Abstract: Salicylic acid（SA）plays a pivotal role in plant resistance to pathogens. Salicylic acid carboxyl methyltransferase（SAMT）is an enzyme for catalyzing the formation of methyl salicylate（MeSA）from SA. Here，we cloned a SAMT gene named as CsSAMT-1，which was associated with Huanglongbing（HLB）infection. In HLB-susceptible Jincheng orange（Citrus sinensis Osbeck），the expression level of CsSAMT-1 was highly induced by HLB infection，in contrast to the constitutive expression of CsSAMT-1 in a HLB-resistant variety，sour pomelo（C. grandis Osbeck）. CsSAMT-1 was also induced by SA and methyl jasmonate（MeJA）in both Jincheng orange and sour pomelo. Moreover，the expression level of CsSAMT-1 in the vein was higher than that in the mesophyll. The contents of SA and MeSA in the vein of sour pomelo were 11.85 and 18.18 ng ? g-1，respectively，which were significantly higher than that（0.82 and 0.0072 ng ? g-1）in the vein of Jingcheng orange. After HLB infection，the contents of both SA and MeSA were markedly increased in Jincheng orange；while in sour pomelo，SA content was decreased and MeSA remained unchanged. Taken together，our results suggest that CsSAMT-1 may confer resistance to HLB through mediating the conversion of SA and MeSA in citrus.

Key words: citrus, HLB, SAMT, MeSA, gene expression