Pseudomonas syringae Increases Water Availability in Leaf Microenvironments via Production of Hygroscopic Syringafactin

Abstract

The epiphytic bacterium Pseudomonas syringae strain B728a produces the biosurfactant syringafactin which is hygroscopic. The water absorbing potential of syringafactin is high. At high relative humidities, syringafactin attracts 250% of its weight in water but is less hygroscopic at lower relative humidities. This suggests that syringafactins benefit to the producing cells is strongly context-dependent. The contribution of syringafactin to the water availability around cells on different matrices was assessed by examining water availability biosensor strains that express gfp via the water-stress activated proU promoter. Wild-type cells exhibited significantly less GFP fluorescence than a syringafactin-deficient strain, on humid but dry filters as well as on leaf surfaces indicating higher water availability. When infiltrated into the leaf apoplast, wild-type cells also subsequently exhibited less GFP fluorescence than a syringafactin-deficient strain. These results suggest that the apoplast is a dry, but humid environment and that, just as on dry but humid leaf surfaces, syringafactin increases liquid water availability and reduces the water stress experienced by P. syringae.\n\nIMPORTANCEMany microorganisms, including the plant pathogen Pseudomonas syringae, produce amphiphilic compounds known as biosurfactants. While biosurfactants are known to disperse hydrophobic compounds and reduce water tension, they have other properties that can benefit the cells that produce them. Leaf colonizing bacteria experience frequent water stress since liquid water is only transiently present on or in leaf sites that they colonize. The demonstration that syringafactin, a biosurfactant produced by P. syringae, is sufficiently hygroscopic to increase water availability to cells, thus relieving water stress, reveals that P. syringae can modify its local habitat both on leaf surfaces and in the leaf apoplast. Such habitat modification may be a common role for biosurfactants produced by other bacterial species that colonize habitats that are not always water saturated such as soil.