Cell signaling under salt, water and cold stresses

Dehydrins expression related to timing of bud burst in Norway spruce

Cold deacclimation and preparation to flushing likely requires rehydration of meristems. Therefore, water stress related genes, such as dehydrins (DHN), might play an important role in providing protection during winter dormancy, deacclimation and bud burst timing processes. Here we report the sequence analysis of several Norway spruce DHN identified in late and early flushing suppressive subtraction hybridization cDNA libraries and in our Norway spruce EST database. We obtained 15 cDNAs, representing eight genes from three distinct types of DHN, and studied differential expression of these genes before and during bud burst in spring, using qRT-PCR. We found the visible reduction in transcript level of most DHN towards the bud burst, supported by a significant down-regulation of the DHN in needles during experimental induction of bud burst applied at three time points during autumn in Norway spruce grafts. For most of the DHN transcripts, their expression levels in late-flushing spruces were significantly higher than in the early flushing ones at the same calendar dates but were remarkably similar at the same bud developmental stage. From our results we may conclude that the difference between the early and the late families is in timing of the molecular processes leading to bud burst due to differences in their response to the increasing temperature in the spring. They are induced much earlier in the early flushing families.

2R,3R-butanediol, a bacterial volatile produced by Pseudomonas chlororaphis O6, is involved in induction of systemic tolerance to drought in Arabidopsis thaliana

Root colonization of plants with certain rhizobacteria, such as Pseudomonas chlororaphis O6, induces tolerance to biotic and abiotic stresses. Tolerance to drought was correlated with reduced water loss in P. chlororaphis O6-colonized plants and with stomatal closure, indicated by size of stomatal aperture and percentage of closed stomata. Stomatal closure and drought resistance were mediated by production of 2R,3R-butanediol, a volatile metabolite of P. chlororaphis O6. Root colonization with bacteria deficient in 2R,3R-butanediol production showed no induction of drought tolerance. Studies with Arabidopsis mutant lines indicated that induced drought tolerance required the salicylic acid (SA)-, ethylene-, and jasmonic acid-signaling pathways. Both induced drought tolerance and stomatal closure were dependent on Aba-1 and OST-1 kinase. Increases in free SA after drought stress of P. chlororaphis O6-colonized plants and after 2R,3R-butanediol treatment suggested a primary role for SA signaling in induced drought tolerance. We conclude that the bacterial volatile 2R,3R-butanediol was a major determinant in inducing resistance to drought in Arabidopsis through an SA-dependent mechanism.

Genetic subtraction profiling identifies genes essential for Arabidopsis reproduction and reveals interaction between the female gametophyte and the maternal sporophyte

Genetic subtraction and expression profiling of wild-type Arabidopsis and a sporophytic mutant lacking an embryo sac identified 1,260 genes expressed in the embryo sac; a total of 527 genes were identified for their expression in ovules of mutants lacking an embryo sac.

Background

The embryo sac contains the haploid maternal cell types necessary for double fertilization and subsequent seed development in plants. Large-scale identification of genes expressed in the embryo sac remains cumbersome because of its inherent microscopic and inaccessible nature. We used genetic subtraction and comparative profiling by microarray between the Arabidopsis thaliana wild-type and a sporophytic mutant lacking an embryo sac in order to identify embryo sac expressed genes in this model organism. The influences of the embryo sac on the surrounding sporophytic tissues were previously thought to be negligible or nonexistent; we investigated the extent of these interactions by transcriptome analysis.

Results

We identified 1,260 genes as embryo sac expressed by analyzing both our dataset and a recently reported dataset, obtained by a similar approach, using three statistical procedures. Spatial expression of nine genes (for instance a central cell expressed trithorax-like gene, an egg cell expressed gene encoding a kinase, and a synergid expressed gene encoding a permease) validated our approach. We analyzed mutants in five of the newly identified genes that exhibited developmental anomalies during reproductive development. A total of 527 genes were identified for their expression in ovules of mutants lacking an embryo sac, at levels that were twofold higher than in the wild type.

Conclusion

Identification of embryo sac expressed genes establishes a basis for the functional dissection of embryo sac development and function. Sporophytic gain of expression in mutants lacking an embryo sac suggests that a substantial portion of the sporophytic transcriptome involved in carpel and ovule development is, unexpectedly, under the indirect influence of the embryo sac.

Signal transduction during cold stress in plants

Cold stress signal transduction is a complex process. Many physiological changes like tissue break down and senescence occur due to cold stress. Low temperature is initially perceived by plasma membrane either due to change in membrane fluidity or with the help of sensors like Ca(2+) permeable channels, histidine kinases, receptor kinases and phospholipases. Subsequently, cytoskeleton reorganization and cytosolic Ca(2+) influx takes place. Increase in cytosolic Ca(2+) is sensed by CDPKs, phosphatase and MAPKs, which transduce the signals to switch on transcriptional cascades. Photosynthetic apparatus have also been thought to be responsible for low temperature perception and signal transduction. Many cold induced pathways are activated to protect plants from deleterious effects of cold stress, but till date, most studied pathway is ICE-CBF-COR signaling pathway. However, the importance of CBF independent pathways in cold acclimation is supported by few Arabidopsis mutants' studies. Cold stress signaling has certain pathways common with other abiotic and biotic stress signaling which suggest cross-talks among these. Most of the economically important crops are sensitive to low temperature, but very few studies are available on cold susceptible crop plants. Therefore, it is necessary to understand signal transducing components from model plants and utilize that knowledge to improve survival of cold sensitive crop plants at low temperature.

A novel WRKY transcriptional factor from Thlaspi caerulescens negatively regulates the osmotic stress tolerance of transgenic tobacco

A novel member of the WRKY gene family, designated TcWRKY53, was isolated from a cadmium (Cd)-treated Thlaspi caerulescens cDNA library by differential screening. WRKY proteins specifically bind to W-boxes, which are found in the promoters of many genes involved in defense and response to environmental stress. TcWRKY53 contains a 975-bp open reading frame encoding a putative protein of 324 amino acids. Homology searches showed that TcWRKY53 resembles similar WRKY domain-containing proteins from rice, parsley and tobacco, especially AtWRKY53 from Arabidopsis thaliana. Semi-quantitative RT-PCR showed that the expression of TcWRKY53 was strongly induced by various environmental stresses, including an excess of NaCl, drought, cold and the signal molecule salicylic acid (SA). The expression of TcWRKY53 in response to NaCl, drought and cold suggested a possible role of TcWRKY53 in abiotic stress response. However, physiological tests indicated that the expression of TcWRKY53 in tobaccos decreases tolerance to sorbitol during seedling root development. This was consistent with PEG6000 treatment of tobacco seedlings, and together these results indicate a negative modulation of TcWRKY53 in response to osmotic stress. Furthermore, two ethylene responsive factor (ERF) family genes, NtERF5 and NtEREBP-1, were negatively induced in TcWRKY53-overexpressing transgenic plants. In contrast, a LEA family gene, NtLEA5, showed no change, suggesting that TcWRKY53 might regulate the plant osmotic stress response by interacting with an ERF-type transcription factor rather than by regulating function genes directly.

