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Pseudomonas fluorescens SBW25 secretes a biosurfactant that facilitates sliding motility and plant growth promotion Alsohim, A.1, Gallie, J.2, Zhang, X.-X.2, Rainey, P.B.2,3 and Jackson, R.W.1 1 School of Biological Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, UK New Zealand Institute for Advanced Study, Massey University at Albany, New Zealand 3 Max Planck Institute for Evolutionary Biology, Ploen, Germany 2 Pseudomonas fluorescens bacteria are common soil inhabitants that favour colonisation of plants, especially the root environment (rhizosphere). P. fluorescens strain SBW25 has been studied to understand the genetic basis of its ecological success in the rhizosphere. The flagellum master regulator, FleQ, is important for negatively regulating wss genes (encoding cellulose extracellular polysaccharide (EPS)) and positively regulating flagellar genes. This indicates that FleQ is probably important for transitional switching of the bacterial lifecycle from the motile planktonic form (in the soil) to the non-motile EPS-producing biofilm form on and within plant tissues. It was discovered that FleQ plays a role in bacterial surface-spreading: mutation of fleQ in SBW25 (SBW25fleQ) revealed a flagellum-independent surface-spreading motility phenotype. Mutagenesis of SBW25fleQ identified several non-motile mutants. PCR analysis identified the mutations to two non-ribosomal synthetase genes known to be involved in production of the biosurfactant viscosin. Complementation of these mutants with fleQ restored surface motility despite a lack of viscosin production. This indicates that SBW25 can move over surfaces by flagellum-dependent swarming and viscosin-dependent sliding motility. Plant growth promotion assays using SBW25fleQ viscosin mutants showed that viscosin is the key bacterial product responsible for suppression of root pathogen effects on plant seedling emergence and development.