PHYSIOLOGICAL/BIOCHEMICAL RESPONSES
Plants adapt themselves to dry environments by undergoing different
biochemical and physiological changes in order to maintain cell turgor.
Synthesis of ABA, accumulation of various osmolytes like proline and
activation of ROS scavengers including peroxidase and superoxide
dismutase (SOD) are some of the common physiological response of plants
to tolerate drought stress (Bari and Jones 2009; Krasensky and Jonak
2012; Y. H Yang et al. 2020; J.-K. Zhu 2002). Enzymes like protein
kinases, phosphatases, ubiquitin ligases, and those involved in
metabolism of phospholipids are also reported to actively participate in
signaling pathways that are triggered during drought stress (J.-H. Lee
and Kim 2011; Vierstra 2009; J.-K. Zhu 2002). Root’s ability to retain
water is a pre-requisite for tolerating prolonged water stress, various
studies have suggested that water retaining ability of roots can be
preserved through waterproof barriers in roots and adjustment of
osmolytes. (L. Hu et al. 2018; Kosma et al. 2014; Krasensky and Jonak
2012).
Increasing evidence suggests that plant hormone ABA play important role
in regulating gene expression and various physiological responses during
drought conditions (Shinozaki and Yamaguchi-Shinozaki 2007; Y. Yang et
al. 2020; J.-K. Zhu 2002). Dehydration of leaf is also responsible for
activating ABA responses and its biosynthesis in roots (Manzi et al.
2017), for instance, Yang et al. (2020) reported that when compared to
roots, relative water content (RWC) in leaf reduced significantly,
meanwhile increasing the ABA concentration in roots, thereby it has been
suggested that loss of turgor in leaves might act as important signal
for accumulation of ABA in roots. In maize, ABA is known to suppress
ethylene production and increases growth of shoot and root in
well-watered condition whereas during dry environment, ABA has drastic
effect on root-shoot ratio by promoting root growth while suppressing
shoot growth (Pierik & Testerink, 2014). Root cortex in the elongation
zone is known to specifically express ABA signaling intermediates
namely, MIZU KUSSEI1 (MIZ1) and Snf1-related kinases (SnRK2s) which are
responsible for hydrotropism (Dietrich et al., 2017; Karlova et al.,
2021). When root tip senses heterogenous presence of water, it triggers
MIZ1 to generate Ca2+ signal which is then transported
to elongation zone through phloem and leads to asymmetrical distribution
of Ca2+ according to gradient of water (Tanaka-Takada
et al., 2019). Ethylene production and K+ transport maintains the
balance between Na+/K+ ions and thus regulates expansion of cells,
turgor and development of lateral roots under osmotic and water stress
(Jiang et al., 2013; Osakabe et al., 2013). Accumulation of ABA under
water stress is also known to activate miRNA 165 (miR165) and miR166 to
suppress transcription factors belonging to class III HD-ZIP, which are
responsible for inhibiting xylem formation, thus, their inhibition
results in formation of additional protoxylem under water stress
conditions (Ramachandran et al., 2018). Zhang et al (2022) reported that
during mild drought stress in tomato plants, length of primary roots
enhanced in wild plants but failed to increase in the mutant plants
lacking genes involved in biosynthetic pathway of ABA, hence concluded
that ABA acts as a positive regulator of primary root growth in drought
conditions.
Plant hormone auxin also promotes growth of primary roots, lateral
roots, and adventitious roots (Pierik & Testerink, 2014). Columella
cells are mainly responsible for controlling root angle as amyloplasts
began to sediment with respect to gravity in root tips. Due to this
asymmetric distribution of amyloplasts, flow of auxin is directed
towards lower side of root (Karlova et al., 2021). Low amount of auxin
towards upper side of roots leads to enhanced elongation and bending of
root in downward direction (Ge and Chen, 2019). Hydro patterning is also
dependent on Auxin Response Factor 7 (ARF7), which gets SUMOylated on
perceiving the dry environment and increases its interaction with
repressor indole-3-acetic acid (IAA3) thereby causing its inhibition and
ultimately preventing lateral root formation (Karlova et al., 2021).
