Genes controlling physiological responses
Genes involved in ABA signaling pathways are most crucial for conferring tolerance against drought, which is then also followed by various other phytohormones such as salicylic acid, gibberellin, auxin, cytokinin, brassinosteroid and jasmonic acid. A root-derived peptide called “CLE25” passes, from roots to leaves and acts as signal for closing of stomata by regulating accumulation of ABA, and thereby increasing drought tolerance (Takahashi et al., 2018). In A. thalianaexpression of CmMYB2 gene triggered ABA pathways, reducing stomatal aperture and thereby leading to enhanced drought tolerance (Shan et al. 2012). In rice, biosynthesis of ABA requires a gene OsNCED3 , whose expression level is known to be enhanced significantly in seedlings on perception of drought stress (Liu et al 2019).
In Chrysanthemum, various transcription factors, such as CgDREBa, are known to activate superoxide dismutase (SOD), peroxide dismutase (POD) and proline accumulation in response to drought (Chen et al., 2011). A transcription factor, MYB, responsible for governing gibberellic acid responses, is shown to increase concentration of osmolytes in seeds under stress conditions by enhancing the accumulation of proline, sugars and production of late embryogenesis abundant (LEA) proteins, establishing water-potential gradient as a result of which water enters into seeds from the soil (Zhao et al., 2019).
Different experiments have been conducted to study molecular responses in plants with respect to drought stress. PtabZIP1-like gene increased formation of lateral roots and growth of biomass under moisture stress conditions (Dash et al., 2017). In poplar, PagWOX11/12a, a WUSCHEL related homeobox gene, enhanced root elongation in response to water stress (Wang et al., 2020). Orthologs of dro1 (gene in rice controlling angle of root growth) are found in both monocot and dicot species (Kulkarni et al., 2017), for instance, Guesman et al., (2017) reported that orthologs of dro1 are present in Arabidopsisand Prunus and the lines overexpressing dro1 had much deeper roots in stress conditions. Drought tolerant crops have been produced by engineering of genes encoding two enzymes, namely, beta aldehyde dehydrogenase and choline mono oygenase which are responsible for expression of glycine betaine, which plays crucial role in configuring drought tolerance to higher plants (Zhang et al., 2008), for example, drought tolerant maize variety DH4866, has been formed by transferring beta gene from E. coli which leaded to improved production of betaine and hence tolerance to drought (Quan et al., 2004). Crops such as tobacco and soybean has also been engineered using the genes involved in biosynthesis of proline, an important osmolyte (Ronde et al., 2004; Gubis et al., 2007).
Roche et al. (2007) tried to study the expression of various genes that were responsible for signal transduction and metabolism in immature embryos and leaves of Helianthus annus with the help of cDNA array and he identified around 409 DEGs out of which 82 were induced in response to drought and were organ specific. Betaine aldehyde dehydrogenase (BADH) gene is also known to be produced in sunflower with respect to salt and drought stress (Liang et al. 2017).
Leaf transcriptome profiles of O.taihangensis have been studied after treating with 5% and 25% PEG6000. The study revealed that differentially expressed genes (DEGs) responsible for coping with stress were being produced (Gu, H.H et al. 2019). Moreover, genes involved in trehalose biosynthesis such as tps6, tps1, tps10 (belonging to trehalose-6-phosphate synthase family) and in sucrose metabolism such assus3 and sus1 also showed increased expression during drought conditions (Y. Yang et al., 2020). Various genes that are involved in biosynthesis of different amino acids such as phenylalanine, alanine, tryptophan, aspartate, glutamate, proline and arginine were also found to be upregulated under water deficient conditions (Y. Yang et al., 2020). Abirad et al., (2022) reported 288 DEGs belonging to families of NAC, AUX/IAA, WRKY, AP2/ERF, MYB and EXPANSIN that are being produced under drought stress in roots of rice.
Genes involved in transport of nutrients including nia2, nrt1.2, nrt3.2, gln1.3 and glt are also upregulated under water deficit situations (Martinez et al., 2020), for example, Qi et al. (2019) reported upregulation of genes responsible for potassium transport including MdHKT1 , MdHAK3.2 and MdCHX4.11 under water stress.