Epiphytic fitness of Pseudomonas syringae pv. syringae on mango trees is increased by 62-Kb plasmids

In this work, we have studied the relationship between presence of indigenous plasmids and copper- and ultraviolet light-resistance in Pseudomonas syringae pv. syringae strains isolated from mango trees. In such strains, plasmids of 62 kb showed cross-hybridisation with specific sequences of the copper-resistance operon copABCD and the UV-resistance determinant rulAB. Copper-resistance was evaluated by determining the minimal inhibitory concentration (MIC) of copper sulphate, as well as analysing the increase of copper-resistant (Cu-r) bacteria in commercial field plots under copper treatments during three years. UV-resistance was evaluated by performing survival curves of P. s. pv. syringae cells exposed to doses of B+A UV-fractions, as well as the persistence of P. s. pv. syringae strains under field conditions studied on mango leaves directly exposed to solar radiation. Survival curves showed that strains harbouring 62-kb plasmids had a higher survival ability under both conditions. The role of the indigenous plasmids of 62 kb in copper- and UV-resistance was confirmed by evaluating survival of transconjugants obtained by mating assays. The most abundant 62-kb plasmid (62.1) carried copper- and UV-resistance determinants homologous to copABCD and rulAB genes, and was shown to be conjugative and very stable. These results strongly suggest an important contribution of 62-kb plasmids to increasing the epiphytic fitness of P. s. pv. syringae on mango plant surfaces.