Centre for Research in Bioscience, Faculty of Health and Life Sciences, The University of the West of England, Frenchay Campus, Bristol BS16 1QY, UK.
Plasmid. 2013 Nov;70(3):420-4. doi: 10.1016/j.plasmid.2013.07.002. Epub 2013 Jul 26.
Pseudomonas syringae pv. phaseolicola (Pph) strain 1302A, a causative agent of halo blight in the common bean Phaseolus vulgaris, contains four native plasmids designated pAV505 (150 kb), pAV506 (50 kb), pAV507 (47 kb) and pAV508 (42 kb). Pph 1302A also contains a 106 kb genomic island PPHGI-1 which shares features with integrative and conjugative elements (ICElands) and carries the effector gene avrPphB (hopAR1) which triggers a defensive response in bean cultivars carrying the matching R3 resistance gene. It has been shown that when Pph 1302A is sequentially inoculated (passaged) through resistant bean cultivar Tendergreen (TG) in which the hypersensitive response (HR) is generated, the three largest plasmids are lost and an extra ∼100 kb plasmid is gained, which tests confirmed to be the 106 kb circular form of PPHGI-1. The aim of the current study was to determine if upon further passaging though bean plants, the plasmid profile of Pph 1302A would alter again and if the missing plasmids had been integrated into the chromosome. Pph 1302A-P6, the strain with the altered plasmid profile was passaged twice through TG and of the four re-isolated strains examined all displayed the plasmid profile associated with wildtype Pph 1302A, that is, all four native plasmids had reappeared and the PPHGI-1 plasmid was absent. This demonstrated that the plasmid composition of Pph 1302A-P6 could indeed change on further exposure to the plant environment and also that the seemingly missing native plasmids were still present within the genome, lending evidence to the theory that they had integrated into the chromosome. Furthermore two of these re-isolated strains had lost PPHGI-1 entirely, meaning they no longer triggered a HR on TG and instead generated a disease response. This study clearly demonstrates the plasticity of the bacterial genome and the extent it can be influenced by the plant environment and conditions generated during the HR.
丁香假单胞菌 pv. phaseolicola（Pph）菌株 1302A 是普通菜豆 Phaseolus vulgaris 叶斑病的病原体，含有四个天然质粒，分别命名为 pAV505（150 kb）、pAV506（50 kb）、pAV507（47 kb）和 pAV508（42 kb）。Pph 1302A 还含有一个 106 kb 的基因组岛 PPHGI-1，它与整合和共轭元件（ICElands）具有相似的特征，并携带效应基因 avrPphB（hopAR1），该基因在携带匹配 R3 抗性基因的菜豆品种中触发防御反应。已经表明，当 Pph 1302A 依次接种（传代）到抗性菜豆品种 Tendergreen（TG）中时，会丢失三个最大的质粒，并获得一个额外的约 100 kb 的质粒，经测试证实为 PPHGI-1 的 106 kb 环状形式。本研究的目的是确定在进一步通过菜豆植物传代后，Pph 1302A 的质粒图谱是否会再次改变，以及缺失的质粒是否已整合到染色体中。Pph 1302A-P6 是质粒图谱发生改变的菌株，通过 TG 传代两次，在重新分离的 4 株菌中，所有菌株均表现出与野生型 Pph 1302A 相关的质粒图谱，即所有 4 个天然质粒均重新出现，而 PPHGI-1 质粒缺失。这表明 Pph 1302A-P6 的质粒组成确实可以在进一步暴露于植物环境时发生变化，并且似乎缺失的天然质粒仍存在于基因组中，这为它们已整合到染色体的理论提供了证据。此外，这两个重新分离的菌株完全丢失了 PPHGI-1，这意味着它们不再在 TG 上引发 HR，而是产生疾病反应。本研究清楚地表明了细菌基因组的可塑性以及它在多大程度上受到植物环境和 HR 过程中产生的条件的影响。
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