Lowered Virulence to Rice Plants and Decreased Biosynthesis of Gibberellins in Mutants of <i>Gibberella fujikuroi</i> Selected with Pefurazoate

Abstract

Mutants of Gibberella fujihuroi were obtained by selection from a large number of conidia of a wild type field isolate of the fungus on agar plates containing lethal concentration of a sterol demethylation inhibitor, pefurazoate.The mutants isolated from the colonies grown on the plates had higher minimal growth-inhibitory concentrations of pefurazoate and the EC95 values were higher than that for the mother isolate but only a small increase was observed in the EC5a values. Though the slope of the dosage-response curve was changed in the mutants, the mechanism of action of the fungicide in a mutant proved to be its inhibition of sterol 14cc-demethylation as shown by an experiment on inhibition of ergosterol biosynthesis using 14C-acetate. Gibberellin production in the mutants in liquid culture was tested by bioassay of gibberellins and assay of gibberellin A3 using HPLC. Mutants were also tested for their virulence to rice plants.Decreased virulence due to decreased production of gibberellins was observed in the mutants.A mutational change in a site related to 14c -demethylation in ergosterol biosynthesis and to kaurene oxidation in gibberellin biosynthesis in common is discussed.

Species composition, gibberellin production and sensitivity to ipconazole of the <i>Fusarium fujikuroi</i> species complex isolates obtained before and after its launch

“…It has been reported that some DMIs inhibit the gibberellin production of F. fujikuroi. 2,18,19) Takenaka et al 20) reported that the less-sensitive mutants selected by pefurazoate showed low virulence to rice and low gibberellin production. They suggested the involvement of the enzyme component or factor for the demethylation of 24-methylenedihydrolanosterol in the pathway to ergosterol and kaurene oxidation steps in the pathway to gibberellins.…”

Gibberellin biosynthesis inGibberella

“…An example in that direction is given by the unexpected phenotype of mutants resistant to pefurazoate, an inhibitor of sterol biosynthesis. The reduced capacity of these mutants to synthesise gibberellins (Takenaka et al, 1992) suggests a relation between a demethylation step in sterol biosynthesis and the oxidation ofkaurene in the gibberellin pathway.…”

Gibberellins and Other Metabolites of Fusarium fujikuroi and Related Fungi

“…A regulatory connection between the two pathways seems unlikely, but both may be related indirectly, for example, by a common dependeance on enzymes and cofactors. This may explain that some inhibitors of one of the pathways are inhibitors of the other, and the reduced capacity to synthesize gibberellins of the mutants resistant to pefurazoate, an inhibitor of sterol biosynthesis [143].…”