Introduction

Drought is one of the most devastating abiotic stresses that impacts growth, development and productivity of agricultural crops worldwide. Plants respond and adapt to this stress through a series of molecular, cellular, biochemical as well as physiological reactions and activate a number of defense mechanisms that function to increase tolerance to all kinds of abiotic stresses. The early events of the adaptation of plants to abiotic stress include the perception of stress signals and subsequent signal transduction, leading to the activation of various physiological and metabolic responses. Within the signal transduction networks that are involved in the conversion of stress signal perception to stress-responsive gene expression, various TFs and cis-acting elements contained in stress-responsive promoters function not only as molecular switches for gene expression, but also as terminal points of signal transduction (Fig. 1).

Fig. 1
figure 1

Abiotic stress signaling network

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Plant engineering strategies for cellular and metabolic reprogramming to increase the efficiency of plant adaptive responses may either concentrate on (1) providing stress tolerance by directly re-programming ion transport processes and primary metabolism or (2) manipulating signaling and regulatory pathways of the adaptive mechanisms. The second approach seems to be more effective because regulatory factors directly orchestrate the transcriptional and translational signaling components (Golldack et al. 2011). Consequently, many regulatory components have been identified. The advent of microarray technology made it possible to investigate stress responsive transcript changes at the whole genome level (Kreps et al. 2002; Seki et al. 2003).

Novel transcription factors regulating stress-responsive gene expression have been identified by various means (Zou et al. 2007). Elegant genetic works like employing a firefly luciferase fused to a stress-inducible promoter (RD29A-LUC) and conventional forward/reverse genetic approaches resulted in the isolation of key stress-signaling components. In many cases, the regulatory components thus identified have been utilized to generate transgenic plants that can tolerate stress to some extent (Zhang et al. 2004). TFs are master regulators that control gene clusters. A single transcription factor (regulatory gene) can control the expression of many target proteins (effector genes) through specific binding of the TF to the cis-acting element in the promoters of respective target genes. This type of transcriptional regulatory system is called regulon (Shinozaki and Shinozaki 2005). Several major regulons that are active in response to abiotic stress have been identified in Arabidopsis. In general, drought responsive transcription factors fall in two categories: abscisic acid (ABA) independent transcription factors, which do not require stress hormone (ABA) to operate and ABA-dependent transcription factors that require stress hormone somewhere in the process of drought responsive signal cascade ending with the final step of structural gene expression (Ali et al. 2011). Dehydration-responsive element binding protein 1 (DREB1)/C repeat binding factor (CBF) and DREB2 regulons function in ABA-independent manner, whereas the ABA-responsive element (ABRE) binding protein (AREB)/ABRE binding factor (ABF) regulon functions in ABA-dependent manner. TFs comprise families of related proteins that share a homologous DNA binding domain. Most common transcription factor families include DREB1/CBF, DREB2, AREB/ABF, MYB/MYC, bHLH, ZFPs, bZIP, WRKY and NAC regulons.

Recent studies demonstrated that DREB1/CBF, DREB2, AREB/ABF, MYB/MYC, bHLH, ZFPs, WRKY and NAC regulons have important roles in abiotic stress responsive gene expression in rice. Expression analysis of these stress inducible genes in Arabidopsis indicated the existence of complex regulatory mechanisms between perception of abiotic stress signals and expression of respective stress responsive gene (Zhu 2002). Over-expression of the genes that regulate the transcription of a number of downstream stress responsive genes seems to be a promising approach in the development of drought, salt and cold resistant/tolerant transgenic plants when compared to engineering individual functional genes. The regulatory genes reported so far not only play a significant role in drought, salinity and cold stresses, but also in submergence tolerance. More recently, an ethylene-response-factor-like gene, named Sub1A, has been identified in rice and the over-expression of Sub1A-1 in a submergence-intolerant variety conferred enhanced submergence tolerance to the plants (Xu et al. 2006), thus confirming the role of this gene in submergence tolerance in rice. Therefore, it is important to enhance regulatory ability of an important transcription factor that activates the expression of many target genes controlling correlated characters. Many key genes have been identified that are involved in the regulatory networks under abiotic stress. Thus transcription factor based transgenic approaches are likely to play a prominent part to develop limited soil moisture, salt and cold tolerant plants. Some of the important transcription factors families involved in abiotic stress tolerance are described below:

DREB family of transcription factors

The best studied group of TFs involved in abiotic stress, particularly in drought and cold tolerance are the DREB1 genes (Reddy and Reddy 2008). Many osmotic stress inducible genes contain a conserved drought responsive element (DRE) in their promoters. Several cDNAs encoding the DRE binding proteins, DREB1A and DREB2A have been isolated from Arabidopsis thaliana shown to specifically bind and activate the transcription of genes containing DRE sequences. In fact, in many studies over-expression of stress inducible DREB transcription factor was found to activate the expression of many target genes having DRE elements in their promoters and the resulting transgenic plants showed improved stress tolerance (Table 1). One important way to achieve tolerance to multiple abiotic stresses is to over-express TFs that control multiple genes from various pathways. The best studied and broadly applied example is the over-expression of the DREB1A driven either by a constitutive (CaMV35S) or the dehydration inducible (rd29A) promoter. This approach has activated the expression of many stress tolerant genes under normal growth conditions and resulted in the generation of plants with increased tolerance to drought, freezing and salinity stress (Lata and Prasad 2011).

Table 1 Function of DREB genes in various abiotic stresses in plants (Lata and Prasad 2011)
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CBF/DREB1 transcription factors have been considered to be the important regulators of the cold acclimation response, controlling the level of COR (cold-regulated) gene expression, which in turn promotes tolerance to freezing (Gilmour et al. 2000). Therefore, transformation of CBF/DREB1 genes has been found to improve environmental stress tolerance to many plants, some of these have the potential to be used as freezing-tolerant verities. Over-expression of the AtDREB1A in Nicotiana tabacum, wheat, rice and groundnut has been shown to lead to increased drought tolerance and enhanced expression of late embryogenesis abundant (LEA) type genes, at least under greenhouse conditions. Recently, transgenic peanut plants over-expressing Arabidopsis DREB1A under stress inducible promoter accumulated considerably high levels of some key enzymes compared to wild type plants under drought stress but these enzymes showed no relationship to transpiration efficiency. The orthologue DREB genes from other plants have also been shown to be functional, for example transgenic rice over-expressing Oryza sativa DREB1A has demonstrated improved tolerance to drought, high salt and low temperature stresses and also caused accumulation of elevated levels of osmo-protectants such as free proline and various soluble sugars. As a matter of fact, ectopic expression of DREB genes in Arabidopsis as well as in heterologous systems such as wheat, barley, soybean, tomato, tobacco, strawberry, rice, oilseed rape, potato, and other grasses has produced enhanced tolerance to one or more types of abiotic stresses.

Transgenic turf grass (Lolium perenne) over-expressing AtDREB1/CBF3 showed enhanced drought and freezing tolerance and the activities of superoxide dismutase (SOD) and peroxidase (POX) were also higher in transgenic plants compared to non-transgenic control plants (Li et al. 2011). Interestingly one of the DREB1 type transcription factor from dwarf apple named MbDREB1 showed increase in tolerance to low temperature, drought and salt stresses via both ABA-dependent and ABA-independent pathways (Yang et al. 2011). The MtDREB1C gene isolated from Medicago truncatula and driven by the Arabidopsis rd29A promoter enhanced freezing tolerance in transgenic China rose significantly without any obvious morphological or developmental abnormality (Chen et al. 2010). However, little information is available about stress tolerance conferred by different members of the DREB1 subfamily. In case of Arabidopsis, DREB1A, DREB1B and DREB1C were tested through their over-expression under the same freezing conditions and no significant differences in freezing tolerance and downstream gene expression induction was observed (Gilmour et al. 2004). Similar results were also observed in rice when comparing the freezing or drought tolerance between OsDREB1A and OsDREB1B over-expressed plants and in Brassica napus, when comparing freezing tolerance between BnCBF5 and BnCBF17 over-expressed plants (Savitch et al. 2005).

