Choline–betaine pathway contributes to hyperosmotic stress and subsequent lethal stress resistance in Pseudomonas protegens SN15-2

Pseudomonas protegens SN15-2, a typical non-spore-forming rhizosphere bacterium, has excellent biocontrol capabilities; thus, it is necessary to explore the stress resistance of SN15-2. The choline-glycine betaine pathway is considered as an important mechanism by which bacteria adapt to stressful environments. In this work, we demonstrated that the expression of the betA and betB genes, which are involved in the choline- glycine betaine pathway in SN15-2, was highly increased by 12-fold and 26-fold, respectively, by hyperosmotic stress and choline treatment. The accumulation of betaine in SN15-2 (5.54 g/L) was significantly higher than that in the mutants Δ betA (3.44 g/L) and Δ betB (2.68 g/L) under hyperosmotic stress and choline treatment. Moreover, choline enhanced the growth of SN15-2 greatly, but it did not enhance the growth of Δ betB under hyperosmotic stress. Choline combined with hyperosmotic adaptation significantly increased the lethal stress resistance of SN15-2 while the resistance of Δ betA and Δ betB was significantly decreased. This research illuminated a strategy underlying the adaptation to osmotic stress in P. protegens and provided an effective method to improve the stress resistance of this species, thus provided a theoretical basis for the practical application of P. protegens SN15-2.