Physical and Functional Interactions between Pathogen-Induced Arabidopsis WRKY18, WRKY40, and WRKY60 Transcription Factors

Limited information is available about the roles of specific WRKY transcription factors in plant defense. We report physical and functional interactions between structurally related and pathogen-induced WRKY18, WRKY40, and WRKY60 transcription factors in Arabidopsis thaliana. The three WRKY proteins formed both homocomplexes and heterocomplexes and DNA binding activities were significantly shifted depending on which WRKY proteins were present in these complexes. Single WRKY mutants exhibited no or small alterations in response to the hemibiotrophic bacterial pathogen Pseudomonas syringae and the necrotrophic fungal pathogen Botrytis cinerea. However, wrky18 wrky40 and wrky18 wrky60 double mutants and the wrky18 wrky40 wrky60 triple mutant were substantially more resistant to P. syringae but more susceptible to B. cinerea than wild-type plants. Thus, the three WRKY proteins have partially redundant roles in plant responses to the two distinct types of pathogens, with WRKY18 playing a more important role than the other two. The contrasting responses of these WRKY mutants to the two pathogens correlated with opposite effects on pathogen-induced expression of salicylic acid-regulated PATHOGENESIS-RELATED1 and jasmonic acid-regulated PDF1.2. While constitutive expression of WRKY18 enhanced resistance to P. syringae, its coexpression with WRKY40 or WRKY60 made plants more susceptible to both P. syringae and B. cinerea. These results indicate that the three WRKY proteins interact both physically and functionally in a complex pattern of overlapping, antagonistic, and distinct roles in plant responses to different types of microbial pathogens.