'Candidatus Liberibacter asiaticus' Effector SDE4250 Targets Citrus Proteasome Protein RPN10 to Inhibit Salicylic...

Ubiquitin-proteasome is a conserved mechanism that regulates cellular responses and disease resistance in plants. However, the regulatory role of ubiquitin-proteasome in the pathogenicity of "Candidatus Liberibacter asiaticus" (CLas), the causal agent of citrus huanglongbing (HLB), one of the most serious citrus diseases, remains poorly defined. In this study, we demonstrated that CLIBASIA_04250 (SDE4250) suppresses 26S proteasome activity in citrus, leading to a reduction in salicylic acid (SA) levels and downregulation of SA-responsive genes. Further investigation revealed that SDE4250 specifically targets the citrus 26S proteasome non-ATPase regulatory subunit 4 homologue (CsRPN10) and enhances its degradation via the 26S proteasome pathway. Moreover, overexpression of CsRPN10 inhibited CLas proliferation in citrus hairy roots, which was accompanied by a significant accumulation of SA content and upregulation of related defence genes. Collectively, our findings indicate that SDE4250 targets the citrus 26S proteasome to disrupt SA-mediated citrus immunity.