Study of nitrogen and carbon transfer from soil organic matter to Tuber melanosporum mycorrhizas and ascocarps using 15N and 13C soil labelling and whole-genome oligoarrays

Abstract

We previously showed by 13CO2 host labelling that almost all of the constitutive carbon allocated to the truffles originated from the host. The objective of this present work was to determine the putative capacity of T. melanosporum ectomycorrhizas and ascocarps to use soil carbon and to uptake or assimilate soil nitrate. The current investigation involved 13C and 15N soil labelling by incorporating labelled leaf litter and expression of genes involved in carbon and nitrogen metabolism in ascocarps and ectomycorrhizas. The ascocarps harvested in the labelled plots were highly enriched in 15N but were almost never enriched in 13C. The main source of soil mineral nitrogen was nitrate. A nitrate transporter, one nitrate reductase and a nitrite reductase were well expressed in ectomycorrhizas. Several genes involved in aminoacid synthesis or in transamination processes were also well expressed in ectomycorrhizas. No nitrate transporter was expressed in ascocarps where the CAZyme genes upregulated were mainly Glycosyltransferases involved in saccharide transfer. Ascocarps did not exhibit saprotrophic capacity for C, supporting previous results from 13CO2 host labelling showing that C is provided by the host tree. The 15N present in the ascocarps after soil labelling is supplied as ammonium or aminoacids by the ectomycorrhizas, which are able to uptake, reduce and metabolize nitrate.