Citrus Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (Las), is one of the most destructive citrus diseases worldwide, yet how Las causes HLB is poorly understood. Here we show that a Las-secreted protein, SDE15 (CLIBASIA_04025), suppresses plant immunity and promotes Las multiplication. Transgenic expression of SDE15 in Duncan grapefruit (Citrus x paradisi) suppresses the hypersensitive response induced by Xanthomonas citri ssp. citri (Xcc) and reduces the expression of immunity-related genes. SDE15 also suppresses the hypersensitive response triggered by the Xanthomonas vesicatoria effector protein AvrBsT in Nicotiana benthamiana, suggesting that it may be a broad-spectrum suppressor of plant immunity. SDE15 interacts with the citrus protein CsACD2, a homolog of Arabidopsis (Arabidopsis thaliana) ACCELERATED CELL DEATH 2 (ACD2). SDE15 suppression of plant immunity is dependent on CsACD2, and overexpression of CsACD2 in citrus suppresses plant immunity and promotes Las multiplication, phenocopying overexpression of SDE15. Identification of CsACD2 as a susceptibility target has implications in genome editing for novel plant resistance against devastating HLB.

柑橘黄龙病 (HLB) 由亚洲自由杆菌 (Las) 引起，是世界范围内最具破坏性的柑橘病害之一，但人们对 Las 是如何引起 HLB 的了解甚少。在这里，我们发现 Las 分泌蛋白 SDE15 (CLIBASIA_04025) 可抑制植物免疫并促进 Las 增殖。邓肯葡萄柚 ( Citrus x paradisi ) 中 SDE15 的转基因表达可抑制由柑橘黄单胞菌 ( Xanthomonas citri ssp) 诱导的过敏反应。 citri ( Xcc ) 并降低免疫相关基因的表达。 SDE15 还抑制本塞姆氏烟草中由泡泡黄单胞菌效应蛋白 AvrBsT 引发的过敏反应，表明它可能是植物免疫的广谱抑制剂。 SDE15 与柑橘蛋白 CsACD2 相互作用，CsACD2 是拟南芥 ( Arabidopsis thaliana ) 加速细胞死亡 2 (ACD2) 的同源物。 SDE15对植物免疫的抑制依赖于CsACD2 ，柑橘中CsACD2的过度表达抑制植物免疫并促进Las增殖，表型复制SDE15的过度表达。将 CsACD2 鉴定为易感性靶点对于基因组编辑对于新型植物对破坏性 HLB 的抗性具有重要意义。