OsASR5 enhances drought tolerance through a stomatal closure pathway associated with ABA and H2O2 signalling in rice

Drought is one of the major abiotic stresses that directly implicate plant growth and crop productivity. Although many genes in response to drought stress have been identified, genetic improvement to drought resistance especially in food crops is showing relatively slow progress worldwide. Here, we reported the isolation of abscisic acid, stress and ripening (ASR) genes from upland rice variety, IRAT109 (Oryza sativa L. ssp. japonica), and demonstrated that overexpression of OsASR5 enhanced osmotic tolerance in Escherichia coli and drought tolerance in Arabidopsis and rice by regulating leaf water status under drought stress conditions. Moreover, overexpression of OsASR5 in rice increased endogenous ABA level and showed hypersensitive to exogenous ABA treatment at both germination and postgermination stages. The production of H₂O₂, a second messenger for the induction of stomatal closure in response to ABA, was activated in overexpression plants under drought stress conditions, consequently, increased stomatal closure and decreased stomatal conductance. In contrast, the loss‐of‐function mutant, osasr5, showed sensitivity to drought stress with lower relative water content under drought stress conditions. Further studies demonstrated that OsASR5 functioned as chaperone‐like protein and interacted with stress‐related HSP40 and 2OG‐Fe (II) oxygenase domain containing proteins in yeast and plants. Taken together, we suggest that OsASR5 plays multiple roles in response to drought stress by regulating ABA biosynthesis, promoting stomatal closure, as well as acting as chaperone‐like protein that possibly prevents drought stress‐related proteins from inactivation.     

OsSUV3 dual helicase functions in salinity stress tolerance by maintaining photosynthesis and antioxidant machinery in rice (Oryza sativa L. cv. IR64)

To overcome the salinity‐induced loss of crop yield, a salinity‐tolerant trait is required. The SUV3 helicase is involved in the regulation of RNA surveillance and turnover in mitochondria, but the helicase activity of plant SUV3 and its role in abiotic stress tolerance have not been reported so far. Here we report that the Oryza sativa (rice) SUV3 protein exhibits DNA and RNA helicase, and ATPase activities. Furthermore, we report that SUV3 is induced in rice seedlings in response to high levels of salt. Its expression, driven by a constitutive cauliflower mosaic virus 35S promoter in IR64 transgenic rice plants, confers salinity tolerance. The T₁ and T₂ sense transgenic lines showed tolerance to high salinity and fully matured without any loss in yields. The T₂ transgenic lines also showed tolerance to drought stress. These results suggest that the introduced trait is functional and stable in transgenic rice plants. The rice SUV3 sense transgenic lines showed lesser lipid peroxidation, electrolyte leakage and H₂O₂ production, along with higher activities of antioxidant enzymes under salinity stress, as compared with wild type, vector control and antisense transgenic lines. These results suggest the existence of an efficient antioxidant defence system to cope with salinity‐induced oxidative damage. Overall, this study reports that plant SUV3 exhibits DNA and RNA helicase and ATPase activities, and provides direct evidence of its function in imparting salinity stress tolerance without yield loss. The possible mechanism could be that OsSUV3 helicase functions in salinity stress tolerance by improving photosynthesis and antioxidant machinery in transgenic rice.     

Drought stress responses in crops

Among the effects of impending climate change, drought will have a profound impact on crop productivity in the future. Response to drought stress has been studied widely, and the model plant Arabidopsis has guided the studies on crop plants with genome sequence information viz., rice, wheat, maize and sorghum. Since the value of functions of genes, dynamics of pathways and interaction of networks for drought tolerance in plants can only be judged by evidence from field performance, this mini-review provides a research update focussing on the current developments on the response to drought in crop plants. Studies in Arabidopsis provide the basis for interpreting the available information in a systems biology perspective. In particular, the elucidation of the mechanism of drought stress response in crops is considered from evidence-based outputs emerging from recent omic studies in crops.     

