Abscisic acid regulation of gene expression during water-deficit stress in the era of the Arabidopsis genome

Differential responses of saltbush Atriplex halimus L. exposed to salinity and water stress in relation to senescing hormones abscisic acid and ethylene

Drought and salinity induce water deficit, but may also have distinct effects on plant metabolism. To compare their impact on leaf senescence in relation to ABA and ethylene synthesis, young plants of Atriplex halimus L. were exposed to iso-osmotic concentrations of NaCl (160mM) or PEG (15%) in nutrient solution. Plant growth and development were more affected by PEG than by NaCl. Stressed plants remained able to reduce their osmotic potential, but the nature of accumulated organic osmocompatible solutes varied according to the stressing agent. Glycinebetaine accumulated to a greater extent in salt-treated plants than in water-stressed plants. Sodium chloride induced the accumulation of non-reducing sucrose, while PEG-treated plants mainly accumulated reducing glucose and fructose. Abscisic acid (ABA) accumulated in response to salt, while ethylene was synthesized mainly by PEG-treated plants and was involved in the induction of early senescence processes characterized by synthesis of reactive oxygen species, peroxidation of membrane lipids and a decrease in chlorophyll content. ABA sensitivity of stressed tissues was markedly different in response to salt and in response to non-ionic osmotic stress, and exogenous ABA (50μM) had contrasting effects on most physiological parameters depending on the stressing agent. Exogenous ABA induced a decrease in root and shoot growth and sucrose content, and an increase in reactive oxygen species content in salt-stressed plants. In contrast, exogenous ABA increased growth in PEG-treated plants in relation to an improvement of water use efficiency resulting from a more efficient stomatal control. Exogenous ABA increased ethylene synthesis in salt-treated plants, but had only marginal impact on PEG-treated ones. The xero-halophyte A. halimus thus responds in a contrasting way to salt and water stress, through accumulation of distinct osmocompatible solutes and hormonal compounds such as ethylene and ABA could play distinct roles in stress-induced senescence processes.

Structural insights into maize viviparous14, a key enzyme in the biosynthesis of the phytohormone abscisic acid

The key regulatory step in the biosynthesis of abscisic acid (ABA), a hormone central to the regulation of several important processes in plants, is the oxidative cleavage of the 11,12 double bond of a 9-cis-epoxycarotenoid. The enzyme viviparous14 (VP14) performs this cleavage in maize (Zea mays), making it a target for the rational design of novel chemical agents and genetic modifications that improve plant behavior through the modulation of ABA levels. The structure of VP14, determined to 3.2-Å resolution, provides both insight into the determinants of regio- and stereospecificity of this enzyme and suggests a possible mechanism for oxidative cleavage. Furthermore, mutagenesis of the distantly related CCD1 of maize shows how the VP14 structure represents a template for all plant carotenoid cleavage dioxygenases (CCDs). In addition, the structure suggests how VP14 associates with the membrane as a way of gaining access to its membrane soluble substrate.

Exploring genetic and expression differences between physiologically extreme ecotypes: comparative genomic hybridization and gene expression studies of Kas-1 and Tsu-1 accessions of Arabidopsis thaliana

Recent studies have documented remarkable genetic variation among Arabidopsis thaliana accessions collected from diverse habitats. Of particular interest are accessions with putatively locally adapted phenotypes - that is, accessions with attributes that are likely adaptive at their sites of origin. These genotypes may provide insight into the genetic basis of adaptive evolution as well as allow the discovery of genes of ecological importance. We studied the physiology, genome content and gene expression of two physiologically extreme accessions (Tsu-1 from Tsushima, Japan and Kas-1 from Kashmir, India). Our study was conducted under two levels of soil moisture and accompanied by physiological measurements to characterize early responses to soil drying. Genomic hybridizations identified 42,503 single feature polymorphisms (SFP) between accessions, providing an initial screen for genetic differences. Transcript profiling identified a large number (5996) of genes exhibiting constitutive differences in expression including genes involved in many biological pathways. Mild soil drying resulted in only subtle physiological responses but resulted in gene expression changes in hundreds of transcripts, including 352 genes exhibiting differential responses between accessions. Our results highlight the value of genomic studies of natural accessions as well as identify a number of candidate genes underlying physiological differences between Tsu-1 and Kas-1.

The role of abscisic acid and auxin in the response of poplar to abiotic stress

The plant hormones auxin and abscisic acid may at first sight appear to be a conflicting pair of plant regulators. Abscisic acid content increases during stress and protects plant water status. The content of free auxin in the developing xylem of poplar declines during stress, while auxin conjugates increase. This indicates that specific down-regulation of a signal transduction chain is important in plant adaptation to stress. Diminished auxin content may be a factor that adapts growth and wood development of poplar during adverse environmental conditions. To allow integration of environmental signals, abscisic acid and auxin must interact. Data are accumulating that abscisic acid-auxin cross-talk exists in plants. However, knowledge of the role of plant hormones in the response of trees to stress is scarce. Our data show that differences in the localisation of ABA synthesis exist between the annual, herbaceous plant Arabidopsis and the perennial woody species, poplar.

Arbuscular mycorrhizal fungi in alleviation of salt stress: a review

BACKGROUND

Salt stress has become a major threat to plant growth and productivity. Arbuscular mycorrhizal fungi colonize plant root systems and modulate plant growth in various ways.

SCOPE

This review addresses the significance of arbuscular mycorrhiza in alleviation of salt stress and their beneficial effects on plant growth and productivity. It also focuses on recent progress in unravelling biochemical, physiological and molecular mechanisms in mycorrhizal plants to alleviate salt stress.

CONCLUSIONS

The role of arbuscular mycorrhizal fungi in alleviating salt stress is well documented. This paper reviews the mechanisms arbuscular mycorrhizal fungi employ to enhance the salt tolerance of host plants such as enhanced nutrient acquisition (P, N, Mg and Ca), maintenance of the K(+) : Na(+) ratio, biochemical changes (accumulation of proline, betaines, polyamines, carbohydrates and antioxidants), physiological changes (photosynthetic efficiency, relative permeability, water status, abscissic acid accumulation, nodulation and nitrogen fixation), molecular changes (the expression of genes: PIP, Na(+)/H(+) antiporters, Lsnced, Lslea and LsP5CS) and ultra-structural changes. Theis review identifies certain lesser explored areas such as molecular and ultra-structural changes where further research is needed for better understanding of symbiosis with reference to salt stress for optimum usage of this technology in the field on a large scale. This review paper gives useful benchmark information for the development and prioritization of future research programmes.

