WRKY transcription factors and their role in biotic and abiotic stress in rice
Authors: Dhammaprakash P. Wankhede
ICAR-National Bureau of Plant Genetic Resources, Pusa campus, New Delhi

Plants in order to meet their various biological requirements have evolved complex and delicate regulatory mechanisms, among which regulation at the transcriptional level pays pivotal role as it determines many of the downstream responses. Transcriptional regulation of gene expression is largely mediated by specific recognition of cis acting elements by trans-acting DNA-binding transcription factor proteins. The DNA-binding transcription factors from various organisms can be generally categorized on the basis of their DNA-binding domains. Amongst transcription factors, WRKY transcription factor proteins possess a WRKY DNA-binding domain, about 60 amino acid residues containing a WRKYGQK sequence in its N-terminal end and a C2H 2 or C2HC zinc finger motif in its C-terminal (Eulgem et al., 2000; Wu et al., 2005). However, great divergence exists outside the WRKY domain. WRKY genes are to be involved in a several plant defence responses as well as developmental processes. It has been found that about two thirds of Arabidopsis WRKY genes are induced by pathogen infection or by salicylic acid, a signal molecule of plant defence responses (Dong et al., 2003). Expression of rice OsWRKY gene was induced by the infection of bacterial blight pathogen (Guo et al., 2004). Accumulating evidence suggest that WRKY genes might be involved in the developmental and metabolic processes, such as in the trichome development of Arabidopsis (Johnson et al., 2002), in the gibberellin signalling pathway in rice aleurone cells (Zhang et al., 2004) and in regulation of sesquiterpene syntheses in cotton (Xu et al., 2004) WRKY transcription factors bind to W-box elements with a core sequence of TGAC (Rushton et al., 1996).

Roles of rice WRKY transcription factors in biotic stresses

Plants have developed two layers of innate immunity system against pathogen attacks, one, PAMP (pathogen associated molecular patterns) triggered immunity (PTI) and the other, effector-triggered immunity (ETI), where immunity is triggered by resistance (R) proteins upon recognition of corresponding effector proteins. Both the processes (PTI and ETI) involve drastic changes in expression pattern of genes related to defense mechanism. Transcription factors are considered to be responsible for such altered expression pattern of defence related genes (Jones and Dangl, 2006). Several reposts have shown that the WRKY transcription factor superfamily play crucial roles in defence signalling pathways. Accumulations of evidences indicate that a common defence pathways in Arabidopsis and rice (Chern et al., 2005). Involvement of WRKY transcription factors in defence responses in rice have been well documented as evident from several reports. Rice WRKY3 and WRKY71 overexpression have resulted in enhanced resistance to Xanthomonas oryzae pv. oryzae (Xoo) and WRKY3 and WRKY71 are shown to functions upstream of OsNH1 (Liu et al., 2005, 2007). Expression profiling of OsWRKY13 activated transgenic lines revealed an altered transcript profile of other members of WRKY family such as up-regulation ofOsWRKY10 and down-regulation of OsWRKY14, OsWRKY24, OsWRKY42, OsWRKY45, OsWRKY51 and OsWRKY68. Down-regulated OsWRKY members were considered to be negative regulator in OsWRKY13 mediated defence signalling. The detailed action mechanism between these OsWRKYs remains to be elucidated. Overexpression of OsWRKY31 also conferred resistance to rice plants againstM. grisea. An interesting OsWRKY member, OsWRKY89 showed strong expression upon MeJA treatment, UV-B radiation as well as by rice blast fungus M. grisea. Over-expression of OsWRKY89 conferred enhanced resistance to rice plants against blast fungus and white backed plant hopper as well as UV irradiation (Wang et al., 2007). OsWRKY89 over-expression lines showed growth retardation at the early stage, reduction of internode length, an increase in wax deposition on leaf surfaces, increase in SA levels and enhanced lignification in culms. OsWRKY89-RNAi lines showed opposite function to that of overexpressed lines. These results suggest that OsWRKY89 plays an important role in response to biotic and abiotic stresses.

Roles of rice WRKY transcription factors in response to abiotic stresses and phytohormones

WRKY transcription factors play a variety of developmental and physiological roles in plants. A dozen rice WRKY genes are induced by heat shock, cold stress, high salinity and polyethylene glycol (Qiu et al., 2004). OsWRKY11 was induced by heat and drought. Over-expression of OsWRKY11 under the control of HSP101 promoter showed enhanced heat and drought tolerance (Wu et al., 2009). OsWRKY31 overexpression lines showed enhanced disease resistance to M. grisea and altered root growth and auxin response in rice transgenics (Zhang et al., 2008). Several rice WRKY genes are induced by phyto-hormone such as ABA and/or GA. The transient expression experiments demonstrated several rice WRKY genes were capable of regulating ABA-inducible HVA22 promoter in positive ( OsWRKY72 and OsWRKY77 ) or negative ( OsWRKY24 and OsWRKY45 ) manner. OsWRKY11 and OsWRKY71 were induced by ABA, but they do not regulate the ABA-inducible HVA22 promoter in aleurone cells (Xie et al., 2005).

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About Author / Additional Info:
Scientist, Division of Genomic Resources, ICAR-National Bureau of Plant Genetics Resources, New Delhi , India