Khadgi Archana, Zayed Omar, Sagawa Cintia H D, Zhang Fei, Seymour Danelle K, Irish Vivian F
Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, Connecticut, USA.
Botany and Plant Sciences Department, University of California, Riverside, Riverside, California, USA.
Mol Plant Pathol. 2025 May;26(5):e70094. doi: 10.1111/mpp.70094.
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.
像黄龙病（HLB）和柑橘溃疡病这样的细菌性病害严重影响柑橘生产。由“亚洲韧皮杆菌”（CLas）引起的黄龙病会导致树木衰退，而由柑橘溃疡病菌（Xanthomonas citri pv. citri，Xcc）引起的柑橘溃疡病会在叶片和果实上造成坏死性病变。许多细菌病原体分泌效应蛋白，通过上调宿主编码的感病基因来抑制宿主植物免疫并促进发病。Xcc利用III型分泌系统引入效应蛋白，如转录因子样（TAL）效应蛋白PthA4，它可以直接激活宿主感病基因的表达。相比之下，CLas缺乏大多数细菌分泌系统，主要依靠Sec分泌系统来致病。虽然在CLas中已经鉴定出一些Sec分泌蛋白，但它们在导致黄龙病症状方面的直接作用仍未得到证实。几个编码蔗糖转运蛋白的蔗糖最终输出转运体（SWEET）基因是候选感病基因。在这里，我们研究了柑橘SWEET10、SWEET12和SWEET15基因的作用，并表明SWEET15的突变导致三个不同柑橘品种对柑橘溃疡病的感病性降低：卡里佐枳橙（Citrus sinensis 'Washington'甜橙×枳壳）、‘Limoneria 8A’里斯本柠檬（Citrus limon）和‘菠萝’甜橙（C. sinensis）。此外，SWEET15发生突变的里斯本柠檬植株在感染植株中也显示出CLas滴度降低。这些结果表明，SWEET15可能作为一个广谱感病基因，破坏SWEET15基因的活性可能是减轻多种柑橘品种中柑橘溃疡病和黄龙病等细菌性病害的可行方法。
