjected to drought were considerably lower than in the corresponding non-mycorrhizal plants, indicating that the accumulation of LEA proteins is not a mechanism by which the AM symbiosis protects their host plant.

Long term transcript accumulation during the development of dehydration adaptation in Cicer arietinum

Cool season crops face intermittent drought. Exposure to drought and other abiotic stresses is known to increase tolerance of the plants against subsequent exposure to such stresses. Storage of environmental signals is also proposed. Preexposure to a dehydration shock improved adaptive response during subsequent dehydration treatment in a cool season crop chickpea (Cicer arietinum). We have identified 101 dehydration-inducible transcripts of chickpea by repetitive rounds of cDNA subtraction; differential DNA-array hybridization followed by northern-blot analysis and analyzed their responses to exogenous application of abscisic acid (ABA). Steady-state expression levels of the dehydration-induced transcripts were monitored during the recovery period between 2 consecutive dehydration stresses. Seven of them maintained more than 3-fold of expression after 24 h and more than 2-fold of expression level even at 72 h after the removal of stress. Noticeably, all of them were inducible by exogenous ABA treatment. When the seedlings were subjected to recover similarly after an exposure to exogenous ABA, the steady-state abundances of 6 of them followed totally different kinetics returning to basal level expression within 24 h. This observation indicated a correlation between the longer period of abundance of those transcripts in the recovery period and improved adaptation of the plants to subsequent dehydration stress and suggested that both ABA-dependent and -independent mechanisms are involved in the maintenance of the messages from the previous stress experience.