3.1 Basic leucine zipper

The basic leucine zipper (bZIP) TF family is a very large class of TF widely distributed in eukaryotes. The characteristic feature of this TF is the presence of a highly conserved bZIP domain, which is 60–80 amino acids long and contains two functional regions: a conserved basic region at N-terminus and a variable leucine zipper region towards the C-terminus (Glover & Harrison, 1995). Genome-wide studies in many plant species have identified numerous bZIP family genes, including 75 in Arabidopsis (Jakoby et al., 2002), 89 in rice (Nijhawan et al., 2008), 125 in maize (Zou et al., 2012), 92 in sorghum (Wang et al., 2011), 89 in barley (Pourabed et al., 2015), 96 in Brachypodium distachyon (Liu & Chu, 2015), 69 in tomato (Li, Fu, et al., 2015a), 160 in soybean (Zhang et al., 2018), 55 in grapevine and 247 in Brassica napus (Zhou et al., 2017).

Extensive studies have revealed the diversified role of the bZIP TF family genes. They play crucial roles in plant developmental processes, like initiation of floral organs, embryogenesis, and seed maturation (Abe et al., 2005; Alonso et al., 2009; Jakoby et al., 2002). However, in plants, they are well known for responding to abiotic stresses, like drought (Xiang et al., 2008), high salinity (Hartmann et al., 2015), and extreme temperature (Liao, Zou, Wang, et al., 2008b) conditions. For instance, transgenic rice plants overexpressing OsbZIP66 conferred drought tolerance (Yoon et al., 2017). Drought, high salinity, and extreme temperature stresses culminate into osmotic stress in the plants. The plant adaptation response to osmotic stress is a complex phenomenon and the precise molecular mechanism behind the water-deficit stress response remains unclear. Nonetheless, the plant hormone abscisic acid (ABA) plays an indispensable role during drought stress by participating in the signal transduction pathway that induces expression of the drought stress-responsive genes (Lee & Luan, 2012). The bZIP genes mainly belonging to group A have been shown to enhance plants' tolerance to abiotic stresses in an ABA-dependent manner. Recent works revealed the roles of group A of bZIP TFs, like AREB2, ABF4, ABF3 ABI5, ABF1, ABF2/AREB1, ABF3, and ABF4 of Arabidopsis thaliana, in positively regulating ABA-dependent gene expression (Choi et al., 2000; Saxena et al., 2013). Similarly, in rice, the bZIP TFs like OsbZIP23, OsbZIP46, TRAB1, OsbZIP2, OsbZIP42, OsbZIP46, OsABF1, and OsABI5 also play an important role in ABA signal transduction and osmotic stress responses (Joo, Lee, & Song, 2019; Yoon et al., 2017; Zou et al., 2008). Furthermore, transgenic Arabidopsis plants overexpressing wheat TabZIP60 or TaZIP14-B were found to impart drought, salt, and freezing stress tolerance in an ABA-dependent manner (Zhang et al., 2015, 2017). Transgenic Arabidopsis and cotton overexpressing cotton GhABF2 have enhanced tolerance to drought and salt stress, which was achieved due to the induction of ABA-, drought- and salt-responsive genes (Liang et al., 2016). Overexpression of sweet potato IbABF4 containing a typical bZIP domain led to improved tolerance to multiple abiotic stresses through the ABA signaling pathway (Ding et al., 2019). Interestingly, the bZIP TFs have also been found to negatively regulate the drought stress response by modulating ABA signaling. For instance, the expression of tomato SlbZIP38 gene was downregulated under salt stress, drought stress, and ABA application and hence, the transgenic tomato plants became drought and salt stress susceptible when SlbZIP38 was overexpressed (Pan et al., 2017). Similarly, the subgroup D bZIP transcription factor CaATBZ1 in pepper negatively modulated ABA signaling and drought stress response and therefore, the CaATBZ1-silenced pepper plants became drought tolerant (Joo, Lim, & Lee, 2019).

