Disruption of the sensor kinase phoQ gene decreases acid resistance in plant growth-promoting rhizobacterium Rahnella aquatilis HX2

Aims: Rahnella aquatilis HX2, a promising plant growth-promoting rhizobacterium (PGPR) in the field, contains genes homologous to the PhoP/PhoQ two-component regulatory system. Although this system regulates stress response in numerous pathogens, PhoP/PhoQ characterization in a PGPR has not received in-depth exploration. Methods and Results: The phoQ gene was mutated in strain HX2 using an in-frame deletion strategy. Compared to the wild type, the phoQ mutant exhibited increased sensitivity to acidic conditions (pH 4.0) in a chemically defined medium and in mild acidic natural soil (pH 5.7). The phoQ mutant also exhibited increased swimming motility under acidic conditions. Acid resistance was restored in the mutant by introducing the phoQ gene on a plasmid. Three acid resistance genes, add, cfa, and fur were downregulated significantly, whereas the chaperone encoding gene, dnak, was upregulated when the phoQ mutant was exposed to acid stress. Conclusions: This study suggested that the PhoP/PhoQ system positively regulates the acid resistance of R. aquatilis HX2.