Towards salinity tolerance in Brassica: an overview

Among the various abiotic stresses limiting the crop productivity, salinity stress is a major problem, which needs to be addressed and answered urgently. Since members of Brassicaceae are important contributor to total oilseed production, there is an immediate need being felt to raise Brassica plants which would be more suitable for saline and dry lands in years to come. One of the suggested way to develop salinity tolerant Brassica plants is to make use of the broad gene pool available within the family. Efforts of breeders have been successful in such endeavors to a large extent and several salinity tolerant Brassica genotypes have been developed within India and elsewhere. On the other hand, transgenic technology will undoubtedly continue to aid the search for the cellular mechanisms that confer tolerance, but the complexity of the trait is likely to mean that the road to engineer such tolerance into sensitive species will not be easy. However, with increasing number of reports available for suitable genetic transformation for various Brassica genotypes, there is a hope that salinity tolerance can be improved in this important crop plant. In this direction, the complete genome sequence of related wild plants such as Arabidopsis or crop plants such as rice can also serve as a platform for identification of "candidate genes". Recently, complete genome sequencing of the Brassica genomes has also been initiated with the view that availability of such useful information can pave way towards raising Brassica with improved tolerance towards these stresses. In the present paper, we discuss the success obtained so far; in raising brassica genotypes with improved salinity tolerance employing both plant breeding and/or genetic engineering tools.

Raising salinity tolerant rice: recent progress and future perspectives

With the rapid growth in population consuming rice as staple food and the deteriorating soil and water quality around the globe, there is an urgent need to understand the response of this important crop towards these environmental abuses. With the ultimate goal to raise rice plant with better suitability towards rapidly changing environmental inputs, intensive efforts are on worldwide employing physiological, biochemical and molecular tools to perform this task. In this regard, efforts of plant breeders need to be duly acknowledged as several salinity tolerant varieties have reached the farmers field. Parallel efforts from molecular biologists have yielded relevant knowledge related to perturbations in gene expression and proteins during stress. Employing transgenic technology, functional validation of various target genes involved in diverse processes such as signaling, transcription, ion homeostasis, antioxidant defense etc for enhanced salinity stress tolerance has been attempted in various model systems and some of them have been extended to crop plant rice too. However, the fact remains that these transgenic plants showing improved performance towards salinity stress are yet to move from 'lab to the land'. Pondering this, we propose that future efforts should be channelized more towards multigene engineering that may enable the taming of this multigene controlled trait. Recent technological achievements such as the whole genome sequencing of rice is leading to a shift from single gene based studies to genome wide analysis that may prove to be a boon in re-defining salt stress responsive targets.

Spatial and temporal quantitative analysis of cell division and elongation rate in growing wheat leaves under saline conditions

Leaf growth in grasses is determined by the cell division and elongation rates, with the duration of cell elongation being one of the processes that is the most sensitive to salinity. Our objective was to investigate the distribution profiles of cell production, cell length and the duration of cell elongation in the growing zone of the wheat leaf during the steady growth phase. Plants were grown in loamy soil with or without 120 mmol/L NaCl in a growth chamber, and harvested at day 3 after leaf 4 emerged. Results show that the elongation rate of leaf 4 was reduced by 120 mmol/L NaCl during the steady growth phase. The distribution profile of the lengths of abaxial epidermal cells of leaf 4 during the steady growth stage shows a sigmoidal pattern along the leaf axis for both treatments. Although salinity did not affect or even increased the length of the epidermal cells in some locations in the growth zone compared to the control treatment, the final length of the epidermal cells was reduced by 14% at 120 mmol/L NaCl. Thus, we concluded that the observed reduction in the leaf elongation rate derived in part from the reduced cell division rate and either the shortened cell elongation zone or shortened duration of cell elongation. This suggests that more attention should be paid to the effects of salinity on those properties of cell production and the period of cell maturation that are related to the properties of cell wall.

The plant growth-promoting fungus Penicillium simplicissimum GP17-2 induces resistance in Arabidopsis thaliana by activation of multiple defense signals

Arabidopsis thaliana grown in soil amended with barley grain inocula of Penicillium simplicissimum GP17-2 or receiving root treatment with its culture filtrate (CF) exhibited clear resistance to Pseudomonas syringae pv. tomato DC3000 (Pst). To assess the contribution of different defense pathways, Arabidopsis genotypes implicated in salicylic acid (SA) signaling expressing the NahG transgene or carrying disruption in NPR1 (npr1), jasmonic acid (JA) signaling (jar1) and ethylene (ET) signaling (ein2) were tested. All genotypes screened were protected by GP17-2 or its CF. However, the level of protection was significantly lower in NahG and npr1 plants than it was in similarly treated wild-type plants, indicating that the SA signaling pathway makes a minor contribution to the GP17-2-mediated resistance and is insufficient for a full response. Examination of local and systemic gene expression revealed that GP17-2 and its CF modulate the expression of genes involved in both the SA and JA/ET signaling pathways. Subsequent challenge of GP17-2-colonized plants with Pst was accompanied by direct activation of SA-inducible PR-2 and PR-5 genes as well as potentiated expression of the JA-inducible Vsp gene. In contrast, CF-treated plants infected with Pst exhibited elevated expression of most defense-related genes (PR-1, PR-2, PR-5, PDF1.2 and Hel) studied. Moreover, an initial elevation of SA responses was followed by late induction of JA responses during Pst infection of induced systemic resistance (ISR)-expressing plants. In conclusion, we hypothesize the involvement of multiple defense mechanisms leading to an ISR of Arabidopsis by GP17-2.

Differential expression of the TFIIIA regulatory pathway in response to salt stress between Medicago truncatula genotypes

Soil salinity is one of the most significant abiotic stresses for crop plants, including legumes. These plants can establish root symbioses with nitrogen-fixing soil bacteria and are able to grow in nitrogen-poor soils. Medicago truncatula varieties show diverse adaptive responses to environmental conditions, such as saline soils. We have compared the differential root growth of two genotypes of M. truncatula (108-R and Jemalong A17) in response to salt stress. Jemalong A17 is more tolerant to salt stress than 108-R, regarding both root and nodulation responses independently of the nitrogen status of the media. A dedicated macroarray containing 384 genes linked to stress responses was used to compare root gene expression during salt stress in these genotypes. Several genes potentially associated with the contrasting cellular responses of these plants to salt stress were identified as expressed in the more tolerant genotype even in the absence of stress. Among them, a homolog of the abiotic stress-related COLD-REGULATEDA1 gene and a TFIIIA-related transcription factor (TF), MtZpt2-1, known to regulate the former gene. Two MtZpt2 TFs (MtZpt2-1 and MtZpt2-2) were found in Jemalong A17 plants and showed increased expression in roots when compared to 108-R. Overexpression of these TFs in the sensitive genotype 108-R, but not in Jemalong A17, led to increased root growth under salt stress, suggesting a role for this pathway in the adaptive response to salt stress of these M. truncatula genotypes.

