A Secretory Protein From Candidatus Liberibacter Asiaticus Targets the Ubiquitin-Related Protein CsRUB2 to Disturb...

Citrus Huanglongbing (HLB), caused by 'Candidatus Liberibacter asiaticus' (CaLas), is the most devastating disease affecting the global citrus industry. Here, we reported that the CaLas effector SDE70 promotes HLB pathogenicity by targeting the citrus ubiquitination pathway. Transgenic expression of SDE70 in Wanjincheng orange (Citrus sinensis Osbeck) accelerated early CaLas proliferation, aggravated HLB symptoms, and increased susceptibility to citrus canker induced by Xanthomonas citri subsp. citri (Xcc). These results demonstrate that SDE70 functions as a broad-spectrum suppressor of citrus immunity. Mechanistically, SDE70 physically interacts with CsRUB2, a citrus ubiquitin-related protein. Furthermore, CsRUB2 overexpression in Wanjincheng oranges reduced resistance to HLB but enhanced resistance to citrus canker. Both SDE70 and CsRUB2 elevated salicylic acid (SA) and hydrogen peroxide (H₂O₂) levels in transgenic plants while lowering methyl salicylate (MeSA) levels. CsRUB2 also decreased jasmonic acid (JA). In contrast to the suppressive effect of SDE70, CsRUB2 enhanced the transcription of citrus immunity genes. Transient expression assays further demonstrated that the SDE70-CsRUB2 interaction dysregulates citrus immunity by perturbing SA, MeSA, JA, and H₂O₂ signals. These findings provide a theoretical basis for understanding citrus-CaLas interactions and breeding citrus varieties with broad-spectrum resistance to both HLB and citrus canker.