Sucrose is a common carbon source in nature, which is utilized in plants as a transport sugar and metabolized by microbes. One microbe which thrives on sucrose during infection is the corn smut fungus Ustilago maydis. U. maydis has a dimorphic lifestyle. It grows saprotrophically in a haploid yeast form, while together with a compatible mate it forms a dikaryotic hyphae, which infects the plant. It infects Zea mays in a biotrophic manner, resulting in the formation of tumors that spread spores. Till now, the sucrose metabolism of U. maydis during the different stages of the life cycle is not described in detail. So far it was assumed that U. maydis uses a high-affinity sucrose transporter (Srt1) for import, a dual localized invertase (Suc2), which can act both intra- and extracellularly and an additional intracellular invertase that we termed Suc1. The overall aim of this study was to understand the sucrose metabolism in the biotrophic plant pathogen Ustilago maydis.

We identified relevant enzymes bioinformatically and confirmed their activities by yeast complementation assays. Deletion mutants were generated to verify the observations. Axenic growth experiments on media supplemented with sucrose were conducted with U. maydis deletion mutants growing in the yeast form to determine the need of transporters and invertases. Maize infections with deletion mutants were conducted to study their relevance during infection.

A combination of bioinformatics and yeast complementation assays revealed that U. maydis employs additional two intracellular invertases. This results in a total of four invertases of which three are only intracellular and one can be secreted but also has an intracellular form. Additionally, to the known transporter Srt1, U. maydis utilizes Agt1 for sucrose import, which was described in S. cerevisiae as a maltose transporter. Growth experiments with deletion mutants uncovered, that Ima1 and Agt1 are the main responsible invertase and transporter, respectively, during axenic growth on sucrose.

We display that U. maydis employs a complex toolbox of invertases and transporters, which may result from its different life stages and reflect the competition between the plant immune system and the infection strategy of the fungus.