A pathogen- and
salicylic acid (SA)-induced
DNA-binding activity has been recently identified in tobacco that is related to a previously identified class of WRKY
DNA-binding proteins. To identify members of the WRKY gene family associated with this
DNA-binding activity, we have attempted to isolate those WRKY genes that are induced by pathogen
infection. Using a domain-specific differential display procedure, we have isolated two tobacco WRKY genes, tWRKY3 and tWRKY4, that are rapidly induced in resistant tobacco plants after
infection by tobacco mosaic virus (TMV). Both tWRK3 and tWRKY4 encode
proteins with a single WRKY domain that contain the conserved WRKYGQK sequence. Unlike other isolated WRKY
proteins that contain the Cys2His2
zinc motif, tWRKY3 and tWRKY4 appear to contain the Cys2HisCys
zinc motif. Nonetheless, both tWRKY3 and tWRKY4 are capable of binding
DNA molecules with the W-box (TTGAC)
element recognized by other WRKY
proteins. Expression of the tWRKY3 and tWRKY4 genes could be rapidly induced not only by TMV
infection but also by SA or its biologically active analogues that are capable of inducing pathogenesis-related genes and enhanced resistance. Interestingly, induction of both genes by TMV
infection was still observed in resistant tobacco plants expressing the bacterial
salicylate hydroxylase gene (nahG), although the levels of induction appeared to be reduced. Identification of pathogen- and SA-induced genes encoding WRKY
DNA-binding proteins should facilitate future studies on the regulation and functions of this novel group of
DNA-binding proteins.