Smut fungi are globally distributed plant pathogens that infect agriculturally important crop plants such as maize or potato. During the major part of their life cycle, these Basidiomycetes grow as endophytes inside the host without causing disease symptoms. Only at late stages of infection, they interfere with seed development by producing large amounts of fungal spores in different plant organs. To date, molecular studies on host species of smut fungi are difficult due to lack of genetically tractable model host plants. Therefore, we set out to investigate the only known smut fungus of Brassicaceae hosts: the Arabis-specific smut fungus Thecaphora thlaspeos.

Spore material was obtained from natural infections. In four consecutive years, we have collected spores from seven different European populations in Arabis hirsuta. Interestingly, in contrast to other smut fungi, these teliospores germinate only in the presence of a yet uncharacterized plant signal. The resulting filaments are able to infect germinating seedlings of the original host, as well as the model plant Arabidopsis thaliana. Upon successful penetration, T. thlaspeos fungal hyphae spread systemically along the vasculature and gives rise to spores in the siliques.

We have sequenced the fungal genome, which in most characteristics is a typical smut genome. Interestingly, there are three NEP-like proteins in the genome. NLPs are conserved effectors that can induce necrosis in dicot plants, but with to date unknown virulence function. In contrast to necrotrophic pathogens, T. thlaspeos does not induce necrosis, which will allow us to characterize the virulence function of these NLPs.

In this novel model pathosystem, we aim to characterize the molecular basis of smut infection and plant responses. Ultimately, we hope to exploit the endophytic phase for plant protection.