Abscisic acid has contrasting effects on salt excretion and polyamine concentrations of an inland and a coastal population of the Mediterranean xero-halophyte species Atriplex halimus

BACKGROUND AND AIMS

Different populations of the Mediterranean xerohalophyte species Atriplex halimus exhibit different levels of resistance to salt and osmotic stress depending on the nature of the osmocompatible solute they accumulate. There is, however, no conclusive description of the involvement of abscisic acid (ABA) in the plant response to NaCl or osmotic stress in this species.

METHODS

Seedlings issued from an inland water-stress-resistant population (Sbikha) and from a coastal salt-resistant one (Monastir) were exposed in nutrient solutions to NaCl (40 or 160 mm) or to 15 % PEG for 1 d and 10 d in the presence or absence of 50 microm ABA.

KEY RESULTS

Plants from Sbikha accumulated higher amounts of ABA in response to osmotic stress than those of Monastir, while an opposite trend was recorded for NaCl exposure. Exogenous ABA improved osmotic stress resistance in Monastir through an improvement in the efficiency of stomatal conductance regulation. It also improved NaCl resistance in Sbikha through an increase in sodium excretion through the external bladders. It is suggested that polyamines (spermidine and spermine) are involved in the salt excretion process and that ABA contributes to polyamine synthesis as well as to the conversion from the bound and conjugated to the free soluble forms of polyamine. Proline accumulated in response to osmotic stress and slightly increased in response to ABA treatment while glycinebetaine accumulated in response to salinity and was not influenced by ABA.

CONCLUSIONS

It is concluded that ABA is involved in both salt and osmotic stress resistance in the xerohalophyte species Atriplex halimus but that it acts on different physiological cues in response to those distinct environmental constraints.

Water relations of the tos1 tomato mutant at contrasting evaporative demand

The tos1 (tomato osmotically sensitive) mutant, isolated from an in vitro screen of root growth during osmotic stress, was less sensitive to exogenous ABA, but accumulated more ABA under osmotic stress than WT plants. We assessed growth and water relations characteristics of hydroponically grown tos1 seedlings (in the absence of osmotic stress) at low and high evaporative demands. Growth of tos1 was severely inhibited at both high and low evaporative demands. Twenty DAS, WT and tos1 genotypes had a similar leaf water and turgor potential, but mature tos1 plants (45 day old) showed a significant diurnal loss of leaf turgor, with recovery overnight. Increased evaporative demand increased turgor loss of tos1 plants. High evaporative demand at the beginning of the day decreased stomatal conductance of tos1, without diurnal recovery, thus whole plant transpiration was decreased. De-topped tos1 seedlings showed decreased root hydraulic conductance and had a 1.4-fold increase in root ABA concentration. Impaired root function of tos1 plants failed to meet transpirational water demand and resulted in shoot turgor loss, stomatal closure and growth inhibition.

AtMKK1 and AtMPK6 are involved in abscisic acid and sugar signaling in Arabidopsis seed germination

Abscisic acid (ABA) and sugars have been well established to be crucial factors controlling seed germination of Arabidopsis. Here we demonstrate that AtMKK1 and AtMPK6 are both critical signals involved in ABA and sugar-regulated seed germination. Wild type plants depended on stratification and after-ripening for seed germination, whereas this dependence on either stratification or after-ripening was not required for mutants of mkk1 and mpk6 as well as their double mutant mkk1 mpk6. While seed germination of wild type plants was sensitively inhibited by ABA and glucose, mkk1, mpk6 and mkk1 mpk6 were all strongly resistant to ABA or glucose treatments, and in contrast, plants overexpressing MKK1 or MPK6 were super-sensitive to ABA and glucose. Glucose treatment significantly induced increases in MKK1 and MPK6 activities. These results clearly indicate that MKK1 and MPK6 are involved in the ABA and sugar signaling in the process of seed germination. Further experiments showed that glucose was capable of inducing ABA biosynthesis by up-regulating NCED3 and ABA2, and furthermore, this up-regulation of NCED3 and ABA2 was arrested in the mkk1 mpk6 double mutant, indicating that the inhibition of seed germination by glucose is potentially resulted from sugar-induced up-regulation of the ABA level.

Antagonistic epistasis for ecophysiological trait differences between Solanum species

Epistasis, the nonadditive interaction between loci, is thought to play a role in many fundamental evolutionary processes, including adaptive differentiation and speciation. Focusing on species differences in ecophysiological traits, we examined the strength and direction of pairwise epistatic interactions between target chromosomal regions from one species, when co-introgressed into the genetic background of a foreign species. A full diallel cross was performed using 15 near-isogenic lines (NILs) constructed between two tomato species (Solanum habrochaites and Solanum lycopersicum) to compare the phenotypic effects of each chromosomal region singly and in combination with each other region. We detected main effect quantitative trait loci (QTLs) for two of our three focal traits. Epistatic effects accounted for c. 25% of detected effects on trait means, depending on the trait. Strikingly, all but two interactions were antagonistic, with the combined effect of chromosomal regions acting in the opposite direction from that of one or both individual chromosomal regions. Our study is one of the few to systematically examine pairwise epistatic effects in a nonmicrobial system. Our results suggest that epistatic interactions can contribute substantially to the genetic basis of traits involved in adaptive species differentiation, especially highly complex, multivariate traits.

Exogenous ABA accentuates the differences in root hydraulic properties between mycorrhizal and non mycorrhizal maize plants through regulation of PIP aquaporins

The arbuscular mycorrhizal (AM) symbiosis has been shown to modulate the same physiological processes as the phytohormone abscisic acid (ABA) and to improve plant tolerance to water deficit. The aim of the present research was to evaluate the combined influence of AM symbiosis and exogenous ABA application on plant root hydraulic properties and on plasma-membrane intrinsic proteins (PIP) aquaporin gene expression and protein accumulation after both a drought and a recovery period. Results obtained showed that the application of exogenous ABA enhanced osmotic root hydraulic conductivity (L) in all plants, regardless of water conditions, and that AM plants showed lower L values than nonAM plants, a difference that was especially accentuated when plants were supplied with exogenous ABA. This effect was clearly correlated with the accumulation pattern of the different PIPs analyzed, since most showed reduced expression and protein levels in AM plants fed with ABA as compared to their nonAM counterparts. The possible involvement of plant PIP aquaporins in the differential regulation of L by ABA in AM and nonAM plants is further discussed.

