Ubiquitin Receptor RPN13-Mediated “Candidatus Liberibacter asiaticus” Virulence Effector Degradation to Positively Regulate Immunity

Ubiquitin–proteasome is a conserved mechanism that regulates cellular responses and disease resistance in plants. However, the regulation role of ubiquitin–proteasome in the pathogenicity of “Candidatus Liberibacter asiaticus” (CLas), the causal agent of citrus Huanglongbing, one of the most serious citrus diseases, remains poorly defined. In this study, we identified a CLas effector, SDE5640 (CLIBASIA_05640), which downregulates salicylic acid signaling pathway genes and partial 26S proteasome genes in SDE5640-transgenic citrus shoots. SDE5640 suppresses proteasome activity to promote bacterial infection. Intriguingly, RPN13, a known component of the proteasome, was first identified as a new ubiquitin receptor. Overexpression of NbRPN13 leads to the degradation of SDE5640 via the 26S proteasome system. We further revealed that SDE5640 can be ubiquitinated in planta and that NbRPN13 promotes the degradation of SDE5640 by binding to ubiquitinated SDE5640. Furthermore, transient coexpression of SDE5640 and CsRPN13 enhances citrus resistance against Xanthomonas. Together, these results demonstrate that RPN13 recognizes the ubiquitination site of SDE5640 and specifically degrades it using the ubiquitin–proteasome system, which provides evidence for the role of ubiquitin–proteasome in CLas–citrus interactions.