Drought and salinity are major abiotic stresses to crop production. Here, we show that overexpression of stress responsive gene SNAC1 (STRESS-RESPONSIVE NAC 1) significantly enhances drought resistance in transgenic rice (22-34% higher seed setting than control) in the field under severe drought stress conditions at the reproductive stage while showing no phenotypic changes or yield penalty. The transgenic rice also shows significantly improved drought resistance and salt tolerance at the vegetative stage. Compared with WT, the transgenic rice are more sensitive to abscisic acid and lose water more slowly by closing more stomatal pores, yet display no significant difference in the rate of photosynthesis. SNAC1 is induced predominantly in guard cells by drought and encodes a NAM, ATAF, and CUC (NAC) transcription factor with transactivation activity. DNA chip analysis revealed that a large number of stress-related genes were up-regulated in the SNAC1-overexpressing rice plants. Our data suggest that SNAC1 holds promising utility in improving drought and salinity tolerance in rice.Oryza sativa ͉ abscisic acid ͉ stomata ͉ dehydration P oor water management, increased competition for limited water resources, and the uncertain threats associated with global warming all highlight the looming water crisis that threatens agricultural productivity worldwide. In China alone, the estimated annual loss of national economy from water shortage alone reaches Ͼ$25 billion (1). In addition to altered water management practices, the ability to enhance the tolerance of crops to drought and salinity stress, particularly at the most sensitive reproductive stage of growth, can have a potentially huge impact on productivity in the years to come.Plants can develop numerous physiological and biochemical strategies to cope with adverse conditions (2, 3). The major events of plant response to dehydration stresses are perception and transduction of the stress signals through signaling components, resulting in activation of a large number of stress-related genes and synthesis of diverse functional proteins that finally lead to various physiological and metabolic responses (4-6). Well characterized proteins involved in the protection of plant cells from dehydration stress damage include molecule chaperons, osmotic adjustment proteins (7), ion channels (8), transporters (9), and antioxidation or detoxification proteins (10). The expression of these functional proteins is largely regulated by specific transcription factors (4, 11).More than 30 families of transcription factors have been predicted for Arabidopsis (12). Members of DREB or CBF, MYB, bZIP, and zinc-finger families have been well characterized with roles in the regulation of plant defense and stress responses (4-6, 13, 14). Most of these transcription factors regulate their target gene expression through binding to the cognate cis-elements in the promoters of the stress-related genes. Two well characterized dehydration stress-related cis-elements bound by transcription factors are...

Stomatal pores are formed by pairs of specialized epidermal guard cells and serve as major gateways for both CO 2 influx into plants from the atmosphere and transpirational water loss of plants. Because they regulate stomatal pore apertures via integration of both endogenous hormonal stimuli and environmental signals, guard cells have been highly developed as a model system to dissect the dynamics and mechanisms of plant-cell signaling. The stress hormone ABA and elevated levels of CO 2 activate complex signaling pathways in guard cells that are mediated by kinases/phosphatases, secondary messengers, and ion channel regulation. Recent research in guard cells has led to a new hypothesis for how plants achieve specificity in intracellular calcium signaling: CO 2 and ABA enhance (prime) the calcium sensitivity of downstream calciumsignaling mechanisms. Recent progress in identification of early stomatal signaling components are reviewed here, including ABA receptors and CO 2 -binding response proteins, as well as systems approaches that advance our understanding of guard cell-signaling mechanisms.

