Differential expression of the hexose-regulated fungal genes AmPAL and AmMst1 within Amanita/Populus ectomycorrhizas

At least three fungal 'networks' are formed in ectomycorrhizal symbiosis: the Hartig net, the sheath and extramatrical hyphae. Since these structures have different functions regarding uptake, storage and partitioning of carbohydrates and nutrients, regulation and coordination of metabolic fluxes are of special interest for ectomycorrhizal functioning. To get some insight into structure-specific functions, the Amanita fungal sheath was separated by microdissection from the remaining Populus roots that still contained the Hartig net. Expression of the genes coding for phenylalanine ammonium lyase (AmPAL) and hexose transporter (AmMst1) was investigated separately for both structures by RT-PCR. When constitutively expressed fungal genes were used as a control, the expression of Amn/Mst1 was enhanced about six-fold in hyphae of the Hartig net compared with those of the fungal sheath. By contrast, AmPAL was only barely detectable in the Hartig net, but highly expressed in the fungal sheath. Since in pure fungal culture both genes are regulated by monosaccharides, it is supposed that an apoplastic hexose gradient, between hyphae of the Hartig net and those of the fungal sheath, might regulate fungal gene expression and thus hyphal physiology within ectomycorrhizas. (C) New Phyfologist (2001).