Arabidopsis mutant deficient in 3 abscisic acid-activated protein kinases reveals critical roles in growth, reproduction, and stress

Abscisic acid (ABA) is an important phytohormone regulating seed dormancy, germination, seedling growth, and plant transpiration. We report here an Arabidopsis triple mutant that is disrupted in 3 SNF1-related protein kinase subfamily 2 (SnRK2s) and nearly completely insensitive to ABA. These SnRK2s, SnRK2.2, SnRK2.3, and SnRK2.6 (also known as OST1), are activated by ABA and can phosphorylate the ABA-responsive element binding factor family of b-ZIP transcription factors, which are important for the activation of ABA-responsive genes. Although stomatal regulation of snrk2.6 and seed germination and seedling growth of the snrk2.2/2.3 double mutant are insensitive to ABA, ABA responses are still present in these mutants, and the growth and reproduction of these mutants are not very different from those of the WT. In contrast, the snrk2.2/2.3/2.6 triple mutant grows poorly and produces few seeds. The triple mutant plants lose water extremely fast when ambient humidity is not high. Even on 50 muM ABA, the triple mutant can germinate and grow, whereas the most insensitive known mutants cannot develop on 10 muM ABA. In-gel kinase assays showed that all ABA-activated protein kinase activities are eliminated in the triple mutant. Also, the expression of ABA-induced genes examined is completely blocked in the triple mutant. These results demonstrate that the protein kinases SnRK2.2, SnRK2.3, and SnRK2.6 have redundant functions, and suggest that ABA signaling is critical for plant growth and reproduction.

Enhanced determination of abscisic acid (ABA) and abscisic acid glucose ester (ABA-GE) in Cistus albidus plants by liquid chromatography-mass spectrometry in tandem mode

An improved, quick and simple method for the extraction and quantification of the phytohormones (+)-abscisic acid (ABA) and its major glucose conjugate, abscisic acid glucose ester (ABA-GE) in plant samples is described. The method includes the addition of deuterium-labeled internal standards to the leaves at the beginning of the extraction for quantification, a simple extraction/centrifugation process and the injection into the liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS-MS) system in multiple reaction monitoring mode (MRM). Quality parameters of the method (detection limits, repeatability, reproducibility and linearity) have been studied. The objective of this work is to show the applicability of this method for quantifying the endogenous content of both ABA and ABA-GE in Cistus albidus plants that have been grown during an annual cycle under Mediterranean field conditions. Leaf samples from winter plants have low levels of ABA which increase in spring and summer showing two peaks that corresponded to April and August. These increases are coincident with the high temperature and solar radiation and the low RWC and RH registered along the year. On the other hand, the endogenous levels of ABA-GE increase until maximum values in July just before the ABA content reaches its highest concentration, decreasing in August and during autumn and winter. Our results suggest that the method is useful for quantifying both compounds in this plant material and represents the advantage of a short-time sample preparation with a high accuracy and viability.

Dynamic changes in concentrations of auxin, cytokinin, ABA and selected metabolites in multiple genotypes of Douglas-fir (Pseudotsuga menziesii) during a growing season

Changes in concentrations of several endogenous phytohormones and metabolites were analyzed in the long shoots of nine genotypes of coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) at five developmental stages: (1) closed buds, (2) flushing buds, (3) rapidly elongating shoots, (4) growing shoots and (5) near full-length shoots during one growing season. When averaged across genotypes, indole-3-acetic acid (IAA) concentration was high at stages 1 and 3. The only pattern that correlated with cone productivity was the one that was unique to IAA, in which high concentrations at stages 3 and 4 were found in all genotypes with high female cone productivity. Concentrations of isopentenyl adenosine (iPA) decreased and zeatin riboside (ZR) concentrations increased as the buds initiated and differentiated; ZR was 30 and 28 ng g(-1) dry weight (DW) at stages 1 and 4, respectively, before increasing to 166 ng g(-1) DW at stage 5. Isopentenyl adenosine peaked at 92 ng g(-1) DW at stage 2 and declined to low concentrations at stages 4 and 5. Zeatin-O-glucoside was 30 ng g(-1) DW at stage 1, declined at stages 2 and 3 and increased at stages 4 and 5. High abscisic acid (ABA) concentrations were positively correlated with rapid shoot elongation (stages 1 and 2), but as growth slowed and terminated, ABA concentrations decreased. Abscisic acid was 7 microg g(-1) DW at stage 1, increased to 13 microg g(-1) DW at stage 2 and then declined. The glucosyl ester (GE) of ABA decreased rapidly in early summer, and increased inversely with an increase in ABA. Between stages 1 and 2, ABA-GE decreased from 10 to 0.2 microg g(-1) DW and then increased. Of the ABA catabolites studied, 7'-hydroxy-ABA was about 2 microg g(-1) DW at stage 1, declined at stages 2 and 3 and increased at stages 4 and 5; phaseic acid concentrations were low at all stages, whereas dihydrophaseic acid was detected only at stages 4 and 5.

Effects of water stress on respiration, photosynthetic pigments and water content in two maize cultivars

Water stress is one of the most important environmental factors that reduce growth, development and production of plants. Stress was applied with polyethyleneglycol (PEG) 6000 and water potentials were: zero (control), -0.15 (PEG 10%), -0.49 (PEG 20%), -1.03 (PEG 30%) and -1.76 (PEG 40%) MPa. The roots and leaves respiration of two maize (Zea mays L.) cultivars -704 and 301- were determined in various concentrations of PEG 6000. Oxygen uptake declined in leaves and roots with increasing PEG concentrations. Decrease of oxygen uptake in roots and leaves of 704 variety were higher than 301 variety. Chlorophyll a, b and total chlorophyll content were significantly decreased (p < 0.05), but carotenoids content increased (p < 0.05) under water stress. Decrease of chlorophyll content in 704 var. was higher than 301 var., but carotenoids content in 301 var. was higher than 704 var. Relative Water Content (RWC) was used to indicate the degree of stress. RWC decreased with increasing PEG concentrations. Lowering of RWC reduced growth and increased shoot/root ratio. Decrease of water content in 704 plants was higher than 301 plants. Shoot/root ratio in 704 var. was higher than 301 var.

GMCHI, cloned from soybean [Glycine max (L.) Meer.], enhances survival in transgenic Arabidopsis under abiotic stress

Plants respond to cold stress by modifying the expression of a battery of cold-responsive genes. Using cDNA-AFLP techniques, GMCHI (G lycine m ax chilling-inducible) (accession no. EU699765) was isolated from the embryonic axis of a chilling-resistant cultivar of soybean seed imbibed at 4 degrees C for 24 h. The full-length GMCHI cDNA which consisted of a single open reading frame (ORF) encoded a putative polypeptide of 129 amino acids. Sequence analysis revealed neither significant similarity of GMCHI to known proteins, nor any conserved domains found. Soybean seed imbibed at 4 degrees C dramatically enhanced transcript level of GMCHI after 1 h, and reached a maximum at 18 h, while the expression was only detected in the embryonic axis. GMCHI expression was strongly induced by treatment with ABA and PEG, but weakly by 250 mM NaCl which suggests that GMCHI is probably regulated by ABA-dependent signal transduction pathway during cold acclimation. Overexpression of GMCHI in Arabidopsis under the control of CaMV35S promoter enhanced the tolerance to cold, drought and NaCl stresses. Therefore, GMCHI may play an important role in the adaptation of chilling-resistant soybean seed to chilling imbibition.

