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.     

Photosynthesis and antioxidative defense mechanisms in deciphering drought stress tolerance of crop plants

Crop plants are regularly exposed to an array of abiotic and biotic stresses, among them drought stress is a major environmental factor that shows adverse effects on plant growth and productivity. Because of this these factors are considered as hazardous for crop production. Drought stress elicits a plethora of responses in plants resulting in strict amendments in physiological, biochemical, and molecular processes. Photosynthesis is the most fundamental physiological process affected by drought due to a reduction in the CO₂ assimilation rate and disruption of primary photosynthetic reactions and pigments. Drought also expedites the generation of reactive oxygen species (ROS), triggering a cascade of antioxidative defense mechanisms, and affects many other metabolic processes as well as affecting gene expression. Details of the drought stress-induced changes, particularly in crop plants, are discussed in this review, with the major points: 1) leaf water potentials and water use efficiency in plants under drought stress; 2) increased production of ROS under drought leading to oxidative stress in plants and the role of ROS as signaling molecules; 3) molecular responses that lead to the enhanced expression of stress-inducible genes; 4) the decrease in photosynthesis leading to the decreased amount of assimilates, growth, and yield; 5) the antioxidant defense mechanisms comprising of enzymatic and non-enzymatic antioxidants and the other protective mechanisms; 6) progress made in identifying the drought stress tolerance mechanisms; 7) the production of transgenic crop plants with enhanced tolerance to drought stress.     

干旱对陆地生态系统生产力的影响

该文综述了干旱对陆地生态系统生产力的影响,分析了其影响机制,并总结了植被对干旱的响应与适应及其机理机制。干旱通过抑制光合作用来降低陆地生态系统总初级生产力,干旱还可以降低生态系统的自养呼吸和异养呼吸。同时干旱还可以通过影响其它干扰形式来间接影响陆地生态系统生产力,如增加火干扰的发生频率和强度,增加植物的死亡率,增加病虫害的发生等。在生态系统水平上干旱可以降低碳固定,减弱碳汇功能,甚至把生态系统从碳汇改变成碳源。目前生态系统水平上的干旱影响研究主要通过两种方法实现,一种是模型模拟,另一种就是大型模拟实验。作为陆地生态系统生产力的实现者,在干旱胁迫条件下,植物也会采取积极的适应策略以减弱干旱对生态系统生产力的影响,其适应策略主要分以下3种:在一些周期性发生干旱的地区,植物会调整生长期以避开干旱或通过休眠来减弱干旱所造成的伤害;还有一些植物会通过调节体内的代谢过程,改变一些生理特性来抵御干旱;而长期生活在干旱条件下的植物则通过进化来改变了自身的生理生化代谢过程,形成耐旱机制。目前,植物对干旱响应的分子学机制,以及生态系统水平上对干旱的响应和适应仍然是薄弱的领域,也必然成为未来研究的重点。     

Brassinosteroid-Mediated Stress Responses

Brassinosteroids (BRs) are a group of naturally occurring plant steroidal compounds with wide-ranging biological activity that offer the unique possibility of increasing crop yields through both changing plant metabolism and protecting plants from environmental stresses. In recent years, genetic and biochemical studies have established an essential role for BRs in plant development, and on this basis BRs have been given the stature of a phytohormone. A remarkable feature of BRs is their potential to increase resistance in plants to a wide spectrum of stresses, such as low and high temperatures, drought, high salt, and pathogen attack. Despite this, only a few studies aimed at understanding the mechanism by which BRs promote stress resistance have been undertaken. Studies of the BR signaling pathway and BR gene-regulating properties indicate that there is cross-talk between BRs and other hormones, including those with established roles in plant defense responses such as abscisic acid, jasmonic acid, and ethylene. Recent studies aimed at understanding how BRs modulate stress responses suggest that complex molecular changes underlie BR-induced stress tolerance in plants. Analyses of these changes should generate exciting results in the future and clarify whether the ability of BRs to increase plant resistance to a range of stresses lies in the complex interactions of BRs with other hormones. Future studies should also elucidate if BRI1, an essential component of the BR receptor, directly participates in stress response signaling through interactions with ligands and proteins involved in plant defense responses.     

