Proteomic analysis of Rhizobium freirei PRF 81T reveals the key role of central metabolic pathways in acid tolerance

Abstract

Soil acidity strongly affects microbial diversity and represents a constraint to legume production. Rhizobium freirei is a common bean symbiont recognized by its tolerance to environmental stresses, including low pH. The protein expression profiles of R. freirei PRF 81T grown in pH 6.8 and 4.8 were analyzed to clarify the mechanisms responsible for acid tolerance in this species. Bacteria were grown up to exponential phase in tryptone-yeast (TY) medium at pH 6.8 and 4.8. Whole-cell protein extracts were separated by two-dimensional electrophoresis (2-DE), and spots that showed statistical difference in their relative volumes (%vol) between the treatments were selected and excised for identification by MALDI-TOF mass spectrometry. Data showed that protein synthesis was increased at pH 4.8, which consequently raises energy demand. Differential expression of membrane-associated proteins suggested an increased proton extrusion and decreased influx, while central metabolism seemed to be through inducing acid-consuming catabolic pathways and preventing fatty acid biosynthesis. Consequently, the respiratory chain was stimulated along with the production of reactive oxygen species (ROS). The broad range of metabolic pathways modulated by acidified pH endorses the adaptive response to environmental stresses as a multigenic character in R. freirei PRF 81. Moreover, our data emphasize the key role of central metabolism in acid stress tolerance.