Fungal and plant gene expression in the Tulasnella calospora - Serapias vomeracea symbiosis provides clues on N pathways in orchid mycorrhizas

• Orchids are highly dependent on their mycorrhizal fungal partners for nutrient supply, especially during early developmental stages. In addition to organic carbon, nitrogen (N) is likely a major nutrient transferred to the plant because orchid tissues are highly N-enriched. We know almost nothing on the N form preferentially transferred to the plant and on the key molecular determinants required for N uptake and transfer. • We identified, in the genome of the orchid mycorrhizal fungus Tulasnella calospora, two functional ammonium transporters and several amino acid transporters but no evidence of a nitrate assimilation system, in agreement with N preference of the free living mycelium grown on different N sources. • Differential expression in symbiosis of a repertoire of fungal and plant genes involved in transport and metabolism of N compounds suggests that organic N may be the main form transferred to the orchid host and that ammonium is taken up by the intracellular fungus from the apoplatic symbiotic interface. • This is the first study addressing the genetic determinants of N uptake and transport in orchid mycorrhiza, and provides a model for nutrient exchanges at the symbiotic interface, which may guide future experiments.