Plant responses to drought stress and exogenous ABA application are modulated differently by mycorrhization in tomato and an ABA-deficient mutant (sitiens)

The aims of the present study are to find out whether the effects of arbuscular mycorrhizal (AM) symbiosis on plant resistance to water deficit are mediated by the endogenous abscisic acid (ABA) content of the host plant and whether the exogenous ABA application modifies such effects. The ABA-deficient tomato mutant sitiens and its near-isogenic wild-type parental line were used. Plant development, physiology, and expression of plant genes expected to be modulated by AM symbiosis, drought, and ABA were studied. Results showed that only wild-type tomato plants responded positively to mycorrhizal inoculation, while AM symbiosis was not observed to have any effect on plant development in sitiens plants grown under well-watered conditions. The application of ABA to sitiens plants enhanced plant growth both under well-watered and drought stress conditions. In respect to sitiens plants subjected to drought stress, the addition of ABA had a cumulative effect in relation to that of inoculation with G. intraradices. Most of the genes analyzed in this study showed different regulation patterns in wild-type and sitiens plants, suggesting that their gene expression is modulated by the plant ABA phenotype. In the same way, the colonization of roots with the AM fungus G. intraradices differently regulated the expression of these genes in wild-type and in sitiens plants, which could explain the distinctive effect of the symbiosis on each plant ABA phenotype. This also suggests that the effects of the AM symbiosis on plant responses and resistance to water deficit are mediated by the plant ABA phenotype.

Do brassinosteroids mediate the water stress response?

Brassinosteroids (BRs) have been suggested to increase the resistance of plants to a variety of stresses, including water stress. This is based on application studies, where exogenously applied bioactive BRs have been shown to improve various aspects of plant growth under water stress conditions. However, it is not known whether changes in endogenous BR levels are normally involved in mediating the plant's response to stress. We have utilized BR mutants in pea (Pisum sativum L.) to determine whether changes in endogenous BR levels are part of the plant's response to water stress and whether low endogenous BR levels alter the plant's ability to cope with water stress. In wild-type (WT) plants, we show that while water stress causes a significant increase in ABA levels, it does not result in altered BR levels in either apical, internode or leaf tissue. Furthermore, the plant's ability to increase ABA levels in response to water stress is not affected by BR deficiency, as there was no significant difference in ABA levels between WT, lkb (a BR-deficient mutant) and lka (a BR-perception mutant) plants before or 14 days after the cessation of watering. In addition, the effect of water stress on traits such as height, leaf size and water potential in lkb and lka was similar to that observed in WT plants. Therefore, it appears that, at least in pea, changes in endogenous BR levels are not normally part of the plant's response to water stress.

Discovery of oxidative burst in the field of plant immunity: Looking back at the early pioneering works and towards the future development

This article is introductory to the series of works presented in this special issue on the homeostasis and the signaling roles of reactive oxygen species (ROS) in plants. Upper half of this article briefly describes the history of the ROS study in the field of plant immunity research initiated by the observation that the attacks by pathogenic microorganisms possibly stimulate the burst of ROS production in the plant tissues. The topics covered in the series of works presented here include the plants' responses to abiotic oxidative stress (atmospheric ozone), regulation of seed germination, chemical interaction between parasitic and host plants and the draught tolerance, all controlled through homeostasis of ROS at biochemical and molecular biological levels. Lastly a discussion forum was proposed to further deepen our understanding of ROS behaviors in plants.

Improving water use in crop production

Globally, agriculture accounts for 80-90% of all freshwater used by humans, and most of that is in crop production. In many areas, this water use is unsustainable; water supplies are also under pressure from other users and are being affected by climate change. Much effort is being made to reduce water use by crops and produce 'more crop per drop'. This paper examines water use by crops, taking particularly a physiological viewpoint, examining the underlying relationships between carbon uptake, growth and water loss. Key examples of recent progress in both assessing and improving crop water productivity are described. It is clear that improvements in both agronomic and physiological understanding have led to recent increases in water productivity in some crops. We believe that there is substantial potential for further improvements owing to the progress in understanding the physiological responses of plants to water supply, and there is considerable promise within the latest molecular genetic approaches, if linked to the appropriate environmental physiology. We conclude that the interactions between plant and environment require a team approach looking across the disciplines from genes to plants to crops in their particular environments to deliver improved water productivity and contribute to sustainability.

Influence of salinity on the in vitro development of Glomus intraradices and on the in vivo physiological and molecular responses of mycorrhizal lettuce plants

Increased salinization of arable land is expected to have devastating global effects in the coming years. Arbuscular mycorrhizal fungi (AMF) have been shown to improve plant tolerance to abiotic environmental factors such as salinity, but they can be themselves negatively affected by salinity. In this study, the first in vitro experiment analyzed the effects of 0, 50, or 100 mM NaCl on the development and sporulation of Glomus intraradices. In the second experiment, the effects of mycorrhization on the expression of key plant genes expected to be affected by salinity was evaluated. Results showed that the assayed isolate G. intraradices DAOM 197198 can be regarded as a moderately salt-tolerant AMF because it did not significantly decrease hyphal development or formation of branching absorbing structures at 50 mM NaCl. Results also showed that plants colonized by G. intraradices grew more than nonmycorrhizal plants. This effect was concomitant with a higher relative water content in AM plants, lower proline content, and expression of Lsp5cs gene (mainly at 50 mM NaCl), lower expression of the stress marker gene Lslea gene, and lower content of abscisic acid in roots of mycorrhizal plants as compared to nonmycorrhizal plants, which suggest that the AM fungus decreased salt stress injury. In addition, under salinity, AM symbiosis enhanced the expression of LsPIP1. Such enhanced gene expression could contribute to regulating root water permeability to better tolerate the osmotic stress generated by salinity.

