Neofunctionalization of the secreted Tin2 effector in the fungal pathogen Ustilago maydis

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Abstract

Plant-pathogenic fungi hijack their hosts by secreting effector proteins. Effectors serve to suppress plant immune responses and modulate the host metabolism to benefit the pathogen. Smut fungi are biotrophic pathogens that also parasitize important cereals, including maize1. Symptom development is usually restricted to the plant inflorescences. Ustilago maydis is an exception in its ability to cause tumours in both inflorescences and leaves of maize, and in inducing anthocyanin biosynthesis through the secreted Tin2 effector2,3. How the unique lifestyle of U. maydis has evolved remains to be elucidated. Here we show that Tin2 in U. maydis has been neofunctionalized. We functionally compared Tin2 effectors of U. maydis and the related smut Sporisorium reilianum, which results in symptoms only in the inflorescences of maize and fails to induce anthocyanin. We show that Tin2 effectors from both fungi target distinct paralogues of a maize protein kinase, leading to stabilization and inhibition, respectively. An ancestral Tin2 effector functionally replaced the virulence function of S. reilianum Tin2 but failed to induce anthocyanin, and was unable to substitute for Tin2 in U. maydis. This shows that Tin2 in U. maydis has acquired a specialized function, probably connected to the distinct pathogenic lifestyle of this fungus.

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Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis.

Jörg Kämper, Regine Kahmann, Michael Bölker … (2006)

Ustilago maydis is a ubiquitous pathogen of maize and a well-established model organism for the study of plant-microbe interactions. This basidiomycete fungus does not use aggressive virulence strategies to kill its host. U. maydis belongs to the group of biotrophic parasites (the smuts) that depend on living tissue for proliferation and development. Here we report the genome sequence for a member of this economically important group of biotrophic fungi. The 20.5-million-base U. maydis genome assembly contains 6,902 predicted protein-encoding genes and lacks pathogenicity signatures found in the genomes of aggressive pathogenic fungi, for example a battery of cell-wall-degrading enzymes. However, we detected unexpected genomic features responsible for the pathogenicity of this organism. Specifically, we found 12 clusters of genes encoding small secreted proteins with unknown function. A significant fraction of these genes exists in small gene families. Expression analysis showed that most of the genes contained in these clusters are regulated together and induced in infected tissue. Deletion of individual clusters altered the virulence of U. maydis in five cases, ranging from a complete lack of symptoms to hypervirulence. Despite years of research into the mechanism of pathogenicity in U. maydis, no 'true' virulence factors had been previously identified. Thus, the discovery of the secreted protein gene clusters and the functional demonstration of their decisive role in the infection process illuminate previously unknown mechanisms of pathogenicity operating in biotrophic fungi. Genomic analysis is, similarly, likely to open up new avenues for the discovery of virulence determinants in other pathogens.

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Resurrecting ancient genes: experimental analysis of extinct molecules.

Aaron Thornton (2004)

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Pathogenicity determinants in smut fungi revealed by genome comparison.

Kerstin Schipper, Philip Wong, Mathias Walter … (2010)

Biotrophic pathogens, such as the related maize pathogenic fungi Ustilago maydis and Sporisorium reilianum, establish an intimate relationship with their hosts by secreting protein effectors. Because secreted effectors interacting with plant proteins should rapidly evolve, we identified variable genomic regions by sequencing the genome of S. reilianum and comparing it with the U. maydis genome. We detected 43 regions of low sequence conservation in otherwise well-conserved syntenic genomes. These regions primarily encode secreted effectors and include previously identified virulence clusters. By deletion analysis in U. maydis, we demonstrate a role in virulence for four previously unknown diversity regions. This highlights the power of comparative genomics of closely related species for identification of virulence determinants.