Expression of an Arabidopsis molybdenum cofactor sulphurase gene in soybean enhances drought tolerance and increases yield under field conditions

LOS5/ABA3 gene encoding molybdenum cofactor sulphurase is involved in aldehyde oxidase (AO) activity in Arabidopsis, which indirectly regulates ABA biosynthesis and increased stress tolerance. Here, we used a constitutive super promoter to drive LOS5/ABA3 overexpression in soybean (Glycine max L.) to enhance drought tolerance in growth chamber and field conditions. Expression of LOS5/ABA3 was up‐regulated by drought stress, which led to increasing AO activity and then a notable increase in ABA accumulation. Transgenic soybean under drought stress had reduced water loss by decreased stomatal aperture size and transpiration rate, which alleviated leaf wilting and maintained higher relative water content. Exposed to drought stress, transgenic soybean exhibited reduced cell membrane damage by reducing electrolyte leakage and production of malondialdehyde and promoting proline accumulation and antioxidant enzyme activities. Also, overexpression of LOS5/ABA3 enhanced expression of stress‐up‐regulated genes. Furthermore, the seed yield of transgenic plants is at least 21% higher than that of wide‐type plants under drought stress conditions in the field. These data suggest that overexpression of LOS5/ABA3 could improve drought tolerance in transgenic soybean via enhanced ABA accumulation, which could activate expression of stress‐up‐regulated genes and cause a series of physiological and biochemical resistant responses.     

Constitutive expression of a meiotic recombination protein gene homolog, OsTOP6A1, from rice confers abiotic stress tolerance in transgenic Arabidopsis plants

Plant productivity is greatly influenced by various environmental stresses, such as high salinity and drought. Earlier, we reported the isolation of topoisomerase 6 homologs from rice and showed that over expression of OsTOP6A3 and OsTOP6B confers abiotic stress tolerance in transgenic Arabidopsis plants. In this study, we have assessed the function of nuclear-localized topoisomerase 6 subunit A homolog, OsTOP6A1, in transgenic Arabidopsis plants. The over expression of OsTOP6A1 in transgenic Arabidopsis plants driven by cauliflower mosaic virus-35S promoter resulted in pleiotropic effects on plant growth and development. The transgenic Arabidopsis plants showed reduced sensitivity to stress hormone, abscisic acid (ABA), and tolerance to high salinity and dehydration at the seed germination; seedling and adult stages as reflected by the percentage of germination, fresh weight of seedlings and leaf senescence assay, respectively. Concomitantly, the expression of many stress-responsive genes was enhanced under various stress conditions in transgenic Arabidopsis plants. Moreover, microarray analysis revealed that the expression of a large number of genes involved in various processes of plant growth and development and stress responses was altered in transgenic plants. Although AtSPO11-1, the homolog of OsTOP6A1 in Arabidopsis, has been implicated in meiotic recombination; the present study demonstrates possible additional role of OsTOP6A1 and provides an effective tool for engineering crop plants for tolerance to different environmental stresses. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

盐芥叶片响应干旱胁迫的蛋白质组学初步分析

盐芥是新兴起的植物非生物逆境研究模式植物,研究盐芥叶片蛋白质组对于干旱胁迫的响应,以推进对植物干旱耐受机制的认识。该研究应用双向电泳技术分析了干旱胁迫对于盐芥叶片蛋白质组的影响,结果共鉴定了63个干旱胁迫差异表达蛋白,包括丰度上调的31个,新出现的蛋白点14个,丰度下调的15个,消失的蛋白点3个。应用生物质谱分析技术确定了包括硫氧还蛋白,铁蛋白-1和凝集素在内的9个干旱胁迫响应蛋白的身份,对这些干旱胁迫响应蛋白的功能分类分析表明,盐芥的耐旱机制可能涉及自由基清除能力的增强、能量代谢的调整以及光合作用的维持。     

