Contribution of orchids to the carbon budget of fungi in germinating seeds of Gymnadenia conopsea.

Orchids critically rely on mycorrhizal fungi for seed germination and seedling development, but the extent to which the fungus benefits from the orchid is less clear. Recent work in arbuscular mycorrhizae has suggested that plants can provide fatty acids (FAs) to fungi, but empirical evidence in orchids remains limited. Here, we combine lipidomic and transcriptomic analyses to test the hypothesis that the germination-promoting fungus Ceratobasidium sp. GS2 receives carbon in the form of FAs from Gymnadenia conopsea seeds during symbiotic germination. Confocal and transmission electron microscopy confirmed the potential of FA transfer from seeds to the fungus. Symbiosis resulted in significant changes in the lipid composition of the fungus, with increased concentrations of FAs in the external mycelium. RNA-seq showed upregulation of genes associated with FA synthesis in seeds and downregulation of de novo FA synthesis genes in fungi 12 d post-symbiosis, indicating that the increased amounts of FAs in the fungus may originate from the seeds. These results indicate that FAs absorbed by hyphae in the colonized inner cortex cells support hyphal growth, providing evidence for directional carbon flow from the orchid seeds to the fungus and supporting a "give now and get now" model of mutualism in orchid-fungus symbioses.