The susceptibility factor Resistance To Phytophthora parasitica 1 negatively regulates Arabidopsis immunity by interacting with the cytochrome P450 protein CYP71B3.

Oomycetes, particularly Phytophthora species, cause destructive plant diseases that severely threaten sustainable crop production. Due to the loss of genotype-specific disease resistance, it is important to identify and understand immune factors that mediate plant susceptibility. Loss-of-function of the susceptibility factor resistance to Phytophthora parasitica 1 (RTP1) leads to broad-spectrum disease resistance in Arabidopsis thaliana (A. thaliana (L.) Heynh.). Through RNA-seq analysis, we determined that CYP71B3, encoding an uncharacterized P450 enzyme, is significantly upregulated in rtp1 mutant plants infected with P. parasitica. Loss-of-function of CYP71B3 led to abolished pathogen-associated molecular pattern (PAMP)-triggered oxidative burst and rendered Arabidopsis more susceptible to diverse pathogens, including the oomycete P. parasitica and bacterial Pseudomonas syringae. Conversely, overexpression of CYP71B3 enhanced plant resistance and PAMP-triggered oxidative burst. CYP71B3 localized in the endoplasmic reticulum and was destabilized by interacting with RTP1 via the I-38 residue, which is essential for its immune function and P450 enzyme activity. The expression of CYP71B3 was regulated by transcription factor bZIP60, which is required for rtp1-mediated resistance to P. parasitica. Our studies indicate that RTP1 mediates plant susceptibility by destabilizing the downstream positive immune factor CYP71B3.