Crop responses to drought and the interpretation of adaptation

Drought is a multidimensional stress affecting plants at various levels of their organization. The effect of and plant response to drought at the whole plant and crop level is most complex because it reflects the integration of stress effects and responses at all underlying levels of organization over space and time. This review discusses some of the major aspects of crop response to drought stress which are relevant for plant breeding. Emphasis is given to whole plant aspects which are too often disregarded when conclusions are drawn from molecular studies towards the genetic improvement of crop drought resistance. Topics discussed are seedling emergence and establishment, plant phenology, leaf area, water deficit and assimilation, osmotic adjustment, the root and the formation of yield. The discussion is concluded with the interpretation of crop adaptation to drought conditions in its agronomic sense. Conclusions are drawn regarding plant breeding for drought-prone conditions.     

缓慢干旱下春小麦叶片质膜脂肪酸组成、H+-ATPase及5′-AMPase活力的变化

 研究了田间缓慢干旱胁迫下,抗旱性不同的两个小麦(Triticum aestivum)品种的生长状况、质膜极性脂脂肪酸组成以及质膜关键酶活力的变化。在小麦生长发育的早期,干旱胁迫使其叶片质膜极性脂脂肪酸不饱和度下降、质膜微囊消耗O2的速率升高、膜蛋白含量降低、H+-ATPase (EC 3.6.1.35)活力下降、5'-AMPase (EC 3.1.3.5)活力大幅度升高;在小麦发育的后期,随着干旱的持续,小麦叶片质膜的极性脂脂肪酸不饱和度不变或升高、质膜微囊消耗O2的速率降低、膜蛋白含量与H+-ATPase活力升高、5'-AMPase活力下降。以上结果表明,小麦在发育的早期阶段对干旱较敏感,其细胞质膜流动性降低、细胞中能荷贮备降低;而在后期,则又表现出对干旱的适应。这些结果将有助于阐明自然干旱条件下植物的抗旱机制。     

植物应对干旱胁迫的阶段性策略

干旱是影响植物生存、生长和分布的最重要的非生物胁迫之一,全球暖干化将加剧干旱胁迫.植物对干旱胁迫的响应和适应机制一直是
学术研究的热点领域.本文综述了植物应对干旱胁迫的生长和生理响应,在已有的研究结果基础上,提出了植物应对干旱胁迫的阶段性响应策略.从干旱开始到干旱致死,植物经历了干旱开始-轻度干旱-中度干旱-严重干旱-极端干旱5个阶段,分别对应着应激响应-主动适应-被动适应3种响应方式和适应机制.不同阶段中植物抗旱机制的核心任务不同.最后提出了研究植物阶段性响应策略需要解决的关键科学问题及研究方向.     

缓慢干旱下春小麦叶片质膜脂肪酸组成、H~+-ATPase肥及5′-AMPase活力的变化

研究了田间缓慢干旱胁迫下,抗旱性不同的两个小麦(Triticum aestivum)品种的生长状况、质膜极性脂脂肪酸组成以及质膜关键酶活力的变化。在小麦生长发育的早期,干旱胁迫使其叶片质膜极性脂脂肪酸不饱和度下降、质膜微囊消耗O_2的速率升高、膜蛋白含量降低、H~+-ATPase(EC 3.6.1.35)活力下降、5′-AMPase(EC 3.1.3.5)活力大幅度升高;在小麦发育的后期,随着干旱的持续,小麦叶片质膜的极性脂脂肪酸不饱和度不变或升高、质膜微囊消耗O_2的速率降低、膜蛋白含量与H~+-ATPase活力升高、5′-AMPase活力下降。以上结果表明,小麦在发育的早期阶段对干旱较敏感,其细胞质膜流动性降低、细胞中能荷贮备降低;而在后期,则又表现出对干旱的适应。这些结果将有助于阐明自然干旱条件下植物的抗旱机制。     

