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.     

Silicon Application to Rice Root Zone Influenced the Phytohormonal and Antioxidant Responses Under Salinity Stress

Silicon (Si) application shows beneficial effects on plant growth; however, its effects on the phytohormone and enzymatic antioxidant regulation have not been fully understood. We studied the effects of short-term (6, 12, and 24 h) silicon (0.5, 1.0, and 2.0 mM) application on salinity (NaCl)-induced phytohormonal [abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA)] and antioxidant regulation in Oryza sativa. The results showed that Si treatments significantly increased rice plant growth compared to controls under salinity stress. Si treatments reduced the sodium accumulation resulting in low electrolytic leakage and lipid peroxidation compared to control plants under salinity stress. Enzymatic antioxidant (catalase, peroxidase and polyphenol oxidase) responses were more pronounced in control plants than in Si-treated plants under salinity stress. Stress- and defense-related phytohormones like JA were significantly downregulated and SA was irregular after short-term Si applications under salinity stress compared to control. Conversely, ABA was significantly higher after 6 and 12 h but insignificant after 24 h in Si-treated plants under salinity stress. After 6 and 12 h, Si and salinity stress resulted in upregulation of zeaxanthin epoxidase and 9-cis-epoxycarotenoid dioxygenase 1 and 4 (NCED1 and 4), whereas 24-h treatments significantly downregulated the expressions of these genes compared to those in the control. NCED3 expression increased after 6 and 24 h but it was insignificant after 12 h of Si application compared to control. The current findings indicate that increasing the Si concentrations for longer periods of time can regulate the salinity-induced stress by modulating phytohormonal and enzymatic antioxidants’ responses.     

Phytohormonal signaling in plant responses to aphid feeding

Aphid feeding induces various defense signaling mechanisms in plants. The recognition of feeding activities by plants occurs through the use of transmembrane pattern recognition receptors (PRRS) or, acting largely inside the cell, polymorphic nucleotide-binding leucine-rich-repeat (NB-LRR) protein products, encoded by most R genes. Activation may induce defensive reactions which are the result of highly coordinated sequential changes at the cellular level comprising, among other changes, the synthesis of signaling molecules. The ensuing plant responses are followed by the transmission of defense response signal cascades. Signals are mediated by bioactive endogenous molecules, i.e. phytohormones, such as jasmonic acid (JA), salicylic acid (SA), ethylene (ET), abscisic acid (ABA), gibberellic acid (GA) and free radicals such as hydrogen peroxide (H₂O₂) and nitric oxide (NO) which independently provide direct chemical resistance. Plant-induced defenses are also regulated by a network of inter-connecting signaling pathways, in which JA, SA, and ET play dominant roles. Both synergistic and inhibitory aspects of the cross-talk among these pathways have been reported. This paper presents molecular mechanisms of plant response to aphid feeding, the precise activation of various endogenous bioactive molecules signaling in the response of many plant species and their participation in the regulation of numerous defense genes, which lead to a specific metabolic effect. Selected important points in signal transduction pathways were also discussed in studies on plant response to aphid feeding.     

植物激素调控丛枝菌根发育的作用机制研究进展

丛枝菌根（arbuscular mycorrhiza，AM）是土壤中AM真菌和绝大多数维管植物根系长期进化过程中相互识别、相互作用形成的互利共生体。AM的发育与功能效应依赖AM真菌-寄主植物之间精准的“分子对话”，同时受到环境条件特别是土壤养分水平、干旱和盐渍化的制约。植物激素作为低浓度的小分子有机物，是参与调控AM共生过程的重要信号分子。其中，主要有9种植物激素参与AM发育过程且分工各有不同：独脚金内酯（strigolactones，SLs）参与AM真菌-寄主植物之间最初的共生识别，脱落酸（abscisic acid，ABA）和油菜素内酯（brassinosteroid，BR）促进前期的菌丝入侵，但水杨酸（salicylic acid，SA）和乙烯（ethylene，ET）抑制前期的菌丝入侵，生长素（auxin，Aux）、ABA和BR促进随后的丛枝形成而ET和赤霉素（gibberellin，GA）的作用则相反，茉莉酸（jasmonic acid，JA）对菌丝入侵与丛枝形成均可能存在正调控或负调控作用。目前细胞分裂素（cytokinin，CTK）在AM发育中的作用尚不明确。更为复杂的是，通常植物激素信号之间的交叉互作决定AM的发育进程。本文针对AM发育过程总结了不同植物激素的调控作用特点和不同植物激素信号之间的互作（协同或拮抗），以及胁迫条件下不同植物激素信号的可能调控机制。深入研究和系统阐明植物激素调控AM真菌-寄主植物共生的生理/分子机制，将有助于促进生物共生学理论研究及菌根技术的应用。     

