Comparative Analysis of Tolerant and Susceptible Citrus Reveals the Role of Methyl Salicylate Signaling in the Response to Huanglongbing

Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (Las), is the most devastating disease of citrus worldwide. Tolerance to HLB has been observed in some citrus varieties, but its molecular mechanisms are not well understood. Methyl salicylate (MeSA), involved in salicylic acid (SA) signaling, is a critical mobile signal for plant systematic acquired resistance (SAR). This study compared the response of tolerant sour pomelo (Citrus grandis Osbeck) and susceptible Jincheng orange (Citrus sinensis Osbeck) to Las infection. During 18 months of resistance evaluation, sour pomelo displayed significantly delayed and milder symptoms, and tolerated higher levels of Las growth, compared with Jincheng orange. High levels of MeSA were detected in sour pomelo and MeSA responded positively to Las infection. Little MeSA was found in Jincheng orange regardless of Las infection. Correspondingly, the SA content in sour pomelo was significantly higher than that in Jincheng orange. During Las infection, SA levels decreased significantly in sour pomelo but increased in Jincheng orange. These data indicated that MeSA was correlated with tolerance to HLB in citrus. Gene expression analysis showed that CsSAMT1 and CsSABP2-1, involved in the interconversion of MeSA and SA, were related to MeSA accumulation in sour pomelo, and sour pomelo possesses a strong SAR response. Our study indicates that MeSA-mediated SAR plays an important role in citrus tolerance to HLB. This study provides new insights into HLB tolerance in citrus and useful guidance for improving citrus resistance to HLB by manipulation of MeSA signaling in the future.