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Drought stress negatively affects crop performance and weakens global food security. It triggers the activation of downstream pathways, mainly through phytohormones homeostasis and their signaling networks, which further initiate the biosynthesis of secondary metabolites (SMs). Roots sense drought stress, the signal travels to the above-ground tissues to induce systemic phytohormones signaling. The systemic signals further trigger the biosynthesis of SMs and stomatal closure to prevent water loss. SMs primarily scavenge reactive oxygen species (ROS) to protect plants from lipid peroxidation and also perform additional defense-related functions.Moreover, drought-induced volatile SMs can alert the plant tissues to perform drought stress mitigating functions in plants. Other phytohormone-induced stress responses include cell wall and cuticle thickening, root and leaf morphology alteration, and anatomical changes of roots, stems, and leaves, which in turn minimize the oxidative stress, water loss, and other adverse effects of drought. Exogenous applications of phytohormones and genetic engineering of phytohormones signaling and biosynthesis pathways mitigate the drought stress effects. Direct modulation of the SMs biosynthetic pathway genes or indirect via phytohormones' regulation provides drought tolerance. Thus, phytohormones and SMs play key roles in plant development under the drought stress environment in crop plants. | INTRODUCTIONA large segment of the population is going to face food scarcity due to climate change and the gradually decreasing arable land area. Plants are confronted to a variable environment while growing from seedling to mature plant. Different abiotic and biotic stresses affect plant growth and development (Cramer et al., 2011;Fujita et al., 2006;Tuteja & Sopory, 2008). Abiotic stress includes drought, submergence, osmotic stress, salinity stress, oxidative stress, ultra-violet irradiation, wounding, and nutrient depletion. The extremity of optimum factors such as high temperature or low temperature and freezing-thawing is also included in abiotic stresses. Biotic stresses include viruses, bacteria, fungi, algae, worms, nematodes, insects, herbivores, and parasitic plants. Plants have to combat these stressors due to their sessile lifestyle (Cramer et al., 2011;Fujita et al., 2006). More than 50% reduction in average yields of major cereal crops has been reported because of various abiotic stresses. Drought is one of the most important abiotic stresses that negatively influence plant performance and causes harsh effects on biomass and yield (Fahad et al., 2017). A

High salinity is one of the major problems in crop productivity, affecting seed germination as well as yield. In order to enhance tolerance of crops towards salinity, it is essential to understand the underlying physiological and molecular mechanisms. In this endeavor, study of contrasting genotypes of the same species differing in their response towards salinity stress can be very useful. In the present study, we have investigated temporal differences in morphological, physiological and proteome profiles of two contrasting genotypes of rice to understand the basis of salt tolerance. When compared to IR64 rice, Pokkali, the salt-tolerant wild genotype, has enhanced capacity to cope with stress, better growth rate and possesses efficient antioxidant system, as well as better photosynthetic machinery. Our proteome studies revealed a higher and an early abundance of proteins involved in stress tolerance and photosynthesis in Pokkali in comparison with IR64, which, in contrast, showed greater changes in metabolic machinery even during early duration of stress. Our findings suggest important differences in physicochemical and proteome profiles of the two genotypes, which may be the basis of observed stress tolerance in the salt-tolerant Pokkali.

BackgroundTo delineate the adaptive mechanisms operative under salinity stress, it is essential to study plant responses at the very early stages of stress which are very crucial for governing plant survival and adaptation. We believe that it is the initial perception and response phase which sets the foundation for stress adaptation in rice seedlings where plants can be considered to be in a state of osmotic shock and ion buildup.ResultsAn isobaric Tags for Relative and Absolute Quantitation (iTRAQ) approach was used to analyze the pre-existing differences as well as the very early salt shock responsive changes in the proteome of seedlings of contrasting rice genotypes, viz salt-sensitive IR64 and salt-tolerant Pokkali. In response to a quick salt shock, shoots of IR64 exhibited hyperaccumulation of Na+, whereas in Pokkali, these ions accumulated more in roots. Interestingly, we could find 86 proteins to be differentially expressed in shoots of Pokkali seedlings under non-stress conditions whereas under stress, 63 proteins were differentially expressed in Pokkali shoots in comparison to IR64. However, only, 40 proteins under non-stress and eight proteins under stress were differentially expressed in Pokkali roots. A higher abundance of proteins involved in photosynthesis (such as, oxygen evolving enhancer proteins OEE1 & OEE3, PsbP) and stress tolerance (such as, ascorbate peroxidase, superoxide dismutase, peptidyl-prolyl cis-trans isomerases and glyoxalase II), was observed in shoots of Pokkali in comparison to IR64. In response to salinity, selected proteins such as, ribulose bisphosphate carboxylase/oxygenase activase, remained elevated in Pokkali shoots. Glutamate dehydrogenase - an enzyme which serves as an important link between Krebs cycle and metabolism of amino acids was found to be highly induced in Pokkali in response to stress. Similarly, other enzymes such as peroxidases and triose phosphate isomerase (TPI) were also altered in roots in response to stress.ConclusionWe conclude that Pokkali rice seedlings are primed to face stress conditions where the proteins otherwise induced under stress in IR64, are naturally expressed in high abundance. Through specific alterations in its proteome, this proactive stress machinery contributes towards the observed salinity tolerance in this wild rice germplasm.Electronic supplementary materialThe online version of this article (10.1186/s12284-018-0259-5) contains supplementary material, which is available to authorized users.

BackgroundRice productivity is adversely affected by environmental stresses. Transcription factors (TFs), as the regulators of gene expression, are the key players contributing to stress tolerance and crop yield. Histone gene binding protein-1b (OsHBP1b) is a TF localized within the Saltol QTL in rice. Recently, we have reported the characterization of OsHBP1b in relation to salinity and drought tolerance in a model system tobacco. In the present study, we over-express the full-length gene encoding OsHBP1b in the homologous system (rice) to assess its contribution towards multiple stress tolerance and grain yield.ResultsWe provide evidence to show that transgenic rice plants over-expressing OsHBP1b exhibit better survival and favourable osmotic parameters under salinity stress than the wild type counterparts. These transgenic plants restricted reactive oxygen species accumulation by exhibiting high antioxidant enzyme activity (ascorbate peroxidase and superoxide dismutase), under salinity conditions. Additionally, these transgenic plants maintained the chlorophyll concentration, organellar structure, photosynthesis and expression of photosynthesis and stress-related genes even when subjected to salinity stress. Experiments conducted for other abiotic stresses such as drought and high temperature revealed improved tolerance in these transgenic plants with better root and shoot growth, better photosynthetic parameters, and enhanced antioxidant enzyme activity, in comparison with WT. Further, the roots of transgenic lines showed large cortical cells and accumulated a good amount of callose, unlike the WT roots, thus enabling them to penetrate hard soil and prevent the entry of harmful ions in the cell.ConclusionCollectively, our results show that rice HBP1b gene contributes to multiple abiotic stress tolerance through several molecular and physiological pathways and hence, may serve as an important gene for providing multiple stress tolerance and improving crop yield in rice.Electronic supplementary materialThe online version of this article (10.1186/s12284-019-0316-8) contains supplementary material, which is available to authorized users.

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