Effect of Validamycin A on the Production of Cellulase, Xylanase and Polygalacturonase by Rhizoctonia solani

Validamycin A decreased the biomass and extracellular protein produced by cultures of Rhizoctonia solani A79 grown on medium containing cellulose as the carbon source. In the presence of the antibiotic, the production of inducible enzymes active on filter paper (FPase), carboxymethylcellulose (CMCase) and (cellobiase) was decreased, but not the production of constitutive enzymes involved in hemicellulose (xylanase) and pectin (polygalacturonase) degradation. When incorporated in the assay mixtures, validamycin A did not affect the activity of the above enzymes, but the antibiotic did act as a substrate for cellobiase. Validamycin A did not affect the distribution of cellulase enzymes in stationary phase cultures of R . solani, and incorporation of inositol in the culture medium did not reverse the effect of the antibiotic on cellulase production. Addition of inositol to the medium decreased the activities of FPase and CMCase in the culture supernatant but had no effect on cellobiase activity. The results are discussed in relation to the way validamycin A controls rice sheath blight.