Understanding drought tolerance in plants

Drought limits plant growth and productivity, and the severity and duration of drought determine its damaging effects. Almost all phenological stages are prone to drought, especially germination and flowering. Global climate change is exacerbating the effects of drought. Understanding the mechanisms underpinning plant responses to drought is challenging due to variations in the traits controlling the changing water availability in soil and the demand for water at evaporation sites, responses to water status, and genetic differences among species. Drought affects water relations, nutrient acquisition and assimilation, photosynthesis, enzyme functioning and assimilates partitioning (Xiong et al. 2020; Ayyaz et al., 2021; Bano et al., 2021). The visible morphological effects of drought contributing to altered water relations and water use efficiency include reduced leaf number, leaf size, stem extension and root growth (Xiong et al., 2020). Drought is often characterised by reductions in cell water content, turgor and tissue water potential, resulting in wilting, stomatal closure and reduced cell enlargement. These effects lead to photosynthetic arrest, triggering drastic changes in normal metabolism that can lead to growth cessation or death. Reductions in the hydraulic conductivity of shoots and roots under drought result from interruptions in the xylem water column (embolism) or modifications in xylem vessels. Leaf hydraulic conductivity is regulated by aquaporin water transport proteins (Patel and Mishra 2021). Drought tolerance involves metabolic and hydraulic readjustment at the cellular and whole plant level, combined with finely regulated signalling events. Several key metabolic and physiological adaptations in response to drought tolerance in xero-halophytic plants have been identified (Panda et al. 2021). Physiological, biochemical and molecular responses at the cellular or whole plant level are triggered by drought to counteract its damaging effects and maintain plant performance. Drought tolerance is a complex process that is regulated by genes. Drought upregulates genes, such as those involved in osmolyte metabolism, secondary metabolite synthesis and hormone synthesis, which facilitate the rapid induction of response mechanisms and crosstalk. Reducing water loss by increasing diffusive resistance, developing a prolific and deep root system for enhanced water uptake, and developing small and succulent leaves for reduced transpirational water loss are important strategies for drought tolerance (Osakabe et al., 2014; van der Vyver and Peters 2017). Mineral ions like potassium assist in osmotic adjustment, thereby contributing to the maintenance of tissue water content, while silicon improves water retention capacity through silicification of endodermal cells. Low molecular weight osmolytes are crucial for sustaining drought-induced injury and protecting metabolism. Among growth hormones, auxins, gibberellic acid, ABA, salicylic acid, jasmonic acid and ethylene contribute to modulating the drought response. Polyamines and secondary metabolites exhibit antioxidant functioning for reducing the adverse effects of drought. Phytohormones, such as Abscisic acid (ABA), act as key regulators under drought conditions, controlling pathways that are either ABA-dependent or ABA-independent. ABA mediated increased Water use efficiency (WUE) and photosynthetic regulation has been reported by Gouveia et al. (2020). The damaging effects of drought largely result from the weak functioning of ROS scavenging mechanisms due to excessive damage to macromolecules, such as proteins, lipids and nucleic acids, which hampers the functioning of key organelles, such as mitochondria and chloroplasts. ROS-induced peroxidation of polyunsaturated fatty acids causes ion leakage, leaving cells deficient in key mineral ions for metabolism. Loss of cellular ion homeostasis directly affects metabolic pathways, disturbing the structural and functional profile of cells and the whole plant. The antioxidant system, compatible osmolytes and secondary metabolites help to minimise the adverse effects of drought. Molecular genetic studies have shown that modulating the above-mentioned tolerance machinery improves plant growth and functioning under drought and other stresses. Among antioxidant components, non-enzymatic components, such as ascorbate, glutathione and tocopherol, have an essential role in maintaining redox homeostasis and protecting the normal functioning of processes such as photosynthetic electron transport. Modern research techniques can further strengthen our understanding of these tolerance mechanisms. The gene sets involved in local and systematic drought sensing need to be identified in model crops (e.g. Arabidopsis) before extending to other crops. The advancement in genome sequencing, assembly and annotation has improved our understanding of molecular genomics related to drought responses and tolerance mechanisms in crop plants. Genome-wide association studies (GWAS), used for studying natural genetic variations, can identify genes and alleles involved in drought tolerance and metabolic regulation, which will pave the way for mapping and cloning gene loci regulating drought tolerance through controlled hormonal regulation, seed development and maintenance of water relations. In addition to current management practices, such as mineral fertiliser application and the use of compost (Hafez et al., 2020), the exogenous application of biostimulants (Nadeem et al. 2020) and phytohormones (Khan et al., 2020) has been tested for their efficacy in improving water use efficiency and the mitigation of drought-induced oxidative effects. Molecular genetic approaches and physio-biochemical evaluations have revealed several key avenues for improving drought tolerance in plants, especially food crops as discussed by several researchers (Jabeen et al., 2020; Razik et al., 2020; Aleem et al., 2020). Modern genetic and functional omics approaches, such as transcriptomics, metabolomics and proteomics, have identified and characterised several drought-responsive genes in plants, including those coding for osmolyte synthesis, water channels, ion transporters, detoxification, late embryogenesis abundant proteins and proteolysis. Signal transduction and gene expression during drought stress are controlled by transcription factors (TFs), phosphatases and protein kinases, including mitogen-activated, Ca-dependent and transcription regulation protein kinases. TFs involved in drought response include AREB, NAC, AP2/ERF, MYB, bZIP and MYC. Some of the transcription factor families like AP2-EFR, Dof-type, MADS-box, bZIP, CPP, ZF-HD and GATA-type have been identified in drought-stressed cowpea (Ferreira-Netoet al., 2020). Transcriptomic study discussing the identification of TFs and the pathways involved in drought stress tolerance in pepper has been reported by Negi et al. (2020). Of note are the dehydration-responsive element binding (DREB) and CRT element binding factors in the AP2/ERF family, which have been well-studied for their role in protein structure stabilisation, DNA binding and post-translational modification through transgenic studies. Transcriptomic studies carried by Aleem et al. (2020) have demonstrated a significant difference in gene expression patterns between sensitive and tolerant crop cultivars. Other genes induced during desiccation include those encoding osmo-protectants, such as proline, glycine betaine, sugars and ABA biosynthetic pathway, signalling proteins, antioxidant components and TFs. TFs regulate several downstream stress-responsive genes by binding cis-regulatory elements in the promoter region of genes. However, further research is needed to understand the physiology, biochemistry and genetics of drought stress tolerance in plants. This special issue covers current advances in drought stress research with experimental evidence confirming the key roles of novel signalling molecules, such as hydrogen sulphide, melatonin and jasmonic acid. Transgenic approaches using microRNAs and other candidate genes for understating and improving the plant performance under drought stress conditions have also been discussed. We hope this special issue provides an excellent set of papers for a greater understanding of drought stress tolerance with novel ideas for future research and development on drought stress biology.