The effect of drought and heat stress on reproductive processes in cereals

As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

Isolation of dehydration-responsive genes in a drought tolerant common bean cultivar and expression of a group 3 late embryogenesis abundant mRNA in tolerant and susceptible bean cultivars

Drought is one of the main constraints for common bean (Phaseolus vulgaris L.) production in Latin America. The aim of this work was to identify upregulated genes in the drought-tolerant common bean cv. Pinto Villa, grown under water-deficit conditions. Twenty-eight cDNAs representing differentially-expressed mRNAs in roots and/or leaves were isolated via suppression subtractive hybridisation. Their expression profiles in plants under intermediate and severe dehydration stress were tested. Three cDNAs corresponded to genes already described as associated to drought stress in P. vulgaris, 12 were known P. vulgaris sequences without previous association with drought response, and 13 were new P. vulgaris sequences. Analysis of the deduced proteins encoded by the cDNAs revealed putative functions in cellular protection, sugar metabolism, and protein synthesis, folding and turnover. Additionally, a new member of group 3 late embryogenesis abundant (LEA) genes (PvLEA3) was cloned and its complete sequence was obtained. Given the lack of reports comparing expression of dehydration-responsive genes in bean cultivars with different response to drought, the expression of PvLEA3 transcript in five bean cultivars from different origin was analysed. The induction of PvLEA3 was directly associated with the level of drought tolerance in the cultivars studied.

Overproduction of abscisic acid in tomato increases transpiration efficiency and root hydraulic conductivity and influences leaf expansion

Overexpression of genes that respond to drought stress is a seemingly attractive approach for improving drought resistance in crops. However, the consequences for both water-use efficiency and productivity must be considered if agronomic utility is sought. Here, we characterize two tomato (Solanum lycopersicum) lines (sp12 and sp5) that overexpress a gene encoding 9-cis-epoxycarotenoid dioxygenase, the enzyme that catalyzes a key rate-limiting step in abscisic acid (ABA) biosynthesis. Both lines contained more ABA than the wild type, with sp5 accumulating more than sp12. Both had higher transpiration efficiency because of their lower stomatal conductance, as demonstrated by increases in delta(13)C and delta(18)O, and also by gravimetric and gas-exchange methods. They also had greater root hydraulic conductivity. Under well-watered glasshouse conditions, mature sp5 plants were found to have a shoot biomass equal to the wild type despite their lower assimilation rate per unit leaf area. These plants also had longer petioles, larger leaf area, increased specific leaf area, and reduced leaf epinasty. When exposed to root-zone water deficits, line sp12 showed an increase in xylem ABA concentration and a reduction in stomatal conductance to the same final levels as the wild type, but from a different basal level. Indeed, the main difference between the high ABA plants and the wild type was their performance under well-watered conditions: the former conserved soil water by limiting maximum stomatal conductance per unit leaf area, but also, at least in the case of sp5, developed a canopy more suited to light interception, maximizing assimilation per plant, possibly due to improved turgor or suppression of epinasty.

Immunoaffinity chromatography of abscisic acid combined with electrospray liquid chromatography-mass spectrometry

Polyclonal antibodies with high specificity for C1-immobilised (+)-cis,trans-abscisic acid (ABA) were raised, characterised by enzyme-linked immunosorbent assay (ELISA) and used for preparation of an immunoaffinity chromatography (IAC) gel. The detection limit of the ELISA was approximately 4.6x10(-10)mol/L. Sensitive electrospray liquid chromatography-mass spectrometry (LC-ESI-MS) methods were also developed with detection limits below 0.1x10(-12)mol. The IAC allowed quick, single-step processing of samples prior to the analyses. The LC-ESI-MS and LC-ELISA techniques were used for comparative estimation of endogenous ABA levels in immunoaffinity purified extracts of normal and water-stressed Nicotiana tabacum L. leaves. The analytical approaches were validated using deuterium- and tritium-labelled internal standards, respectively. The IAC method was found to be highly effective, sensitive and convenient for isolating the target analyte from plant material.

Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis

Abscisic acid (ABA) is an important phytohormone regulating various plant processes, including seed germination. Although phosphorylation has been suggested to be important, the protein kinases required for ABA signaling during seed germination and seedling growth remain elusive. Here, we show that two protein kinases, SNF1-RELATED PROTEIN KINASE2.2 (SnRK2.2) and SnRK2.3, control responses to ABA in seed germination, dormancy, and seedling growth in Arabidopsis thaliana. A snrk2.2 snrk2.3 double mutant, but not snrk2.2 or snrk2.3 single mutants, showed strong ABA-insensitive phenotypes in seed germination and root growth inhibition. Changes in seed dormancy and ABA-induced Pro accumulation consistent with ABA insensitivity were also observed. The snrk2.2 snrk2.3 double mutant had a greatly reduced level of a 42-kD kinase activity capable of phosphorylating peptides from ABF (for ABA Response Element Binding Factor) transcription factors. ABA-induced expression of several genes whose promoters contain an ABA response element (ABRE) was reduced in snrk2.2 snrk2.3, suggesting that the mechanism of SnRK2.2 and SnRK2.3 action in ABA signaling involves the activation of ABRE-driven gene expression through the phosphorylation of ABFs. Together, these results demonstrate that SnRK2.2 and SnRK2.3 are redundant but key protein kinases that mediate a major part of ABA signaling in Arabidopsis.

The Arabidopsis tetratricopeptide repeat-containing protein TTL1 is required for osmotic stress responses and abscisic acid sensitivity

Mutations in the Arabidopsis (Arabidopsis thaliana) TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE 1 (TTL1) cause reduced tolerance to NaCl and osmotic stress that is characterized by reduced root elongation, disorganization of the root meristem, and impaired osmotic responses during germination and seedling development. Expression analyses of genes involved in abscisic acid (ABA) biosynthesis and catabolism suggest that TTL1 is not involved in the regulation of ABA levels but is required for ABA-regulated responses. TTL1 regulates the transcript levels of several dehydration-responsive genes, such as the transcription factor DREB2A, and genes encoding dehydration response proteins, such as ERD1 (early response to dehydration 1), ERD3, and COR15a. The TTL1 gene encodes a novel plant protein with tetratricopeptide repeats and a region with homology to thioredoxin proteins. Based on homology searches, there are four TTL members in the Arabidopsis genome with similar intron-exon structure and conserved amino acid domains. Proteins containing tetratricopeptide repeat motifs act as scaffold-forming multiprotein complexes and are emerging as essential elements for plant hormonal responses (such as gibberellin responses and ethylene biosynthesis). In this report, we identify TTL1 as a positive regulator of ABA signaling during germination and seedling development under stress.

Quantification of abscisic acid in grapevine leaf (Vitis vinifera) by isotope-dilution liquid chromatography-mass spectrometry

A specific, sensitive, precise, and accurate method for the determination of abscisic acid (ABA) in grapevine leaf tissues is described. The method employs high-performance liquid chromatography and electrospray ionization-mass spectrometry (LC-ESI-MS) in selected ion monitoring mode (SIM) to analyze ABA using a stable isotope-labeled ABA as an internal standard. Absolute recoveries ranged from 72% to 79% using methanol/water pH 5.5 (50:50 v/v) as an extraction solvent. The best efficiency was obtained when the chromatographic separation was carried out by using a porous graphitic carbon (PGC) column. The statistical evaluation of the method was satisfactory in the work range. A relative standard deviation (RDS) of < 5.5% and < 6.0% was obtained for intra-batch and inter-batch comparisons, respectively. As for accuracy, the relative error (%Er) was between -2.7 and 4.3%, and the relative recovery ranged from 95% to 107%.

