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).