A Large Tn7-like Transposon Confers Hyperresistance to Copper in Pseudomonas syringae pv. syringae

摘要

Copper resistance mechanisms provide an important adaptive advantage to plant-pathogenic bacteria under exposure to copper treatments. Copper resistance determinants have been described in Pseudomonas syringae pv. syringae (Pss) strains isolated from mango intimately associated with 62-kb plasmids belonging to the pPT23A family (PFP). It has been previously described that the indiscriminate use of copper-based compounds promotes the selection of copper-resistant bacterial strains and constitutes a selective pressure in the evolution of copper resistance determinants. Hence, we have explored in this study the copper resistance evolution and the distribution of specific genetic determinants in two different Pss mango populations isolated from the same geographical regions, mainly from southern Spain, with an average of 20?years of difference. The total content of plasmids, in particular the 62-kb plasmids, and the number of copper-resistant Pss strains were maintained at similar levels over time. Interestingly, the phylogenetic analysis indicated the presence of a phylogenetic subgroup (PSG) in the Pss mango phylotype mostly composed of the recent Pss population analyzed in this study that was strongly associated with a hyperresistant phenotype to copper. Genome sequencing of two selected Pss strains from this PSG revealed the presence of a large Tn 7 -like transposon of chromosomal location, which harbored putative copper and arsenic resistance genes (COARS Tn 7 -like). Transformation of the copper-sensitive Pss UMAF0158 strain with some putative copper resistance genes and reverse transcription-quantitative PCR experiments brought to light the role of COARS Tn 7 -like transposon in the hyperresistant phenotype to copper in Pss.IMPORTANCE Copper compounds have traditionally been used as standard bactericides in agriculture in the past few decades. However, the extensive use of copper has fostered the evolution of bacterial copper resistance mechanisms. Pseudomonas syringae is a plant-pathogenic bacterium used worldwide as a model to study plant-pathogen interactions. The adaption of P. syringae to plant surface environment is the most important step prior to an infection. In this scenario, copper resistance mechanisms could play a key role in improving its epiphytic survival. In this work, a novel Tn 7 -like transposon of chromosomal location was detected in P. syringae pv. syringae strains isolated from mango. This transposon conferred the highest resistance to copper sulfate described to date for this bacterial phytopathogen. Understanding in depth the copper resistance mechanisms and their evolution is an important step for the agricultural industry to improve disease management strategies.