Rhizobial Secretion of Truncated Exopolysaccharides Severely Impairs the Symbiosis.

The symbiosis between R7A and Gifu is an important model system for investigating the role of bacterial exopolysaccharides (EPS) in plant-microbe interactions. Previously, we showed that R7A mutants that are affected at an early stage of EPS synthesis and in lipopolysaccharide (LPS) synthesis induce effective nodules on Gifu after a delay, whereas mutants affected in the biosynthesis of the EPS side chain induce small uninfected nodule primordia and are impaired in infection. The presence of a halo around the mutant when grown on Calcofluor-containing media suggested the mutant secreted a truncated version of R7A EPS. A nonpolar Δ mutant defective in the addition of the first glucose residue to the EPS backbone was also severely impaired symbiotically. Here, we used a suppressor screen to show that the severe symbiotic phenotype of the mutant was due to the secretion of an acetylated pentasaccharide, as both monomers and oligomers, by the same Wzx/Wzy system that transports wild-type exopolysaccharide. We also present evidence that the Δ mutant secretes an oligosaccharide by the same transport system, contributing to its symbiotic phenotype. In contrast, Δ and polar and mutants have a similar phenotype to mutants, forming effective nodules after a delay. These studies provide substantial evidence that secreted incompatible EPS is perceived by the plant, leading to abrogation of the infection process. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.