3.2 Dehydration responsive element binding (DREB)

The dehydration responsive element (DRE) is a part of the promoter sequence in many stress-responsive genes. The proteins that bind to this DRE were termed as DREB (Yamaguchi-Shinozaki & Shinozaki, 1994). Many DREBs have been identified in various plant species and take part in abiotic stress tolerance in both ABA-independent and ABA-dependent ways. DREB genes comprise of highly conserved AP2/ERF (of 50 to 60 amino acids length) DNA-binding domain (Lata & Prasad, 2011). Though DREB is usually found to recognize the DRE, some of its members recognize the AGCCGCC cis-acting element commonly known as the GCC box (Sakuma et al., 2002). The DREB family TFs have been recognized in various plant species, like Arabidopsis (Sharoni et al., 2010), rice (Nakano et al., 2006), mangrove (Peng et al., 2013), soybean (Marcolino-Gomes et al., 2013) and potato (Bouaziz et al., 2013).

Drought, salt and cold stresses ultimately culminate into a state of dehydration in plant cells resulting in the up-regulation of many TFs, including DREB proteins. Arabidopsis and rice have 57 and 52 DREB TFs, respectively, and have been found to respond to a wide range of abiotic stresses that finally lead to dehydration stress in the plants (Nakano et al., 2006). For instance, Arabidopsis DREB2A has a dual role in combating both water and heat-shock stress responses (Sakuma et al., 2006). When OsDREB1A from rice was overexpressed in Arabidopsis, various stress-inducible genes were upregulated, resulting in the plant's improved tolerance to drought, salt, and freezing stresses. Different DREB proteins are induced by different abiotic stress treatments. For example, AtDREB1 was expressed within ten minutes of cold treatment and provided cold stress tolerance (Liu et al., 1998). But, AtCBF4, a gene encoding DREB TF, was rapidly induced during ABA treatment and drought stress, while cold stress did not induce the gene (Haake et al., 2002). OsDREB1A and OsDREB1B were induced by low temperature treatment but not by ABA treatment. OsDREB1C was constitutively-expressed throughout the stress exposure, whereas OsDREB1D expression was not induced by any stress treatment (Agarwal et al., 2007). OsDREBL accumulated quickly after exposure to low temperature, but it did not respond to ABA, dehydration and NaCl treatments (Chen et al., 2003). WCBF2 gene from wheat was up-regulated by both cold and drought treatments, but ABA treatment had no such effect (Kume et al., 2005). In groundnut, PNDREB1 gene was induced by low temperature and dehydration treatments (Zhang et al., 2009). When DREB TFs are overexpressed in transgenic plants, certain stress-responsive genes are also induced, resulting in overall fitness advantages of the transgenic plants during various environmental stress conditions. For example, Arabidopsis plants over-expressing AtDREB1A revealed that the expression of 12 stress-responsive genes was much higher than in the wild-type plants (Liu et al., 1998). Similarly, transgenic Arabidopsis plants over-expressing the rice OsDREB1A witnessed the increased expression of six stress-related genes (Kasuga et al., 1999). Therefore, DREB TFs are the principal regulators of abiotic stress tolerance in plants. Hence, plants' abiotic stress tolerance level can be significantly enhanced by overexpressing DREB TFs.

3.3 DNA binding with one finger (DOF)

DNA binding with one finger (DOF) transcription factor is a plant-specific transcription factor family widely known to regulate numerous biological processes including vascular tissue formation, abiotic stress, cell cycle, photoperiod regulation, floral organ abscission, redox homeostasis and secondary metabolite production (Diaz et al., 2002; Mena et al., 2002; Skirycz et al., 2008; Yanagisawa, 2002). DOF protein comprises a highly conserved N-terminal DNA binding domain. This DOF domain is a Cys2Cys2 (C2C2) type zinc finger motif of 50–53 amino acid and remained conserved throughout the process of evolution (Yanagisawa, 2002). Besides the highly conserved DOF domain, the transcriptional regulatory C-terminal of this protein is composed of a highly variable amino acid sequence. DOF-TFs specifically bind to a core sequence of (AT)/AAAG at the cis-regulatory element present in their respective target gene (Plesch et al., 2001). Like some other zinc finger, DOF-TF also exhibits a bifunctional role by having both DNA binding and protein binding capability. Several DOF proteins are known to interact with DOF-TF or other biologically important proteins. The intrinsic property of DOF-TF to homo- and heterodimerize and its interaction with other transcription factors (like bZIP, Myb-TF, and TCP-TF) reflect its complex regulatory network (Gupta et al., 2015; Shigyo et al., 2007).

