Molecular Plant Pathology – Pathogen Profiles

Ustilago maydis, model system for analysis of the molecular basis of fungal pathogenicity

Christoph W. Basse and Gero Steinberg

Max-Planck-Institute for Terrestrial Microbiology, Department of Organismic Interactions, Karl-von-Frisch Strasse, 35043 Marburg, Germany

Summary:Ustilago maydis, a facultative biotrophic basidiomycete fungus, causes smut disease in maize. A hallmark of this disease is the induction of large plant tumours that are filled with masses of black-pigmented teliospores. During the last 15 years U. maydis has become an important model system to unravel molecular mechanisms of fungal phytopathogenicity. This review highlights recent insights into molecular mechanisms of complex signalling pathways that are involved in the transition from budding to filamentous growth and operate during the pathogenic growth phase. In addition, we describe recent progress in understanding the structural basis of morphogenesis and polar growth in different stages of U. maydis development. Finally, we present an overview of recently identified genes related to pathogenic development and summarize novel molecular and genomic approaches that are powerful tools to explore the genetic base of pathogenicity.

Taxonomy:Ustilago maydis (DC) Corda (synonymous with Ustilago zeae Ung.)-Kingdom Eukaryota, Phylum Fungi, Order Basidiomycota, Family Ustilaginomycetes, Genus Ustilago.

Host range:Infects aerial parts of corn plants (Zea mays) and its progenitor teosinte (Zea maysssp. parviglumis). Maize smut is distributed throughout the world.

Disease symptoms: U. maydis causes chlorotic lesions in infected areas, the formation of anthocyanin pigments, necrosis, hyperplasia and hypertrophy of infected organs. Infection by U. maydis can inhibit development and lead to stunting of infected plants. A few days after infection plant tumours develop in which massive fungal proliferation and the formation of the black-pigmented, diploid teliospores occurs. Under natural conditions tumours predominantly develop on sexual organs (tassels and ears), stems and nodal shoots. Tumours may vary in size from minute pustules to several centimetres in diameter and contain up to 200 billion spores.

U. maydis induced maize tumours. (A) Leaf tumours 10 days after inoculation of a compatible mixture of sporidia into 7-day-old maize plants. (B) Tumors caused by U. maydis in a maize field. Tumors developed in individual kernels and are partially burst, releasing black teliospores. Photograph kindly provided by Jörg Kämper.