Abstract: Citrus Huanglongbing（HLB）is the most devastating disease of citrus. The main pathogenic species is the Asian species（‘Candidatus Liberibacter asiaticus’，Las）in citrus industry. Las has a complete Sec-dependent secretion system which could secrete effectors to target proteins or genes of host to regulate the plant resistance. In order to explore the pathogenesis of Las effectors and their host target genes，a Sec-dependent effector gene SDE70（CLIBASIA_02470）of Las was cloned from infected Jincheng. Informatics analysis showed that the SDE70 protein has a total length of 132 amino acids，and a putative 20 amino acids signal peptide was detected in the N-terminal. The analysis of E. coli alkaline phosphatase activity showed that the signal peptide was functional in transferring protein to outside cell. Subcellular localization analysis showed that the SDE70：：GFP fusion protein accumulated in the onion cytoplasm and nucleus. Real-time quantitative PCR analysis found that the expression of SDE70 in the symptomatic veins of tolerant Sour Pomelo was significantly lower than that in the asymptomatic veins，on the contrary，its expression in the symptomatic veins of susceptible Jincheng orange was significantly higher than that in the asymptomatic veins. In Jincheng orange，the expression of SDE70 in roots was significantly higher than that in veins，while its expression in young leaves was significantly lower than that in old leaves. The data showed that SDE70 exhibited significant differential expressions in different disease-resistant varieties，different developmental stages of symptoms，and different tissues. Twenty-six citrus proteins that interact with SDE70 were screened out by yeast two-hybrid assay. Gene Ontology rich analysis indicated that 42.3%，34.6% and 65.4% of these proteins were involved in the cellular biosynthetic process，the molecular function of ribonucleotide binding，and located on the cytoplasmic matrix or membrane，respectively. Further yeast two-hybrid experiments showed that three target proteins interacted strongly with SDE70 and they were involved in plant vesicle transport and ubiquitination-like pathway. These results suggested that SDE70 effector regulate plant response to Las infection through manipulating the signal transduction and/or cytoplasmic transport pathways in citrus.

Key words: citrus, HLB, Sec-dependent secreted protein, Yeast two-hybrid, transient expression, real-time quantitative PCR