Overexpression of a putative maize calcineurin B-like protein in Arabidopsis confers salt tolerance

The calcineurin B-like proteins (CBLs) represent a unique family of calcium sensors in plants. Although extensive studies and remarkable progress have been made in Arabidopsis (Arabidopsis thaliana) CBLs, their functions in other plant species are still quite limited. Here, we report the cloning and functional characterization of ZmCBL4, a novel CBL gene from maize (Zea mays). ZmCBL4 encodes a putative homolog of the Arabidopsis CBL4/SOS3 protein, with novel properties. ZmCBL4 has one copy in maize genome and harbors seven introns in its coding region. ZmCBL4 expressed differentially in various organs of the maize plants at a low level under normal condition, and its expression was regulated by NaCl, LiCl, ABA and PEG treatments. Expression of 35S::ZmCBL4 not only complemented the salt hypersensitivity in Arabidopsis sos3 mutant, but also enhanced the salt tolerance in Arabidopsis wild type at the germination and seedling stages. Moreover, the LiCl tolerance in all of the ZmCBL4-expressing lines increased more significantly as compared with the NaCl tolerance, and in consistent with this, it was found that the expression of Arabidopsis AtNHX8, a putative plasma membrane Li+/H+ antiporter gene identified recently, was induced in these transgenic lines under LiCl stress. The ZmCBL4-expressing Arabidopsis lines accumulated less Na+ and Li+ as compared with the control plants. This study has identified a putative maize CBL gene which functions in the salt stress-elicited calcium signaling and thus in the tolerance to salinity.

Characterization of AtALMT1 expression in aluminum-inducible malate release and its role for rhizotoxic stress tolerance in Arabidopsis

Malate transporters play a critical role in aluminum (Al) tolerance responses for some plant species, such as Arabidopsis (Arabidopsis thaliana). Here, we further characterize AtALMT1, an Arabidopsis aluminum-activated malate transporter, to clarify its specific role in malate release and Al stress responses. Malate excretion from the roots of accession Columbia was sharply induced by Al, which is concomitant with the induction of AtALMT1 gene expression. The malate release was specific for Al among rhizotoxic stressors, namely cadmium, copper, erbium, lanthanum, sodium, and low pH, which accounts for the specific sensitivity of a null mutant to Al stress. Al-specific malate excretion can be explained by a combined regulation of AtALMT1 expression and activation of AtALMT1 protein, which is specific for Al. Although low pH treatment slightly induced gene expression, other treatments did not. In addition, malate excretion in Al-activated seedlings was rapidly stopped by removing Al from the solution. Other rhizotoxic stressors were not effective in maintaining malate release. Protein kinase and phosphatase inhibitor studies indicated that reversible phosphorylation was important for the transcriptional and posttranslational regulation of AtALMT1. AtALMT1 promoter-beta-glucuronidase fusion lines revealed that AtALMT1 has restricted expression within the root, such that unnecessary carbon loss is likely minimized. Lastly, a natural nonsense mutation allele of AtALMT1 was identified from the Al-hypersensitive natural accession Warschau-1.

The CCCH-type zinc finger proteins AtSZF1 and AtSZF2 regulate salt stress responses in Arabidopsis

The molecular mechanism governing the response of plants to salinity stress, one of the most significant limiting factors for agriculture worldwide, has just started to be revealed. Here, we report AtSZF1 and AtSZF2, two closely related CCCH-type zinc finger proteins, involved in salt stress responses in Arabidopsis. The expression of AtSZF1 and AtSZF2 is quickly and transiently induced by NaCl treatment. Mutants disrupted in the expression of AtSZF1 or AtSZF2 exhibit increased expression of a group of salt stress-responsive genes in response to high salt. Significantly, the atszf1-1/atszf2-1 double mutant displays more sensitive responses to salt stress than the atszf1-1 or atszf2-1 single mutants and wild-type plants. On the other hand, transgenic plants overexpressing AtSZF1 show reduced induction of salt stress-responsive genes and are more tolerant to salt stress. We also showed that AtSZF1 is localized in the nucleus. Taken together, these results demonstrated that AtSZF1 and AtSZF2 negatively regulate the expression of salt-responsive genes and play important roles in modulating the tolerance of Arabidopsis plants to salt stress.

Isolation of cold stress-responsive genes in the reproductive organs, and characterization of the OsLti6b gene from rice (Oryza sativa L.)

During their reproductive stage, rice crops often are exposed to cold stress, which leads to sterility and reduced yields. To understand the cold response mechanism at that stage, we used an mRNA differential display method to isolate cold-responsive genes from pre-anthesis flowers. Approximately 5,000 transcripts were identified here, of which 123 were found to be displayed differentially between the control (30 degrees C) and cold-treated (12 degrees C) flowers. Among them, 26 were analyzed by northern analysis; 8 of those clones were confirmed as cold-responsive. OsLti6b, encoding a hydrophobic protein homologous to Arabidopsis RCI2, was analyzed in detail. RNA blot analysis revealed that its transcript is increased by cold, salt, drought, or ABA treatments. In situ hybridization indicated that this transcript is highly accumulated in the ovaries and stamens of cold-treated flowers, particularly in the anther walls and vascular tissues of the filaments. Over-expression of OsLti6b increased cold tolerance as revealed by seedling wilting rates and ion leakages of mature leaves, demonstrating that the extent of the tolerance correlates well with its expression level.

Effect of different levels of NaCl and KCl on growth and some biological indexes of wheat plant

In this study, it was aimed to determine the effects of different levels (0, 15, 30 and 60 mM) of NaCl and KCl salt on seedling growth and some biological indexes of wheat. Shoot height, stem diameter, leaves number of plant, fresh weight of shoot and dry matter weight index were investigated. The results showed that biological index of wheat decreased with increasing salt in comparison to control. The adverse affect of NaCl on wheat plant was obtained higher than that of KCl. This should carefully be considered if wheat is grown under saline soil condition.

Regulation and functional analysis of ZmDREB2A in response to drought and heat stresses in Zea mays L

DREB1/CBFs and DREB2s are transcription factors that specifically interact with a cis-acting element, DRE/CRT, which is involved in the expression of genes responsive to cold and drought stress in Arabidopsis thaliana. The function of DREB1/CBFs has been precisely analyzed and it has been found to activate the expression of many genes responsive to cold stress containing a DRE/CRT sequence in their promoters. However, the regulation and function of DREB2-type transcription factors remained to be elucidated. In this research, we report the cloning of a DREB2 homolog from maize, ZmDREB2A, whose transcripts were accumulated by cold, dehydration, salt and heat stresses in maize seedlings. Unlike Arabidopsis DREB2A, ZmDREB2A produced two forms of transcripts, and quantitative real-time PCR analyses demonstrated that only the functional transcription form of ZmDREB2A was significantly induced by stresses. Moreover, the ZmDREB2A protein exhibited considerably high transactivation activity compared with DREB2A in Arabidopsis protoplasts, suggesting that protein modification is not necessary for ZmDREB2A to be active. Constitutive or stress-inducible expression of ZmDREB2A resulted in an improved drought stress tolerance in plants. Microarray analyses of transgenic plants overexpressing ZmDREB2A revealed that in addition to genes encoding late embryogenesis abundant (LEA) proteins, some genes related to heat shock and detoxification were also upregulated. Furthermore, overexpression of ZmDREB2A also enhanced thermotolerance in transgenic plants, implying that ZmDREB2A may play a dual functional role in mediating the expression of genes responsive to both water stress and heat stress.

