The developmental history of Ustilago maydis: A saprophytic yeast, a mycelial fungus, mushroom-like, and a smut

Abstract

Ustilago maydis is a basidiomycete fungus of the subphylum Ustilaginomycotina. U. maydis is a biotrophic phytopathogen that causes common smut in maize (Zea mays L) and its ancestor teozintle (Zea mays ssp. parviglumis and ssp. mexicana). The economical importance of U. maydis is not as significant as other smuts because it does not cause heavy losses in agriculture but is considered a classic fungal model for studying the mechanism of DNA recombination and mating, other important aspects of fungal development, and the molecular mechanisms of fungal pathogenesis. The life cycle of U. maydis involves one saprophytic phase of haploid yeast form (sporidia) and a hyphal virulent dikaryon formed by mating of two sexually compatible sporidia. This invades the host growing in the hyphal form and finally forms tumors full of diploid teliospores that germinate with the formation of a phragmobasidium and four basidiospores. This cycle has been analyzed at the molecular level. Importantly, it was found that U. maydis may be pathogenic under axenic conditions for plants unrelated to maize, but does not complete the sexual cycle. The dimorphic yeast-to-hypha transition occurs also in vitro induced by growth with fatty acids or acetate and at acidic pH, developing into multicellular individuals, and unexpectedly forms basidiocarps in vitro completing the sexual cycle with the formation of holobasidia instead of phragmobasidia, septal pores, and fibulae. Interestingly, laboratory and natural strains of U. maydis harbor a bacterial symbiont that fixes N2.