Drought and Salt Tolerance in Plants

Agricultural productivity worldwide is subject to increasing environmental constraints, particularly to drought and salinity due to their high magnitude of impact and wide distribution. Traditional breeding programs trying to improve abiotic stress tolerance have had some success, but are limited by the multigenic nature of the trait. Tolerant plants such as Craterostigma plantagenium, Mesembryanthemum crystallinum, Thellungiella halophila and other hardy plants could be valuable tools to dissect the extreme tolerance nature. In the last decade, Arabidopsis thaliana, a genetic model plant, has been extensively used for unravelling the molecular basis of stress tolerance. Arabidopsis also proved to be extremely important for assessing functions for individual stress-associated genes due to the availability of knock-out mutants and its amenability for genetic transformation. In this review, the responses of plants to salt and water stress are described, the regulatory circuits which allow plants to cope with stress are presented, and how the present knowledge can be applied to obtain tolerant plants is discussed.     

Identification and profiling of salinity stress-responsive proteins in Sorghum bicolor seedlings

Sorghum bicolor, a drought tolerant cereal crop, is not only an important food source in the semi arid/arid regions but also a potential model for studying and gaining a better understanding of the molecular mechanisms of drought and salt stress tolerance in cereals. In this study, seeds of a sweet sorghum variety, MN1618, were planted and grown on solid MS growth medium with or without 100mM NaCl. Heat shock protein expression immunoblotting assays demonstrated that this salt treatment induced stress within natural physiological parameters for our experimental material. 2D PAGE in combination with MS/MS proteomics techniques were used to separate, visualise and identify salinity stress responsive proteins in young sorghum leaves. Out of 281 Coomassie stainable spots, 118 showed statistically significant responses (p<0.05) to salt stress treatments. Of the 118 spots, 79 were selected for tandem mass spectrometric identification, owing to their good resolution and abundance levels, and of these, 55 were positively identified. Identified proteins were divided into six functional categories including both known and novel/putative stress responsive proteins. Molecular and physiological functions of some of our proteins of interest are currently under investigation via bioinformatic and molecular biology approaches.     

Down‐regulation of CBP80 gene expression as a strategy to engineer a drought‐tolerant potato

Developing new strategies for crop plants to respond to drought is crucial for their innovative breeding. The down‐regulation of nuclear cap‐binding proteins in Arabidopsis renders plants drought tolerant. The CBP80 gene in the potato cultivar Desiree was silenced using artificial microRNAs. Transgenic plants displayed a higher tolerance to drought, ABA‐hypersensitive stomatal closing, an increase in leaf stomata and trichome density, and compact cuticle structures with a lower number of microchannels. These findings were correlated with a higher tolerance to water stress. The level of miR159 was decreased, and the levels of its target mRNAs MYB33 and MYB101 increased in the transgenic plants subjected to drought. Similar trends were observed in an Arabidopsis cbp80 mutant. The evolutionary conservation of CBP80, a gene that plays a role in the response to drought, suggests that it is a candidate for genetic manipulations that aim to obtain improved water‐deficit tolerance of crop plants.     

Overexpression of the leucine‐rich receptor‐like kinase gene LRK2 increases drought tolerance and tiller number in rice

Drought represents a key limiting factor of global crop distribution. Receptor‐like kinases play major roles in plant development and defence responses against stresses such as drought. In this study, LRK2, which encodes a leucine‐rich receptor‐like kinase, was cloned and characterized and found to be localized on the plasma membrane in rice. Promoter–GUS analysis revealed strong expression in tiller buds, roots, nodes and anthers. Transgenic plants overexpressing LRK2 exhibited enhanced tolerance to drought stress due to an increased number of lateral roots compared with the wild type at the vegetative stage. Moreover, ectopic expression of LRK2 seedlings resulted in increased tiller development. Yeast two‐hybrid screening and bimolecular fluorescence complementation (BiFC) indicated a possible interaction between LRK2 and elongation factor 1 alpha (OsEF1A) in vitro. These results suggest that LRK2 functions as a positive regulator of the drought stress response and tiller development via increased branch development in rice. These findings will aid our understanding of branch regulation in other grasses and support improvements in rice genetics.     

