Recent methods of drought stress tolerance in plants

In the climate change scenario the drought has been diagnosed as major stress affecting crop productivity. This review demonstrates some recent findings on the amelioration of drought stress. Nanoparticles, synthetic growth regulators viz. Trinexapac-ethyl, and Biochar addition helps to economize the water budget of plants, enhances the bioavailability of water and nutrients as well as overcomes drought induced osmotic and oxidative stresses. Besides ABA, SA and JA are also involved in inducing tolerance to drought stress through modulation of physiological and biochemical processes in plants. Plant growth promoting rhizobacteria (PGPR) offer new opportunities in agricultural biotechnology. These beneficial microorganisms colonize the rhizosphere/endo-rhizosphere of plants and impart drought tolerance by improving root architechture, enhancing water use efficiency, producing exopolysaccharides, phytohormones viz, ABA, SA and IAA and volatile compounds. Further PGPR also play positive role in combating osmotic and oxidative stresses induced by drought stress through enhancing the accumulation of osmolytes, antioxidants and upregulation or down regulation of stress responsive genes. In transgenic plants stress inducible genes enhanced abiotic stress tolerance by encoding key enzymes regulating biosynthesis of compatible solutes. The role of genes/cDNAs encoding proteins involved in regulating other genes/proteins, signal transduction process and strategies to improve drought stress tolerance have also been discussed.     

Interaction between PGPR and PGR for water conservation and plant growth attributes under drought condition

Drought is one of the key restraints to agricultural productivity worldwide and is expected to increase further. Drought stress accompanied by reduction in precipitation pose major challenges to future food safety. Strategies should be develop to enhance drought tolerance in crops like chickpea and wheat, in order to enhance their growth and yield. Drought tolerance strategies are costly and time consuming however, recent studies specify that plant growth promoting rhizobacteria (PGPR) and plant growth regulators (PGRs) can help plants to withstand under harsh environmental condition and enable plants to cope with drought stress. PGPR can act as biofertilizer and bioenhancer for different legumes and non-legumes. The use of PGPR and symbiotic microorganisms, may be valuable in developing strategies to assist water conservation in plants. The use of PGPR has been confirmed to be an ecologically sound way of enhancing crop yields by facilitating plant growth through direct or indirect mechanism. The mechanisms of PGPR for water conservation include secretion of exopolysaccharides, biofilm formation, alternation in phytohormone content, improvement in sugar concentration, enhancing availability of micro- and macronutrients and changes in plant functional traits. Similarly, plant growth regulators (PGRs) are specially noticed in actively growing tissues under stress conditions and have been associated in the control of cell division, embryogenesis, root formation, fruit development and ripening, and reactions to biotic and abiotic stresses and upholding water conservation status in plants. Previous studies also suggest that plant metabolites interact with plant physiology under stress condition and impart drought tolerance. Metabolites like, sugars, amino acids, organic acid and polyols play a key role in drought tolerance of crop plants grown under stress condition. It is concluded from the present study that PGRs in combination with PGPR consortium can be an effective formulation to promote plant growth and maintenance of plant turgidity under drought stress. This review is a compilation of the effect of drought stress on crop plants and described interactions between PGPR/PGRs and plant development, knowledge of water conservation and stress release strategies of PGPR and PGRs and the role of plant metabolites in drought tolerance of crop plants. This review also bridges the gaps that summarizes the mechanism of action of PGPR for drought tolerance of crop plants and sustainability of agriculture and applicability of these beneficial rhizobacteria in different agro-ecosystems under drought stress.     