棉花WRKY基因GhWRKY25的克隆和表达分析

棉花是一种重要的经济作物,在国民生产中起着重要的作用。然而干旱盐碱等不良环境严重地制约着棉花的种植区域及其产量和品质。近年来,利用分子育种改良棉花对这些逆境的耐性的研究成为热点。本文从棉花中克隆了一个对干旱和盐碱具有应答的WRKY基因,定名为GhWRKY25。该基因编码的蛋白包含两个典型的WRKY结构域,每个WRKY结构域的C端含有一个C2H2类型的锌指蛋白结构,进化树分析表明其属于Ⅰ类WRKY蛋白,与可可的WRKY3同源性最高。利用qPCR分析发现该基因在棉花里为组成型表达,但在花中表达量较高。GhWRKY25在施用SA、JA、ABA、GA 中表达表现为上调。干旱和NaCl处理时发现GhWRKY25基因表达均显著上调。通过上述结果可以推测GhWRKY25在棉花对于逆境的适应中具有重要的作用,为该基因功能的进一步研究提供基础。     

Overexpression of a nascent polypeptide associated complex gene (SaβNAC) of Spartina alterniflora improves tolerance to salinity and drought in transgenic Arabidopsis

Salinity and drought are the most important environmental constraints limiting crop growth and productivity. Here, we have characterized a gene 'SaβNAC' encoding the β subunit of nascent polypeptide associated complex from a halophyte Spartina alterniflora and investigated its role toward abiotic stress regulation. Expression of SaβNAC was differentially regulated by abiotic stresses, including salinity, drought, cold, and ABA in leaves and roots of S. alterniflora. Constitutive over-expression of SaβNAC in Arabidopsis exhibited normal growth under non-stress conditions but enhanced tolerance to salt and drought stresses. Transgenic SaβNAC Arabidopsis retained more chlorophyll, proline, and showed improved ionic homeostasis with less damage under stress conditions compared to WT plants. This is a first report to demonstrate the involvement of βNAC in imparting abiotic stress tolerance which might be due to protection of the newly synthesized polypeptides involved in various stress tolerance mechanisms from abiotic stress induced damage and inhibition of cell death in plant.     

High temperatures change the perspective: Integrating hormonal responses in citrus plants under co‐occurring abiotic stress conditions

Plants growing in the field are subjected to multiple stress factors acting simultaneously. Abnormally high temperatures are expected to affect wild plants and crops in the next years due to global warming. In this work, we have studied physiological, hormonal and molecular responses of the citrus rootstock, Carrizo citrange (Poncirus trifoliata L. Raf. × Citrus sinensis L. Osb.) subjected to wounding or high salinity occurring individually or in combination with heat stress. According to our results, combination of high salinity and heat stress aggravated the negative effects of salt intoxication in Carrizo. The high transpiration rate caused by high temperatures counteracted physiological responses of plants to salt stress and increased Cl^? intake in leaves. In addition, 12‐oxo‐phytodienoic acid accumulated specifically under combination of wounding and heat stress, whereas at low temperatures, wounded plants accumulated jasmonic acid (JA) and JA‐isoleucine (JA‐Ile). Moreover, an antagonism between salicylic acid (SA) and JA was observed, and wounded plants subjected to high temperatures did not accumulate JA nor JA‐Ile whereas SA levels increased (via isochorismate synthase biosynthetic pathway). Wounded plants did not accumulate abscisic acid (ABA) but its catabolite phaseic acid. This could act as a signal for the upregulation of (ABA)‐RESPONSIVE ELEMENT (ABRE)‐BINDING TRANSCRIPTION FACTOR 2 (CsAREB2) and RESPONSIVE TO DISSECATION 22 (CsRD22) in an ABA‐independent way. This work uncovers some mechanisms that explain Carrizo citrange tolerance to high temperatures together with different hormonal signals in response to specific stresses. It is suggested that co‐occurring abiotic stress conditions can modify (either enhance or reduce) the hormonal response to modulate specific responses.     

