Induction Medium to Analyze In Vitro Morphological Changes and Biotrophic Features in Sporidia of Ustilago maydis

Biotrophic fungal pathogens such as Ustilago maydis rely on a host to complete their lifecycle. U. maydis has the advantage, in contrast to obligate biotrophs such as rust fungi, that the saprophytic yeast-like sporidia can be cultivated in artificial media, while the dikaryotic filamentous and infectious stage depends on the host to complete the sexual phase of the lifecycle. Key phenotypes during plant infection include filamentous growth after mating, an increase of fungal biomass in developing tumors, production of an extracellular matrix (ECM) around fungal cells, and melanin formation during sporulation. Furthermore, effector proteins are only highly expressed and secreted during plant infection and function to manipulate plant immunity and metabolism to enable colonization of plant tissue. In general, the display of virulence traits only during in planta growth hampers research progress, and existing artificial media containing glucose don't mimic the plant environment. We noted that other carbon sources (e.g., amino acids, fatty acids, organic acids, secondary plant metabolites, vitamins, and plant hormones) are all present in planta in combination with preferred carbon sources such as glucose or sucrose. Considering these other carbon sources, we developed an artificial medium that can be used to phenocopy in planta development of the fungus. The use of this specialized medium reveals that U. maydis can exploit a variety of different host carbon sources as signals and to support proliferation during pathogenic development.