Comparative Genomics of Plant Fungal Pathogens: The Ustilago-Sporisorium Paradigm

The closely related smut fungi Ustilago maydis, U. hordei, and Sporisorium reilianum f. sp. zeae are facultatively biotrophic basidiomycetes that occur ubiquitously. Teliospores germinate to produce sporidia of different mating type that grow saprophytically and multiply mitotically by budding [1]. For mass proliferation and sexual genetic exchange, successful colonization of economically important crop plants like maize, barley, and oats is a prerequisite. Mating of compatible haploid yeast cells leads to the formation of dikaryotic filaments that are infection competent. These filaments enter their hosts by penetration of the leaf surface [2]. Once inside the plant, filaments multiply in the affected tissue and induce spore formation in tumors near the penetration site (U. maydis) [3] or spread through the entire plant and form spores in inflorescences (S. reilianum and U. hordei) [4,5]. Although presence of the fungus is clearly detected [6], defense reactions of native host plants are very limited, allowing fungal spread initially without major plant tissue damage. In fact, a living host plant is required to provide nutrients for massive fungal proliferation and successful spore formation.