Gluconacetobacter diazotrophicus exopolysaccharide protects bacterial cells against oxidative stress in vitro and during rice plant colonization

The endophytic bacterium Gluconacetobacter diazotrophicus produces exopolysaccharides (EPS) that are required for biofilm formation and colonization of rice seedlings. Here we investigated whether EPS produced by the G. diazotrophicus strain Pal5 protects the bacteria against free radicals. EPS-mediated protection of Pal5 cells against oxidative damage was evaluated by cell counting and fluorescence microscopy. Rice seedling inoculation studies were performed to investigate the antioxidant activity of EPS during plant colonization. The expression of three bacterial antioxidant genes during plant colonization was also monitored. Free radical activities were reduced in vitro by the addition of Pal5 EPS. An EPS-defective Pal5 mutant was hypersensitive to H2O2 and addition of purified EPS reversed this phenotype. Addition of EPS at the inoculation time increased colonization efficiency by the mutant strain and a similar effect was observed after addition of the antioxidant ascorbic acid. qPCR profiles of sodA, gor, and katE gene expression in the mutant confirmed the role of EPS during the initial plant colonization. Our results indicated that EPS produced by G. diazotrophicus protects the bacterial cells against oxidative stress in vitro and during colonization of rice plants.