The sigma factor AlgU (AlgT) controls exopolysaccharide production and tolerance towards desiccation and osmotic stress in the biocontrol agent Pseudomonas fluorescens CHA0

A variety of stress situations may affect the activity and survival of plan t-beneficial pseudomonads added to soil to control root diseases. This stud y focused on the roles of the sigma factor AlgU (synonyms, AlgT, RpoE, and sigma (22)) and the anti-sigma factor MucA in stress adaptation of the bioc ontrol agent Pseudomonas fluorescens CHA0. The algU-mucA-mucB gene cluster of strain CHAO was similar to that of the pathogens Pseudomonas aeroginosa and Pseudomonas syringae. Strain CHAO is naturally nonmucoid, whereas a muc A deletion mutant or algU-overexpressing strains were highly mucoid due to exopolysaccharide overproduction. Mucoidy strictly depended on the global r egulator GacA. An algU deletion mutant was significantly more sensitive to osmotic stress than the wild-type CHAO strain and the mucA mutant were. Exp ression of an algU'-'lacZ reporter fusion was induced severalfold in the wi ld type and in the mucA mutant upon exposure to osmotic stress, whereas a l ower, noninducible level of expression was observed in the algU mutant. Ove rexpression of algU did not enhance tolerance towards osmotic stress. AlgU was found to be essential for tolerance of P.fluorescens towards desiccatio n stress in a sterile vermiculite-sand mixture and in a natural sandy loam soil. The size of the population of the algU mutant declined much more rapi dly than the size of the wild-type population at soil water contents below 5%. In contrast to its role in pathogenic pseudomonads, AlgU did not contri bute to tolerance of P.fluorescens towards oxidative and heat stress. In co nclusion, AlgU is a crucial determinant in the adaptation of P. fluorescens to dry conditions and hyperosmolarity, two major stress factors that limit bacterial survival in the environment.