Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois 60637, USA.
Plant Physiol. 2012 Apr;158(4):1803-18. doi: 10.1104/pp.111.190686. Epub 2012 Feb 7.
The bacterium Pseudomonas syringae pv syringae B728a (PsyB728a) uses a type III secretion system (T3SS) to inject effector proteins into plant cells, a process that modulates the susceptibility of different plants to infection. Analysis of GREEN FLUORESCENT PROTEIN-expressing PsyB728a after spray inoculation without additives under moderate relative humidity conditions permitted (1) a detailed analysis of this strain's survival and growth pattern on host (Nicotiana benthamiana) and nonhost (tomato [Solanum lycopersicum]) leaf surfaces, (2) an assessment of the role of plant defenses in affecting PsyB728a leaf surface (epiphytic) growth, and (3) the contribution of the T3SS and specific effectors to PsyB728a epiphytic survival and growth. On host leaf surfaces, PsyB728a cells initially persist without growing, and show an increased population only after 48 h, unless plants are pretreated with the defense-inducing chemical benzothiazole. During the persistence period, some PsyB728a cells induce a T3SS reporter, whereas a T3SS-deficient mutant shows reduced survival. By 72 h, rare invasion by PsyB728a to the mesophyll region of host leaves occurs, but endophytic and epiphytic bacterial growths are not correlated. The effectors HopZ3 and HopAA1 delay the onset of epiphytic growth of PsyB728a on N. benthamiana, whereas they promote epiphytic survival/growth on tomato. These effectors localize to distinct sites in plant cells and likely have different mechanisms of action. HopZ3 may enzymatically modify host targets, as it requires residues important for the catalytic activity of other proteins in its family of proteases. Thus, the T3SS, HopAA1, HopZ3, and plant defenses strongly influence epiphytic survival and/or growth of PsyB728a.
丁香假单胞菌 pv 丁香（PsyB728a）利用 III 型分泌系统（T3SS）将效应蛋白注入植物细胞，从而调节不同植物对感染的易感性。在适度相对湿度条件下（1）对无添加剂喷雾接种后表达绿色荧光蛋白的 PsyB728a 进行详细分析，（2）评估植物防御在影响 PsyB728a 叶片表面（附生）生长中的作用，（3）T3SS 和特定效应物对 PsyB728a 附生存活和生长的贡献。在宿主叶片表面，PsyB728a 细胞最初不生长而持续存在，除非植物用诱导防御的化学物质苯并噻唑预处理，否则在 48 小时后才会出现种群增加。在持续期内，一些 PsyB728a 细胞诱导 T3SS 报告基因，而 T3SS 缺陷突变体显示存活率降低。到 72 小时时，PsyB728a 很少侵入宿主叶片的叶肉区，但内生和附生细菌的生长没有相关性。效应物 HopZ3 和 HopAA1 延迟 PsyB728a 在 N. benthamiana 上的附生生长开始，而在番茄上则促进附生存活/生长。这些效应物定位于植物细胞的不同部位，可能具有不同的作用机制。HopZ3 可能通过酶促修饰宿主靶标，因为它需要其蛋白酶家族中其他蛋白质的催化活性所必需的残基。因此，T3SS、HopAA1、HopZ3 和植物防御强烈影响 PsyB728a 的附生存活和/或生长。
