Chloroplast protein StFC-II was manipulated by a effector to enhance host susceptibility.

Oomycete secretes a range of RxLR effectors into host cells to manipulate plant immunity by targeting proteins from several organelles. In this study, we report that chloroplast protein StFC-II is hijacked by a pathogen effector to enhance susceptibility. RxLR effector is activated during the early stages of colonization. Stable overexpression of in plants suppresses flg22-triggered reactive oxygen species (ROS) burst and enhances leaf colonization by . A potato ferrochelatase 2 (FC-II, a nuclear-encoded chloroplast-targeted protein), a key enzyme for heme biosynthesis in chloroplast, was identified as a target of Pi22922 in the cytoplasm. The pathogenicity of Pi22922 in plants is partially dependent on FC-II. Overexpression of decreases resistance of potato and against , and silencing of in reduces colonization Overexpression of increases heme content and reduces chlorophyll content and photosynthetic efficiency in potato leaves. Moreover, ROS accumulation both in chloroplast and cytoplasm is attenuated and defense-related genes are down-regulated in overexpression transgenic potato and leaves. Pi22922 inhibits E3 ubiquitin ligase StCHIP-mediated StFC-II degradation in the cytoplasm and promotes its accumulation in chloroplasts. In summary, this study characterizes a new mechanism that an oomycete RxLR effector suppresses host defenses by promoting StFC-II accumulation in chloroplasts, thereby compromising the host immunity and promoting susceptibility.