A VvWRKY8-salicylic acid pathway amplification loop confers resistance to Plasmopara viticola in grapevine

To defend against pathogen invasion, plants deploy a variety of strategies, among which salicylic acid (SA), a key plant defense hormone, plays crucial roles in enhancing host resistance to biotrophic and semibiotrophic microbes. Although numerous studies elucidated mechanisms of the SA signaling pathway, many questions remain. In this study, we show that the group IIc WRKY transcription factor VvWRKY8 is involved in grape (Vitis vinifera) defense responses to the oomycete pathogen Plasmopara viticola, as indicated by transcriptome analyses. VvWRKY8 increases the expression of defense-related genes and SA accumulation, thereby promoting grape resistance to P. viticola. Further analyses reveal that VvWRKY8 is recruited to the promoters of VvCBP60g and VvSARD1, which encode two key regulators of SA biosynthesis, and upregulates their transcription. Moreover, VvWRKY8 transcription is induced by SA and by two bZIP transcription factors, VvTGA2a and VvTGA2b, which cooperate with the SA receptors VvNPR1 and VvNPR3 to modulate the expression of SA-responsive genes. Collectively, our results indicate that a positive feedback loop involving VvWRKY8 and the SA pathway components VvCBP60g, VvSARD1, and VvTGA2a/2b functions in response to P. viticola attack in grapevine.