An α-glucan elicitor from the cell wall of a biocontrol binucleate Rhizoctonia isolate

Abstract

Binucleate Rhizoctonia (BNR) isolate (232-C6) is an effective biocontrol agent for protection of potato from Rhizoctonia canker, a disease caused by Rhizoctonia solani. Production of hydrolytic enzymes is one of the best known inducible defense responses following microbial infection. We isolated and characterized a cell wall α-glucan from BNR, which induces β-1,3 glucanase activities in potato sprouts, the primary site of infection by R. solani. An autoclaving method, previously reported for isolation of oligosaccharide elicitors was used, and the glucan purified by chromatographic techniques. Maximal induction of β-1,3 glucanase activity in potato sprouts was obtained with 250 μg of the α-glucan elicitor after 6 days from inoculation time. Both, BNR mycelium and the α-glucan produced a similar kinetic response of β-1,3 glucanase. However, the α-glucan did not induce phytoalexin accumulation, previously correlated with the defense response. Uronic acids (∼10% with respect to total neutral sugars) were determined and identified as glucuronic acid by high-pH anion-exchange chromatography. Methylation analysis showed that the glucan consists of (1→3) and (1→4)-linked glucose units with preponderance of the first ones. Some of the (1→4) linkages were branched at position 6. The glucan was partially degraded with amyloglucosidase. This, together with the NMR spectra data and the high optical rotation of the original (+195°) and degraded glucans (+175°) proved the α configuration. Further methylation of the amyloglucosidase degraded glucans indicated that they consist of (1→3)-linked glucoses. The present study is the first report on the isolation and characterization of an α-glucan from Rhizoctonia, that may be important as a biocontrol factor.