In the arbuscular mycorrhizas (AM), the symbiotic interface is the site of nutrient exchange where the mycobiont receives up to 20% of the photosynthates of the photobiont as carbohydrates. We have, for the first time, functionally characterised a monosaccharide transporter type, represented by GpMST1, putatively playing a role in this process (Schüßler, A., Martin, H., Cohen, D., Fitz, M., and Wipf, D. 2006. Characterization of a carbohydrate transporter from symbiotic glomeromycotan fungi. Nature 444:933-936.). Together with the arbuscular mycorrhizal (AM) fungi, forming endomycorrhiza with the majority of land plants, Geosiphon pyriformis belongs to the Glomeromycota. Phylogenetically it is deeply embedded within the AM forming fungi. However, Geosiphon associates symbiotically with cyanobacteria instead of – or in parallel to – plants. This unique association therefore represents a symbiosis of an ‘AM fungus’ with a photoautotrophic prokaryote. Due to this unique life style, the Geosiphon-symbiosis offers fundamental advantages for basic research, when compared to the AM. Especially this holds true for investigations of the symbiotic stage: for gene expression studies, the fungal poly(A) mRNA can easily and specifically be isolated. The symbiotic stage corresponds to the intraradical phase in the AM, where fungal mRNA can hardly be separated from the plant mRNA. We use this advantage for the identification of symbiotically expressed ‘AM fungal’ genes. A fungal cDNA yeast-expression library was constructed and used for functional complementation of hexose transport null-mutants of Saccharomyces cerevisiae. The functionally characterised GpMST1 belongs to a new type of transporters, potentially involved in the immense C-transfer from about 80% of all land plants to AM fungi. Interestingly, the transporter probably also transfers cell wall derived pentoses; the uptake of cell wall derived sugars will be discussed. We suppose that the same approach can and should be used to characterise other key genes playing putative roles in the AM and AM-like symbioses.