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。     

Drought-induced responses of photosynthesis and antioxidant metabolism in higher plants

Environmental stresses trigger a wide variety of plant responses, ranging from altered gene expression and cellular metabolism to changes in growth rates and crop yields. A plethora of plant reactions exist to circumvent the potentially harmful effects caused by a wide range of both abiotic and biotic stresses, including light, drought, salinity, high temperatures, and pathogen infections. Among the environmental stresses, drought stress is one of the most adverse factors of plant growth and productivity. Understanding the biochemical and molecular responses to drought is essential for a holistic perception of plant resistance mechanisms to water-limited conditions. Drought stress progressively decreases CO2 assimilation rates due to reduced stomatal conductance. Drought stress also induces reduction in the contents and activities of photosynthetic carbon reduction cycle enzymes, including the key enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase. The critical roles of proline and glycine-betaine, as well as the role of abscisic acid (ABA), under drought stress conditions have been actively researched to understand the tolerance of plants to dehydration. In addition, drought stress-induced generation of active oxygen species is well recognized at the cellular level and is tightly controlled at both the production and consumption levels in vivo, through increased antioxidative systems. Knowledge of sensing and signaling pathways, including ABA-mediated changes in response to drought stress, is essential to improve crop management. This review focuses on the ability and strategies of higher plants to respond and adapt to drought stress.     

Building stress tolerance through over-producing trehalose in transgenic plants

Trehalose is a rare sugar with unique abilities to protect biomolecules from environmental stresses and is present in many bacteria, fungi and some desiccation-tolerant higher plants. Increasing trehalose accumulation in crop plants could improve drought and salinity tolerance. Transgenic plants have been developed with trehalose biosynthetic genes--a recent study on the stress-inducible overexpression of the bifunctional TPSP fusion gene in transgenic rice could offer novel strategies for improving abiotic stress tolerance in crop plants.     

Overexpression of cotton GhMKK4 enhances disease susceptibility and affects abscisic acid,gibberellin and hydrogen peroxide signalling in transgenic Nicotiana benthamiana

Mitogen‐activated protein kinase (MAPK) cascades are involved in plant development, stress responses and hormonal signal transduction. MAPK kinases (MAPKKs), as the key nodes in these cascades, link MAPKs and MAPKK kinases (MAPKKKs). In this study, GhMKK4, a novel group C MAPKK gene from cotton (Gossypium hirsutum), was isolated and identified. Its expression can be induced by various stresses and signalling molecules. The overexpression of GhMKK4 in Nicotiana benthamiana enhanced its susceptibility to bacterial and fungal pathogens, but had no significant effects on salt or drought tolerance. Notably, the overexpressing plants showed increased sensitivity to abscisic acid (ABA) and gibberellin A3 (GA3), and ABA and gibberellin (GA) signalling were affected on infection with Ralstonia solanacearum bacteria. Furthermore, the overexpressing plants showed more reactive oxygen species (ROS) accumulation and stronger inhibition of catalase (CAT), a ROS‐scavenging enzyme, than control plants after salicylic acid (SA) treatment. Interestingly, two genes encoding ornithine decarboxylase (ODC) and S‐adenosylmethionine decarboxylase (SAMDC), the key enzymes in polyamine synthesis, exhibited reduced R. solanacearum‐induced expression in overexpressing plants. These findings broaden our knowledge about the functions of MAPKKs in diverse signalling pathways and the negative regulation of disease resistance in the cotton crop.     

The pepper late embryogenesis abundant protein CaLEA1 acts in regulating abscisic acid signaling,drought and salt stress response

As sessile organisms, plants are constantly challenged by environmental stresses, including drought and high salinity. Among the various abiotic stresses, osmotic stress is one of the most important factors for growth and significantly reduces crop productivity in agriculture. Here, we report a function of the CaLEA1 protein in the defense responses of plants to osmotic stress. Our analyses showed that the CaLEA1 gene was strongly induced in pepper leaves exposed to drought and increased salinity. Furthermore, we determined that the CaLEA1 protein has a late embryogenesis abundant (LEA)_3 homolog domain highly conserved among other known group 5 LEA proteins and is localized in the processing body. We generated CaLEA1‐silenced peppers and CaLEA1‐overexpressing (OX) transgenic Arabidopsis plants to evaluate their responses to dehydration and high salinity. Virus‐induced gene silencing of CaLEA1 in pepper plants conferred enhanced sensitivity to drought and salt stresses, which was accompanied by high levels of lipid peroxidation in dehydrated and NaCl‐treated leaves. CaLEA1‐OX plants exhibited enhanced sensitivity to abscisic acid (ABA) during seed germination and in the seedling stage; furthermore, these plants were more tolerant to drought and salt stress than the wild‐type plants because of enhanced stomatal closure and increased expression of stress‐responsive genes. Collectively, our data suggest that CaLEA1 positively regulates drought and salinity tolerance through ABA‐mediated cell signaling.