植物逆境胁迫抗性的功能基因组研究策略

植物对逆境胁迫抗性的功能基因组研究主要是寻找胁迫抗性位点在相关物种基因组中的保守位置,发现胁迫反应中的高度保守序列,确定植物胁迫反应的调控机理,进而得到植物对逆境胁迫抗性的关键代谢途径和其中的关键调控因子,为进一步选择用于改良植物对逆境胁迫抗性的关键基因奠定基础。本文从主要模式植物(苔藓类植物、复苏植物、盐土植物和甜土植物)、主要技术策略(基因的差异表达分析、基因表达序列标签、cDNA芯片技术。基因表达序列分析和基因敲除和突变体筛选分析)和生物信息学方法(数据分析的生物信息学方法设计到序列比较、比较基因组学、电子克隆)等三个方面对国内外植物逆境胁迫抗性的功能基因组研究策略作了全面综述。     

植物水通道蛋白的干旱应答机制研究进展

干旱胁迫是严重影响全球作物生产的非生物胁迫之一,研究植物耐旱机制已成为一个重要领域。水通道蛋白是一类特异、高效转运水及其它小分子底物的膜通道蛋白,在植物中具有丰富的亚型,参与调节植物的水分吸收和运输。近10年来,水通道蛋白在植物不同生理过程中的作用,一直受到研究人员的关注,特别是在非生物胁迫方面,而研究表明水通道蛋白在干旱胁迫下对植物的耐旱性起着至关重要的作用,能维持细胞水分稳态和调控环境胁迫快速响应。水通道蛋白在植物耐旱过程中的调控机制及功能较复杂,而关于其应答机制和不同亚型功能性研究的报道甚少。该文综述了植物水通道蛋白的分类、结构、表达调控和活性调节,分别从植物水通道蛋白响应干旱表达调控机制、水通道蛋白基因表达的时空特异性、水通道蛋白基因的表达与蛋白丰度,水通道蛋白基因的耐旱转化四个方面阐明干旱胁迫下植物水通道蛋白的表达,重点阐述其参与植物干旱胁迫应答的作用机制,并提出水通道蛋白研究的主要方向。     

Abiotic stress-associated microRNAs in plants: discovery, expression analysis, and evolution

Abiotic stresses such as drought, cold, and high salinity are among the most adverse factors that affect plant growth and yield in the field. MicroRNAs are small RNA molecules that regulate gene expression in a sequence-specific manner and play an important role in plant stress response. Identifying abiotic stress-associated microRNAs and understanding their function will help develop new strategies for improvement of plant stress tolerance. Here we highlight recent advances in our understanding of abiotic stress-associated miRNAs in various plants, with focus on their discovery, expression analysis, and evolution.     

Regulation in Plant Stress Tolerance by a Potential Plant Growth Regulator, 5-Aminolevulinic Acid

Exogenous application of different plant growth regulators is a well-recognized strategy to alleviate stress-induced adverse effects on different crop plants by regulating a variety of physiobiochemical processes such as photosynthesis, chlorophyll biosynthesis, nutrient uptake, antioxidant metabolism, and protein synthesis, which are directly or indirectly involved in the mechanism of stress tolerance. Of various environmental factors, salinity, drought, and extreme temperature (low or high) considerably diminish plant growth and yield by modulating endogenous levels as well as signaling pathways of plant hormones. Of various plant hormones/regulators, a potential plant growth regulator, 5-aminolevulinic acid (ALA), is known to be effective in counteracting the injurious effects of various abiotic stresses in plants. Until now the mechanisms behind ALA regulation of growth under stress have not been fully elucidated. It is also not yet clear how far growth and yield in different crops can be promoted by exogenous application of ALA and whether this ALA-induced growth and yield promotion is cost-effective. Thus, in this review we discuss at length the effects of ALA in regulating growth and development in plants under a variety of abiotic stress conditions, including salinity, drought, and temperature stress. Furthermore, advances in the functional and regulatory interactions of this plant growth regulator with plant stress tolerance, as well as the effective mode of exogenous application of ALA in inducing stress tolerance in plants are also comprehensively discussed in this review. In the future, overaccumulation of ALA in plants through manipulation of gene(s) could enhance plant stress tolerance. Thus, genetic manipulation of plants with the goal of attaining increased synthesis/accumulation of ALA and hence improved stress tolerance under stress conditions is an important area for research.     

