Cyclic Amp Regulates the Dimorphic Switch in Ustilago Hordei

Abstract

Ustilago hordei,a heterothallic basidiomycete, is the causal pathogen of the covered smut disease of barley [1]. It is a dimorphic fungus whose haploid cells are yeast-like and nonpathogenic, while dikaryons produced upon mating of compatible haploid strains are filamentous and pathogenic [2]. This dimorphic switch is believed to be controlled primarily by the b genes [3]. However, little is known about the signals regulating the switching process from nonpathogenic sporidia to filamentous, infectious hyphae in U. hordei. We report here genetic and biochemical properties of a filamentous, haploid mutant that was obtained by heat shock treatment. Cells expressing the mutant phenotype can be readily reverted to wild type sporidial cells in the presence of cyclic AMP (cAMP). These results indicate an important role for cAMP in the regulation of dimorphic growth of U. hordei, and molecular characterization of this mutation should provide a better understanding of the intracellular mechanisms and signal transduction pathways that control filamentous morphology and pathogenicity in U. hordei.