In Arabidopsis, the overexpression and down-regulation (antisense) of AtDOF4.2 depicted a role in the phenylpropanoid pathway by limiting the flavonoid synthesis (Zou et al., 2012). The expression level of several DOF genes is unambiguously upregulated after abiotic stress, including drought stress (Wang et al., 2017). An Arabidopsis DOF-TF (CDF3) is highly upregulated during drought stress and its overexpression is associated with the modulation of some other transcription factors such as CBFs, DREB2A and ZAT12 (Corrales et al., 2017). The T-DNA insertion mutant cdf3 is hypersensitive to the drought stress. The metabolite profiling of overexpression lines also revealed the accumulation of protective molecules like proline, glutamine, sucrose etc. (Corrales et al., 2017). Further, the protein–protein interaction of DOF-TF with MYB, bZIP, or TCP is considered to be important for drought stress response. The comprehensive analysis of the molecular mechanisms underlying the interaction of DOF with drought-responsive TF will be insightful to identify the key player under drought stress. Further, it could provide an immense opportunity to investigate their downstream molecular target and their physiological and molecular response during drought stress.

3.4 Heat shock factor (HSF)

Heat Shock Factors (HSFs) are important transcription factors that are mostly induced by heat stress along with other abiotic stresses like drought, salt and cold. Though HSFs have been identified in a wide range of species, they are widely distributed in plant species. The first plant HSF was identified from tomato and later in many other plant species at genome wide scale. For example, 21 and 24 HSF genes have been identified in Arabidopsis and tomato, respectively (Scharf et al., 2012). HSF is a modular structure; it includes a DNA binding domain (DBD), which is the most conserved domain characterized by a central helix-turn-helix motif. Another important domain is an oligomerization domain (OD) that connects the DBD through a flexible linker of 15–80 amino acid residues. In addition, a nuclear localization signal (NLS), nuclear export signal (NES), activator motifs, repressor domain and functional and signature sequence are also included in the structure of HSF. Based on the flexible linker length and type of amino acid residues between DBD and OD, HSFs have been classified into three categories namely, HSFA, HSFB and HSFC (Kotak et al., 2004). HSFs function by binding to the heat stress promoter element containing a palindromic binding motif (5′-AGAANNTTCT-3′) present upstream to the TATA box of eukaryotic HSF-inducible genes (Scharf et al., 2012). As explicated by their name, they are involved in heat stress, but recent studies have illustrated their potential role in drought stress. Besides, in a field condition, drought stress is usually accompanied with heat stress. Thus, it is reasonable to ponder that plants have evolved with an intriguing transcriptional regulation mechanism to simultaneously mediate the drought- and thermo-tolerance. Also, HSFs can simultaneously regulate abiotic and phytohormone signaling pathways and, therefore, play a crucial role in the regulation of stress-responsive genes.

Plant HSFs are either positive or negative transcriptional regulators for drought stress-responsive genes depending on several factors, including their post-translation modification, interacting proteins or binding target. ABA signaling also crosstalks with HSF-mediated gene expression during abiotic stresses. The expression of Arabidopsis thaliana AtHSFA6a was enhanced in response to external ABA application, salt, drought and salt treatment. The ABA mediated expression of AtHSF6b also increased plant resistance to salt and drought stresses. The transgenic lines overexpressing HsfA2 had a higher accumulation of several dehydration-responsive genes like GALACTINOL SYNTHASE (AtGolS) and RAFFINOSE SYNTHASE (RafS) of the raffinose family oligosaccharide (RFO) pathway. The enhanced mRNA level of GolS and RafS led to the over-accumulation of galactinol and raffinose, which were implicated in the scavenging of hydroxyl radicle generated via oxidative damage (Nishizawa-Yokoi et al., 2008; Panikulangara et al., 2004).