Silicon: a duo synergy for regulating crop growth and hormonal signaling under abiotic stress conditions

Abiotic stresses, such as salinity, heavy metals and drought, are some of the most devastating factors hindering sustainable crop production today. Plants use their own defensive strategies to cope with the adverse effects of these stresses, via the regulation of the expression of essential phytohormones, such as gibberellins (GA), salicylic acid (SA), jasmonates (JA), abscisic acid (ABA) and ethylene (ET). However, the efficacy of the endogenous defensive arsenals of plants often falls short if the stress persists over an extended period. Various strategies are developed to improve stress tolerance in plants. For example, silicon (Si) is widely considered to possess significant potential as a substance which ameliorate the negative effects of abiotic stresses, and improves plant growth and biomass accumulation. This review aims to explain how Si application influences the signaling of the endogenous hormones GA, SA, ABA, JA and ET during salinity, wounding, drought and metal stresses in crop plants. Phytohormonal cross talk plays an important role in the regulation of induced defences against stress. However, detailed molecular and proteomic research into these interactions is needed in order to identify the underlying mechanisms of stress tolerance that is imparted by Si application and uptake.     

Osmoregulation in Plants: Implications for Agriculture

Drought and salinity stress are the major causes of historicand modern agricultural productivity losses throughout the world.Both drought and salinity result in osmotic stress that maylead to inhibition of growth. Salinity causes additional iontoxicity effects mainly through perturbations in protein andmembrane structure. In contrast to animals, which rely on Na⁺/K⁺-ATPasesfor the expulsion of osmotica, plants rely on plasma membraneand endosomal ATPase activities to generate proton gradientsto drive ion extrusion and intracellular sequestration. Consequently,most angiosperms, including all major crop species, have a diminishedcapacity for Na⁺ transport and tolerance to high salinity. Newinsights into the molecular mechanisms of Na⁺/K⁺ discrimination,Na⁺ extrusion and compartmentation, water transport, and osmolytebiosynthesis and function have led to genetically engineeredplants with improved salt, drought, and cold tolerance. A deeperunderstanding of the complex signal transduction and regulatoryresponses to osmotic stress promises novel strategies for improvingsalinity and drought tolerance that will be of practical benefitto agriculture.     

Tetrapyrrole‐based drought stress signalling

Tetrapyrroles such as chlorophyll and heme play a vital role in primary plant metabolic processes such as photosynthesis and respiration. Over the past decades, extensive genetic and molecular analyses have provided valuable insights into the complex regulatory network of the tetrapyrrole biosynthesis. However, tetrapyrroles are also implicated in abiotic stress tolerance, although the mechanisms are largely unknown. With recent reports demonstrating that modified tetrapyrrole biosynthesis in plants confers wilting avoidance, a component physiological trait to drought tolerance, it is now timely that this pathway be reviewed in the context of drought stress signalling. In this review, the significance of tetrapyrrole biosynthesis under drought stress is addressed, with particular emphasis on the inter‐relationships with major stress signalling cascades driven by reactive oxygen species (ROS) and organellar retrograde signalling. We propose that unlike the chlorophyll branch, the heme branch of the pathway plays a key role in mediating intracellular drought stress signalling and stimulating ROS detoxification under drought stress. Determining how the tetrapyrrole biosynthetic pathway is involved in stress signalling provides an opportunity to identify gene targets for engineering drought‐tolerant crops.     