Genome-wide transcriptional analysis of the Arabidopsis thaliana interaction with the plant pathogen Pseudomonas syringae pv. tomato DC3000 and the human pathogen Escherichia coli O157:H7

Pseudomonas syringae pv. tomato DC3000 (Pst) is a virulent pathogen that causes disease on tomato and Arabidopsis. The type III secretion system (TTSS) plays a key role in pathogenesis by translocating virulence effectors from the bacteria into the plant host cell, while the phytotoxin coronatine (COR) contributes to virulence and disease symptom development. Recent studies suggest that both the TTSS and COR are involved in the suppression of host basal defenses. However, little is known about the interplay between the host gene expression changes associated with basal defenses and the virulence activities of the TTSS and COR during infection. In this study, we used the Affymetrix full genome chip to determine the Arabidopsis transcriptome associated with basal defense to Pst DC3000 hrp mutants and the human pathogenic bacterium Escherichia coli O157:H7. We then used Pst DC3000 virulence mutants to characterize Arabidopsis transcriptional responses to the action of hrp-regulated virulence factors (e.g. TTSS and COR) during bacterial infection. Additionally, we used bacterial fliC mutants to assess the role of the pathogen-associated molecular pattern flagellin in induction of basal defense-associated transcriptional responses. In total, our global gene expression analysis identified 2800 Arabidopsis genes that are reproducibly regulated in response to bacterial pathogen inoculation. Regulation of these genes provides a molecular signature for Arabidopsis basal defense to plant and human pathogenic bacteria, and illustrates both common and distinct global virulence effects of the TTSS, COR, and possibly other hrp-regulated virulence factors during Pst DC3000 infection.

Methods and concepts in quantifying resistance to drought, salt and freezing, abiotic stresses that affect plant water status

The abiotic stresses of drought, salinity and freezing are linked by the fact that they all decrease the availability of water to plant cells. This decreased availability of water is quantified as a decrease in water potential. Plants resist low water potential and related stresses by modifying water uptake and loss to avoid low water potential, accumulating solutes and modifying the properties of cell walls to avoid the dehydration induced by low water potential and using protective proteins and mechanisms to tolerate reduced water content by preventing or repairing cell damage. Salt stress also alters plant ion homeostasis, and under many conditions this may be the predominant factor affecting plant performance. Our emphasis is on experiments that quantify resistance to realistic and reproducible low water potential (drought), salt and freezing stresses while being suitable for genetic studies where a large number of lines must be analyzed. Detailed protocols for the use of polyethylene glycol-infused agar plates to impose low water potential stress, assay of salt tolerance based on root elongation, quantification of freezing tolerance and the use of electrolyte leakage experiments to quantify cellular damage induced by freezing and low water potential are also presented.

Role of abscisic acid (ABA) and Arabidopsis thaliana ABA-insensitive loci in low water potential-induced ABA and proline accumulation

The mechanisms by which plants respond to reduced water availability (low water potential) include both ABA-dependent and ABA-independent processes. Pro accumulation and osmotic adjustment are two important traits for which the mechanisms of regulation by low water potential, and the involvement of ABA, is not well understood. The ABA-deficient mutant, aba2-1, was used to investigate the regulatory role of ABA in low water potential-induced Pro accumulation and osmotic adjustment in seedlings of Arabidopsis thaliana. Low water potential-induced Pro accumulation required wild-type levels of ABA, as well as a change in ABA sensitivity or ABA-independent events. Osmotic adjustment, in contrast, occurred independently of ABA accumulation in aba2-1. Quantification of low water potential-induced ABA and Pro accumulation in five ABA-insensitive mutants, abi1-1, abi2-1, abi3, abi4, and abi5, revealed that abi4 had increased Pro accumulation at low water potential, but a reduced response to exogenous ABA. Both of these responses were modified by sucrose treatment, indicating that ABI4 has a role in connecting ABA and sugar in regulating Pro accumulation. Of the other abi mutants, only abi1 had reduced Pro accumulation in response to low water potential and ABA application. It was also observed that abi1-1 and abi2-1 had increased ABA accumulation. The involvement of these loci in feedback regulation of ABA accumulation may occur through an effect on ABA catabolism or conjugation. These data provide new information on the function of ABA in seedlings exposed to low water potential and define new roles for three of the well-studied abi loci.

LEA proteins in higher plants: Structure, function, gene expression and regulation

Late embryogenesis abundant (LEA) proteins are mainly low molecular weight (10-30 kDa) proteins, which are involved in protecting higher plants from damage caused by environmental stresses, especially drought (dehydration). These findings and the fact that the breeding of drought tolerant varieties would be of great value in agriculture, form the basis of search for anti-drought inducible genes and their characterization. LEA proteins are generally classified into six groups (families) according to their amino acid sequence and corresponding mRNA homology, which are basically localized in cytoplasm and nuclear region. LEA protein synthesis, expression and biological activities are regulated by many factors (e.g. developmental stages, hormones, ion change and dehydration), signal transduction pathways and lea genes. No tissue-specific lea gene expression has been considered as one main regulatory mechanism on the basis of extensive studies with the model plant, Arabidopsisthaliana. The study of the regulatory mechanism of lea gene expression is an important feature of modern plant molecular biology.

Effects of water stress on respiration in soybean leaves

The effect of water stress on respiration and mitochondrial electron transport has been studied in soybean (Glycine max) leaves, using the oxygen-isotope-fractionation technique. Treatments with three levels of water stress were applied by irrigation to replace 100%, 50%, and 0% of daily water use by transpiration. The levels of water stress were characterized in terms of light-saturated stomatal conductance (g(s)): well irrigated (g(s) > 0.2 mol H(2)O m(-2) s(-1)), mildly water stressed (g(s) between 0.1 and 0.2 mol H(2)O m(-2) s(-1)), and severely water stressed (g(s) < 0.1 mol H(2)O m(-2) s(-1)). Although net photosynthesis decreased by 40% and 70% under mild and severe water stress, respectively, the total respiratory oxygen uptake (V(t)) was not significantly different at any water-stress level. However, severe water stress caused a significant shift of electrons from the cytochrome to the alternative pathway. The electron partitioning through the alternative pathway increased from 10% to 12% under well-watered or mild water-stress conditions to near 40% under severe water stress. Consequently, the calculated rate of mitochondrial ATP synthesis decreased by 32% under severe water stress. Unlike many other stresses, water stress did not affect the levels of mitochondrial alternative oxidase protein. This suggests a biochemical regulation (other than protein synthesis) that causes this mitochondrial electron shift.