Further, the expression of HSFA3 in response to drought and heat stress are dependent on the expression of DREB2A. Also, AtHSFA3 and AtHSFA1b take part in different signaling pathways to enhance the plant's tolerance to drought stress (Scharf et al., 2012). Overexpression of chickpea CarHSFB2 improved drought tolerance by enhancing the expression of some stress-responsive genes at the seedling stage in Arabidopsis (Ma et al., 2016). Li, Zhang, et al. (2015) demonstrated that overexpression of the drought- and heat-inducible maize ZmHSF06 in Arabidopsis improved drought and heat stress tolerance. Moreover, Triticum aestivum TaHSFs induced by ABA plays a crucial role during drought (Huang et al., 2016). The expression of Chenopodium quinoa CqHSF5 and CqHSF12 were upregulated during drought stress treatment. Likewise, CqHSF8 and CqHSF17 also play an important role in drought resistance. Some genes out of 25 SbHSF genes identified in sorghum were expressed differentially during drought, cold and salt stresses (Tashi et al., 2018). The Oryza sativa OsHSFB2b negatively regulated HSF genes during salt and drought tolerance (Yoshida et al., 2011). Thus, HSFs are one of the most important TFs influencing the expression of various stress-responsive genes, thereby enhancing the adaptation to multiple environmental stress conditions. However, the functions of some HSFs in many abiotic stresses continue to be mostly unknown (Wu et al., 2018). So, to improve stress tolerance and productivity of crops, more research on HSFs should be conducted.

3.5 MYB superfamily

The MYB superfamily of proteins is a part of a large gene family of TFs distributed in almost all eukaryotes and perform varied functions (Dubos et al., 2010). The MYB TF was first discovered and named after the avian myeloblastosis virus (Paz-Ares et al., 1987). The first plant-specific MYB TF (ZmMYBC1 gene) was isolated from maize and it primarily regulates the anthocyanin biosynthesis (Salih et al., 2016). The complete MYB protein has three regions: the DNA binding domain (DBD), the transcriptional activation domain and the negative regulatory domain (Ogata et al., 1996; Thompson & Ramsay, 1995). The Arabidopsis and rice genomes have over 198 and 183 MYB genes, respectively (Yanhui et al., 2006).

Expression studies have validated the upregulation of several MYB genes under drought stress conditions (Abe et al., 2003). For instance, MYB4 TF from rice imparted drought tolerance to Arabidopsis overexpressing the transgene and these transgenic plants accumulated several osmolytes. In addition, the genes of the proline biosynthesis pathway were also upregulated (Mattana et al., 2005). About 43 MYB genes discovered in soybean responded to desiccation (Liao, Zou, Wang, et al., 2008a). Drought stress leads to ABA accumulation in plant cells, which in turn upregulate several TFs and other proteins. MYB TFs take part in ABA-dependent response to drought stress (Lata & Prasad, 2011). For example, MYB2 TF interacted with the promoter region of a dehydration responsive rd22 gene in Arabidopsis (Abe et al., 1997) and induced several ABA-responsive genes in Arabidopsis (Abe et al., 2003). Another MYB TF of Arabidopsis, AtMYB44, down-regulated the expression of PP2C genes that are the negative regulators of ABA signaling (Jung et al., 2008). AtMYB60 plays a crucial role in regulating stomatal movement and desiccation survival (Cominelli et al., 2005) and AtMYB96 takes part in ABA and auxin signaling pathway for imparting drought tolerance to plants (Seo et al., 2009). The overexpression of OsMYB48-1 in rice triggered the upregulation of many ABA-related genes, indicating MYB's role in conferring drought and salt tolerance in an ABA-dependent manner (Xiong et al., 2014). In finger millet (Eleusine coracana), the transcript level of EcMyb1 was induced during drought stress response in drought-tolerant cultivars (Jadhav et al., 2018; Salvi et al., 2012). There are very few instances where MYB conferred drought tolerance in an ABA-independent manner. For example, overexpression of chrysanthemum CmMYB2 in Arabidopsis improved plant's survival to desiccation and high salinity but the plants became more sensitive to ABA application suggesting it functions in an ABA-independent manner (Shan et al., 2012). In conclusion, MYB TFs largely function as positive regulators of drought stress tolerance mainly in an ABA-dependent manner.

