Jain Mukesh, Fleites Laura A, Gabriel Dean W
Department of Plant Pathology, University of Florida, Gainesville, FL 32611-0680, U.S.A.
Mol Plant Microbe Interact. 2015 Dec;28(12):1330-7. doi: 10.1094/MPMI-07-15-0145-R. Epub 2015 Nov 5.
'Candidatus Liberibacter asiaticus' is transmitted by psyllids and causes huanglongbing (HLB), a lethal disease of citrus. Most pathogenic 'Ca. L. asiaticus' strains carry two nearly identical prophages similar to SC1 and SC2 in strain UF506. SC2 was observed to replicate as a moderately high-copy excision plasmid encoding a reactive oxygen species-scavenging peroxidase (SC2_gp095), a predicted lysogenic conversion factor. SC2_gp095 was expressed at significantly higher levels in periwinkle than in citrus and was suppressed in psyllids. SC2_gp095 was cloned in a shuttle vector and transformed into Escherichia coli and Liberibacter crescens, a culturable proxy for 'Ca. L. asiaticus'. Transformed L. crescens cells showed 20 to 25% enhanced resistance to H₂O₂on agar plates, 47% greater enzymatic activity, and enhanced growth in liquid cultures. A nonclassical secretion potential was predicted for SC2_gp095 and secretion from L. crescens was confirmed by enzymatic and Western blot analyses. Transient expression of SC2_gp095 in planta resulted in strong transcriptional downregulation of RbohB, the key gatekeeper of the H₂O₂-mediated defense signaling in plants, helping explain the surprisingly long incubation period (years) before HLB symptoms appear in 'Ca. L. asiaticus'-infected citrus. 'Ca. L. asiaticus' peroxidase is likely a secreted, horizontally acquired effector that suppresses host symptom development, a tactic used by most biotrophic plant pathogens.
“亚洲韧皮杆菌候选种”（'Candidatus Liberibacter asiaticus'）由木虱传播，可引发柑橘黄龙病（HLB），这是一种柑橘的致命病害。大多数致病的“亚洲韧皮杆菌候选种”菌株携带两个与UF506菌株中的SC1和SC2相似的近乎相同的原噬菌体。观察到SC2作为一种中等高拷贝的切除质粒进行复制，该质粒编码一种活性氧清除过氧化物酶（SC2_gp095），这是一种预测的溶原转变因子。SC2_gp095在长春花中的表达水平显著高于柑橘，而在木虱中受到抑制。SC2_gp095被克隆到穿梭载体中，并转化到大肠杆菌和渐狭蜡蚧杆菌（Liberibacter crescens）中，渐狭蜡蚧杆菌是“亚洲韧皮杆菌候选种”的一种可培养替代物。转化后的渐狭蜡蚧杆菌细胞在琼脂平板上对过氧化氢的抗性增强了20%至25%，酶活性提高了47%，并且在液体培养中生长增强。预测SC2_gp095具有非经典分泌潜力，通过酶分析和蛋白质免疫印迹分析证实了其从渐狭蜡蚧杆菌中的分泌。SC2_gp095在植物中的瞬时表达导致植物中过氧化氢介导的防御信号传导的关键守门人RbohB的转录强烈下调，这有助于解释在“亚洲韧皮杆菌候选种”感染的柑橘中出现HLB症状之前令人惊讶的长时间潜伏期（数年）。“亚洲韧皮杆菌候选种”过氧化物酶可能是一种分泌的、水平获得的效应子，可抑制宿主症状的发展，这是大多数活体营养型植物病原体所采用的策略。
