13C pulse-labeling suggests separate fast and slow dynamics for two amino acid pools in ectomycorrhizal fungi

Abstract Background and Aims Ectomycorrhizal fungi depend on recent photosynthate from their host plants but can also assimilate soil-derived amino acids into fungal protein. The dynamics of different pools of amino acids may differ because their energy cost for acquisition may differ. Methods To study carbon dynamics between Pinus sylvestris and ectomycorrhizal fungi, 13CO2 was traced for 97 days into amino acids and structural carbohydrates of Cortinarius semisanguineus and Lactarius rufus sporocarps. Results Prior to 13C labeling, amino acid δ13C was higher in Cortinarius than Lactarius, which suggests uptake of older soil-derived amino acids by Cortinarius. 13C labeling in amino acids was 58 ± 3% of that in structural carbohydrates. In stepwise regression, 58% of amino acid carbon tracked the δ13C of the structural carbon which peaked on day 16. The other 42% of amino acid carbon peaked later, on day 30, at 19% of the 13C enrichment of structural carbon, indicating two pools of amino acids. Both structural carbohydrates and the ‘fast’ pool of amino acids derived from recent plant sugars. Conclusion It is hypothesized that fast cycling amino acids had simple synthesis pathways such as alanine, aspartic acid and glutamic acid, whereas slow cycling amino acids had multiple enzymatic steps to synthesis, such as lysine, leucine, and isoleucine. This slow pool is more likely than the fast pool to reflect soil-derived amino acids. Studies of amino acid use by ectomycorrhizal fungi should account for the large differences in dynamics and energetic costs between slow and fast cycling amino acids.