Increased expression of phospholipase Dα1 in guard cells decreases water loss with improved seed production under drought in Brassica napus

The activation of phospholipase Dα1 (PLDα1) produces lipid messenger phosphatidic acid and promotes stomatal closure in Arabidopsis. To explore the use of the PLDα1‐mediated signalling towards decreasing water loss in crop plants, we introduced Arabidopsis PLDα1 under the control of a guard cell–specific promoter AtKatI_pro into two canola (Brassica napus) cultivars. Multiple AtKatI_pro::PLDα1 lines in each cultivar displayed decreased water loss and improved biomass accumulation under hyperosmotic stress conditions, including drought and high salinity. Moreover, AtKatI_pro::PLDα1 plants produced more seeds than did WT plants in fields under drought. The results indicate that the guard cell–specific expression of PLDα1 has the potential to improve crop yield by enhancing drought tolerance.     

Incorporating thresholds into understanding salinity tolerance: A study using salt‐tolerant plants in salt marshes

Although salinity in many ecosystems such as salt marshes can be extremely high, an asymmetry in salinity range between experimental studies (relatively narrow) and field conditions (potentially broad) has strongly affected current understanding of plant salinity tolerance. To improve understanding, it is thus important to examine plant tolerances over a broad range of salinities and identify potential tolerance thresholds. We examine tolerances of two widely distributed marsh plants, Suaeda salsa and Salicornia europaea, to salinities ranging from 0 to 100 g/kg, and determine survival, above‐ and belowground biomass after 8 weeks of salinity treatment. Both species, Sa. europaea in particular, have much broader salinity tolerances than other plants previously examined, (2) plant survival, above‐ and belowground biomass have remarkably different responses to salinity, and (3) there is a nonlinear, threshold response of S. salsa to salinity, above which S. salsa survivorship drastically decreases. These results provide multiple important insights. Our study suggests that the potential for using these halophytes to revegetate and restore salt‐affected land may be greater than previously thought, and highlights the importance of studying multiple plant responses. Importantly, our study calls for a better integration of thresholds into understanding plant salinity tolerances and their applications.     

Responses of antioxidant enzymes to cold and high light are not correlated to freezing tolerance in natural accessions of Arabidopsis thaliana

Low temperatures and high light cause imbalances in primary and secondary reactions of photosynthesis, and thus can result in oxidative stress. Plants employ a range of low‐molecular weight antioxidants and antioxidant enzymes to prevent oxidative damage, and antioxidant defence is considered an important component of stress tolerance. To figure out whether oxidative stress and antioxidant defence are key factors defining the different cold acclimation capacities of natural accessions of the model plant Arabidopsis thaliana, we investigated hydrogen peroxide (H₂O₂) production, antioxidant enzyme activity and lipid peroxidation during a time course of cold treatment and exposure to high light in four differentially cold‐tolerant natural accessions of Arabidopsis (C24, Nd, Rsch, Te) that span the European distribution range of the species. All accessions except Rsch (from Russia) had elevated H₂O₂ in the cold, indicating that production of reactive oxygen species is part of the cold response in Arabidopsis. Glutathione reductase activity increased in all but Rsch, while ascorbate peroxidase and superoxide dismutase were unchanged and catalase decreased in all but Rsch. Under high light, the Scandinavian accession Te had elevated levels of H₂O₂. Te appeared most sensitive to oxidative stress, having higher malondialdehyde (MDA) levels in the cold and under high light, while only high light caused elevated MDA in the other accessions. Although the most freezing‐tolerant, Te had the highest sensitivity to oxidative stress. No correlation was found between freezing tolerance and activity of antioxidant enzymes in the four accessions investigated, arguing against a key role for antioxidant defence in the differential cold acclimation capacities of Arabidopsis accessions.