Metabolic Shifts and Nutrient Transfer Patterns in Orchid Seeds During Symbiotic Germination

Symbiotic germination in orchids is a complex biological process driven by a unique dependence on mycorrhizal fungi. It is generally assumed that, due to a lack of endosperm, orchids fully rely on fungi for carbon sources. However, orchid seed embryos store substantial nutrient reserves, but the metabolic functions of these reserves and their roles in establishing symbiosis during germination remain unclear. Here, we used time-series transcriptomics and ultrastructural morphological analyses to investigate the early stages of symbiotic germination in Gymnadenia conopsea, a terrestrial orchid widely distributed across Eurasia. We identify three distinct phases during early seed germination (seed imbibition, fungal invasion and symbiotic establishment) that correspond with pronounced changes in gene expression and energy metabolism. During imbibition, lipid metabolism was already active, leading the oleosomes to fuse into large lipid vacuoles, whereas carbohydrate metabolism became dominant after fungal invasion, with lipid droplets appearing within fungal hyphae. Based on this transcriptomic and morphological evidence, we propose a potential model in which lipid vacuoles facilitate lipid transfer from the seeds to the fungi during the initial invasion, after which the fungi supply carbohydrates to the seeds for further development.