Drought and salinity are among the most important environmental factors that hampered agricultural productivity worldwide. Both stresses can induce several morphological, physiological, biochemical, and metabolic alterations through various mechanisms, eventually influencing plant growth, development, and productivity. The responses of plants to these stress conditions are highly complex and depend on other factors, such as the species and genotype, plant age and size, the rate of progression as well as the intensity and duration of the stresses. These factors have a strong effect on plant response and define whether mitigation processes related to acclimation will occur or not. In this review, we summarize how drought and salinity extensively affect plant growth in agriculture ecosystems. In particular, we focus on the morphological, physiological, biochemical, and metabolic responses of plants to these stresses. Moreover, we discuss mechanisms underlying plant-microbe interactions that confer abiotic stress tolerance.

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Drought and Salinity Stress Responses and Microbe-Induced Tolerance in Plants.

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant–insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant–microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses.

Global climate change is identified as a major threat to survival of natural ecosystems. Climate change is a dynamic, multifaceted system of alterations in environmental conditions that affect abiotic and biotic components of the world. It results in alteration in environmental conditions such as heat waves, intensity of rainfall, CO2 concentration and temperature that lead to rise in new pests, weeds and pathogens. Climate change is one of the major constraints limiting plant growth and development worldwide. It impairs growth, disturbs photosynthesis, and reduces physiological responses in plants. The variations in global climate have gained the attention of researchers worldwide, as these changes negatively affect the agriculture by reducing crop productivity and food security. With this background, this review focuses on the effects of elevated atmospheric CO2 concentration, temperature, drought and salinity on the morphology, physiology and biochemistry of plants. Furthermore, this paper outlines an overview on the reactive oxygen species (ROS) production and their impact on the biochemical and molecular status of plants with increased climatic variations. Also additionally, different tolerance strategies adopted by plants to combat environmental adversities have been discussed.

Climate change regulated abiotic stress mechanisms in plants: a comprehensive review

Increasing population and subsequent food requirement puts a lot of onus on agriculture worldwide. However, agricultural output is constantly influenced by several factors such as small land holdings, lack of proper mechanization, and occurrence of several abiotic and biotic stresses. Drought stress is one such abiotic stress which causes major setbacks to agricultural productivity every year. Water scarcity due to decline in rainfall and increased frequency of dry spells give rise to drought conditions. Often drought is accompanied by other detrimental effects like salinity, heat and attack of pathogens. In response plants undergo several physiological and morphological modifications like reduced transpiration and photosynthesis rate, osmotic adjustments, repressed root and shoot growth, overproduction of reactive oxygen species (ROS), modified stress signalling pathways, and senescence. These modifications can cause permanent injury to the plant and hence there is a need to develop mitigation strategies. Employment of nanoparticles, film farming, use of drought resistant plant varieties, super-absorbent hydrogels, and biochar are some techniques used to ameliorate drought stress. Though, most of these practices are labour-intensive and expensive with limited benefits. Application of plant-growth-promoting rhizobacteria (PGPR), however, is proving to be a preferential strategy that encompasses many direct and indirect benefits. PGPRs impart induced systemic tolerance (IST) to plants towards drought stress through a variety of mechanisms like improvement of antioxidant system, production of ACC-deaminase and phytohormones, nitrogen fixation, phosphate solubilization, siderophore and exopolysaccharides production, enhanced root and shoot system, amplified photosynthesis rates and carotenoid production. The commercial applicability of PGPRs as biostimulants will still be dependent on the selection and performance of robust strains in various environmental conditions.

A review on drought stress in plants: Implications, mitigation and the role of plant growth promoting rhizobacteria

Abiotic stresses, including drought, detrimentally affect the growth and productivity of many economically important crop plants, leading to significant yield losses, which can result in food shortages and threaten the sustainability of agriculture. Balancing plant growth and stress responses is one of the most important functions of agricultural application to optimize plant production. In this study, we initially report that copper nanoparticle priming positively regulates drought stress responses in maize. The copper nanoparticle priming plants displayed enhanced drought tolerance indicated by their higher leaf water content and plant biomass under drought as compared with water-treated plants. Moreover, our data showed that the treatment of copper nanoparticle on plants increased anthocyanin, chlorophyll and carotenoid contents compared to water-treated plants under drought stress conditions. Additionally, histochemical analyses with nitro blue tetrazolium and 3,3′-diaminobenzidine revealed that reactive oxygen species accumulation of priming plants was decreased as a result of enhancement of reactive oxygen species scavenging enzyme activities under drought. Furthermore, our comparative yield analysis data indicated applying copper nanoparticles to the plant increased total seed number and grain yield under drought stress conditions. Our data suggest that copper nanoparticle regulates plant protective mechanisms associated with drought tolerance, which is a promising approach for the production of drought-tolerant crop plants.

Copper Nanoparticle Application Enhances Plant Growth and Grain Yield in Maize Under Drought Stress Conditions

In our study, the effects of water stress on photosynthesis and photosynthetic electron transport chain (PETC) were studied in several ways, including monitoring the change of gas exchange parameters, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, reactive oxygen species (ROS), antioxidant enzyme activities and D1 protein levels in apple leaves. Our results show that when leaf water potential ( ψ w ) is above –1.5 MPa, the stomatal limitation should be the main reason for a drop of photosynthesis. In this period, photosynthetic rate ( P N ), stomatal conductance ( G s ), transpiration rate ( E ) and intercellular CO 2 concentration ( C i ) all showed a strong positive correlation with ψ w . Modulated chlorophyll fluorescence parameters related to photosynthetic biochemistry activity including maximum photochemical efficiency (F v /F m ), actual photochemical efficiency of PSII (Φ PSII ), photochemical quenching coefficient ( q P ) and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae ( q L ) also showed a strong positive correlation as ψ w gradually decreased. On the other hand, in this period, Stern-Volmer type non-photochemical quenching coefficient (NPQ) and quantum yield of light-induced non-photochemical fluorescence quenching [ Y (NPQ) ] kept going up, which shows an attempt to dissipate excess energy to avoid damage to plants. When ψ w was below –1.5 MPa, P N continued to decrease linearly, while C i increased and a ‘V’ model presents the correlation between C i and ψ w by polynomial regression. This implies that, in this period, the drop in photosynthesis activity might be caused by non-stomatal limitation. F v /F m , Φ PSII , q P and q L in apple leaves treated with water stress were much lower than in control, while NPQ and Y (NPQ) started to go down. This demonstrates that excess energy might exceed the tolerance ability of apple leaves. Consistent with changes of these parameters, excess energy led to an increase in the production of ROS including H 2 O 2 and O 2 • − . Although the activities of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) increased dramatically and ascorbate peroxidase (APX) decreased in apple leaves with drought stress, it was still not sufficient to scavenge ROS. Consequently, the accumulation of ROS triggered a reduction of net D1 protein content, a core protein in the PSII reaction center. As D1 is responsible for the photosynthetic electron transport from plastoquinone A (Q A ) to plastoquinone B (Q B ), the capacity of PETC between Q A and Q B was considerably downregulated. The decline of photosynthesis and activity of PETC may result in the shortage of adenosine triphosphate (ATP) and limitation the regeneration of RuBP ( J max ), a key enzyme in CO 2 assimilation. These are all non-stomatal factors and together contributed to decreased CO 2 assimilation under severe water stress.

Guofang Li

Papers

Effects of drought stress on photosynthesis and photosynthetic electron transport chain in young apple tree leaves.