Rhizobial secretion of truncated exopolysaccharides severely impairs the Mesorhizobium-Lotus symbiosis

• Published in: Molecular plant-microbe interactions Vol. 37; no. 9; pp. 662 - 675
• Main Authors: Wightman, Todd, Muszyński, Artur, Kelly, Simon J, Sullivan, John T, Smart, Caitlan J, Stougaard, Jens, Ferguson, Shaun, Azadi, Parastoo, Ronson, Clive W
• Format: Journal Article
• Language: English
• Published: United States American Phytopathological Society 01.09.2024; The American Phytopathological Society
• Subjects: Biosynthesis; exo mutants; exopolysaccharide secretion; Exopolysaccharides; Genotype & phenotype; Lipopolysaccharides; Mesorhizobium; Mesorhizobium-Lotus symbiosis; Mutants; Nodules; Oligosaccharides; Phenotypes; Primordia; Secretion; Symbiosis; Transportation systems; truncated exopolysaccharide
• ISSN: 0894-0282; 1943-7706
• DOI: 10.1094/MPMI-03-24-0024-R

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 exoB 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 non-polar 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 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.