Global genome expression analysis of rice in response to drought and high-salinity stresses in shoot, flag leaf, and panicle

To elucidate genome-level responses to drought and high-salinity stress in rice, a 70 mer oligomer microarray covering 36,926 unique genes or gene models was used to profile genome expression changes in rice shoot, flag leaf and panicle under drought or high-salinity conditions. While patterns of gene expression in response to drought or high-salinity stress within a particular organ type showed significant overlap, comparison of expression profiles among different organs showed largely organ-specific patterns of regulation. Moreover, both stresses appear to alter the expression patterns of a significant number of genes involved in transcription and cell signaling in a largely organ-specific manner. The promoter regions of genes induced by both stresses or induced by one stress in more than one organ types possess relative enrichment of two cis-elements (ABRE core and DRE core) known to be associated with water stress. An initial computational analysis indicated that novel promoter motifs are present in the promoters of genes involved in rehydration after drought. This analysis suggested that rice might possess a mechanism that actively detects rehydration and facilitates rapid recovery. Overall, our data supports a notion that organ-specific gene regulation in response to the two abiotic stresses may primarily be mediated by organ-specific transcription responses.

Identification of expressed sequence tags in an alkali grass (Puccinellia tenuiflora) cDNA library

Alkali grass (Puccinellia tenuiflora), a monocotyledonous halophyte, can serve as a model of salt tolerance in monocotyledon crops. To elucidate the molecular events associated with salt tolerance in alkali grass, we generated a directional cDNA library from leaves treated with the alkali salt, NaHCO3. Large-scale sequencing of the cDNA library identified 2942 ESTs representing 2366 non-redundant transcripts. These have been deposited in the dbEST division of GenBank. BLASTX evaluation indicated that 1274 of the ESTs were homologous to various known genes/proteins in a broad range of organisms, especially gramineae species. The other 1092 ESTs, which showed little if any homology to known sequences, were considered novel. Based on the encoded proteins, the 1274 identified ESTs fell into 12 functional categories, of which four were abundant, namely metabolism (18.84%), transcription (12.48%), unclassified (11.22%) and cell rescue/defense (9.66%). The 162 unique transcripts corresponding to possible salt-related genes were also identified. This study provides an overview of gene expression in NaHCO3-stressed alkali grass, as well as useful information for further investigation of salt resistance in plants.

A novel drought-inducible gene, ATAF1, encodes a NAC family protein that negatively regulates the expression of stress-responsive genes in Arabidopsis

NAC proteins are plant-specific transcriptional regulators. ATAF1 was one of the first identified NAC proteins in Arabidopsis. In present study, we characterized the ATAF1 expression and biological function in response to water deficit stress. ATAF1 mRNA expression was strongly induced by dehydration and abscisic acid (ABA) treatment, but inhibited by water treatment, suggesting a general role in drought stress responses. Transient expression analysis in onion epidermal cells indicated the nuclear localization for the ATAF1::GFP fusion protein. Yeast transactivation analysis showed that ATAF1 had ability to activate reporter gene expression. Furthermore, domain deletion analysis revealed that the ATAF1 transactivation activity was conferred by its C-terminal domain. When ATAF1 gene was knocked out by T-DNA insertions, Arabidopsis ataf1-1 and ataf1-2 mutants displayed a recovery rate about seven times higher than wild-type plants in drought response test. This ataf1 phenotype was coincident with the enhanced expression of stress responsive marker genes, such as COR47, ERD10, KIN1, RD22 and RD29A under drought stress. Above evidences suggest that ATAF1, as a transcriptional regulator, negatively regulates the expression of stress responsive genes under drought stress in Arabidopsis.

Deciphering the regulatory mechanisms of abiotic stress tolerance in plants by genomic approaches

Environmental constraints that include abiotic stress factors such as salt, drought, cold and extreme temperatures severely limit crop productivity. Improvement of crop plants with traits that confer tolerance to these stresses was practiced using traditional and modern breeding methods. Molecular breeding and genetic engineering contributed substantially to our understanding of the complexity of stress response. Mechanisms that operate signal perception, transduction and downstream regulatory factors are now being examined and an understanding of cellular pathways involved in abiotic stress responses provide valuable information on such responses. This review presents genomic-assisted methods which have helped to reveal complex regulatory networks controlling abiotic stress tolerance mechanisms by high-throughput expression profiling and gene inactivation techniques. Further, an account of stress-inducible regulatory genes which have been transferred into crop plants to enhance stress tolerance is discussed as possible modes of integrating information gained from functional genomics into knowledge-based breeding programs. In addition, we envision an integrative genomic and breeding approach to reveal developmental programs that enhance yield stability and improve grain quality under unfavorable environmental conditions of abiotic stresses.

Genome-wide analysis of the stress associated protein (SAP) gene family containing A20/AN1 zinc-finger(s) in rice and their phylogenetic relationship with Arabidopsis

Proteins with the A20/AN1 zinc-finger domain are present in all eukaryotes and are well characterized in animals, but little is known about their function in plants. Earlier, we have identified an A20/AN1 zinc-finger containing stress associated protein 1 gene (SAP1) in rice and validated its function in abiotic stress tolerance. In this study, genome-wide survey of genes encoding proteins possessing A20/AN1 zinc-finger, named SAP gene family, has been carried out in rice and Arabidopsis. The genomic distribution and gene architecture as well as domain structure and phylogenetic relationship of encoded proteins numbering 18 and 14 in rice and Arabidopsis, respectively, have been studied. Expression analysis of the rice SAP family was done to investigate their response under abiotic stress conditions. All the genes were inducible by one or the other abiotic stresses indicating that the OsSAP gene family is an important component of stress response in rice. Manipulation of their expression and identification of their superior alleles should help confer stress tolerance in target crops.

Transgenic tobacco plants overexpressing chitinases of fungal origin show enhanced resistance to biotic and abiotic stress agents

Genes encoding defense-related proteins have been used to alter the resistance of plants to pathogens and other environmental challenges, but no single fungal gene overexpression has produced broad-spectrum stress resistance in transgenic lines. We have generated transgenic tobacco (Nicotiana tabacum) lines that overexpress the endochitinases CHIT33 and CHIT42 from the mycoparasitic fungus Trichoderma harzianum and have evaluated their tolerance to biotic and abiotic stress. Both CHIT33 and CHIT42, individually, conferred broad resistance to fungal and bacterial pathogens, salinity, and heavy metals. Such broad-range protective effects came off with no obvious detrimental effect on the growth of tobacco plants.

