Abstract
The phytopathogenic bacterium Pectobacterium atrosepticum (Pba), one of the members of the soft rot Pectobacteriaceae, 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 Pba-produced exopolysaccharides (Pba EPS) as the bacterial emboli “mature”. No information about the properties of Pba EPS and their possible roles in Pba-plant interactions has so far been obtained. We have shown that Pba 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 Pba EPS aggregates is provided (at least partly) by the acetyl groups of the Pba EPS molecules. Besides, Pba EPS scavenge reactive oxygen species (ROS), the accumulation of which is known to be associated with the formation of bacterial emboli. In addition, Pba EPS act as suppressors of the quantitative immunity of plants, repressing PAMP-induced reactions; this property is partly lost in the deacetylated form of Pba EPS. Overall, our study shows that Pba EPS play structural, protective, and immunosuppressive roles during Pba–plant interactions and thus should be considered as virulence factors of these bacteria.
Highlights
Plant diseases caused by the members of the soft rot Pectobacteriaceae (SRP) usually manifest as the destruction of the outer parenchymatous tissues of the host plant
In contrast to typical biofilms, the matrix of which consists of bacterial exopolysaccharides (EPS), the initial matrix of bacterial emboli is composed of the plant cell wall polysaccharide fragments
Initial matrix of of bacterial emboli the RG-I of of the thehost hostplant) plant)that thatconstitutes constitutesthe the initial matrix bacterial emis destroyed as the bacterial emboli
Summary
Plant diseases caused by the members of the soft rot Pectobacteriaceae (SRP) usually manifest as the destruction of the outer parenchymatous tissues of the host plant. In the primary xylem vessels, Pectobacterium atrosepticum (Pba), has been shown to form specific multicellular biofilm-like structures known as bacterial emboli, in which bacteria reside in an extracellular matrix [4,6]. Blockage or a reduction in water flow by the EPS/biofilm can enable pathogens to effectively colonize vessels and to interact with the host plant. In addition to their structure-forming capacity, EPS carry out protective functions. We gave special attention to those features of EPS that are of special importance for bacterial embolus development, namely their structure-forming capacity and their detoxification of ROS, the level of which is increased during the formation of bacterial emboli [7]. We assessed the phytoimmune properties (both inducing and suppressive) of the target polymers
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