**4. Molecular symphony of plant adaptation: innate shield to aridity**

The intrinsic ability of plant system to respond against drought stress involves a complex cascade of highly regulated genes and signal transduction pathways. Under drought prone conditions, competent stimuli are perceived and captured by uncharacterized membrane sensors, and the signals are then passed down through multiple signal transduction pathways, resulting in the expression of drought-responsive genes and drought adaptation. Secondary messengers (such as Ca2+, ROS, phosphoglycerol, ABA, and diacylglycerol) and transcriptional regulators all play important roles in signalling pathways.

Drought stress increase ABA accumulation in plants, and exogenous ABA application, such as gene induction, can have similar effects to osmotic stress. According to Mittler and Blumwald (2015), drought causes the production of ABA in roots, which is then transferred to the shoots and causes stomatal closure, ultimately limiting development [64]. Additionally, ABA is produced in leaf cells and distributed throughout the plant. According to recent data, xylem/apoplastic pH affects ABA compartmentation, which in turn affects the quantity of ABA that reaches stomata. As a result of less ABA being removed from the xylem and leaf apoplast to the symplast in drought-stressed plants (a process known as alkaline trapping of ABA), more ABA reaches the guard cells, allowing for the modulation of stomatal aperture in response to various environmental factors.

Transcription factors are early genes that are activated within minutes of being stressed. Some of the gene families including RD29A contains both ABRE and DRE/ CRT elements [65]. The RD29A gene has served as a model for both ABA-dependent and ABA-independent gene regulation. Although ABA does not activate the DRE element, it is required for the DRE to be fully activated by osmotic stress.

Both cis-acting and trans-acting regulatory elements involved in droughtinduced ABA-independent/ ABA-responsive gene expression have been thoroughly

#### **Figure 2.**

*Two intrinsic immune strategies in plant system for survival against aridity. ABA: Abscisic acid; ABRE: ABAresponsive element; DRE/CRT: Dehydration-responsive element/C-repeat; bZIP: Basic-leucine zipper.*

investigated at the molecular level [66]. Several drought-inducible genes, on the other hand, do not respond to ABA treatment, implying the existence of an ABAindependent pathway during the dehydration stress response. Exogenous ABA treatment increases the expression of many osmotic stress inducible genes. ABRE is a key cis-acting element in ABA responsive gene expression. Two ABRE motifs are critical cis-acting elements that regulate RD29B-mediated ABA responsive expression [67]. ABA up-regulates three members of the AREB/ABF subfamily, AREB1, AREB2, and ABF3, and their full activation requires ABA. The triple mutant areb1 areb2 abf3 exhibits increased ABA resistance and decreased drought tolerance, indicating that the three factors co-ordinately govern ABRE-dependent gene expression under water stress conditions. Involvement of ABA in drought responsive system has been well depicted in the **Figure 2**.

Some ROS genes have been used to create drought-tolerant plants. The formation of ROS, also referred to as the "oxidative burst," is a primary defence response of plants to water stress and serves as a secondary messenger to start additional defence responses in plants [68]. Overexpression of a pea manganese superoxide dismutase (*MnSOD*) gene in rice chloroplasts under the control of an oxidative stress-inducible promoter SWPA2 improved transgenic rice drought tolerance. Cytosolic APX1 has been shown to play an important role in the response to a combination of drought and heat stress.

Multiple mechanisms increase ROS generation when there is a drought stress. The Mehler process leaks more electrons to O2 when photosynthesis is occurring. One of the main risks to the chloroplast during a drought is the Fenton reaction's creation of the hydroxyl radical in the thylakoids. Since it has the strongest oxidising potential

and the shortest half-life, the hydroxyl radical is the ROS that reacts with the majority of biological molecules.

LEA proteins are expressed at specific stages of late embryonic development and play critical roles in desiccation tolerance by capturing water, stabilising and protecting protein and membrane structure and function, and acting as molecular chaperons and hydrophilic solutes to protect cells from water stress damage [69].

