Molecular mechanism of OsPUB44-mediated activation of WRKY45 in rice immunity

Xanthomonas oryzae pv. oryzae (Xoo) delivers a variety of effectors into rice cells. Expression of Xoo effector XopP in rice cells strongly inhibited immune responses induced by the LysM-type receptor-like kinase OsCERK1 that involves recognition of bacterial peptidoglycan (PGN) and fungal chitin. We found that XopP targets OsPUB44, a rice ubiquitin ligase with a unique U-box domain. XopP interacted with the U-box domain of OsPUB44, and inhibited its ubiquitin ligase activity. Two amino-acid residues specific for the OsPUB44 U-box domain are responsible for the interaction with XopP. Since silencing of OsPUB44 reduced chitin- and PGN-induced immunity, it is likely that OsPUB44 functions as a positive regulator downstream of OsCERK1. We identified PBI1 as an interactor of OsPUB44. Upon chitin perception, PBI1 was ubiquitinated and then degraded, which was suppressed by silencing of OsPUB44 or expression of XopP. These data indicated that PBI1 degradation is mediated by OsPUB44. PBI1 interacted with rice WRKY-type transcription factor WRKY45, a key regulator of rice immunity. In addition, PBI1 suppressed the transcriptional activity of WRKY45. Therefore, it is possible that OsPUB44-mediated degradation of PBI1 leads to activation of WRKY45. (1) Ishikawa et al. (2014) Bacterial effector modulation of host E3 ligase activity suppresses PAMP-triggered immunity in rice. Nat. Commun. 5:5430. (2) Ichimaru et al. (2022) Cooperative regulation of PBI1 and MAPKs controls WRKY45 transcription factor in rice immunity. Nat. Commun. 12:2397.