Mycorrhizal symbiosis between fungal microorganism and plants is an important feature in many ecosystems. Ectomycorrhizal fungi that remain outside of plants cells are important as partners for trees of temperate and boreal forests. The fungal partners are mostly ascomycetes or basidiomycetes that can be generalists or specialists. During symbiosis, the fungus transfers water and minerals to the plant, while the plant transfers carbohydrates to the fungus. Although many aspects of ectomycorrhizal fungi have been studied in detail, molecular genetic work and functional characterization of genes remains scarce.

Here, we aim to establish transformation methods for two ectomycorrhizal fungi, the ascomycete Cenococcum geophilum and the basidiomycete Paxillus involutus, which both undergo symbiosis with different tree species, including the genetically tractable poplar. We test selection markers for transformation, including different antibiotics and carboxin, and show strategies for cloning of the corresponding genes into suitable vectors. An important aspect in genetic transformation of basidiomycetes is the promoter; thus, we further show data on Golden Gate vectors for easy promoter exchange. Concerning transformation methods, we work on both, protoplast-mediated and Agrobacterium-mediated transformation. Since P. involutus is dikaryotic, we aim to generate monokaryons, which are easier to transform in some basidiomycete species due to their single nucleus and the formation of vegetative spores. With a robust transformation system for ectomycorrhizal fungi, we will be able to study gene functions and to learn more about the molecular mechanisms underlying ectomycorrhizal symbiosis, which is of major importance for temperate and boreal forests.