The continuing rise in atmospheric CO2 causes closing of stomatal pores in leaves and thus globally affects CO2 influx into plants, water use efficiency and leaf heat stress1–4. However, the CO2-binding proteins that control this response remain unknown. Moreover, the cell type that responds to CO2, mesophyll or guard cells, and whether photosynthesis mediates this response are matters of debate5–8. We demonstrate that Arabidopsis double mutant plants in the β-carbonic anhydrases, βCA1 and βCA4, display impaired CO2-regulation of stomatal movements and increased stomatal density, but retain functional abscisic-acid and blue-light responses.βCA-mediated CO2-triggered stomatal movements are not, in-first-order, linked to leaf-photosynthesis and can function in guard cells. Furthermore, guard cell βCA-over-expression plants exhibit enhanced water use efficiency. Guard cell-expression of mammalian αCAII complements ca1ca4 shows that carbonic anhydrase-mediated catalysis is an important mechanism for βCA-mediated CO2-induced stomatal closing and patch clamp analyses indicate that CO2/HCO3−transfers the signal to anion channel regulation. These findings, together with ht1-29 epistasis analysis demonstrate that carbonic anhydrases function early in the CO2 signalling pathway that controls gas-exchange between plants and the atmosphere.

The plant hormone abscisic acid (ABA) is produced in response to abiotic stresses and mediates stomatal closure in response to drought via recently identified ABA receptors (pyrabactin resistance/regulatory component of ABA receptor; PYR/RCAR). SLAC1 encodes a central guard cell S-type anion channel that mediates ABA-induced stomatal closure. Coexpression of the calcium-dependent protein kinase 21 (CPK21), CPK23, or the Open Stomata 1 kinase (OST1) activates SLAC1 anion currents. However, reconstitution of ABA activation of any plant ion channel has not yet been attained. Whether the known core ABA signaling components are sufficient for ABA activation of SLAC1 anion channels or whether additional components are required remains unknown. The Ca 2+ -dependent protein kinase CPK6 is known to function in vivo in ABA-induced stomatal closure. Here we show that CPK6 robustly activates SLAC1-mediated currents and phosphorylates the SLAC1 N terminus. A phosphorylation site (S59) in SLAC1, crucial for CPK6 activation, was identified. The group A PP2Cs ABI1, ABI2, and PP2CA down-regulated CPK6-mediated SLAC1 activity in oocytes. Unexpectedly, ABI1 directly dephosphorylated the N terminus of SLAC1, indicating an alternate branched early ABA signaling core in which ABI1 targets SLAC1 directly (downregulation). Furthermore, here we have successfully reconstituted ABA-induced activation of SLAC1 channels in oocytes using the ABA receptor pyrabactin resistant 1 (PYR1) and PP2C phosphatases with two alternate signaling cores including either CPK6 or OST1. Point mutations in ABI1 disrupting PYR1-ABI1 interaction abolished ABA signal transduction. Moreover, by addition of CPK6, a functional ABA signal transduction core from ABA receptors to ion channel activation was reconstituted without a SnRK2 kinase.Arabidopsis | chloride channel T he perception of the phytohormone abscisic acid (ABA) is achieved by the recently discovered 14-member START protein family of ABA receptors named pyrabactin resistance (PYR), or regulatory component of ABA receptor (RCAR) (1, 2). PYR/RCARs have been shown to bind to clade A PP2Cs and inhibit the activity of these PP2Cs in the presence of ABA (1-5). Structural studies show that PYR1, PYL1, and PYL2 function as ABA receptors, with ABA binding in a protein cavity that locks down the ABA molecule (6-10).ABA reduces transpirational water loss of plants by inducing stomatal closure (11). ABA can cause an increase in guard cell intracellular Ca 2+ concentration (12-17), which leads to the down-regulation of inward-rectifying K + channels and activation of both slow-sustained (S-type) and rapid-transient (R-type) anion channels (18)(19)(20). Previous findings have led to the model that S-type anion channels play a key role in controlling stomatal closure (18,21,22). slac1 mutant plants have greatly reduced S-type anion channel activity (23) and display impaired stomatal closure in response to ABA, elevated CO 2 , ozone, reactive oxygen species, calcium, and reduced humidity, underlining that SLAC1 repres...

Drought is one of the most important environmental stresses affecting the productivity of most field crops. Elucidation of the complex mechanisms underlying drought resistance in crops will accelerate the development of new varieties with enhanced drought resistance. Here, we provide a brief review on the progress in genetic, genomic, and molecular studies of drought resistance in major crops. Drought resistance is regulated by numerous small-effect loci and hundreds of genes that control various morphological and physiological responses to drought. This review focuses on recent studies of genes that have been well characterized as affecting drought resistance and genes that have been successfully engineered in staple crops. We propose that one significant challenge will be to unravel the complex mechanisms of drought resistance in crops through more intensive and integrative studies in order to find key functional components or machineries that can be used as tools for engineering and breeding drought-resistant crops.