bZIP class transcription factors

The bZIP proteins are characterized by a 40–80 amino-acid long conserved domain (bZIP domain) that is composed of two motifs: a basic region responsible for specific binding of the TF to its target DNA, and a leucine zipper required for TF dimerization. Genetic, molecular and biochemical analyses have indicated that bZIPs are regulators of important plant processes, such as organ and tissue differentiation, cell elongation, nitrogen/carbon balance control, pathogen defense, energy metabolism, unfolded protein response, hormone and sugar signaling, light response, osmotic control and seed storage protein gene regulation (Nijhawan et al. 2008). Initially, 50 plant bZIP proteins were classified into five families, taking into account similarities of their bZIP domain. An original investigation of the complete A. thaliana genome sequence indicated the presence of 81 putative bZIP genes (Correa et al. 2008). Basic leucine zipper proteins contain a DNA binding domain, rich in basic residues and a leucine zipper dimerization domain. Several b-ZIP factors that bind to ABA responsive elements (ABREs—PyACGTGGC) are AREB1, AREB2, and AREB3. ABREs were first identified in the wheat Em gene and rice rab16 gene. These genes are expressed in both dehydrated vegetative tissues as well as maturing seeds during late embryogenesis. Region between −294 and +27 in the 5′ upstream region of the rab-16A gene is sufficient to confer ABA-responsive, transient expression upon the CAT reporter gene in transfected rice protoplasts. Two motifs have been found to be conserved in all rab-16 genes discovered in rice. Motif I has the consensus RTACGTGGR (R is an unspecified purine nucleoside), which is similar to the cAMP-responsive element (TGACGTCA) that binds to the transcription factor CREB. More importantly, motif I has been reported in the 5′ upstream regions of five of six LEA genes and in that of the ABA-responsive wheat Em gene. Motif II, which is found in two copies (IIa and IIb) in rab-16A and once in rab-16B-D, has the consensus CGSCGCGCT, in which S is either G or C. It occurs in the rab-16 genes as part of sequences that are similar to the degenerate deca-nucleotide binding site of SP1, an auxiliary mammalian transcription factor. AREB1 and AREB2 proteins are up-regulated by ABA (Kang et al. 2002).

Till now, 89 bZIP transcription factor-encoding genes have been identified in the rice (O. sativa) genome. Their chromosomal distribution and sequence analyses suggest that the bZIP transcription factor family has evolved via gene duplication. The phylogenetic relationship among rice bZIP domains as well as bZIP domains from other plants, suggests that homologous bZIP domains exist in plants. Similar intron/exon structural patterns were observed in the basic and hinge regions of their bZIP domains (Nijhawan et al. 2008). Wlip19 (a bZIP transcription factor type gene) expressing in transgenic tobacco showed a significant increase in abiotic stress tolerance, especially freezing tolerance by activating cor/lea genes (Kobayashi et al. 2008). Over-expressing a bZIP transcription factor SlAREB1 cloned from tomato and introduced in tobacco using Agrobacterium-mediated transformation showed up-regulation of stress-responsive genes such as RD29B, thus mediating the stress response (Yanez et al. 2009). Transgenic Arabidopsis plants, over-expressing a bZIP type transcription factor ABF3, conferred enhanced tolerance to drought stress using CaMV35S constitutive promoter (Abdeen et al. 2010). The homozygous T-DNA insertional mutants Osabf1-1 and Osabf1-2 (bZIP transcription factor) were more sensitive in response to drought and salinity treatments compared to wild type plants (Hossain et al. 2010). A novel bZIP gene ThbZIP1, cloned from Tamarix hispida and introduced in tobacco plant, showed differential regulation in response to treatment with NaCl, polyethylene glycol (PEG) 6000, NaHCO3 and CdCl2, suggesting that ThbZIP1 is involved in abiotic stress responses. ThbZIP1 transgenic plants conferred stress tolerance by enhancing reactive oxygen species (ROS) scavenging, facilitating the accumulation of compatible osmolytes and inducing the biosynthesis of soluble proteins (Wang et al. 2010).

