<i>Phytophthora</i> RxLR effector Pi18609 suppresses host immunity by promoting turnover of a positive immune regulator StBBX27

Oomycete pathogens deliver a number of RxLR effector proteins into host plant cells to manipulate immunity by targeting diverse host proteins. Here, we reveal that a Phytophthora infestans RxLR effector Pi18609 targets a potato B-box transcription factor (BBX), StBBX27, which was identified to positively regulate late blight resistance. Silencing the ortholog NbBBX27 in Nicotiana benthamiana or RNA interfering StBBX27 in potato increases host colonization by P. infestans, whereas transient expression of StBBX27 in N. benthamiana or stable overexpression of StBBX27 in potato enhances late blight resistance. Overexpression of StBBX27 in potato activates plant immune responses including upregulation of defense-related genes such as StWRKY8, StOSML13, StCHTB4, and StSERK2, and burst of reactive oxygen species (ROS). In addition, StBBX27 directly binds to the G-box of the WRKY8 and SERK2 promoters and activates their expression, which could be suppressed by Pi18609. Furthermore, we revealed that a ubiquitin E3 ligase COP1 promotes StBBX27 turnover by a proteasome-mediated system. Moreover, Pi18609 promotes the degradation of StBBX27 mediated by COP1. Collectively, this study reveals that StBBX27 positively regulates potato immunity and that is suppressed by effector Pi18609 to promote its turnover. This research extends our knowledge on BBXs function and the mechanisms behind P. infestans effectors suppression of host immunity.