Plant cellular responses to water deficit

Water availability and drought limit crop yields worldwide. The responses of plants to drought vary greatly depending on species and stress severity. These responses include changes in plant growth, accumulation of solutes, changes in carbon and nitrogen metabolism, and alterations in gene expression. In this article, we review cellular and molecular responses to water deficit, and their influence on plant dehydration tolerance.     

Expression analysis of stress responsive pathway genes linked to drought hardiness in an adapted crop, finger millet (Eleusine coracana)

Drought is one of the major stresses limiting plant growth and productivity. Drought tolerance is regulated by multiple plant traits and examining the tolerance mechanisms from adapted species would assist in identification of novel pathways and superior genes. Since cellular tolerance is one of the major traits in drought acclimation we made in this study, an attempt to prospect candidate genes associated with the trait in drought hardy crop plant, finger millet (Eleusine coracana (L.) Gaertn). A novel gravimetric approach was employed to simulate field level drought stress for examining stress responsiveness of a few selected genes implicated in different stress response pathways. Gene expression was studied initially by e-northern analysis, and subsequently in leaf tissues experiencing different levels of drought stress by semi-quantitative and quantitative RT-PCR. A few stress responsive genes identified include metallothionein, farnesylated protein ATFP6, protein phosphatase 2A, RISBZ4 and farnesyl pyrophosphate synthase which probably have crucial roles in imparting hardiness to finger millet. Taken together the results suggest that multiple cellular tolerance pathways operate in a coordinated manner in drought tolerant crops.     

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro‐organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

Significance and Impact of the Study

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress‐tolerant plants.     

Redox regulation of water stress responses in field-grown plants. Role of hydrogen peroxide and ascorbate

Abiotic stresses, such as drought, can increase the production of reactive oxygen species (ROS) in plants. An increase in ROS levels can provoke a partial or severe oxidation of cellular components inducing redox status changes, so continuous control of ROS and therefore of their metabolism is decisive under stress conditions. The present work focuses on the contribution of one pro-oxidant, hydrogen peroxide (H₂O₂) and one antioxidant, ascorbate (AA) and its redox status, in the control of plant responses to drought-oxidative stress in resistant plants growing in field conditions. After a general introduction to the concept of drought and oxidative stress and its relationship, we describe the role of H₂O₂ in drought stress responses, emphasizing the importance of studies in H₂O₂ subcellular localization, needed for a better understanding of its role in plant responses to stress. Although more studies are needed in the study of changes of redox status in plants subjected to stress, the AA pools and its redox status can be indicative of its involvement as a part of cellular mechanisms by which the plant respond to drought-induced oxidative stress. The mechanism of resistance and/or tolerance to drought-oxidative stress is complex, especially when studies are carried out in plants growing in field conditions, where an interaction of stresses occurs. This study sheds light on the mechanisms of plant responses to water-oxidative stress in plants growing in the field.     

藜对干旱胁迫的生理生化反应

干旱是植物最易遭受的胁迫之一,每年由于干旱胁迫给农业造成损失几乎相当于其他所有环境因子胁迫所造成的损失的总和。通过人工控制水分模拟干旱来研究生长期的藜对干旱胁迫的生理生化反应,以期望为干旱农业的高效生产提供理论依据。以盆栽的藜为材料,用控制浇水的方法分对照、轻度胁迫、中度胁迫、重度胁迫4个组,研究了不同程度干旱胁迫对藜叶片的水分状况、渗透调节物质、活性氧代谢以及内生保护系统的影响。结果表明:在干旱胁迫下,藜叶片相对含水量(RW C)、自由水含量(FW C)下降,束缚水含量(BW C)上升;可溶性糖、脯氨酸、K 、C a2 含量增加,表现出藜对适度干旱有一定的适应性。但重度干旱胁迫,O2.产生速率和丙二醛(M DA)含量显著提高,导致膜损伤,质膜透性上升;超氧化物歧化酶(SOD)、过氧化物酶(POD)活性先上升,后下降;抗坏血酸(A SA)含量降低。过分干旱胁迫对藜会造成一定伤害。     

