A Phytophthora infestansRXLR effector AVR8 suppresses plant immunity by targeting a desumoylating isopeptidase DeSI2

SUMMARY The potato's most devastating disease is late blight, which is caused by Phytophthora infestans . Whereas various resistance ( R ) genes are known, most are typically defeated by this fast‐evolving oomycete pathogen. However, the broad‐spectrum and durable R8 is a vital gene resource for potato resistance breeding . To support an educated deployment of R8 , we embarked on a study on the corresponding avirulence gene Avr8 . We overexpressed Avr8 by transient and stable transformation, and found that Avr8 promotes colonization of P. infestans in Nicotiana benthamiana and potato, respectively. A yeast‐two‐hybrid (Y2H) screen showed that AVR8 interacts with a desumoylating isopeptidase (StDeSI2) of potato. We overexpressed DeSI2 and found that DeSI2 positively regulates resistance to P. infestans , while silencing StDeSI2 downregulated the expression of a set of defense‐related genes. By using a specific proteasome inhibitor, we found that AVR8 destabilized StDeSI2 through the 26S proteasome and attenuated early PTI responses. Altogether, these results indicate that AVR8 manipulates desumoylation, which is a new strategy that adds to the plethora of mechanisms that Phytophthora exploits to modulate host immunity, and StDeSI2 provides a new target for durable resistance breeding against P. infestans in potato.