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: t he Hartig net, the sheath and extramatrical hyphae. Since these structures have different functions regarding uptake, storage and partitioning of carb ohydrates and nutrients, regulation and coordination of metabolic fluxes ar e of special interest for ectomycorrhizal functioning. To get some insight into structure-specific functions, the Amanita fungal s heath was separated by microdissection from the remaining Populus roots tha t still contained the Hartig net. Expression of the genes coding for phenyl alanine ammonium lyase (AmPAL) and hexose transporter (AmMst1) was investig ated separately for both structures by RT-PCR. When constitutively expressed fungal genes were used as a control, the expr ession 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 barel y detectable in the Hartig net, but highly expressed in the fungal sheath. Since in pure fungal culture both genes are regulated by monosaccharides, i t is supposed that an apoplastic hexose gradient, between hyphae of the Har tig net and those of the fungal sheath, might regulate fungal gene expressi on and thus hyphal physiology within ectomycorrhizas. (C) New Phyfologist ( 2001).