Salicylic Acid Application Mitigates Oxidative Damage and Improves the Growth Performance of Barley under Drought Stress

Drought is a severe environmental constraint, causing a significant reduction in crop productivity across the world. Salicylic acid (SA) is an important plant growth regulator that helps plants cope with the adverse effects induced by various abiotic stresses. The current study investigated the potential effects of SA on drought tolerance efficacy in two barley (Hordeum vulgare) genotypes, namely BARI barley 5 and BARI barley 7. Ten-day-old barley seedlings were exposed to drought stress by maintaining 7.5% soil moisture content in the absence or presence of 0.5, 1.0 and 1.5 mM SA. Drought exposure led to severe damage to both genotypes, as indicated by phenotypic aberrations and reduction of dry biomass. On the other hand, the application of SA to drought-stressed plants protected both barley genotypes from the adverse effects of drought, which was reflected in the improvement of phenotypes and biomass production. SA supplementation improved relative water content and proline levels in drought-stressed barley genotypes, indicating the osmotic adjustment functions of SA under water-deficit conditions. Drought stress induced the accumulation of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and superoxide (O₂ ^•− ), and the lipid peroxidation product malondialdehyde (MDA) in the leaves of barley plants. Exogenous supply of SA reduced oxidative damage by restricting the accumulation of ROS through the stimulation of the activities of key antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX). Among the three-applied concentrations of SA, 0.5 mM SA exhibited better mitigating effects against drought stress considering the phenotypic performance and biochemical data. Furthermore, BARI barley 5 showed better performance under drought stress than BARI barley 7 in the presence of SA application. Collectively, our results suggest that SA played a crucial role in improving water status and antioxidant defense strategy to protect barley plants from the deleterious effects of water deficiency.     

Comparative physiological,ultrastructural and proteomic analyses reveal sexual differences in the responses of Populus cathayana under drought stress

Drought is a major abiotic stress, limiting the survival and growth of young plants. However, little is known about sex‐dependent responses to drought at the proteome level. In this study, we carried out investigations on comparative proteomics, combined with physiological and organelle structure analyses, in males and females of Populus cathayana Rehd. Three‐month‐old poplar cuttings were treated at 30% of field capacity and at 100% field capacity as a control in a greenhouse for 40 days. Drought greatly inhibited plant growth, damaged the photosynthetic system and destructed the structures of chloroplasts, mitochondria and cellular membranes. However, males suffered less from the adverse effects of drought than did females. Using 2‐DE, 563 spots were detected, of which 64 spots displayed significant drought effect and 44 spots displayed a significant sex by drought interaction effect. The results suggest that the different responses to drought stress detected between the sexes have a close relationship to the changes in the expression of sex‐dependent proteins, including, e.g. photosynthesis‐related proteins, homeostasis‐related proteins and stress response proteins. These proteins could contribute to a physiological advantage under drought, giving potential clues for understanding sexual differences in the performance of plants in different environments.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

泛素连接酶E3介导的植物干旱胁迫反应

干旱胁迫严重影响农作物的产量和质量,制约全球的农业生产。泛素连接酶E3是一个种类繁多的大家族,涉及对植物生长发育和逆境胁迫响应等过程中关键步骤的控制。该文概述了植物干旱胁迫的调控机制和植物的泛素连接酶E3,并着重阐述了泛素连接酶E3介导的植物干旱胁迫反应及其作用机制。