Exopolysaccharide is Required by PsJN to Confer Drought-stress Tolerance in Pea

PsJN is a plant symbiotic bacterium that can colonize a broad spectrum of plant hosts and frequently shows beneficial effects on plant growth. Exopolysaccharide (EPS) is known to be important in plant-bacteria interactions. Previously, we reported that EPS is required for PsJN to survive from drought stress and colonize in pea () under drought condition. However, whether EPS is necessary for PsJN to promote plant growth remains unknown. In this work, a comparative study was conducted between the wild-type PsJN and its ∆ mutant that lacks EPS to investigate the role of EPS in PsJN to confer drought-stress tolerance on pea plant. Our results showed that wild type PsJN, but not the ∆ mutant, promoted pea seed germination and seedlings growth under drought stress. Pea plants inoculated with the wild type PsJN had a higher level of drought tolerance, as shown by a better vegetative growth and enhanced nodule formation, than plants inoculated with the ∆ mutant. Moreover, EPS plays a role in the plant colonization under drought stress, because the ∆ mutant was unable to colonize pea seeds and roots as effectively as the wild type PsJN. Further, expression of the EPS biosynthesis genes in the operon of the wild type PsJN was induced by the presence of glucose. Overall, this study demonstrated that PsJN can promote pea plant growth under drought conditions and EPS is required for PsJN to confer beneficial effects to host plant.