Diverse functions and molecular properties emerging for CAX cation/H+ exchangers in plants

Steep concentration gradients of many ions are actively maintained, with lower concentrations typically located in the cytosol, and higher concentrations in organelles and outside the cell. The vacuole is an important storage organelle for many ions. The concentration gradient of cations is established across the plant tonoplast, in part, by high-capacity cation/H+ (CAX) exchange activity. While plants may not be green yeast, analysis of CAX regulation and substrate specificity has been greatly aided by utilizing yeast as an experimental tool. The basic CAX biology in ARABIDOPSIS has immediate relevance toward understanding the functional interplay between diverse transport processes. The long-range applied goals are to identify novel transporters and express them in crop plants in order to "mine" nutrients out of the soil and into plants. In doing so, this could boost the levels of essential nutrients in plants.

Maize DBF1-interactor protein 1 containing an R3H domain is a potential regulator of DBF1 activity in stress responses

The maize dehydration-responsive element (DRE)-binding factor, DBF1, is a member of the Apetala 2/Ethylene Response Factor transcription factors family and is involved in the regulation of the ABA-responsive gene rab17 through the DRE in an ABA-dependent pathway. In this study we analysed the functionality of DBF1 in abiotic stress responses and found that Arabidopsis plants over-expressing DBF1 were more tolerant to osmotic stress than control plants. In yeast two-hybrid analyses, DBF1 interacted with DBF1-interactor protein 1 (DIP1), a protein containing a conserved R3H single-strand DNA-binding domain. Subcellular localization of DIP1 showed that the protein fusion DIP1-Red Flourescent Protein (RFP) was mainly localized in the cytoplasm. However, after co-transformation of DBF1-GFP and DIP1-RFP, both proteins co-localized in the nucleus. Interestingly, when the N-terminal DBF1-GFP was co-expressed with DIP1-RFP, both proteins co-localized predominantly in the cytoplasmic speckles observed for N-terminal DBF1-GFP fusion protein. These results clearly show in vivo interaction of DBF1 with DIP1 in the cell and that this interaction is necessary for the nuclear localization of DIP1 protein. Analysis of the regulatory effect of the DBF1 and DIP1 interaction on the maize rab17 promoter activity indicated that co-transfection of DBF1 with DIP1 enhances promoter activity in the absence of ABA treatment. We suggest that the regulated association of DBF1 and DIP1 may control the levels of target gene expression during stress conditions.

Identification of genes induced upon water-deficit stress in a drought-tolerant rice cultivar

Among the abiotic stresses, the availability of water is the most important factor that limits the productive potential of higher plants. The identification of novel genes, determination of their expression patterns, and the understanding of their functions in stress adaptation is essential to improve stress tolerance. Amplified fragment length polymorphism analysis of cDNA was used to identify rice genes differentially expressed in a tolerant rice variety upon water-deficit stress. In total, 103 transcript-derived fragments corresponding to differentially induced genes were identified. The results of the sequence comparison in BLAST database revealed that several differentially expressed TDFs were significantly homologous to stress regulated genes/proteins isolated from rice or other plant species. Most of the transcripts identified here were genes related to metabolism, energy, protein biosynthesis, cell defence, signal transduction, and transport. New genes involved in the response to water-deficit stress in a tolerant rice variety are reported here.

Identification of plant stress-responsive determinants in Arabidopsis by large-scale forward genetic screens

All plants sense and adapt to adverse environmental conditions, however, crop plants exhibit less genetic diversity for abiotic stress tolerance than do wild relatives indicating that a genetic basis exists for stress adaptability. Model plant genetic systems and the plethora of molecular genetic resources that are currently available are greatly enhancing our ability to identify abiotic stress-responsive genetic determinants. Forward genetic screens of T-DNA mutagenized Arabidopsis thaliana populations in the genetic background of ecotypes C24(RD29a-LUC) and Col-0 gl1 sos3-1 were carried out to begin an exhaustive search for such determinants. The C24(RD29a-LUC) screens identified mutants with altered salt/osmotic stress sensitivity or mutants with altered expression of the salt/osmotic/cold/ABA-responsive RD29a gene. Also, mutations that alter the NaCl sensitivity of sos3-1 were screened for potential genetic suppressors or enhancers of salt-stress responses mediated by SOS3. In total, more than 250 000 independent insertion lines were screened and greater than 200 individual mutants that exhibited altered stress/ABA responses were recovered. Although several of these mutants have been reported, most have not yet been studied in detail. Notable examples include novel alleles of SOS1 and mutations to genes encoding the STT3a subunit of the oligosaccharyltransferase, syntaxin, RNA polymerase II CTD phosphatases, transcription factors, ABA biosynthetic enzyme, Na+ transporter HKT1, and SUMO E3 ligase. The stress-specific phenotypes of mutations to genes that are involved in many basic cellular functions provide indication of the wide range of control mechanisms in cellular homeostasis that are involved in stress adaptation.

Osmotic stress sensing in Populus: components identification of a phosphorelay system

To study the Populus response to an osmotic stress, we have isolated one cDNA encoding a histidine-aspartate kinase (HK1) and four cDNAs encoding histidine-containing phosphotransfer proteins (HPts), HPt1-4. The predicted HK1 protein shares a typical structure with ATHK1 and SLN1 osmosensors. The 4 HPTs are characterized by the histidine phosphotransfer domain. We have shown that HK1 is upregulated during an osmotic stress in hydroponic culture. We have detected an interaction between HK1 and HPt2, using the yeast two-hybrid system. These results suggest the existence of a multi-step phosphorelay pathway probably involved in osmotic stress sensing in Populus.

Root-to-shoot signalling: apoplastic alkalinization, a general stress response and defence factor in barley (Hordeum vulgare)

We used a noninvasive microprobe technique to record in substomatal cavities of barley leaves the apoplastic pH response to different stress situations. When K+ (or Na+) activity at the roots of intact plants was increased from 1 to 50 mM, the leaf apoplastic pH increased by 0.4 to 0.6 units within 8 to 12 min when stomata were open, and within 15 to 20 min when stomata were closed. This reaction was accompanied by a correlative increase in K+ activity. Addition of 1 microM abscisic acid caused an apoplastic alkalinization of 0.5 to 0.8 units, and low temperatures (4 degrees C) increased pH by 0.2 to 0.3 units. Addition of 100 mM sorbitol or pH changes in the range 4.0 to 7.9 had no effect, ruling out that osmotic potential and/or pH is the carried signal. On detached leaves, the same treatments yielded qualitatively similar results, suggesting that the xylem is the most likely signal path. Following the attack of powdery mildew, the apoplastic pH of barley leaves substantially increases. We demonstrate that in susceptible barley, pretreatment (soil drench) with the resistance-inducing chemical benzo- (1,2,3)thiadiazole-7-carbothioic acid S-methyl ester markedly enhances this pH response. This is consistent with previous finding that apoplastic alkalinization is related to the degree of resistance towards this fungus.

