Antagonism by sugars of the effects of validamycin A on growth and morphology of Rhizoctonia cerealis

Abstract

The effects of 0·2 m validamycin A in reducing internode length (the distance between adjacent branches on a hypha) and colony radial growth rate of Rhizoctonia cerealis were antagonized by some carbon sources (cellobiose, fructose, glucose, lactose, maltose, mannose and sucrose) but not by others (acetate, casamino acids, citrate, galactose, gluconate, glucuronate, glycerol, l -arabinose, mannitol, melibiose, ribose, sorbitol, and xylitol). Antagonism of the effects of the antibiotic by 5 m m sucrose and cellobiose (but not 5 m m fructose, glucose, lactose, maltose or mannose) from the onset of inoculation was correlated with the production by the fungus of extracellular invertase and cellobiase activities. The addition of 3- O -methyl- d -glucose to medium containing 5 m m glucose increased the sensitivity of R. cerealis to the antibiotic, suggesting that antagonism of validamycin A by glucose was dependent on the sugar being metabolized, rather than the sugar inhibiting uptake of the antibiotic. The onset of the response of the fungus to validamycin A appeared to be related to the length of time of exposure to the antibiotic, and mycelium grown on medium containing antibiotic continued to display the ‘inhibited’ phenotype even after growing for several hours on medium lacking antibiotic.