A novel polysaccharide secreted by pal/rim mutants of the phytopathogen fungus Ustilago maydis

Linear (1 → 3)-β-d-glucans and side-chain-branched β-d-glucans are major constituents of capsular materials, with roles in bacterial aggregation, virulence and carbohydrate storage; whereas branched (1 → 3),(1 → 6)-β-d-glucans constitute the most abundant components in the cell walls of fungi. In the present manuscript we describe the chemical characterization of a linear β-d-glucan secreted by mutants of the Basidiomycota biotrophic fungus Ustilago maydis affected in the Pal/Rim pathway involved in pH responses, that in contrast is not made by the wild type strain, at least in measurable amounts. The polysaccharide was obtained from the culture medium of a Δrim13::Cbx mutant of the fungus by ethanol precipitation, and analyzed using FT-IR spectroscopy, NMR spectroscopy in solid and liquid states, HPAEC-PAD chromatography, thin layer chromatography, enzymatic digestion and immunodetection. Our data revealed that, according to its characteristics, the polysaccharide is a (1 → 3)-β-d-glucan, and that the mutants used represent a relevant resource of this polysaccharide with wide applications.