WRKY33-mediated indole glucosinolate metabolic pathway confers host resistance against Alternaria brassicicola

The tryptophan (Trp)-derived plant secondary metabolites, including camalexin, 4-hydroxy-indole-3-carbonylnitrile (4OH-ICN), and indole glucosinolate (IGS), show broad-spectrum antifungal activity. However, the upstream regulators of these metabolic pathways among different plant species in response to fungus infection are rarely studied. In this study, our results revealed a positive role of WRKY33 in host resistance to Alternaria brassicicola by directly regulating the transcription of genes involved in the biosynthesis and atypical hydrolysis of IGS both in Arabidopsis and Chinese kale. Indole-3-yl-methylglucosinolate (I3G) and 4-methoxyindole-3-yl-methylglucosinolate (4MI3G) are the main components of IGS. WRKY33 induces the expression of MYB51 and CYP83B1 which promotes the biosynthesis of I3G, the precursor of 4MI3G. Moreover, it also directly activates the expression of CYP81F2, IGMT1, and IGMT2 to drive side chain modification of I3G to produce 4MI3G, which is in turn hydrolyzed by PEN2. However, Chinese kale showed a more severe symptom than Arabidopsis when infected by Alternaria brassicicola. Comparative analyses of the origin and evolution of Trp-metabolism indicate that the loss of camalexin biosynthesis in Brassica crops during evolution might attenuate the resistance of crops to Alternaria brassicicola. As a result, IGS metabolic pathway mediated by WRKY33 becomes essential for Chinese kale to deter Alternaria brassicicola. Our results highlight the differential regulation of Trp-derived camalexin and IGS biosynthetic pathways in plant immunity between Arabidopsis and Brassica crops.One-sentence SummaryPathogen-responsive WRKY33 directly regulates indole glucosinolates biosynthesis and atypical hydrolysis, conferring to host resistance to Alternaria brassicicola in Arabidopsis and Brassica crops.