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.