Dissecting the WRKY Web of Plant Defense Regulators

During the past ten years, a large body of circumstantial evidence has accumulated, implicating WRKY factors in transcriptional reprogramming during plant immune responses [1–3]. Encoded by complex gene families in higher plants [4], these transcription factors share a DNA-binding domain (WRKY domain) comprising approximately 60 amino acids [5]. Additional conserved features of WRKYs are limited to separate subgroups of this family and include putative leucine zippers, nuclear localization signals, calmodulin binding sites, and several domains of unknown function [5–7]. Multiple studies have demonstrated the ability of WRKYs to bind to promoters of defense-associated genes via specific interactions of their WRKY domains with pathogen response elements termed W boxes (TTGACC/T) [8–11]. Both activating and repressing effects of WRKYs and W boxes on transcription have been observed [12–14]. It has also been shown that stable and transient overexpression of several WRKYs in the model plant Arabidopsis thaliana (Arabidopsis) conveys enhanced resistance to various bacterial or fungal pathogens [15–17].