Differential responses of maize MIP genes to salt stress and ABA

Salt stress is known to reduce root hydraulic conductivity and growth. To examine a concomitant regulation of aquaporins, the expression of the maize MIP gene family in response to NaCl was analysed by DNA array hybridization. Plants responded differentially to 100 versus 200 mM NaCl treatments. Leaf water content was reduced rapidly and persistently after the application of 200 mM NaCl in contrast to 100 mM NaCl. Endogenous ABA strongly accumulated in roots after 2 h; it remained at a highly elevated level for 48 h after the addition of 200 mM NaCl, but rapidly declined in plants treated with 100 mM NaCl, indicating an early recovery from water deficit. Interestingly, 2 h after the addition of 100 mM NaCl, when maize regained the osmotic potential allowing water uptake, three highly expressed, specific isoforms ZmPIP1;1, ZmPIP1;5, and ZmPIP2;4 were transiently induced. They were preferentially transcribed in the outer root tissue suggesting a role in cellular water transport. None of the ZmTIP genes was altered. By contrast, after the addition of 200 mM NaCl these responses were missing. Instead, multiple ZmPIP and ZmTIP genes were repressed by 200 mM NaCl after 24 h. After 48 h, deregulations were overridden in both cases indicating homeostasis. ABA (1 muM) exogenously applied to the roots transiently induced ZmPIP2;4 similar to 100 mM NaCl as well as ZmPIP1;2. Thus, the early induction of ZmPIP2;4 by NaCl may be mediated by ABA. Previously, an increase in root hydraulic conductivity had been observed upon ABA application. By contrast, 100 muM ABA led to a complete, possibly non-specific repression of all detected ZmPIP and ZmTIP genes after 24 h.

Functional characterization of NtCEF1, an AP2/EREBP-type transcriptional activator highly expressed in tobacco callus

Using PCR-select cDNA subtraction, we identified the genes that are predominantly expressed in the shooty callus induced by suppression of the CHRK1 receptor-like kinase gene. One of the identified genes encoded a novel AP2/EREBP-type transcription factor, and it was highly expressed in various types of tobacco callus including the CHRK1 transgenic callus, hence designated as Nicotiana tabacum Callus-Expressing Factor 1 NtCEF1. The NtCEF1-GFP fusion protein was localized in the nucleus. The full length and the C-terminal acidic region of NtCEF1 could function as a transactivator in yeast, when fused to the LexA DNA binding domain. Expression of the NtCEF1 gene was induced by ethylene and by various abiotic stresses. Gel retardation assay revealed that NtCEF1 could bind specifically to the GCC box as well as to the C/DRE motif, albeit less strongly. Interestingly, NtCEF1 overexpression in Arabidopsis resulted in constitutive expression of various ethylene-responsive and defense genes that contain the GCC box in the promoter-but none of the genes containing the upstream C/DRE elements-indicating that NtCEF1 preferentially recognizes the GCC box in vivo. Furthermore, the NtCEF1-overexpressing Arabidopsis plants exhibited enhanced resistance to a bacterial pathogen, Pseudomonas syringae pv. tomato DC3000. Taken together, these results suggest that NtCEF1 is a transcription factor preferentially activating the GCC box-containing defense genes, and that it modulates increased resistance against the biotic stress by activation of the downstream gene expression.

Expressing TERF1 in tobacco enhances drought tolerance and abscisic acid sensitivity during seedling development

Previously, we reported on a tomato ERF transcription activator, TERF1, which was concluded to act as a linker between ethylene and osmotic signal pathways. We now report on the regulatory role of TERF1 in ABA sensitivity and drought response during seedling development. Northern blotting analysis indicated that the transcripts of TERF1 were significantly accumulated in response to drought, cold and ABA. TERF1 activated GCC box- or DRE-driven reporter gene expression in transient expression assay, subsequently increasing the tolerance to drought and the osmoticum, PEG6000, in tobacco expressing TERF1. Further tests showed that TERF1 did not affect the seed germination, but greatly enhanced the sensitivity during tobacco seedling development under ABA treatment. This ABA hypersensitivity in transgenic TERF1 tobacco is both indirect ethylene action and expressions of ABA responsive genes, demonstrating that TERF1 is a multifunctional ERF protein that can integrate different stress signal pathways.

Stressed worms: Responding to the post-genomics era

Nematodes are among the most successful organisms in withstanding stress conditions associated with water loss, and viable individuals have been recovered from dry desert soils. Little is known about the biochemical and molecular events underpinning nematodes' physiological responses to dehydration. Post-genomics research in Caenorhabditis elegans may offer an opportunity to understand the stress response better. This review focuses on recent progress in understanding the molecular mechanisms of water-loss associated stress response in the model nematode C. elegans and in parasitic nematodes and discusses the scope for applying the knowledge and tools derived from a model organism for the study of wild, environmentally-adapted, parasitic nematodes, in the light of the emergence of genomics research of non-model organisms.

Sorghum bicolor's transcriptome response to dehydration, high salinity and ABA

Genome wide changes in gene expression were monitored in the drought tolerant C4 cereal Sorghum bicolor, following exposure of seedlings to high salinity (150 mM NaCl), osmotic stress (20% polyethylene glycol) or abscisic acid (125 microM ABA). A sorghum cDNA microarray providing data on 12,982 unique gene clusters was used to examine gene expression in roots and shoots at 3- and 27-h post-treatment. Expression of approximately 2200 genes, including 174 genes with currently unknown functions, of which a subset appear unique to monocots and/or sorghum, was altered in response to dehydration, high salinity or ABA. The modulated sorghum genes had homology to proteins involved in regulation, growth, transport, membrane/protein turnover/repair, metabolism, dehydration protection, reactive oxygen scavenging, and plant defense. Real-time PCR was used to quantify changes in relative mRNA abundance for 333 genes that responded to ABA, NaCl or osmotic stress. Osmotic stress inducible sorghum genes identified for the first time included a beta-expansin expressed in shoots, actin depolymerization factor, inositol-3-phosphate synthase, a non-C4 NADP-malic enzyme, oleosin, and three genes homologous to 9-cis-epoxycarotenoid dioxygenase that may be involved in ABA biosynthesis. Analysis of response profiles demonstrated the existence of a complex gene regulatory network that differentially modulates gene expression in a tissue- and kinetic-specific manner in response to ABA, high salinity and water deficit. Modulation of genes involved in signal transduction, chromatin structure, transcription, translation and RNA metabolism contributes to sorghum's overlapping but nonetheless distinct responses to ABA, high salinity, and osmotic stress. Overall, this study provides a foundation of information on sorghum's osmotic stress responsive gene complement that will accelerate follow up biochemical, QTL and comparative studies.

Gene expression profiling of potato responses to cold, heat, and salt stress

In order to identify genes involved in abiotic stress responses in potato, seedlings were grown under controlled conditions and subjected to cold (4 degrees C), heat (35 degrees C), or salt (100 mM NaCl) stress for up to 27 h. Using an approximately 12,000 clone potato cDNA microarray, expression profiles were captured at three time points following initiation of the stress (3, 9, and 27 h) from two different tissues, roots and leaves. A total of 3,314 clones could be identified as significantly up- or down-regulated in response to at least one stress condition. The genes represented by these clones encode transcription factors, signal transduction factors, and heat-shock proteins which have been associated with abiotic stress responses in Arabidopsis and rice, suggesting similar response pathways function in potato. These stress-regulated clones could be separated into either stress-specific or shared-response clones, suggesting the existence of general response pathways as well as more stress-specific pathways. In addition, we identified expression profiles which are indicative for the type of stress applied to the plants.