MYB family of transcription factors

The myeloblastosis (MYB) family of proteins is large, functionally diverse and represented in all eukaryotes. Most MYB proteins function as transcription factors with varying numbers of MYB domain repeats conferring their ability to bind DNA. MYB proteins are key factors in regulatory networks controlling development, metabolism and responses to biotic and abiotic stresses. In plants, the MYB family has selectively expanded, particularly through the large family of R2R3-MYB (Dubos et al. 2010). So far, large numbers of MYB genes have been identified in different plant species, comprising 204 members in Arabidopsis, 218 members in rice, 279 members in grapevine, 197 members in populus, and 180 members in Brachypodium (Zhang et al. 2012). MYB proteins are characterized by a highly conserved DNA-binding domain: the MYB domain. This domain generally consists of four imperfect amino acid sequence repeats (R) of about 52 amino acids, each forming three α helices. The second and third helices of each repeat build a helix-turn-helix (HTH) structure with three regularly spaced tryptophan (or hydrophobic) residues, forming a hydrophobic core in the 3D HTH structure. The third helix of each repeat is the “recognition helix” that makes direct contact with DNA and intercalates in the major groove. During DNA contact, two MYB repeats are closely packed in the major groove, so that the two recognition helices bind cooperatively to the specific DNA sequence motif. MYC and MYB recognition sites are present in the promoter region of RD22 gene (Abe et al. 2003).

The expression patterns of 60 wheat MYB genes under different abiotic stress conditions have been studied till date. Among 60 genes studied, TaMYB32 encoded an R2R3-MYB-type protein and transgenic Arabidopsis over-expressing this gene showed improved tolerance to high salt (Zhang et al. 2012). Several R2R3MYB genes play important role in the stress responses to environmental stimuli. For example, AtMYB2 in co-operation with AtMYC2 functions as a transcriptional activator in the dehydration and ABA inducible gene expression of RD22 (Abe et al. 2003). AtMyb41 gene from Arabidopsis has been found to be transcriptionally regulated in response to salinity, limited soil moisture, cold as well as the endogenous plant hormone ABA. Over-expression of AtMyb41 in transgenic plants resulted in a dwarf phenotype (Lippold et al. 2009). In another study, a rice R2R3-type MYB gene, OsMYB2, was up-regulated by salt, cold, and dehydration stress. The OsMYB2-over-expressing plants accumulated greater amounts of soluble sugars and proline and less amounts of H2O2 and malondialdehyde than wild type plants. There was greater up-regulation of stress-related genes, including OsLEA3, OsRab16A, and OsDREB2A and enhanced activities of antioxidant enzymes including peroxidase, superoxide dismutase and catalase (Yang et al. 2012).

Three TaMYB2 TFs have been identified from wheat and designated as TaMYB2A, TaMYB2B, and TaMYB2D. Among them, TaMYB2A was early responsive TF and its over-expression in transgenic Arabidopsis conferred enhanced tolerance to multiple abiotic stresses, as revealed by induction of abiotic stress responsive genes, while having no obvious negative effects on phenotype under well-watered and stressed conditions (Mao et al. 2011). Rice MYB transcription factor gene Osmyb4 expression improved adaptive responses to drought and cold stress in transgenic apples (Pasquali et al. 2008). Transgenic Arabidopsis plants over-expressing MYB96 exhibited enhanced drought resistance with reduced lateral roots (Seo et al. 2009). Transgenic Arabidopsis plants over-expressing R2R3-MYB transcription factor gene MdMYB-10 (a regulatory gene of anthocyanin biosynthesis in apple fruit) showed better tolerance to osmotic stress compared to wild type plants (Gao et al. 2011).

NAC family of transcription factors

In recent years, great achievements have been made in increasing the tolerance of plants to abiotic stresses by identifying potential stress related genes. Among them, N-acetyl-l-cysteine (NAC) family, viz., NAM (no apical meristem), ATAF1/2 and CUC (cup-shaped cotyledon) domain proteins comprise of one of the largest plant-specific transcription factors, represented by ~117 genes in Arabidopsis, ~151 genes in rice and ~101 genes in soybean genome. NAC TFs are mostly involved in salinity tolerance. They act downstream of auxin and ethylene signaling pathways in addition to ABA pathway (He et al. 2005). Proteins of this family are characterized by a highly conserved DNA binding domain, known as NAC domain in the N-terminal region and a transcriptional activation domain in the C-terminal region, which is highly diversified both in length and sequence. NAC domain comprises nearly 160 amino acid residues that are divided into five sub-domains (A–E). A sequence of 60 residues within the NAC domain contains a unique TF fold consisting of a twisted-sheet bounded by a few helical elements. NAC domain has been implicated in nuclear localization, DNA binding and the formation of homo-dimers or hetero-dimers with other NAC domain proteins. The C-terminal regions of NAC proteins confer the regulation diversities of transcriptional activation activity (Hu et al. 2006; Jeong et al. 2010).

