Ustilago maydis Infection of the Nonnatural Host Arabidopsis thaliana

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Abstract
The experimental infection of Arabidopsis thaliana by the maize phytopathogenic hemibasidiomycete Ustilago maydis under axenic conditions is described. When plantlets were inoculated with mixtures of compatible haploids, the fungus was able to grow on the plant surface of inoculated seedlings in the form of white mycelium and invade the tissues, probably penetrating through stomata; however, it did not form teliospores. Symptoms of disease were increased anthocyanin formation, development of chlorosis, increased formation of secondary roots, induction of malformations in the leaves and petioles, induction of tissue necrosis, and stunting. In several cases, death of the invaded plants occurred. Interestingly, inoculation of single U. maydis haploid strains produced similar symptoms in Arabidopsis plantlets. In contrast, several mutants avirulent to maize also were avirulent or less virulent than wildtype strains on Arabidopsis. Collectively, the reported data suggest that the U. maydis-Arabidopsis pathosystem may constitute a useful experimental model for the analysis of some aspects of the virulence factors of the fungus. With the study of nonhost responses and their comparison to those occurring during maize infection, we will be able to elucidate some obscure aspects of U. maydis pathogenicity in the future.