The lucky few?

Abstract

In contrast to most biotrophic plant pathogens, smut fungi belonging to the family of Ustilaginaceae are facultative parasites that have a yeastlike, saprophytic phase contrasting the filamentous, parasitic growth during plant infection. This dimorphic lifestyle allows easy culturing, reverse genetic modifications and cell biological analyses in the saprophytic stage; impact on virulence or pathogenicity can then been monitored easily in infection assays. Such excellent preconditions made the maize smut fungus Ustilago maydis the most advanced model organism for fungal biotrophs. A recent key step in Ustilago research was the high-quality sequencing and manual annotation of its genome, which identified a wealth of novel virulence genes that are arranged in physical gene clusters. In next steps, the genomes of the maize head smut Sporisorium reilianum f. sp. zeae and the barley covered smut Ustilago hordei have been sequenced. These closely related pathogens exhibit significant differences regarding genome organization and gene equipment, likely reflecting different infection styles compared to U. maydis, whose unique feature is to cause tumours locally to sites of infection in vegetative tissue. Comparative analysis of these genomes as well as transcriptome profiling approaches allowed the identification of regulatory elements controlling pathogenic development as well as of secreted effector genes with important functions in plant interaction. In this chapter, we summarize the most important findings gained by genome analyses of smut fungi and define important questions that need to be addressed in the future.