NAC TFs are multifunctional proteins and involved in wide range of processes including biotic and abiotic stress responses, lateral root development, flowering, anther dehiscence etc. Evidence for involvement of NAC TFs with the regulation of drought stress response in plants was first reported in Arabidopsis, where over-expression of ANAC019, ANAC055 and ANAC072 altered the expression of many stress-inducible genes in the transgenic plants and conferred a constitutive increase in drought tolerance (Tran et al. 2004). One of the NAC gene, named CsNAC1 has been investigated in citrus and found up-regulated during drought, cold, salt stress and in response to ABA (Oliveira et al. 2011). Recently, the role of two rice NAC genes in rice stress adaptation has been characterized. SNAC1 was induced mainly in guard cells under drought conditions and over-expression of this gene in rice resulted in significant increase in drought resistance under field condition at the stage of anthesis. Over-expression of another NAC gene OsNAC6/SNAC2 in rice resulted in enhanced tolerance to drought, salt, and cold during seedling development (Hu et al. 2006; Zheng et al. 2009).

Over-expression of OsNAC10 in rice under the control of the constitutive promoter GOS2 and the root-specific promoter RCc3 increased the plant tolerance to drought, high salinity and low temperature at the vegetative stage (Jeong et al. 2010). Transgenic rice plants over-expressing OsNAC52 have been found to be highly sensitive to ABA, and over-expressing 35S-OsNAC52 transgenic lines activated expression of downstream genes in transgenic Arabidopsis, resulting in enhanced tolerance to drought stresses without causing any growth retardation. This result suggested that OsNAC52 gene may function as an important transcriptional activator in ABA inducible gene expression and maybe useful in improving the tolerance to abiotic stresses in plants (Gao et al. 2010). Over-expression of ZmSNAC1 in Arabidopsis led to hypersensitivity to ABA and osmotic stress, and conferred tolerance to dehydration (Lu et al. 2012). In another study, TaNAC2, a NAC transcription factor member from common wheat, was cloned and induced under water deficiency, high salinity, low temperature, and ABA. Transgenic experiments indicated that TaNAC2 increases the tolerance to drought, salt, and freezing stresses in Arabidopsis without any obvious negative effects on morphology by TaNAC2 over-expression, suggesting a potential for utilization of such NAC TF gene in crop improvement (Mao et al. 2012). A full-length cDNA of DgNAC1, containing a typical NAC domain, has been isolated from chrysanthemum and it was observed that 35S:DgNAC1 transgenic tobacco exhibited a markedly greater tolerance to salt with no detectable phenotype defects under normal growth conditions (Liu et al. 2011).

Zn finger protein family of transcription factors

C2H2 zinc finger proteins (ZFPs) are characterized by the presence of two cysteine (Cys2) and two histidine (His2) residues in what is called a zinc finger domain, which stabilizes the three-dimensional structure consisting of a two-stranded anti-parallel β-sheet and α-helix surrounding a central zinc ion (Kam et al. 2008). ZFPs constitute one of the largest families of transcription factors in eukaryotes. Compared to mammals, the size of the ZFP family in plant genomes is relatively small 176 members in Arabidopsis and 189 members in rice (Agarwal et al. 2007). Many plant ZFPs contain a highly conserved QALGGH amino acid motif within the zinc finger domain, which forms a plant specific Q-type C2H2 zinc finger sub-family. The QALGGH motif is located at the N-terminus of the α-helix of the zinc finger domain. This α-helix region in animal ZFPs consisting of hyper-variable residues are responsible for recognition of specific DNA bases. The Q-type ZFP family has been implicated in plant response or adaptation to abiotic stresses (Kam et al. 2008; Figueiredo et al. 2012).

