Overexpression of a " Liberibacter Asiaticus" Effector Gene Contributes to Early Colonization in .

Huanglongbing (HLB), caused by " liberibacter asiaticus" (Las), is one of the most devastating diseases in citrus but its pathogenesis remains poorly understood. Here, we reported the role of the LasSDE115 (CLIBASIA_05115) effector, encoded by Las, during pathogen-host interactions. Bioinformatics analyses showed that LasSDE115 was 100% conserved in all reported Las strains but had sequence differences compared with orthologs from other " liberibacter." Prediction of protein structures suggested that the crystal structure of LasSDE115 was very close to that of the invasion-related protein B (IalB), a virulence factor from . Alkaline phosphatase (PhoA) assay in further confirmed that LasSDE115 was a Sec-dependent secretory protein while subcellular localization analyses in tobacco showed that the mature protein of SDE115 (mSDE115), without its putative Sec-dependent signal peptide, was distributed in the cytoplasm and the nucleus. Expression levels of in Las-infected Asian citrus psyllid (ACP) were much higher (∼45-fold) than those in Las-infected Wanjincheng oranges, with the expression in symptomatic leaves being significantly higher than that in asymptomatic ones. Additionally, the overexpression of favored Las proliferation during the early stages (2 months) of infection while promoting the development of symptoms. Hormone content and gene expression analysis of transgenic plants also suggested that overexpressing modulated the transcriptional regulation of genes involved in systemic acquired resistance (SAR) response. Overall, our data indicated that LasSDE115 effector contributed to the early colonization of citrus by the pathogen and worsened the occurrence of Huanglongbing symptoms, thereby providing a theoretical basis for further exploring the pathogenic mechanisms of Huanglongbing disease in citrus.