Mutations in the SWEET15 Sugar Transporter Gene Affect Response of Citrus to Huanglongbing Disease and Citrus Canker.

Bacterial diseases like huanglongbing (HLB) and citrus canker severely impact citrus production. HLB, caused by "Candidatus Liberibacter asiaticus" (CLas), leads to tree decline, while citrus canker, caused by Xanthomonas citri pv. citri (Xcc) causes necrotic lesions on leaves and fruit. Many bacterial pathogens secrete effector proteins that suppress host plant immunity and promote pathogenesis through the upregulation of host-encoded susceptibility genes. Xcc uses the type III secretion system to introduce effector proteins such as the transcription factor-like (TAL) effector PthA4 that can directly activate host susceptibility gene expression. In contrast, CLas lacks most bacterial secretion systems and relies predominantly on the Sec secretion system for pathogenesis. While some Sec-secreted proteins have been identified in CLas, their direct role in causing HLB symptoms remains unproven. Several Sugars Will Eventually be Exported Transporter (SWEET) genes, encoding sucrose transporters, are candidate susceptibility genes. Here we investigate the roles of the citrus SWEET10, SWEET12 and SWEET15 genes and show that mutations of SWEET15 resulted in reduced susceptibility to citrus canker in three different citrus cultivars: Carrizo citrange (Citrus sinensis 'Washington' sweet orange × Poncirus trifoliata), 'Limoneria 8A' Lisbon lemon (Citrus limon) and 'Pineapple' sweet orange (C. sinensis). Furthermore, Lisbon lemon plants mutated for SWEET15 also showed reduced CLas titre in infected plants. These results suggest that SWEET15 may act as a broad-spectrum susceptibility gene, and disruption of SWEET15 gene activity could be a viable approach to mitigating bacterial diseases such as citrus canker and HLB in a variety of citrus cultivars.