Halophytes as a source of genes for abiotic stress tolerance

Agricultural productivity is majorly impacted due to various abiotic stresses, particularly salinity and drought. Halophytes serve as an excellent resource for identifying and developing new crop systems, as these grow very luxuriously in very high saline soils. Understanding salinity stress tolerance mechanisms in such plants is an important step towards generating crop varieties that can cope with environmental stresses. Use of modern tools of ‘omics’ analyses and small RNA sequencing has helped to gain insights into the complex plant stress responses. Salinity tolerance being a multigenic trait requires a combination of strategies and techniques to successfully develop improved crops varieties. Many transgenic crops are being developed through genetic transformation. Besides marker-assisted breeding/QTL approaches are also being used to improve abiotic stress tolerance. In this review, we focus on the recent developments in the utilization of halophytes as a source of genes for genetic improvement in abiotic stress tolerance of crops.     

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.     

CaPUB1, a Hot Pepper U-box E3 Ubiquitin Ligase,Confers Enhanced Cold Stress Tolerance and Decreased Drought Stress Tolerance in Transgenic Rice (Oryza sativa L.)

Abiotic stresses such as drought and low temperature critically restrict plant growth, reproduction, and productivity. Higher plants have developed various defense strategies against these unfavorable conditions. CaPUB1 (Capsicum annuum Putative U-box protein 1) is a hot pepper U-box E3 Ub ligase. Transgenic Arabidopsis plants that constitutively expressed CaPUB1 exhibited drought-sensitive phenotypes, suggesting that it functions as a negative regulator of the drought stress response. In this study, CaPUB1 was over-expressed in rice (Oryza sativa L.), and the phenotypic properties of transgenic rice plants were examined in terms of their drought and cold stress tolerance. Ubi:CaPUB1 T3 transgenic rice plants displayed phenotypes hypersensitive to dehydration, suggesting that its role in the negative regulation of drought stress response is conserved in dicot Arabidopsis and monocot rice plants. In contrast, Ubi:CaPUB1 progeny exhibited phenotypes markedly tolerant to prolonged low temperature (4°C) treatment, compared to those of wild-type plants, as determined by survival rates, electrolyte leakage, and total chlorophyll content. Cold stress-induced marker genes, including DREB1A, DREB1B, DREB1C, and Cytochrome P450, were more up-regulated by cold treatment in Ubi:CaPUB1 plants than in wild-type plants. These results suggest that CaPUB1 serves as both a negative regulator of the drought stress response and a positive regulator of the cold stress response in transgenic rice plants. This raises the possibility that CaPUB1 participates in the cross-talk between drought and low-temperature signaling pathways.     

Insights into the cellular mechanisms of desiccation tolerance among angiosperm resurrection plant species

Water is a major limiting factor in growth and reproduction in plants. The ability of tissues to survive desiccation is commonly found in seeds or pollen but rarely present in vegetative tissues. Resurrection plants are remarkable as they can tolerate almost complete water loss from their vegetative tissues such as leaves and roots. Metabolism is shut down as they dehydrate and the plants become apparently lifeless. Upon rehydration these plants recover full metabolic competence and ‘resurrect’. In order to cope with desiccation, resurrection plants have to overcome a number of stresses as water is lost from the cells, among them oxidative stress, destabilization or loss of membrane integrity and mechanical stress. This review will mainly focus on the effect of dehydration in angiosperm resurrection plants and some of the strategies developed by these plants to tolerate desiccation. Resurrection plants are important experimental models and understanding the physiological and molecular aspects of their desiccation tolerance is of great interest for developing drought‐tolerant crop species adapted to semi‐arid areas.     

内源小RNAs在植物胁迫反应中的作用

世界范围内, 农作物的产量都容易受到各种生物和非生物因素的影响, 对植物逆境适应性反应机制的深入研究有助于我们采取新的措施, 以提高作物的逆境适应性。以前通常认为植物适应逆境胁迫的机制主要涉及相关基因在转录水平的调节, 然而, 近来发现部分内源小RNAs(siRNAs), 如miRNAs、 nat-siRNAs和 lsiRNAs不仅可以调节植物的生长发育,而且在植物逆境反应中具有重要作用。文章就这些内源小RNAs在氧、矿质元素、干旱、低温、脱落酸、机械、重金属、生物及其他环境因素胁迫中的作用机制做一概述。