Gene expression profiling of potato responses to cold, heat, and salt stress

In order to identify genes involved in abiotic stress responses in potato, seedlings were grown under controlled conditions and subjected to cold (4 degrees C), heat (35 degrees C), or salt (100 mM NaCl) stress for up to 27 h. Using an approximately 12,000 clone potato cDNA microarray, expression profiles were captured at three time points following initiation of the stress (3, 9, and 27 h) from two different tissues, roots and leaves. A total of 3,314 clones could be identified as significantly up- or down-regulated in response to at least one stress condition. The genes represented by these clones encode transcription factors, signal transduction factors, and heat-shock proteins which have been associated with abiotic stress responses in Arabidopsis and rice, suggesting similar response pathways function in potato. These stress-regulated clones could be separated into either stress-specific or shared-response clones, suggesting the existence of general response pathways as well as more stress-specific pathways. In addition, we identified expression profiles which are indicative for the type of stress applied to the plants.

Osmotic shrinkage of cells of Synechocystis sp. PCC 6803 by water efflux via aquaporins regulates osmostress-inducible gene expression

Osmotic stress causes water molecules to efflux from cells through the cytoplasmic membrane. This study reveals that targeted mutation of the aqpZ gene, encoding an aquaporin water channel protein, in the cyanobacterium Synechocystis sp. PCC 6803 prevents the osmotic shrinkage of cells, suggesting that it is the water channel rather than the lipid bilayer that is primarily responsible for water transition through the membrane of this organism. The observations suggest that the aquaporin-mediated shrinkage of the Synechocystis cells plays an important role in changes of gene expression in response to hyperosmotic stress.

Physcomitrella patens is highly tolerant against drought, salt and osmotic stress

In order to determine the degree of tolerance of the moss Physcomitrella patens to different abiotic stress conditions, we examined its tolerance against salt, osmotic and dehydration stress. Compared to other plants like Arabidopsis thaliana, P. patens exhibits a high degree of abiotic stress tolerance, making it a valuable source for the identification of genes effecting the stress adaptation. Plants that had been treated with NaCl tolerated concentrations up to 350 mM. Treatments with sorbitol revealed that plants are able to survive concentrations up to 500 mM. Furthermore, plants that had lost 92% water on a fresh-weight basis were able to recover successfully. For molecular analyses, a P. patens expressed sequence tag (EST) database was searched for cDNA sequences showing homology to stress-associated genes of seed plants and bacteria. 45 novel P. patens genes were identified and subjected to cDNA macroarray analyses to define their expression pattern in response to water deficit. Among the selected cDNAs, we were able to identify a set of genes that is specifically up-regulated upon dehydration. These genes encode proteins exerting their function in maintaining the integrity of the plant cell as well as proteins that are known to be members of signaling networks. The identified genes will serve as molecular markers and potential targets for future functional analyses.

Poly(ADP-ribose) polymerase in plants affects energy homeostasis, cell death and stress tolerance

Plants contain two genes that code for poly(ADP-ribose) polymerase (PARP): parp1 and parp2. Both PARPs are activated by DNA damage caused by, example reactive oxygen species. Upon activation polymers of ADP-ribose are synthesized on a range of nuclear enzymes using NAD(+) as substrate. Here, we show that in plants stresses such as drought, high light and heat activate PARP causing NAD(+) breakdown and ATP consumption. When the PARP activity is reduced by means of chemical inhibitors or by gene silencing, cell death is inhibited and plants become tolerant to a broad range of abiotic stresses like high light, drought and heat. Plant lines with low poly(ADP-ribosyl)ation activity maintain under stress conditions their energy homeostasis by reducing NAD(+) breakdown and consequently energy consumption. The higher energy-use efficiency avoids the need for a too intense mitochondrial respiration and consequently reduces the formation of reactive oxygen species. From these results it can be concluded that breeding or engineering for a high energy-use efficiency under stress conditions is a valuable, but until today nearly unexploited, approach to enhance overall stress tolerance of crops.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

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.

The MKK2 pathway mediates cold and salt stress signaling in Arabidopsis

The Arabidopsis mitogen-activated protein kinase (MAPK) kinase 2 (MKK2) and the downstream MAPKs MPK4 and MPK6 were isolated by functional complementation of osmosensitive yeast mutants. In Arabidopsis protoplasts, MKK2 was specifically activated by cold and salt stress and by the stress-induced MAPK kinase kinase MEKK1. Yeast two-hybrid, in vitro, and in vivo protein kinase assays revealed that MKK2 directly targets MPK4 and MPK6. Accordingly, plants overexpressing MKK2 exhibited constitutive MPK4 and MPK6 activity, constitutively upregulated expression of stress-induced marker genes, and increased freezing and salt tolerance. In contrast, mkk2 null plants were impaired in MPK4 and MPK6 activation and were hypersensitive to salt and cold stress. Full genome transcriptome analysis of MKK2-overexpressing plants demonstrated altered expression of 152 genes involved in transcriptional regulation, signal transduction, cellular defense, and stress metabolism. These data identify a MAP kinase signaling cascade mediating cold and salt stress tolerance in plants.

Regulation of phosphatidylcholine biosynthesis under salt stress involves choline kinases in Arabidopsis thaliana

Increasing evidence suggests a major role for phosphatidylcholine (PC) in plant stress adaptation. The present work investigated the regulation of choline, PC and interconnected phosphatidylethanolamine biosynthesis in Arabidopsis thaliana L. as a function of cold- and salt- or mannitol-mediated hyperosmotic stresses. While PC synthesis is accelerated in both salt- and cold-treated plants, the choline kinase (CK) and phosphocholine cytidylyltransferase genes are oppositely regulated with respect to these abiotic treatments. Salt stress also stimulates CK activity in vitro. A possible regulatory role of CK in stimulating PC biosynthesis rate in salt-stressed plants is discussed.

Salt cress. A halophyte and cryophyte Arabidopsis relative model system and its applicability to molecular genetic analyses of growth and development of extremophiles

Salt cress (Thellungiella halophila) is a small winter annual crucifer with a short life cycle. It has a small genome (about 2 x Arabidopsis) with high sequence identity (average 92%) with Arabidopsis, and can be genetically transformed by the simple floral dip procedure. It is capable of copious seed production. Salt cress is an extremophile native to harsh environments and can reproduce after exposure to extreme salinity (500 mm NaCl) or cold to -15 degrees C. It is a typical halophyte that accumulates NaCl at controlled rates and also dramatic levels of Pro (>150 mm) during exposure to high salinity. Stomata of salt cress are distributed on the leaf surface at higher density, but are less open than the stomata of Arabidopsis and respond to salt stress by closing more tightly. Leaves of salt cress are more succulent-like, have a second layer of palisade mesophyll cells, and are frequently shed during extreme salt stress. Roots of salt cress develop both an extra endodermis and cortex cell layer compared to Arabidopsis. Salt cress, although salt and cold tolerant, is not exceptionally tolerant of soil desiccation. We have isolated several ethyl methanesulfonate mutants of salt cress that have reduced salinity tolerance, which provide evidence that salt tolerance in this halophyte can be significantly affected by individual genetic loci. Analysis of salt cress expressed sequence tags provides evidence for the presence of paralogs, missing in the Arabidopsis genome, and for genes with abiotic stress-relevant functions. Hybridizations of salt cress RNA targets to an Arabidopsis whole-genome oligonucleotide array indicate that commonly stress-associated transcripts are expressed at a noticeably higher level in unstressed salt cress plants and are induced rapidly under stress. Efficient transformation of salt cress allows for simple gene exchange between Arabidopsis and salt cress. In addition, the generation of T-DNA-tagged mutant collections of salt cress, already in progress, will open the door to a new era of forward and reverse genetic studies of extremophile plant biology.

