Assessment of Ustilago maydis as a fungal model for root infection studies.

Ustilago maydis is a fungus infecting aerial parts of maize to form smutted galls. Due to its interest as a genetic tool in plant pathology, we evaluated its ability to penetrate into plant roots. The fungus can penetrate between epidermic root cells, forming inter and intracellular pseudohyphae. Root infection didn't provoke gall formation on the maize lines tested, and targeted PCR detection showed that U. maydis, unlike the other maize smut fungus Sporisorium reilianum, has a weak aptitude to grow from the roots up to the aerial part of maize. We also observed that U. maydis can infect Medicago truncatula hairy roots as an alternative host. This plant species is a model host to study root symbiosis, and this pathosystem can provide new insights on root-microbe interactions. Considering that U. maydis could be a soil fungus, we tested its responsiveness to GR24, a strigolactone analogue. Strigolactones are root exuded molecules which activate mitochondrial metabolism of arbuscular mycorrhizal (AM) fungi. Physiologic and molecular analysis revealed that GR24 also increases cell respiration of U. maydis. This result points out that strigolactones could have an incidence on several rhizospheric microbes. These data provide evidences that the biotrophic pathogen U. maydis has to be considered for studying root infection.