No abstract

Although the function of the extramatrical mycelium of ectomycorrhizal fungi is considered essential for the acquisition of nitrogen by forest trees, gene regulation in this fungal compartment is poorly characterized. In this study, the expression of the nitrate transporter gene nrt2 from the ectomycorrhizal basidiomycete Hebeloma cylindrosporum was shown to be regulated by plant host and carbon sources. In the presence of a low fructose concentration, nrt2 expression could not be detected in the free-living mycelium but was high in the extramatrical symbiotic mycelium associated to the host plant Pinus pinaster. In the absence of nitrogen or in the presence of nitrate, high sugar concentrations in the medium were able to enhance nrt2 expression. Nevertheless, in the presence of high fructose concentration, high ammonium concentration still completely repressed nrt2 expression indicating that the nitrogen repression overrides sugar stimulation. This is the first report revealing an effect of host plant and of carbon sources on the expression of a fungal nitrate transporter-encoding gene.

The symbiotic basidiomycete Hebeloma cylindrosporum is a model fungal species used to study ectomycorrhizal symbiosis at the molecular level. In order to have a vital marker, we developed a green fluorescent protein (GFP) reporter system efficiently expressed in H. cylindrosporum using the sgfp coding region bordered by two introns fused to the saprophytic basidiomycete Coprinopsis cinerea cgl1 promoter. Expression of this reporter system was tested under different environmental conditions in two transformants, and glucose was shown to repress gfp expression. Such a reporter system will be used in plant-fungus interaction to evaluate sugar supply by the plant to the compatible mycorrhizal symbiont and to compare the expression of various genes of interest in the free-living mycelia, in the symbiotic (mycorrhizas) and the reproductive (fruit bodies) structures formed by H. cylindrosporum.