The impact of ectomycorrhiza formation on monosaccharide transporter gene expression in poplar roots.

•  By using degenerate primers, five putative poplar monosaccharide transporter genes were isolated from ectomycorrhizas by RT-PCR. The expression profiles of the three most strongly expressed ones are presented in detail. •  Two transporter genes (PttMST1.2 and PttMST2.2) were down-regulated by ectomycorrhiza formation. However, PttMST3.1, which showed 10-times higher expression rates in noninfected roots than any other transporter gene, was up-regulated 12-fold in mycorrhizas. •  While changes in PttMST1.2 and PttMST2.2 expression might be regulated by a fungal metabolite present in axenically grown hyphae, the strong increase of PttMST3.1 expression in mycorrhizas required active plant-fungus interaction. •  Up-regulation of PttMST3.1 by mycorrhiza formation suggests that root cells are able to compete with fungal hyphae for hexoses from the common apoplast during symbiosis, redirecting the sugar-flux back into plant cells whenever the fungal partner does not supply sufficient mineral nutrients. Such a mechanism would enable the plant to link nutrient supply and fungal carbon support at a local level.