Before and beyond ABA: upstream sensing and internal signals that determine ABA accumulation and response under abiotic stress

Sensing and signalling events that detect abiotic stress-induced changes in plant water status and initiate downstream stress responses such as ABA (abscisic acid) accumulation and osmoregulation remain uncharacterized in plants. Although conclusive results are lacking, recent results from plants, and analogies to signalling in other organisms, suggest possible mechanisms for sensing altered water status and initial transduction of that signal. Internal signals that act downstream of ABA and modulate stress responses to reflect the type and severity of the stress and the metabolic status of the plant are also not well understood. Two specific types of signalling, sugar sensing and reactive oxygen signalling, are likely to be modulators of ABA response under stress. For both upstream sensing and signalling of plant water status as well as downstream modulation of ABA response, present results suggest several genetic strategies with high potential to increase our understanding of the molecular basis by which plants sense and respond to altered water status.

A rapid method for analysis of abscisic acid (ABA) in crude extracts of water stressed Arabidopsis thaliana plants by liquid chromatography--mass spectrometry in tandem mode

We describe a quick, simple method for the extraction and quantification of the phytohormone (+)-abscisic acid (ABA) in samples of plants subjected to different water deficit treatments. The method includes an extraction with acetone/water/acetic acid (80:19:1, v/v), evaporation of the extracts and finally injection into the liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS-MS) system in multiple reaction monitoring (MRM) mode. The objective of this work has been to show the applicability of the method to quantify the endogenous content of ABA in Arabidopsis thaliana (Columbia, Col-0) leaves at three different degrees of water stress. Control plants, had almost constant low levels of ABA (2-3 ng g-1, f.w.) throughout the 3 weeks of the experiment. Nevertheless, stressed plants increase the ABA content between the first and the second week (from 10 to 21 ng g-1, f.w.). The results suggest that this method is useful for quantifying ABA from plant material and that it avoids tedious and time-consuming extraction, purification and/or derivatization processes.

A single amino acid substitution in the Arabidopsis FIERY1/HOS2 protein confers cold signaling specificity and lithium tolerance

Low temperature induces the expression of many plant genes through undefined signaling pathways. To gain insight into cold signal transduction mechanisms, we isolated Arabidopsis mutants that exhibited altered regulation of low temperature-induced gene expression. One such mutant, hos2, was shown previously to have an enhanced induction of stress-responsive genes by cold, whereas the expression of these genes under osmotic stress or the phytohormone absciscic acid (ABA) treatments was not affected. Here we further define the targets of HOS2 by examining the regulation of upstream cold-specific CBF transcription factor genes. It was found that the transcript levels of CBF2 and CBF3 were significantly higher in hos2 mutant plants than in the wild type under cold treatments, suggesting that HOS2 may act upstream of CBFs. The HOS2 gene was cloned using a map-based strategy. Surprisingly, HOS2 is identical to the FIERY1 gene that we had described previously. FIERY1 is a general negative regulator that controls cold, osmotic stress, and ABA signal transduction and possesses inositol polyphosphate 1-phosphatase activity. The hos2 mutation rendered the HOS2/FIERY1 recombinant protein completely inactive in the cold but did not substantially affect its activity at warm temperatures. Interestingly, the hos2 mutant protein is extremely tolerant to Li+. This study provides a unique example of a single amino acid substitution in a critical regulator that can lead to conditional changes in protein functions and distinct plant phenotypes. The results reinforce the notion that phosphoinositols are important second messengers in cold signal transduction, and shed light on how the diversity of plant tolerance to cold and other abiotic stresses may evolve due to variations in a common molecular switch.

Uncoupling the effects of abscisic acid on plant growth and water relations. Analysis of sto1/nced3, an abscisic acid-deficient but salt stress-tolerant mutant in Arabidopsis

We have identified a T-DNA insertion mutation of Arabidopsis (ecotype C24), named sto1 (salt tolerant), that results in enhanced germination on both ionic (NaCl) and nonionic (sorbitol) hyperosmotic media. sto1 plants were more tolerant in vitro than wild type to Na(+) and K(+) both for germination and subsequent growth but were hypersensitive to Li(+). Postgermination growth of the sto1 plants on sorbitol was not improved. Analysis of the amino acid sequence revealed that STO1 encodes a 9-cis-epoxicarotenoid dioxygenase (similar to 9-cis-epoxicarotenoid dioxygenase GB:AAF26356 [Phaseolus vulgaris] and to NCED3 GB:AB020817 [Arabidopsis]), a key enzyme in the abscisic acid (ABA) biosynthetic pathway. STO1 transcript abundance was substantially reduced in mutant plants. Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress, although their basal ABA level was only moderately altered. Either complementation of the sto1 with the native gene from the wild-type genome or supplementation of ABA to the growth medium restored the wild-type phenotype. Improved growth of sto1 mutant plants on NaCl, but not sorbitol, medium was associated with a reduction in both NaCl-induced expression of the ICK1 gene and ethylene accumulation. Osmotic adjustment of sto1 plants was substantially reduced compared to wild-type plants under conditions where sto1 plants grew faster. The sto1 mutation has revealed that reduced ABA can lead to more rapid growth during hyperionic stress by a signal pathway that apparently is at least partially independent of signals that mediate nonionic osmotic responses.

A novel inhibitor of 9-cis-epoxycarotenoid dioxygenase in abscisic acid biosynthesis in higher plants

Abscisic acid (ABA) is a major regulator in the adaptation of plants to environmental stresses, plant growth, and development. In higher plants, the ABA biosynthesis pathway involves the oxidative cleavage of 9-cis-epoxycarotenoids, which may be the key regulatory step in the pathway catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). We developed a new inhibitor of ABA biosynthesis targeting NCED and named it abamine (ABA biosynthesis inhibitor with an amine moiety). Abamine is a competitive inhibitor of NCED, with a Ki of 38.8 microm. In 0.4 m mannitol solution, which mimics the effects of osmotic stress, abamine both inhibited stomatal closure in spinach (Spinacia oleracea) leaves, which was restored by coapplication of ABA, and increased luminescence intensity in transgenic Arabidopsis containing the RD29B promoter-luciferase fusion. The ABA content of plants in 0.4 m mannitol was increased approximately 16-fold as compared with that of controls, whereas 50 to 100 microm abamine inhibited about 50% of this ABA accumulation in both spinach leaves and Arabidopsis. Abamine-treated Arabidopsis was more sensitive to drought stress and showed a significant decrease in drought tolerance than untreated Arabidopsis. These results suggest that abamine is a novel ABA biosynthesis inhibitor that targets the enzyme catalyzing oxidative cleavage of 9-cis-epoxycarotenoids. To test the effect of abamine on plants other than Arabidopsis, it was applied to cress (Lepidium sativum) plants. Abamine enhanced radicle elongation in cress seeds, which could be due to a decrease in the ABA content of abamine-treated plants. Thus, it is possible to think that abamine should enable us to elucidate the functions of ABA in cells or plants and to find new mutants involved in ABA signaling.