Aquaporins (AQPs) which belongs to family of major intrinsic proteins
(MIP), plays important role in maintaining RSA by regulating hydraulic
conductivity thereby facilitating easy passage of water across membranes
(Li et al., 2014). AQPs provide enough water supply to developing
primary root primordia helping them to break through endodermis and
grow. Transport of water through AQPs for the emergence of lateral roots
requires auxin (Péret et al., 2012a). During drought stress, reduced
water potential primarily regulates expression of different AQPs;
tonoplast intrinsic proteins (TIPs) and plasma membrane intrinsic
proteins (PIPs) at various levels (Hachez et al. 2006; (Peng et al.,
2007).
Secondary metabolites such as phenolics also play significant role under
drought conditions (Hessini et al., 2022). When the sugar transport to
other plant parts is reduced, it results in accumulation of excess
carbohydrates which is then converted to phenolics to maintain source
and sink balance. With increasing concentration of phenolics,
antioxidant activity also increases significantly (Rehman et al., 2022).
Phenols also prevent loss of water from the cell by forming covalent
bond with carbohydrates of cell wall (Hura et al., 2012). Osmolytes like
proline helps to maintain water potential and turgor pressure in cells,
thus preventing loss of water from cells during stress conditions
(Krasensky and Jonak 2012; Y. Yang et al. 2020). It rapidly accumulates
in various plants, such as wheat and watermelon in order to survive
drought stress (L. Hu et al. 2018; Li et al. 2019).
Drought also has drastic effect on mineral uptake (Gunes et al., 2011;
Samarah et al., 2004) and significantly decrease fixation of nitrogen in
legumes such as pea (Gonzalez et al., 2001), and soybean (Serraj, 2003).
These effects collectively reduce production of assimilates and their
transport to maturing seeds in crops (Zare et al., 2012). Some
micronutrients such as zinc plays important role in resistance to water
deficit conditions (Khan et al., 2003; Yavas & Unay, 2016). Nitrogen is
known to increase water absorption ability of roots and helps to
maintain optimum level of leaf water content in water scarce
environments (Tran et al., 2014; Kumari et al., 2022). In the presence
of low moisture conditions, phosphorous (P) maintains RSA and increases
proliferation of roots in soil which enhances hydraulic conductivity in
roots thereby increased uptake of minerals and nutrients (Sun et al.,
2016; Tariq et al., 2017). It was reported that when phosphorous (P) was
applied to wheat during the initial stages it resulted in increased
production of fertile tillers, grains per spike and spikelet because of
enhanced photosynthetic rate, cell elongation and division and hence
also showed 28% increase in yield (Ahmed et al., 2018; Kumari et al.,
2022). Potassium is also known to increase the accumulation of proline
during drought stress and helps plants to cope with the situation (Yadav
et al., 2019). Under abiotic stresses cytosolic Ca2+ level increases,
activating calcium binding proteins such as Ca-dependent protein kinases
(CDPK) and triggers various stress-responsive genes regulating different
responses such as stomatal movement and increased potassium uptake (Choi
et al., 2005; Yu et al., 2007). Increased concentration of magnesium
(Mg) in leaves maintains water balance in leaves under drought stress,
for instance, in Musa acuminata , plants affected with drought
showed increased accumulation of Mg (about 28%) than control plants
(Kumari et al., 2022; Mahouachi, 2009). Boron is highly beneficial for
plants during drought stress as it is involved in ROS detoxification
process and therefore protects plants against oxidative damage to
membranes and thus plants with increased amount of boron show more
resistance to drought stress and enhanced nutrient uptake (Venugopalan
et al., 2021). As drought stress is known to decrease the plumule length
and ultimately leading to restricted transport of nutrients to embryo,
it has been reported that, application of Zinc enhanced the synthesis of
hormones such as gibberellic acid and auxin under stress conditions and
hence elevate plumule characteristics under water stress, for instance,
Harris et al (2005) reported that seed priming with Zinc resulted in
improved yield and germination in chickpea, maize and wheat. Zinc also
leads to accumulation of various osmolytes, expands leaf area, improves
production of photosynthetic pigments and increases leaf water content,
resulting in better yield and growth (Kumari et al., 2022).