Cooperative regulation of PBI1 and MAPKs precisely controls the master transcription factor WRKY45 in rice immunity

The U-box type ubiquitin ligase PUB44 is targeted by the Xanthomonas oryzae XopP effector and positively regulates pattern-triggered immunity in rice. Here we identified PBI1, a protein that interacts with PUB44. Crystal structure analysis indicated that PBI1 forms a four-helix bundle structure. PBI1 also interacts with WRKY45, a key transcriptional activator of rice immunity, and negatively regulates its activity. PBI1 is degraded during the chitin response, and this is suppressed by silencing of PUB44 or expression of XopP, indicating that PBI1 degradation depends on PUB44. These data suggest that PBI1 suppresses WRKY45 activity when cells are in the unelicited state, and during chitin signaling, PUB44-mediated degradation of PBI1 leads to activation of WRKY45. In addition, phosphorylation of WRKY45 by MAP kinases releases WRKY45 from the PBI1-mediated inhibitory effect. These results demonstrate that chitin-induced activation of WRKY45 is regulated by the cooperation between MAP kinase-mediated phosphorylation and PUB44-mediated PBI1 degradation.