Downy mildew, caused by the biotrophic oomycete Hyaloperonospora parasitica, is one of the most devastating diseases affecting global Brassica production. Despite its significant impact, the molecular and cellular mechanisms underlying both compatible and incompatible interactions of H. parasitica and Brassica rapa remain poorly understood. In this study, we identified an H. parasitica RXLR effector, DM459, which demonstrates the ability to induce autophagy by targeting BraATG8i, a key component of autophagosome formation, as confirmed by multiple in vivo and in vitro assays. BraATG8i is a positive regulator of defense against downy mildew, which was determined by the BraATG8i overexpression and RNA interference in B. rapa. Furthermore, the effector DM459 interacts with BraATG8i as well as BraATG4, BraATG3, and BraATG7—core proteins required for autophagosome assembly. This interaction-enhanced autophagy contributed to elevated disease resistance. Moreover, pathogen inoculation or DM459 presence stimulated salicylic acid (SA) biosynthesis, which in turn activated BraATG8i expression and further elevated autophagy. Collectively, our results demonstrated that the effector DM459 triggers autophagy by directly targeting BraATG proteins and simultaneously activates SA signaling, which consequently enhances plant resistance to downy mildew.

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