3.6 NAC transcription factor family

The NAC-TFs are named after the characterization of three functional genes containing NAC domain: NAM, ATAF, and CUC. NAC-TF family is one of the largest plant-specific TF families involved in the multitude of biological processes. Although a small proportion of these proteins is explored, NAC-TFs reprogram diverse pathways associated with plant growth and development and respond to (a)biotic stresses. These NAC proteins comprise a 150–160 amino acid-long N-terminal DNA binding domain (BD) that aids in DNA-protein interaction and nuclear localization. The transcriptional regulatory region resides in the C-terminus, which is post-translationally regulated under specific conditions (Kim et al., 2012; Lee et al., 2012). The NAC TFs generally bind to cis-regulatory elements: CGT (G/A) or CACG (Tran et al., 2004).

In transgenic rice, overexpression of stress-responsive NAC confers salinity, low temperature, and dehydration stress tolerance. Similarly, the overexpression of root-specific NAC-TF (OsNAC5, OsNAC6, OsNAC9, and OsNAC10) in rice plants altered root architecture and thereby imparted drought tolerance (Hu et al., 2006; Nakashima et al., 2007; Redillas et al., 2012; Tran et al., 2004). A transcriptomic study in rice revealed that OsNAC1 is upregulated during drought stress and ABA exposure. The overexpression of OsNAC1 entailed protection against drought stress and resulted in more fertile spikelets under drought stress. Under drought stress, the OsNAC1-OE plants maintained high water retention capability by mediating the stomatal closure without affecting the photosynthetic rate. Interestingly, the comparative transcriptomic analysis of the wild-type and SNAC1-overexpressing plants indicated the upregulation of about 40 genes associated with drought stress tolerance mechanisms in transgenic plants (Hu et al., 2006). These candidate genes facilitated osmolyte accumulation, signal transduction, and redox homeostasis. Similarly, OsNAC6 overexpression significantly improved the dehydration stress response in rice (Nakashima et al., 2007). Furthermore, expression analysis of GFP driven by OsNAC1 promoter revealed that the OsNAC1 promoter is active in the stomatal guard cells after drought stress exposure (Hu et al., 2006). In Arabidopsis, the NAC-TF AtJUB1 (JUNGBRUNNEN 1; ANAC042) is associated with the regulation of ROS signaling and mediates the drought stress response (Wu et al., 2012). The AtJUB1 directly triggers the expression of DREB2A, an AP2 type TF associated with drought stress response. AtJUB1 also acts as a negative regulator of GA or BR biosynthesis genes, thereby resulting in a decline in the cellular level of these phytohormones. The reduced level of these phytohormones resulted in the stabilization of DELLA proteins. Similarly, a homolog of AtJUB1 in tomato (SlJUB1, Solyc05G021090) directly binds to the promoter region of genes encoding SlDREB and DELLLA to regulate ROS homeostasis and stress-associated gene modulation with the evident role of SlJUB1 in drought stress tolerance (Thirumalaikumar et al., 2018). Being a transcriptional regulator, NAC-TFs are located in the nucleus. However, there are some NAC-TFs localizing at the membrane region of the endoplasmic reticulum or plasma membrane due to the presence of a transmembrane domain; they are thus called NTLs (NAC-TF associated with transmembrane motif-1- Like). Interestingly, during stress progression, these NTLs are channelized to the nucleus through different post-translational modifications, including phosphorylation, membrane proteolysis, alternative splicing, so on. (Liang et al., 2015). Duan et al. (2017) reported that in Medicago falcata, MfNACsa was found in the membrane under normal condition, while it translocated to the nucleus via de-S-palmitoylation during drought stress. This MfNACsa preserves the reduced cellular milieu during drought stress exposure by regulating the transcriptional module of GLYOXALASE1. In conclusion, these NAC-TFs integrate with diverse pathways and could be candidates to improve drought stress tolerance in different crop plants.