Expression analyses have shown that a number of ZFP genes in Arabidopsis are responsive to drought and cold stresses. Some Q-type ZFP genes, such as Arabidopsis Zat12, are involved in oxidative and abiotic stress signaling, indicating that the oxidative stress is associated with abiotic stress (Huang et al. 2009). ZAT12 has been described as a negative regulator of the DREB1/CBF regulon (Vogel et al. 2005), while the ZAT10/STZ gene expression has been shown to be dependent on DREB1A/CBF3 (Maruyama et al. 2004). C2H2-type TFs are therefore, signaling components that can be located either up or downstream of the DREB1/CBF genes. Transgenic Arabidopsis plants, constitutively expressing the Cys2/His2 zinc finger protein Zat7, had suppressed growth and were more tolerant to salinity stress. Tobacco transgenic plants over-expressing the AlSAP gene (a novel A20/AN1 zinc-finger transcription factor gene isolated from the halophyte grass Aeluropus littoralis) under the control of duplicated CaMV35S promoter, exhibited an enhanced tolerance to abiotic stresses, such as salinity, drought, heat and freezing (Saad et al. 2010). Regulation of OsDREB1B through zinc finger TFs have also been reported recently (Figueiredo et al. 2012).

Basic helix-loop-helix family of transcription factors

The bHLH proteins, a super-family of functionally diverse transcription factors, have been intensively studied in plants and animals. This family is defined by the bHLH signature domain, which consists of 60 amino acids with two functionally distinct regions: the basic region at the N-terminal end, involved in DNA binding and the HLH region at the C-terminal end, involved in formation of homo-dimers or hetero-dimers. These proteins have been well characterized and function as important regulating components in transcriptional networks, controlling cell proliferation, determination and differentiation in plants, animals and yeast. In several rice studies, genes from this family have been found to play distinct roles in stress response; examples are OsbHLH1 in cold response, RERJ1 in wound and drought response (Kiribuchi et al. 2005), OsPTF1 in tolerance to phosphate starvation and OsIRO2 responsible for Fe-deficient conditions (Ogo et al. 2007).

Over-expression of a basic helix-loop-helix transcription factor OrbHLH2 isolated from wild rice, which encodes a homologue protein of ICE1, conferred increased tolerance to salt and osmotic stress in Arabidopsis (Zhou et al. 2009). Over-expression of OrbHLH2 in Arabidopsis conferred increased tolerance to salt and osmotic stresses and up-regulated the stress-responsive genes, i.e., DREB1A/CBF3, RD29A, COR15A and KIN1 in transgenic plants. ABA treatment showed a similar effect on the seed germination or transcriptional expression of stress responsive genes in both wild-type and OrbHLH2-over-expressed plants, which implies that OrbHLH2 did not depend on ABA in responding to salt stress. OrbHLH2 may function as a transcription factor and positively regulate salt-stress signaling independent of ABA in Arabidopsis, which provides some useful data for improving salt tolerance in crops (Zhou et al. 2009).

WRKY family of transcription factors

WRKY transcription factors, with a conserved WRKYGQK sequence in their DNA binding domain, bind to the W-box (TTGAC) on the promoter region of target gene. They encode for a wide family of transcription factors, characterized by the presence of approximately 60 amino acids containing the amino acid sequence WRKY at its amino terminal end and a putative zinc finger motif at its carboxyl terminal end. WRKY transcription factor proteins, known to mediate the pathogen-induced defense programme, have been induced by ABA treatment in rice callus, suggesting the cross talk between ABA response pathway and pathogen-induced defense programme (Shimono et al. 2007). Till date, approximately 112 members of WRKY gene family in rice and 74 members in Arabidopsis have been reported. Recently 46 WRKY genes have been isolated from canola (B. napus L.) in response to fungal pathogens and hormone treatments (Yang et al. 2009). The role of WRKY transcription factors in abiotic stress response remains obscure and little information is available in limited crops only (Table 2). However some reports have demonstrated that WRKY TFs are also involved in abiotic stress responses. For example OsWRKY11 gene under control of heat shock induced promoter HSP101 encoded a transcription factor with WRKY domain that is induced by both heat shock and drought stresses in seedlings of rice (Wu et al. 2009). Involvement of an ABA inducible WRKY gene in abiotic stresses has been reported in creosote bush (Larrea tridentate) and Hv-WRKY3 has been found to be involved in cold and drought response in barley (Zou et al. 2007; Mare et al. 2004).

