Abstract: Excessive deposition of callose in sieve pores induced by pathogen is one of main factors causing the phloem plugging. The Phloem Protein（PP）plays an important role in the callose deposition. To explore functions of citrus PP2s in response to Candidatus Liberibacter asiaticus（CLas），which was associated with citrus Huanglongbing（HLB），the encoding sequence and promoter of the CsPP2B15 were cloned. Expression characteristics of CsPP2B15 in the susceptible cultivar Jincheng orange（Citrus sinensis Osbeck）and the resistant cultivar sour pomelo（C. grandis Osbeck）were investigated by qPCR. The results showed that the expression of CsPP2B15 in the young leaf tissues of Jincheng orange was significantly induced by CLas infecton，while it was repressed in sour pomelo. Moreover，the expression level of CsPP2B15 in the mesophyll was higher than that in the midrib. Generally，young leaf and flush was the early infection sites of CLas pathogen. Thus，our results suggest that CsPP2B15 play a role in the early stages of CLas infection. Promoter cis-acting element analysis showed CsPP2B15 promoter has many domains responding to stress and hormone induction. Furthermore，the plant expression vectors containing GUS reporter gene driven by CsPP2B15 promoter were constructed，and introduced into the Poncirus trifoliata（L.）Raf. by Agrobacterium-mediated transformation. Histochemical GUS and quantitative activity analysis showed that CsPP2B15 promoter displayed strong and strict vascular-specific expression pattern in citrus.

Key words: citrus, Citrus Huanglongbing, CsPP2B15, phloem-specificity promoter, gene expression