3.7 TCP transcription factor family

TCP family is plant-specific and regulates a plethora of plant metabolic processes. Phylogenetic analysis of TCP-TF from diverse species revealed the presence of non-canonical basic bHLH-containing DNA-binding domain known as TCP domain. Based on the conserved TCP domain, this TF family is mainly categorized into two classes named TCP-P (Class I) and TCP-C (Class II) (Cubas et al., 1999; Kosugi & Ohashi, 1997). TCP-TF exhibits protein–protein interaction that imparts functional diversity. TCP proteins are largely known to affect the phytohormonal regulation, including the jasmonic acid, auxin, cytokinin, and strigolactone pathways, directly or indirectly influencing cellular functions (Lucero et al., 2015; Nicolas & Cubas, 2016; Resentini et al., 2015). Several studies have documented their role in leaf shape and pattern regulation (Kieffer et al., 2011), flower and shoot development (Doebley et al., 1995), circadian cycle (Giraud et al., 2010), mitochondrial biogenesis (Welchen & Gonzalez, 2006). The TCP TFs generally bind to cis-regulatory elements: GGNCCCAC, G (T/C)GGNCCC (Kosugi & Ohashi, 2002).

Apart from all the roles mentioned above, few studies indicated the role of specific TCP-TF in drought stress response (Ding et al., 2019; Lei et al., 2017; Mukhopadhyay & Tyagi, 2015; Zhou et al., 2013). For instance, the overexpression of OsTCP19 imparted drought stress tolerance in Arabidopsis. The interaction of OsTCP19 and OsABI4 played a significant role during the ABA-induced drought stress response (Mukhopadhyay & Tyagi, 2015). Similarly, TCP14, a positive regulator of seed germination, acts antagonist to DOF6 TF, and regulates a specific set of ABA-related genes to ensure seed germination (Rueda-Romero et al., 2011). In Agrostis stolonifera, the Osa-miR319-mediated downregulation of TCP resulted in enhanced drought stress tolerance. The Osa-miR319-mediated stress tolerance is supported by higher leaf wax content and improved water retention capacity (Zhou et al., 2013). In a recent study, Ding et al. (2019) performed a genome-wide analysis of the TCP-family gene in Zea mays and identified a positive role of ZmTCP42 in drought stress tolerance. However, a more comprehensive analysis is needed to understand the role and regulation of the TCP-TF family in drought stress.

3.8 WRKY transcription factor family

Plants have a big family of WRKY transcriptional regulators. Their activity is not only vital for plant growth and development but also for their stress adaptation during the myriad of environmental stresses encountered during their life cycle. The first WRKY gene was identified in sweet potato (Ishiguro & Nakamura, 1994). The whole genome sequencing of various plant species has led to the identification of a variety of other WRKY genes (He et al., 2016; Wei et al., 2017). The WRKY protein domain of 60 amino acids contains highly conserved sequences: WRKYGQK along with the zinc-finger-like motifs: Cys(2)-His(2) or Cys(2)-HisCys. WRKY TFs bind to W-box [or, TTGAC(C/T)] cis-element present in the promoter of the desired genes (Bakshi & Oelmüller, 2014). There are other binding sites of WRKY TFs in addition to W-box. For example, OSWRKY13 from Oryza sativa binds to pathogen-responsive element (PRE4; TGCGCTT) and HvWRKY46 from Hordeum vulgare binds to SUgar-Responsive Element (SURE; TAAAGATTACTAATAGGAA) (Cai et al., 2008; Sun et al., 2003). WRKY TFs can either activate or repress the expression of a gene in their various homo and heterodimer combinations (Bakshi & Oelmüller, 2014). WRKY TFs are classified into three groups (namely, I, II, and III) based on protein structural characteristics (Li et al., 2010).

