The hygroscopic biosurfactant syringafactin produced by Pseudomonas syringae enhances fitness on leaf surfaces during fluctuating humidity

Biosurfactant production by bacteria on leaf surfaces is poorly documented, and its role in this habitat has not been explored. Therefore, we investigated the production and fitness benefits of syringafactin by Pseudomonas syringae pv. syringae B728a on leaves. Syringafactin largely adsorbed to the waxy leaf cuticle both when topically applied and when produced by cells on plants. Syringafactin increased the rate of diffusion of water across isolated cuticles and attracted water to hydrophobic surfaces exposed to high relative humidity due to its hygroscopic properties. While a wild-type and syringafactin mutant exhibited similar fitness on bean leaves incubated in static conditions, the fitness of the wild-type strain was higher under fluctuating humidity conditions typical of field conditions. When co-inoculated onto either the host plant bean or the non-host plant romaine lettuce, the proportion of viable wild-type cells recovered from plants relative to that of a mutant unable to produce syringafactin increased 10% over 10 days. The number of disease lesions incited by the wild-type strain on bean was also significantly higher than that of the syringafactin mutant. The production of hygroscopic biosurfactants on waxy leaf surfaces apparently benefits bacteria by both attracting moisture and facilitating access to nutrients.