Changes in carbon allocation and expression of carbon transporter genes inBetula pendula Roth. colonized by the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr.

Comparative analyses of aspects of the carbon (C) physiology and the expres sion of C transporter genes in birch (Betula pendula Roth.) colonized by th e ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. were performed usi ng mycorrhizal (M) and non-mycorrhizal (NM) plants of similar foliar nutrie nt status. After six months of growth, the biomass of M plants was signific antly lower than that of NM plants. Diurnal C budgets of both sets of plant s revealed that M plants exhibited higher rates of photosynthesis and root respiration expressed per unit dry weight. However, the diurnal net C gain of M and NM plants remained similar. Ectomycorrhizal roots contained higher soluble carbohydrate pools and increased activity of cell wall invertase, suggesting that additional C was allocated to these roots and their ectomyc orrhizal fungi consistent with an increased sink demand for C due to the pr esence of the mycobiont. In M roots, the expression of two hexose and one s ucrose transporter genes of birch were reduced to less than one-third of th e expression level observed in NM roots. Analysis using a probe against the birch ribosomal internal transcribed spacer region revealed that M roots c ontained 22% less plant RNA than NM roots. As the expression of birch hexos e and sucrose transporter genes was reduced to a much greater extent, this suggests that these specific genes were down-regulated in response to alter ations in C metabolism within M roots.