Many transcription factor families, including APETALA2/Ethylene-responsive element binding protein (AP2/EREBP), basic leucine zipper (bZIP), MYB, NAM-ATAF1/2-CUC2 (NAC), and zinc finger, have been implicated in drought responses (as shown in **Figure 3**). Zinc finger proteins (bZIPs), a big family with 75 members identified in the *Arabidopsis* genome, are among the transcription factors dependent on ABA. Two basic leucine zipper (bZIP) transcription factors that are ABAresponsive element-binding proteins/factors (AREBs/ABFs) best known for their roles in ABRE-dependent ABA signalling during drought stress. The ABA-responsive

#### **Figure 3.**

*Schematic diagram showing genetic cross-talk as an important part of drought responsive system in plants.*

*Molecular Basis of Plant Adaptation against Aridity DOI: http://dx.doi.org/10.5772/intechopen.110593*

elements-binding (AREB) proteins react to drought at the transcriptional and post-transcriptional level, enhancing tolerance to drought stress. AREB/ABF, bind to ABRE and activate ABA-dependent gene expression [67]. The AREB/ABF proteins require an ABA-mediated signal to be activated, as evidenced by their decreased activity in *Arabidopsis* ABA deficient aba2 and ABA insensitive eabi1 mutants and increased activity in *Arabidopsis* ABA hypersensitive era1 mutant [66]. Several rice bZIP proteins, including OsbZIP23 and the constitutive active form of OsbZIP46, have also been identified as having a high potential for improving rice drought resistance [70, 71].

In transgenic petunia, constitutive over-expression of a Cys2/His2 (C2H2)-type zinc finger protein encoding the ZPT2-3 gene improved tolerance to dehydration stress. DST, another zinc finer protein, has been shown to act as a negative regulator of drought and salt tolerance in rice by controlling the genes involved in H2O2 mediated stomatal movement. *ATGPX3*, a gene encoding an *Arabidopsis thaliana* glutathione peroxidase, was discovered to function as a scavenger and an oxidative signal transducer in ABA and drought stress signalling, as well as a key player in H2O2 homeostasis [72]. Several other genes, including *OsSKIPa* and *OsSRO1c*, have been shown to modulate drought resistance in plants by controlling ROS metabolism and regulating ROS homeostasis.

Numerous factors including heat-shock proteins, and other key enzymes involved in protein folding make up the most prevalent functional group of proteins responding to drought. Additionally, in order to generate drought-tolerant crop plants, aquaporin proteins could be used as possible targets. In *Arabidopsis*, constitutive over-expression of the aquaporin gene *GoPIP1* enhanced the rosette/root ratio while lowering drought resistance due to stunted development. It was discovered that the expression of stress-responsive genes, particularly genes of a large set of antioxidant enzymes that directly affect water stress-related traits in rice, was regulated by the plant-specific protein OsGRAS23.

Numerous candidate genes identified through mutant screening or expression profiling studies have been studied further for their roles in drought response. Regulatory proteins have been shown to play critical roles in plant responses to drought stress. Protein phosphorylation and dephosphorylation are common events in plants caused by drought stress. Several kinases have been implicated in drought response, including calcium dependent protein kinases (CDPKs), CBL (calcineurin B-like) interacting protein kinase (CIPK), mitogen-activated protein kinases (MAPKs), and sucrose nonfermenting protein (SNF1)-related kinase 2 (SnRK2). In response to drought stress, the Arabidopsis CDPK gene CPK10 was found to mediate stomatal movement via the ABA and Ca2+ signalling pathways [73].

### **5. Conclusion**

The frequency and severity of agricultural aridity are predicted to increase in the near future due to a warming environment. Under intermittent drought situations, it will be crucial to provide sustainable agricultural production so that plants can retain physiological activities at low plant water status and swiftly recover once the stress is eliminated. In this scenario, selection of individuals with better water use efficiency, stronger antioxidant defences, and ability to produce important osmolytes as well as secondary metabolites seems potential approach to minimise yield loss under limited water conditions. Currently, the use of genetically modified agricultural plants to

introduce and/or overexpress candidate genes appears to be a promising alternative for accelerating the breeding of improved adaptable and high-yielding crop genotypes. The introduction of genomic technology and gene mapping methods like as genome-wide association studies (GWAS) and precision genome editing with the CRISPR/Cas9 system has aided in the development of alleles that can increase plant yield and performance under a variety of conditions. The sincere research of drought response networks that may be targeted by diverse strategies has currently been possible by molecular studies that combine tissue- or cell-specific promoters with live imaging methods for real-time monitoring of cellular processes. In addition, transand multidisciplinary research is urgently required to develop pertinent answers for all the environmental issues affecting agricultural yields and guaranteeing food security. Together, research projects targeted at revealing the physiology of plant responses to water scarcity in model systems and employing innovative discoveries to agriculture are believed to find out some effective avenue to deal with aridity.
