How does a symbiotic fungus modulate expression of its host-plant nitrite reductase?

In the mycorrhizal association, changes in the metabolic activities expressed by the plant and fungal partners could result from modulations in the quantity and nature of nutrients available at the plant-fungus interface. This hypothesis was tested for the nitrite reductase gene in the association Hebeloma cylindrosporum x Pinus pinaster. Transcripts from plant and fungal nitrite reductases and a fungal ammonium transporter were quantified in control uninoculated roots, extraradical mycelia and mycorrhizas formed by either wild-type or nitrate reductase deficient fungal strains. The fungal genes were downregulated in mycorrhizas compared with extraradical hyphae. The plant nitrite reductase was induced only transiently by NO3- in the association with a wild-type strain, but permanently expressed at a high level in mycorrhizas formed by the deficient mutant. These results suggest that reduced nitrogen compounds transferred from the fungus to the root cortical cells repress the plant nitrite reductase, thus highlighting a plant gene regulation by the nutrients available in the Hartig net.