A salt-responsive receptor-like kinase gene regulated by the ethylene signaling pathway encodes a plasma membrane serine/threonine kinase

NTHK1 is a salt-inducible ethylene receptor gene in tobacco. Transgenic tobacco plants for this gene show reduced ethylene sensitivity. Using cDNA microarray analysis, we were able to identify those genes that have different expression levels between NTHK1 transgenic plants and wild-type plants under salt stress conditions. One of these, AtLecRK2, which encodes a receptor-like kinase with an extracellular lectin-like domain, was characterized in detail in the present study. AtLecRK2 contains a signal peptide, an extracellular lectin-like domain, a single transmembrane domain and a cytoplasmic protein kinase domain. AtLecRK2 is transcribed in the root, flower and leaf but not in the stem. In wild-type Arabidopsis, salt stress induced the transcription level of AtLecRK2, whereas in the transgenic NTHK1 Arabidopsis induction of the AtLecRK2 transcript was inhibited and retarded. AtLecRK2 was constitutively overexpressed in the ethylene-overproducer mutant, eto1-1, and could be induced by ethylene. However, in the ethylene-insensitive mutant, ein2-1, the salt-induced expression pattern of AtLecRK2 was the same as that in wild-type plants. The results demonstrate that the induction of AtLecRK2 in response to salt stress is regulated by the ethylene signaling pathway. The induction was inhibited by the ethylene receptor, NTHK1, while it was independent of EIN2. The kinase activity of AtLecRK2 was also studied. We found that that AtLecRK2 can be autophosphorylated and has serine/threonine kinase activities. The subcellular localization of AtLecRK2-GFP in onion epidermal cells indicates that AtLecRK2 is localized on the plasma membrane.

Modulation by cytosolic components of proton pump activities in plasma membrane and tonoplast from Cucumis sativus roots during salt stress

The effect of NaCl on the plasma membrane and tonoplast ATPases measured as the hydrolytic and H(+)-pumping activity was studied. Treatment of cucumber seedlings with salt increased the membrane-bound ATPases of the plasma membrane as well as the tonoplast. In both types of membranes the stimulation of ATP-hydrolysis was much higher than the stimulation of H(+)-transport suggesting that the salt- treatment of plants partially uncoupled the membrane proton pumps. It was shown that the soluble fraction obtained from the unstressed or NaCl-stressed roots stimulated the ATPase activities in both membranes isolated from unstressed plants. A stimulatory effect of the soluble fraction on the proton pump activities was considerably enhanced in the salt conditions indicating the presence of a salt-inducible factor (s) in the soluble fraction, which could rapidly modulate the membrane-bound ATPases. Staurosporine, a specific protein kinase inhibitor, totally abolished the stimulatory action of the soluble fractions on the membrane proton pumps, whereas okadaic acid, a phosphatase inhibitor, had no effect. Inclusion of calcium in the mixture of membranes and the soluble fraction from unstressed roots elevated the ATPase activities to the levels determined with the soluble fraction isolated from NaCl-stressed roots. Cation chelators (EGTA), as well as calmodulin antagonist (W7) cancelled the stimulatory effect of calcium ions. The above results strongly suggest the involvement of specific calcium-calmodulin-dependent protein kinases in the activation of the membrane ATPases under salt-stress conditions.

Comparative study of the expression profiles of the Cor/Lea gene family in two wheat cultivars with contrasting levels of freezing tolerance

Expression profiles of a set of Cor/Lea genes were assessed during early stages of cold acclimation in seedlings of two wheat cultivars, which showed contrasting levels of freezing tolerance. These Cor/Lea family members consisted of three EST clones and 13 previously identified cDNA clones of wheat and rye. Northern blot analysis using RNA extracted from seedling leaves and roots showed that most of the genes exhibited a quite similar time-course of expression, although with different expression levels: They rapidly responded to low temperature and their transcript levels reached high plateaus within 3-5 days. The overall gene expression profiles were correlated with the time-dependent development and the level of freezing tolerance under low temperature in the two cultivars. Western blot analysis of protein accumulation further verified this observation. Abscissic acid response was proved for at least four genes. Light was stimulatory to most of the genes, and this positive light response associated with low temperature occurred not only in leaf-specific genes but also in leaf/root-expressed genes. Taken together, the present results suggest that the Cor/Lea gene family represents a major group of downstream genes involved in the ABA-dependent and -independent signal pathways and that most of them are co-regulated in determining freezing tolerance in wheat seedlings.

Self-reporting Arabidopsis expressing pH and [Ca2+] indicators unveil ion dynamics in the cytoplasm and in the apoplast under abiotic stress

For noninvasive in vivo measurements of intra- and extracellular ion concentrations, we produced transgenic Arabidopsis expressing pH and calcium indicators in the cytoplasm and in the apoplast. Ratiometric pH-sensitive derivatives of the green fluorescent protein (At-pHluorins) were used as pH indicators. For measurements of calcium ([Ca(2+)]), luminescent aequorin variants were expressed in fusion with pHluorins. An Arabidopsis chitinase signal sequence was used to deliver the indicator complex to the apoplast. Responses of pH and [Ca(2+)] in the apoplast and in the cytoplasm were studied under salt and "drought" (mannitol) stress. Results are discussed in the frame of ion flux, regulation, and signaling. They suggest that osmotic stress and salt stress are differently sensed, compiled, and processed in plant cells.

Arabidopsis kinome: after the casting

Arabidopsis thaliana is used as a favourite experimental organism for many aspects of plant biology. We capitalized on the recently available Arabidopsis genome sequence and predicted proteome, to draw up a genome-scale protein serine/threonine kinase (PSTK) inventory. The PSTKs represent about 4% of the A. thaliana proteome. In this study, we provide a description of the content and diversity of the non-receptor PSTKs. These kinases have crucial functions in sensing, mediating and coordinating cellular responses to an extensive range of stimuli. A total of 369 predicted non receptor PSTKs were detailed: the Raf superfamily, the CMGC, CaMK, AGC and STE families, as well as a few small clades and orphan sequences. An extensive relationship analysis of these kinases allows us to classify the proteins in superfamilies, families, sub-families and groups. The classification provides a better knowledge of the characteristics shared by the different clades. We focused on the MAP kinase module elements, with particular attention to their docking sites for protein-protein interaction and their biological function. The large number of A. thaliana genes encoding kinases might have been achieved through successive rounds of gene and genome duplications. The evolution towards an increasing gene number suggests that functional redundancy plays an important role in plant genetic robustness.