WRKY TFs are strongly induced by various abiotic stress conditions like drought, heat stress, excessive saline soil, water stress, so on. (Schluttenhofer & Yuan, 2015). The WRKY TFs are associated with plant hormone signaling, MAP kinase signal transduction pathway and self-regulation of WRKY (Jiang et al., 2017). WRKY TF genes are activators of ABA signaling and the phytohormone further controls downstream stress responses by integrating various stress signals. WRKY TF positively regulates ABA-mediated closure of stomatal aperture during drought stress. However, several WRKY TFs are negative regulators of abiotic stresses. In Boea hygromectrica, BhWRKY1 TF regulates the activation of BhGO1S1 (GALACTINOL SYNTHASE 1), an ABA-induced gene, to increase the dehydration tolerance of the plants (Wang et al., 2009). Several studies have highlighted the role of GolS in drought stress endurance via limiting the excess accumulation of detrimental ROS (Salvi et al., 2018). When cotton GhWRKY41 and Solanum pimpinellifolium SpWRKY1 TFs were overexpressed in tobacco, they regulated the stomatal conduction and ROS levels in plants, thereby conferring drought stress tolerance to the transgenic plants (Li, Luan, & Liu, 2015c). Contrarily, overexpression of cotton GhWRKY25 in Arabidopsis could not impart drought stress tolerance (Liu et al., 2016). Similarly, overexpression of GhWRKY68 in N. benthamiana decreased tolerance to drought and salt (Jia et al., 2015). In Arabidopsis, the expression of the sunflower HaWRKY76 induced drought stress adaptation (Raineri et al., 2015). Similarly, the overexpression of OsWRKY45 and OsWRKY72 in rice was associated with improved tolerance to salt and drought stresses (Qiu & Yu, 2009; Song et al., 2010). Interestingly, when the expression of a drought stress-inducible WRKY gene (OsWRKY11) was driven by a promoter of heat shock protein encoding gene (OsHSP101), the transgenic lines exhibited both drought- and thermo- tolerance phenotype. The tolerance mechanism in the transgenic lines was associated with the improved desiccation tolerance and reduced water loss in the transgenics during stress exposure (Wu et al., 2009). Also, overexpression of GmWRKY13/54 in Arabidopsis thaliana boosted the plant's tolerance to both salt and drought stresses (Zhou et al., 2008). Furthermore, Fei et al. (2019) identified 38 WRKY TFs in Zanthoxylum bungeanum, out of which ZbWRKY33 (homologous sequence of AtWRKY33) was the most upregulated TF during drought conditions. BnaWRKY210 TF of Brassica napus was highly upregulated during drought stress (He et al., 2016). In the presence of exogenous ABA application as well as drought stress, the expression of CsWRKY2 in tea plants was enhanced (Wang et al., 2016). The TaWRKY44 TF positively regulated drought stress response either by eliminating ROS or by activating the expression of stress-linked genes (Wang et al., 2015). Gossypium hirsutum WRKY TFs GhWRKY25 and GhWRKY27 positively influenced the salt stress-mediated dehydration tolerance and Rhizoctonia solani infection, respectively; however, both of them negatively regulated the drought tolerance response (Liu et al., 2016; Yan et al., 2015). In Vitis vinifera, VvWRKY11 was responsible for regulating drought stress tolerance when overexpressed in Arabidopsis (Liu et al., 2011). WRKYs can regulate multiple abiotic stresses simultaneously and, therefore, play a central role in response to abiotic stresses of plants. So, the regulation by WRKY in relation to both biotic and abiotic stresses should be explored in detail. Because of this tight regulation, WRKYs have become a promising target for crop improvement.

All the above molecular players directly or indirectly interact with each other differently to precisely activate the downstream effector, the TFs. The TFs act synergistically or antagonistically to ensure suitable actions that will help plants to circumvent the adverse state (Figure 2).