Fungal metabolism and free amino acid content may predict nitrogen transfer to the host plant in the ectomycorrhizal relationship between Pisolithus spp. and Eucalyptus grandis

Ectomycorrhizal (ECM) fungi are crucial for tree nitrogen (N) nutrition, however, mechanisms governing N transfer from fungal tissues to the host plant are not well understood. ECM fungal isolates, even from the same species, vary considerably in their ability to support tree N nutrition resulting in a range of often unpredictable symbiotic outcomes. In this study, we used isotopic labelling to quantify the transfer of N to the plant host by isolates from the ECM genus Pisolithus known to have significant variability in colonisation and transfer of nutrients to a host. We considered the metabolic fate of N acquired by the fungi and found that the percentage of plant N acquired through symbiosis significantly correlated to the concentration of free amino acids present in the ECM extra-radical mycelium. Transcriptomic analyses complemented these findings with isolates having high amino acid content and N transfer showing increased expression of genes in amino acid transport and catabolic pathways. These results suggest that fungal N metabolism drives transfer to the host plant in this interaction and that relative N transfer may be possible to predict through basic biochemical analyses. Overall design: In total 43 samples were analysed by RNA-sequencing of which 22 samples included 3-4 biological replicates each of 1 timepoint of symbiosis between Eucalyptus grandis and Pisolithus microcarpus (2 week post contact). The remaining samples included biological replicates of six genotypes of P. microcarpus grown axenically as a control and three biological replicates of axenically grown E. grandis. All organisms and symbiotic compartment was performed on ½ strength Modified Melin Norkans Medium (with 0.1g/L of glucose).