Table 2 Induction of WRKY genes and their response to abiotic stresses (Chen et al. 2012)
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Soybean WRKY-type transcription factor genes GmWRKY13, GmWRKY21 and GmWRKY54, when introduced into Arabidopsis, conferred differential tolerance against abiotic stresses in transgenic Arabidopsis plants (Zhou et al. 2008). ABA overly sensitive mutant of Arabidopsis (abo3), which encodes a WRKY transcription factor AtWRKY63, when disrupted by a T-DNA insertion in At1g66600, the mutant showed hypersensitivity to ABA and was less tolerant to drought than the wild type, thus uncovering an important role for a WRKY transcription factor in plant responses to ABA and drought stress (Ren et al. 2010). Over-expression of WRKY25 and WRKY33 in Arabidopsis moderately enhanced the tolerance of transgenic plants to 100 mM NaCl solution (Jiang and Deyholos 2009). BcWRKY46 gene has been reported to play an important role in regulation of ABA concentration and abiotic stress (Wang et al. 2012). Constitutive expression of BcWRKY46 gene under the control of the CaMV35S promoter in tobacco showed the susceptibility of transgenic tobacco plants to freezing, ABA, salt and dehydration stresses (Wang et al. 2012). This finding is in contrast with the previous results obtained with transgenic Arabidopsis plants hosting GmWRKY gene, which showed increased tolerance to cold, salt, and drought stresses. Transgenic seedlings over-expressing VvWRKY11 showed that its gene product affects the expression of two stress responsive genes namely AtRD29A and AtRD29B, resulting in drought tolerance. The VvWRKY11 protein may interact with DREB and/or ABEB, resulting in the increased expression of AtRD29A and AtRD29B (Wang et al. 2012).

Conclusion

Adaptation to salinity and drought is undoubtedly one of the complex processes, involving numerous changes including attenuated growth, activation/increased expression or induction of genes, transient increases in ABA levels, and accumulation of compatible solutes and protective proteins, increased levels of antioxidants and suppression of energy consuming pathways. However, no consensus has been reached in defining the key processes determining tolerance and the secondary follow-up processes. With the advancement of high throughput DNA technologies, several hundred stress-induced or up-regulated genes have been identified. However, the functions of only a limited number of gene products are known.

Many plant genes are regulated in response to abiotic stresses, such as drought, high salinity, heat and cold, and their gene products function in stress response and tolerance. Understanding the molecular mechanisms of plant stress responses to these abiotic stresses will help in manipulating plants to improve stress tolerance and productivity. The use of transgenes to improve the tolerance of crops to abiotic stresses remains an attractive option and targeting multiple gene regulation through transcription factors appear better than targeting single gene. Temporal control of many stress responsive genes is regulated mainly by a combination of TFs and cis-acting elements in stress inducible promoters of plants. They play a key role in providing tolerance to multiple stresses in ABA-dependent and ABA-independent manner. The TFs can genetically be engineered to produce transgenic plants having greater tolerance against drought, salinity, heat and cold stresses using different promoters. Some of the recent studies have shown the importance of DREB TFs as candidate genes in marker-assisted breeding programme and developing proper functional markers, which could be used for allele-mining in crop improvement. Functional markers, based on DREB1 locus on the long arm of chromosome 3B, may be useful in wheat breeding programme for drought tolerance (Lata and Prasad 2011).

It is reasonable to believe that WRKY genes provide selective advantages for plants to withstand environmental constraints, to cope with increased complexity in developmental and metabolic pathways and to modulate gene expression in a tissue-specific manner. The creation of taxa-specific WRKY proteins is likely to be implicated in controlling defined pathways linked to adaptation to different environmental stimuli or species-specific metabolic pathways.