López-Solanilla Emilia, Bronstein Philip A, Schneider Anna R, Collmer Alan
Department of Plant Pathology, Cornell University, Ithaca, NY 14853, USA.
Mol Microbiol. 2004 Oct;54(2):353-65. doi: 10.1111/j.1365-2958.2004.04285.x.
Pseudomonas syringae pv. tomato DC3000 causes bacterial speck disease in tomato, and it elicits the hypersensitive response (HR) in non-host plants such as Nicotiana tabacum and Nicotiana benthamiana. The compatible and incompatible interactions of DC3000 with tomato and Nicotiana spp., respectively, result in plant cell death, but the HR cell death occurs more rapidly and is associated with effective plant defense. Both interactions require the Hrp (HR and pathogenicity) type III secretion system (TTSS), which injects Hop (Hrp outer protein) effectors into plant cells. Here, we demonstrate that HopPtoN is translocated into tomato cells via the Hrp TTSS. A hopPtoN mutant produced eightfold more necrotic 'speck' lesions on tomato leaves than did DC3000, but the mutant and the wild-type strain grew to the same level in infected leaves. In non-host N. tabacum leaves, the hopPtoN mutant produced more cell death, whereas a DC3000 strain overexpressing HopPtoN produced less cell death and associated electrolyte leakage in comparison with wild-type DC3000. Transient expression of HopPtoN via infection with a PVX viral vector enabled tomato and N. benthamiana plants to tolerate, with reduced disease lesions, challenge infections with DC3000 and P. syringae pv. tabaci 11528, respectively. HopPtoN showed cysteine protease activity in vitro, and hopPtoN mutants altered in the predicted cysteine protease catalytic triad (C172S, H283A and D299A) lost HR suppression activity. These observations reveal that HopPtoN is a TTSS effector that can suppress plant cell death events in both compatible and incompatible interactions.
丁香假单胞菌番茄致病变种DC3000可引发番茄细菌性斑点病，且能在非寄主植物如烟草和本氏烟草中引发超敏反应（HR）。DC3000分别与番茄和烟草属植物的亲和性和非亲和性互作都会导致植物细胞死亡，但HR细胞死亡发生得更快且与有效的植物防御相关。这两种互作都需要Hrp（超敏反应和致病性）III型分泌系统（TTSS），该系统将Hop（Hrp外蛋白）效应子注入植物细胞。在此，我们证明HopPtoN可通过Hrp TTSS转运至番茄细胞中。与DC3000相比，hopPtoN突变体在番茄叶片上产生的坏死“斑点”病斑数量多出八倍，但该突变体和野生型菌株在受感染叶片中的生长水平相同。在非寄主烟草叶片中，hopPtoN突变体导致更多细胞死亡，而与野生型DC3000相比，过表达HopPtoN的DC3000菌株导致的细胞死亡和相关电解质渗漏更少。通过感染PVX病毒载体瞬时表达HopPtoN，可使番茄和本氏烟草植株分别耐受DC3000和丁香假单胞菌烟草致病变种11528的挑战感染，病斑减少。HopPtoN在体外表现出半胱氨酸蛋白酶活性，预测的半胱氨酸蛋白酶催化三联体发生改变（C172S、H283A和D299A）的hopPtoN突变体失去了HR抑制活性。这些观察结果表明，HopPtoN是一种TTSS效应子，可在亲和性和非亲和性互作中抑制植物细胞死亡事件。