A new lead compound for abscisic acid biosynthesis inhibitors targeting 9-cis-epoxycarotenoid dioxygenase

9-cis-Epoxycarotenoid dioxygenase (NCED), a key enzyme in abscisic acid (ABA) biosynthesis, cleaves the olefinic double bond of 9-cis-epoxycarotenoid. Several analogues of nordihydroguaiaretic acid (NDGA) were designed and synthesized, and their efficacy as inhibitors of NCED was examined. One of the synthesized compounds (20) was found to be an inhibitor of this enzyme, and inhibited ABA accumulation and stomatal closing, suggesting that 20 should be ABA biosynthesis inhibitor.

Differential mRNA translation contributes to gene regulation under non-stress and dehydration stress conditions in Arabidopsis thaliana

Translational regulation was evaluated for over 2000 genes by measurement of the proportion of individual mRNA species in polysomal (PS) complexes in leaves of non-stressed and moderately dehydration-stressed Arabidopsis. The amount of each mRNA in polysomes ranged from 23 to 97% in non-stressed leaves and was significantly reduced for a large portion of the genes (71%) in response to dehydration. The effect of dehydration on translational status varied extensively between mRNA species. Sixty per cent of the dehydration-inducible mRNAs with twofold or greater increase in abundance maintained PS levels in response to water-deficit stress, while 40% showed impaired ribosome loading (RL). PS association declined significantly for 92% of the mRNAs that displayed a strong decrease in abundance, indicating a relationship between translation and decreased gene transcription and/or mRNA stability. Interestingly, many mRNAs that encode proteins of similar biological function displayed coordinate translational regulation. Thus, the abundance of PS mRNA may provide a more accurate estimate of gene expression than total cellular mRNA because of extensive differential translational regulation.

A novel calmodulin-binding protein functions as a negative regulator of osmotic stress tolerance in Arabidopsis thaliana seedlings

A clone for a novel Arabidopsisthaliana calmodulin (CaM)-binding protein of 25 kDa (AtCaMBP25) has been isolated by using a radiolabelled CaM probe to screen a cDNA expression library derived from A. thaliana cell suspension cultures challenged with osmotic stress. The deduced amino acid sequence of AtCaMBP25 contains putative nuclear localization sequences and shares significant degree of similarity with hypothetical plant proteins only. Fusion of the AtCaMBP25 coding sequence to reporter genes targets the hybrid protein to the nucleus. Bacterially expressed AtCaMBP25 binds, in a calcium-dependent manner, to a canonical CaM but not to a less conserved isoform of the calcium sensor. AtCaMBP25 is encoded by a single-copy gene, whose expression is induced in Arabidopsis seedlings exposed to dehydration, low temperature or high salinity. Transgenic plants overexpressing AtCaMBP25 exhibits an increased sensitivity to both ionic (NaCl) and non-ionic (mannitol) osmotic stress during seed germination and seedling growth. By contrast, transgenic lines expressing antisense AtCaMBP25 are significantly more tolerant to mannitol and NaCl stresses than the wild type. Thus, the AtCaMBP25 gene functions as a negative effector of osmotic stress tolerance and likely participates in stress signal transduction pathways.

Function of a plant stress-induced gene, HVA22. Synthetic enhancement screen with its yeast homolog reveals its role in vesicular traffic

Expression of the barley (Hordeum vulgare) HVA22 gene is induced by environmental stresses, such as dehydration, salinity, and extreme temperatures, and by a plant stress hormone, abscisic acid. Genes sharing high level of sequence similarities with HVA22 exist in diverse eukaryotic organisms, including animals, plants, and fungi, but not in any prokaryotic organisms. The yeast (Saccharomyces cerevisiae) HVA22 homolog, Yop1p, has been shown to interact with the GTPase-interacting protein, Yip1p. Deletion of YOP1 led to only a modest reduction of the stationary phase titer at 37C. A synthetic enhancement mutant screen was performed in the yop1 deletion background to identify genes interacting with YOP1. The open reading frame YOR165W (renamed SEY1 for synthetic enhancement of YOP1) was identified as a YOP1-dependent complementation gene. The yeast SEY1 is a homolog of the Arabidopsis RHD3 gene whose mutations cause the accumulation of transport vesicles near the tips of defective root hairs. The yeast double mutant of yop1 and sey1 is defective in vesicular traffic as evidenced by the accumulation of transport vesicles and the decrease in invertase secretion. Based on these observations, we suggest that Yop1p/HVA22 regulates vesicular traffic in stressed cells either to facilitate membrane turnover, or to decrease unnecessary secretion.

CpR18, a novel SAP-domain plant transcription factor, binds to a promoter region necessary for ABA mediated expression of the CDeT27-45 gene from the resurrection plant Craterostigma plantagineum Hochst

CDeT27-45 is a lea-like gene from the resurrection plant Craterostigma plantagineum (Scrophulariaceae) which is strongly expressed in vegetative tissues in response to dehydration or treatment with abscisic acid (ABA). Expression of the gene is correlated with the acquisition of desiccation tolerance. Nuclear proteins bind to a 29-bp cis-regulatory region of the promoter which is essential for transcriptional activation of the CDeT27-45 gene by ABA. Using a yeast one-hybrid screen, the cDNA clone CpR18 was isolated, which encodes a protein with specific binding activity for the cis-regulatory element in the CDeT27-45 promoter. The protein contains an acidic region, a SAP-domain, a zinc finger of the C3H-type, and two motifs which are conserved in proteins from several plant species. One of the conserved regions is rich in basic residues and is predicted to form a helix-loop-helix structure. The R18 gene shows high similarities to genomic sequences and ESTs from other plant species. The tissue-specific expression pattern of the rare R18 mRNA and the distribution of nuclear protein binding activity for the CDeT27-45 promoter fragment are compared. The R18 protein is indeed localized in the nucleus, and activates transcription of CDeT27-45 promoter-GUS fusion constructs in tobacco protoplasts. DNA blot analysis and isolation of genomic clones reveal that two copies of R18 are present in the C. plantagineum genome.