Exploring the dual roles of sec-dependent effectors from Candidatus Liberibacter asiaticus in immunity of citrus plants

Key message The three SDEs of C Las were expressed in citrus leaves by AuNPs-PEI mediated transient expression system, and promoted the proliferation of C Las and inhibited citrus immunity. Huanglongbing (HLB) is the most severe bacterial disease of citrus caused by Candidatus Liberibacter asiaticus ( C Las). C Las suppress host immune responses and promote infection by sec-dependent effectors (SDEs), thus insight into HLB pathogenesis is urgently needed to develop effective management strategies. In this study, we focused on the roles of SDE4310 , SDE4435 and SDE4955 in citrus. We found that the expression of SDE4310 , SDE4435 and SDE4955 to increase with increasing citrus immune genes CsPR1 , CsPR2 , CsPR5 , CsNPR1 , CsRBOHD , CsMAP3K and CsBIK1 , suggesting that the level of citrus immunity could be judged by the expression of SDE. To further explore the relationship between these three SDEs and citrus immunity, we established a transient expression system in citrus leaves, using gold nanoparticle-polyethyleneimine (AuNPs-PEI) to deliver recombinant plasmid containing SDE4310 , SDE4435 or SDE4955 respectively into citrus leaves. Results showed that SDE4310 , SDE4435 and SDE4955 were successfully expressed in citrus leaves using this transient expression system, and found that SDE4310 , SDE4435 and SDE4955 could promote the C Las proliferation by decreasing the immune gene expression of the citrus. Additionally, we used AuNPs-PEI to deliver siRNA 4310 to citrus cells, significantly reducing the expression of SDE4310 within 3 days. Although the suppression of SDE4310 expression did not inhibit the C Las proliferation, it increased the expression level of CsPR1 , CsNPR1 and CsBIK1 . This is also the first time that AuNPs-PEI has been found to be able to deliver exogenous plasmids into citrus cells and express the target protein, providing a new method for future studies on citrus HLB.