"Review of literaturelocalization signal and a highly variable transactivation domain at the C-terminalregion. In some NAC proteins, the C-terminal domains exhibit protein- bindingactivity and others contain transmembrane (TM) motifs (Seo et al., 2008; Kato et al.,2010). The structure of ANAC019, NAC domain was determined by X-raycrystallography (Ernst et al., 2004).The NAC proteins are able to form homodimersand heterodimers. The dimerization ability is localized to NAC domain but it isinfluenced the C-terminal regions of a protein (Yamaguchi et al., 2010). NAC genes are broadly classified into six major groups based on the. OtherNAC homologous genes like ATAF1, ATAF2 and CUC2 are derived from Arabidopsis(Jensen et al., 2008; Hu et al., 2010). More than 100 members of this family have beenidentified (Hu et al., 2015). However, only a few of them are functionallycharacterized. NACs play important roles in plant development process includingformation of embryos and flowers ( Duvalet al., 2002; Ning et al., 2015), secondarywalls (Yoshida et al., 2013) lateral root development (Hong et al., 2016), embryos andseed quality (Sperotto et al., 2009), senescence (Guo and Gan, 2006; Balazadeh et al.,2008; Oda-Yamamizo et al., 2016) fibre development (Ko et al., 2007) and cell death(Niu et al., 2014).NAC proteins are also thought to participate in pathogens response, viralinfections and environmental stimuli (Sun et al., 2013; Yokotani et al., 2014). A widerange of NAC TFs involving in abiotic stress tolerance was identified from variousplants (Chen et al., 2014; Shao et al., 2015). ANAC019, ANAC055, and ANAC072,three NAC members are have been derived from Arabidopsis. They were found tobind to the ERD1 promoter region to enhance drought tolerance (Tran et al., 2004).Some NAC TFs are known to involve in both biotic and abiotic stress response. ONACa rice NAC gene was found to be expressing in both biotic and abiotic stress was34 Review of literaturereported by sun et al., (2015a).In wheat, NAC genes related to abiotic and bioticstresses was studied by Xia et al., (2010) when wheat plants are infected with striperust pathogen infection and under abiotic stresses.Hu et al., (2008) reported that theoverexpression of the stress NAC responsive gene SNAC2 increased drought andsalinity tolerance in rice plant. Jeong et al., (2010) have reported that theoverexpression of OsNAC10 in rice enhances the abiotic stress tolerance under fieldconditions. Overexpression of MuNAC4 TF in groundnut has increased the toleranceto drought with increased root length was reported by Pandugrangaiah et al., (2014).Overexpression of NAC1 gene has improved drought and salinity stress tolerance incotton with increased root length and reduced transpiration (Liu et al., 2014a). Theexpression of other NAC TFs like AtNAC019, AtNAC072, ONAC045 and OsNAC5providing abiotic stress tolerance in transgenic rice and Arabidopsis was alsoreported (Tran et al.,2004; Takasaki et al., 2010)WRKY Transcription factor genesWRKY TFs family is one among the ten largest families of TFs in higher plantswhich is also found in some green algae (Ulker and Somssich 2004).WRKY TFs areevolved in the green lineage and then a number of lateral gene transfer events haveoccurred in diplomonads, social amoebae, fungi incertae sedis, and amoebozoa. Thesenon-plant WRKY genes do not belong to any seven groups of WRKY genes found inflowering plants (Rinerson et al., 2015a).The first WRKY cDNAs were cloned from sweet potato, wild oat, parsley andArabidopsis, based on the ability to bind specifically to the DNA sequence motifTTGACCT, which is known as the W box (Rushton, et al., 1996). The name WRKY isderived due to the presence of highly conserved WRKY domain amongst familymembers. The conserved domain is a 60 amino acid DNA -binding region, comprising35 Review of literatureof the conserved WRKYGQK heptapeptide and a zinc-finger motif (Rushton, et al.,2010).The WRKY domain and W boxThe most important feature of WRKY TFs is the presence of conserved DNAbinding domain. The WRKY domain is defined by the conserved WRKYGQK aminoacid sequence at its N-terminal and a C2-H2 zinc-finger-like motif. The presence ofzinc finger structure was identified using 2-phenanthroline that chelates zinc ions. Theaddition of 2-phenenthroline to WRKY proteins in gel retardation assays resulted in aloss of binding to the W-box target sequence in DNA (Rushton et al., 1995). WRKYdomain has sequence-specific DNA-binding activity. The WRKY domain consists of afour-stranded ß-sheet, with the zinc coordinating Cys/His residues forming a zinc- binding pocket. All known WRKY TFs has been classified into three groups based onboth the number of WRKY domains and the type of Zinc finger motif (Rushton et al.,2010; Huang et al., 2012). Group I WRKY TFs contains two WRKY conserved domains (N-terminal andC-terminal WRKY domains) and a classic zinc finger motif C2-H2. However, most ofthe WRKY TFs belongs to group II, with a single WRKY domain and C2-H2 zincfinger motif. The WRKY domains of group I and group II members have the same typeof finger motif, whose pattern of potential zinc ligands (C–X4–5–C–X22–23–H–X1– H), which is unique among zinc finger-like motif (Berg and Shi, 1996).The group IIIWRKY TFs contains a single WRKY domain and a C2–HC motif (C–X7–C–X23–H– X1–C) instead of a C2–H2 pattern. Group II genes were further divided into IIa, IIb,IIc, IId and IIe based on the primary amino acid sequence and group I was divided into , IIa + IIb, IIc, IId + IIe, and III, based purely on phylogenetic analysis and structure oftheir WRKY domains (Eulgem et al., 2000; Rushton et al., 2008; Huang et al., 2012;36 "