Role of WRKY family proteins in innate immunity in pepper
By: Manisha Mangal and Arpita Srivastav | Corresponding author: manishamangal@rediffmail.com

Plants activate multiple defense mechanisms against pathogen attack in order to protect themselves from infection. This kind of response is often triggered by the recognition of specific pathogens and the activation of highly regulated signal transduction pathways particularly in the cell nucleus, where signals lead to the transcriptional activation of a large array of defense genes. This transcription reprogramming during plant immune responses involves different transcription factors which are sometimes called a sequence-specific DNA-binding factor and is a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to messenger RNA. Transcription factors perform this function alone or with other proteins in a complex, by promoting , or blocking the recruitment of RNA polymerase to specific genes. A defining feature of transcription factors is that they contain one or more DNA-binding domains (DBDs), which attach to specific sequences of DNA adjacent to the genes that they regulate

WRKY family proteins are a class of plant-specific transcription factors involved in stress response signaling. WRKY family of transcription factors bind at W-box element which refers to promoter elements with the sequence TTGACC or TTGACT and WRKY domains are defined by the conserved amino acid sequence WRKYGQK at their N-terminal ends and a novel zinc finger motif at the C termini. WRKY proteins have been identified from a number of plants and appear to be encoded by a large gene family, with more than 70 members in Arabidopsis and 73 members in pepper.

Many WRKY genes are rapidly induced after treatment with elicitors associated with infection. Plants over expressing a number of WRKY factors have been shown to have heightened resistance toward different plant pathogens.

There are a number of reports which describe WRKY proteins with essential roles in mediating defense responses to different pathogens in pepper. These include involvement of Ca-WRKY 40 in tolerance to heat stress and to Ralstonia solanacearum (RS) infection, CaWRKY27 in resistance to RS infection, CaWRKY in resistance to Tobacco mosaic virus (TMV) and Xanthomonas campestris and Ca-WRKY 30 against various pathogens including avirulent M. incognita, TMV, RS and Phytophthora capsici. Expression profiles of hot pepper (Capsicum annuum) genes under cold stress conditions, has also been studied and it was reported that CaWRKY1 and PR10 were upregulated.

Recently, the gene encoding Capsicum annuum WRKY transcription factor d (CaWRKYd) isolated from microarray analysis in the Tobacco mosaic virus (TMV)-P0â€"inoculated hot pepper plants was characterized and it was observed that CaWRKYd transcripts were induced by TMV-P0 inoculation and silencing of CaWRKYd affected TMV-P0-mediated hypersensitive response (HR), cell death and accumulation of TMV-P0 coat protein in local and systemic leaves. Furthermore, expression of some pathogenesis-related (PR) genes and HR-related genes was reduced in the CaWRKYd-silenced plants compared with TRV2 vector control plants upon TMV-P0 inoculation. CaWRKYd was also confirmed to bind to the W-box.

Further, a recent report by Lim et al., 2011 suggests that Capsicum annuum WRKYb transcription factor binds to the CaPR-10 promoter and functions as a positive regulator in innate immunity upon TMV infection, similarly another Chili Pepper Transcription Factor CaWRKY2, has also been found to be rapidly Induced by incompatible plant pathogens.

References:

1. Dang F, Wang Y, She J, Lei Y, Liu Z, Eulgem T, Lai Y, Lin J, Yu L, Lei D, Guan D, Li X, Yuan Q, He S. 2014. Overexpression of CaWRKY27, a subgroup IIe WRKY transcription factor of Capsicum annuum, positively regulates tobacco resistance to Ralstonia solanacearum infection. Physiol Plant. 150(3):397-411.

2. Dang F, Wang Y, Yu, L; Eulgem T; Lai Y, Liu, ZQ, wang X, Qiu A, Zhang TX, Lin J, Chen Y S, Guan D, Cai H, Mou S and He S. 2013. CaWRKY40, a WRKY protein of pepper, plays an important role in the regulation of tolerance to heat stress and resistance to Ralstonia solanacearum infection. Plant, Cell & Environment. 36(4): 757â€"774.

3. Huh SU, Choi LM, Lee GJ, Kim YJ, Paek KH. 2012. Capsicum annuum WRKY transcription factor d (CaWRKYd) regulates hypersensitive response and defense response upon Tobacco mosaic virus infection. Plant Sci. 197:50-58.

4. Lim JH, Park CJ, Huh SU, Choi LM, Lee GJ, Kim YJ, Paek KH (2011) Capsicum annuum WRKYb transcription factor that binds to the CaPR-10 promoter functions as a positive regulator in innate immunity upon TMV infection. Biochem Bioph Res Co 411 (3):613-619


About Author / Additional Info:
Senior Scientist ( Biotechnology) IARI-New Delhi