Plants respond to adverse environment by initiating a series of signaling processes including activation of transcription factors that can regulate expression of arrays of genes for stress response and adaptation. NAC (NAM, ATAF, and CUC) is a plant specific transcription factor family with diverse roles in development and stress regulation. In this report, a stress-responsive NAC gene (SNAC2) isolated from upland rice IRA109 (Oryza sativa L. ssp japonica) was characterized for its role in stress tolerance. SNAC2 was proven to have transactivation and DNA-binding activities in yeast and the SNAC2-GFP fusion protein was localized in the rice nuclei. Northern blot and SNAC2 promoter activity analyses suggest that SNAC2 gene was induced by drought, salinity, cold, wounding, and abscisic acid (ABA) treatment. The SNAC2 gene was over-expressed in japonica rice Zhonghua 11 to test the effect on improving stress tolerance. More than 50% of the transgenic plants remained vigorous when all WT plants died after severe cold stress (4-8°C for 5 days). The transgenic plants had higher cell membrane stability than wild type during the cold stress. The transgenic rice had significantly higher germination and growth rate than WT under high salinity conditions. Over-expression of SNAC2 can also improve the tolerance to PEG treatment. In addition, the SNAC2-overexpressing plants showed significantly increased sensitivity to ABA. DNA chip profiling analysis of transgenic plants revealed many up-regulated genes related to stress response and adaptation such as peroxidase, ornithine aminotransferase, heavy metal-associated protein, sodium/hydrogen exchanger, heat shock protein, GDSL-like lipase, and phenylalanine ammonia lyase. Interestingly, none of the up-regulated genes in the SNAC2-overexpressing plants matched the genes up-regulated in the transgenic plants over-expressing other stress responsive NAC genes reported previously. These data suggest SNAC2 is a novel stress responsive NAC transcription factor that possesses potential utility in improving stress tolerance of rice.

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Rapid stomatal closure induced by changes in the environment, such as elevation of CO2, reduction of air humidity, darkness, and pulses of the air pollutant ozone (O3), involves the SLOW ANION CHANNEL1 (SLAC1). SLAC1 is activated by OPEN STOMATA1 (OST1) and Ca2+-dependent protein kinases. OST1 activation is controlled through abscisic acid (ABA)-induced inhibition of type 2 protein phosphatases (PP2C) by PYRABACTIN RESISTANCE/REGULATORY COMPONENTS OF ABA RECEPTOR (PYR/RCAR) receptor proteins. To address the role of signaling through PYR/RCARs for whole-plant steady-state stomatal conductance and stomatal closure induced by environmental factors, we used a set of Arabidopsis (Arabidopsis thaliana) mutants defective in ABA metabolism/signaling. The stomatal conductance values varied severalfold among the studied mutants, indicating that basal ABA signaling through PYR/RCAR receptors plays a fundamental role in controlling whole-plant water loss through stomata. PYR/RCAR-dependent inhibition of PP2Cs was clearly required for rapid stomatal regulation in response to darkness, reduced air humidity, and O3. Furthermore, PYR/RCAR proteins seem to function in a dose-dependent manner, and there is a functional diversity among them. Although a rapid stomatal response to elevated CO2 was evident in all but slac1 and ost1 mutants, the bicarbonate-induced activation of S-type anion channels was reduced in the dominant active PP2C mutants abi1-1 and abi2-1. Further experiments with a wider range of CO2 concentrations and analyses of stomatal response kinetics suggested that the ABA signalosome partially affects the CO2-induced stomatal response. Thus, we show that PYR/RCAR receptors play an important role for the whole-plant stomatal adjustments and responses to low humidity, darkness, and O3 and are involved in responses to elevated CO2.