Forests are of outstanding significance for mankind. Not only are they of great economic importance (wood production), but even more they have a great share in carbon sequestration leading to substantial reduction in anthropogenic CO2 release. To guarantee an optimal activity of boreal and temperate forest ecosystems, the mutualistic interaction of tree roots with certain soil fungi (ectomycorrhiza) is a key element of forest ecosystem processes. Elucidation of ectomycorrhizal development and function is thus of major interest for understanding and modulating of tree growth and wood formation.

In ectomycorrhizal symbiosis, plant fine roots as well as ectomycorrhizal fungi undergo a yet only poorly understood developmental process to generate a new, anatomical distinct organ. When soil growing hyphae of ectomycorrhizal fungi recognize an emerging tree fine root, they grow towards it and form a hyphal network that is unsheathing the root. As a consequence of this process the infected host fine root becomes isolated from the surrounding soil. Fungal hyphae also grow into the apoplast of root cortex cells and form a highly branched hyphal network (Hartig net) where plant and fungal cells are tightly connected (Fig. 1).

Plant growth is nutrient dependent but in forest ecosystems large portion of nitrogen and phosphorus is stored in organic form in the litter layer and thus not directly available for trees as they have only a limited degradation capability. In contrast to plant roots, many fungi can utilize organic matter for their nutrient demand. Among other types of mutualistic interactions ectomycorrhizal symbiosis is thus one way to overcome this nutritional limitation of forest trees. Therefore, we are investigating how the nutrient transfer from ectomycorrhizal fungi towards plant fine roots is organized.

Improved tree nutrition by mycorrhizal fungal interaction has its prize that could also affect tree growth. Up to 30% of the net photosynthetic products are necessary to support the fungal partner in symbiosis. We are therefore addressing the questions a) how fungal carbohydrate support is organized in ectomycorrhizas and b) whether and how plant mineral nutrition and fungal carbohydrate support are linked.

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