The biosurfactant viscosin produced by Pseudomonas fluorescens SBW25 aids spreading motility and plant growth promotion

Abstract

Food security depends on enhancing production and reducing loss to pests and pathogens. A promising alternative to agrochemicals is the use of plant growth-promoting rhizobacteria (PGPR), which are commonly associated with many, if not all, plant species. However, exploiting the benefits of PGPRs requires knowledge of bacterial function and an in-depth understanding of plant-bacteria associations. Motility is important for colonization efficiency and microbial fitness in the plant environment, but the mechanisms employed by bacteria on and around plants are not well understood. We describe and investigate an atypical mode of motility in Pseudomonas fluorescens SBW25 that was revealed only after flagellum production was eliminated by deletion of the master regulator fleQ. Our results suggest that this 'spidery spreading' is a type of surface motility. Transposon mutagenesis of SBW25ΔfleQ (SBW25Q) produced mutants, defective in viscosin production, and surface spreading was also abolished. Genetic analysis indicated growth-dependency, production of viscosin, and several potential regulatory and secretory systems involved in the spidery spreading phenotype. Moreover, viscosin both increases efficiency of surface spreading over the plant root and protects germinating seedlings in soil infected with the plant pathogen Pythium. Thus, viscosin could be a useful target for biotechnological development of plant growth promotion agents.

FleQ Coordinates Flagellum-Dependent and -Independent Motilities in Pseudomonas syringae pv. tomato DC3000

“…tomato DC3000 fleQ mutant is similar to that reported for different bacteria after blocking flagellum production (7,8,13,14) and could be considered sliding, a passive form of surface spreading which relies on bacterial growth and biosurfactant production (3). As has been mentioned before, P. syringae pv.…”

“…Several genera of bacteria, including pseudomonads, exhibit swarming, a complex form of motility on semisolid surfaces which is absolutely dependent on flagella but also influenced by specific environmental signals, the presence of other surface structures, and the production of surfactants (8,13,72). In this work, we present evidence that P. syringae pv.…”

“…tomato DC3000 surface motility, other mechanisms may facilitate its displacement over surfaces, as suggested by the movement exhibited by the fleQ mutant. In this regard, a P. fluorescens SBW25 fleQ mutant exhibited a spreading surface motility that required the expression of viscB and viscC, which encode a nonribosomal peptide synthetase involved in the production of the lipopeptide surfactant viscosin (13). Likewise, we hypothesized that the inactivation of fleQ in P. syringae pv.…”

“…Sliding or spreading by expansion has been observed in a diverse group of bacteria, such as mycobacteria, Bacillus subtilis, Vibrio cholerae, Serratia marcescens, Pseudomonas aeruginosa, or Legionella pneumophila (7-12), in which a strong correlation between sliding and production of surfactants has been established. Furthermore, sliding is easily mistaken for swarming motility and can occur when flagella are disrupted in bacteria that normally would swarm (7,8,13,14). Swarming is a rapid and coordinated movement of bacterial populations over a surface like semisolid agar (0.5% to 1%) and depends on flagella, pili, and the presence of a water film and/or surfactants to enable motility (2-4).…”

“…Recently it has been shown that FleQ is a negative regulator of exopolysaccharide biosynthesis in P. aeruginosa and P. fluorescens SBW25, and its activity is modulated by c-di-GMP (42)(43)(44)(45). Mutation of fleQ in P. fluorescens SBW25 is linked to the loss of flagella and swimming motility but also promotes sliding motility, which is dependent on the biosurfactant viscosin (13). P. aeruginosa exhibited similar behavior on swarming plates when both pili and flagella were deleted, and biosurfactant expression was suggested as a major factor regulating this sliding motility (8).…”