The Role of Exopolysaccharides in Plant-Pathogen Interactions.

The phytopathogenic bacterium (), one of the members of the soft rot , forms biofilm-like structures known as bacterial emboli when colonizing the primary xylem vessels of the host plants. The initial extracellular matrix of the bacterial emboli is composed of the host plant's pectic polysaccharides, which are gradually substituted by the -produced exopolysaccharides ( EPS) as the bacterial emboli "mature". No information about the properties of EPS and their possible roles in -plant interactions has so far been obtained. We have shown that EPS possess physical properties that can promote the maintenance of the structural integrity of bacterial emboli. These polymers increase the viscosity of liquids and form large supramolecular aggregates. The formation of EPS aggregates is provided (at least partly) by the acetyl groups of the EPS molecules. Besides, EPS scavenge reactive oxygen species (ROS), the accumulation of which is known to be associated with the formation of bacterial emboli. In addition, EPS act as suppressors of the quantitative immunity of plants, repressing PAMP-induced reactions; this property is partly lost in the deacetylated form of EPS. Overall, our study shows that EPS play structural, protective, and immunosuppressive roles during -plant interactions and thus should be considered as virulence factors of these bacteria.