"Review of literaturePrior knowledge about the existing genes, their involvement and expressionsites are necessary to elucidate the function and regulation of several genes at themolecular level. The promoter is upstream DNA sequence containing a binding site forRNA polymerase and modulates regulation of gene expression (Griffiths et al., 2000;Wu et al., 2009). Promoters often help in the identification of gene expressed either bytissue-specific or under physiological stress. Many promoter databases are availablewhich provides the information for structural analysis of promoters and their control ongrowth and development (Yamamoto and Obokata, 2008). Promoter regulates through DNA recognition sequence, which includes a coreand an upstream enhancer. Core promoter contains two important genetic elements likeTATA box and Inr (initiator element). It is located ~40 bases upstream of thetranscription start site where RNA pol II machinery initiates transcription. Theinitiation also depends on other factors like DNA motif (Juven-Gershon and Kadonaga,2010). The enhancer is imperfectly conserved Cis-elements DNA sequence, which isalso called enhancer elements. They help in the binding of sequence targeted TFs orenhancer binding protein. The rate of transcription depends on the elements that arelocated even far from the core. Cis-acting elements are considered as importantmolecular switches in the regulation of transcription of genes (Yamasaki et al., 2012).Hence the identification and analysis of Cis-elements are crucial for investigation of theregulatory mechanism (Gu et al., 2013). Promoters are classified into three typesdepending on the activity (I) constitutive promoters, (II) tissue specific promoter and(III) Inducible promoters (Jeong and Jung, 2015).Constitutive promotersA constitutive promoter is the one which expresses the gene all time. Eventhough the gene is continuously expressing, the level of expression depends on the43 Review of literaturetissue type (Park et al., 2010). A number of constitutive promoters have been isolatedfrom the plants. The commonly used promoters include CaMV35S, maize ubiquitin, rice actin, APX, R1G1B and PGD1 (Odell et al., 1985; Christensen and Quil, 1996;McElory et al., 1990; Park et al., 2012). The most common promoter used for theoverexpression of transgene is CaMV35S as it is active in all types of tissue and in alldevelopmental stages (Porto et al., 2014). The expression of the gene under constitutivepromoter shows some negatives effects on plant growth due to the accumulation oftransgene (Jeong and Jung, 2015). The reduction in the growth was reported byPriyanka et al., (2010) in Arabidopsis transgenic plants expressing CcHyPRP geneunder constitutive promoter CaMV35S. Tissue-specificMany tissue-specific promoters have been identified in plants which include apromoter from rbcS gene a light inducible tissue-specific expression in transgenicplants and an ubiquitin extension protein promoter UEP1 from chrysanthemumendogenous (Gilmartin and Chau, 1990; Annadana et al., 2002).A root specificOsETHE1 gene promoter was isolated from the rice. The GUS fusion with theOsETHE1 gene has shown the highest level of expression in root tissue (Kaur et al.,2014). Another root specific promoter TobRB7 was derived from tobacco whoseexpression is specific to membrane channel aquaporins located on the root (Yamamotoet al., 1991). The Transgenic rice plants containing a construct with GUS-GFPbifunctional reporter genes under LP2 promoter has shown the strongest expression inthe mesophyll cells containing chloroplast (Thilmony et al., 2009).Inducible promotersAn inducible promoter is the one which is induced by the external factors like,biotic and abiotic stress, chemical or mechanical damage (Hernandez-Garcia and finer,44 Review of literature2014). They can control the gene expression either positively or negatively. Numerousinducible genes are reported by researchers under NaCl or PEG (Sun et al., 2010),dehydration, low temperature and abscisic acid treatments (Yamaguchi-Shinozaki andShinozaki, 1994). Xu et al., (2010) has reported the expression of PsPR10 gene ofPinus strobes was induced by multiple abiotic stress factors such as PEG, NaCl, andMannitol. They also reported that the same gene was also induced by the application ofhormones like salicylic acid, jasmonic acid and abscisic acid. From the literature cited above, it is clear that the stress genes from stressadapted species are functionally more superior in bringing about tolerance to drought. Itneeds a clear understanding of the molecular and physiological processes involved indrought stress tolerance. With the use of molecular biology tools such as cloning,sequencing and annotation and transformation techniques, it is possible to validate thefunctions of genes, there by imparting drought tolerance.WRKY TFs are known to involve in various plant developmental events andstress response. All the above evidence shown that WRKY gene is a key regulatormediating stress tolerance and provides a link between a stress inducible WRKY factorand a downstream target gene that plays a vital role in drought responses (Zhu et al.,2012). Elucidating the functional mechanism of these proteins and identifying their rolein stress responsive pathways are necessary for improving crops stress tolerance bytransgenic teachniques. Therefore, in the present study an attempt is made to understand the role ofstress responsive WRKY TF gene isolated from a drought tolerant horsegram and tovalidate its role in groundnut transgenics for improved drought tolerance.45 "