Recent advances in exploring physiology and biodiversity of ectomycorrhizas highlight the functioning of these symbioses in ecosystems

Ectomycorrhizas. the dominating mycorrhizal symbiosis in boreal, temperate and some tropical forests; are formed by 5000-6000 species of the asco- and basidiomycetes. This high diversity of fungal partners allows optimal fora ging and mobilisation of various nitrogen and phosphorus forms from organic soil layers. In this review, two approaches to study the functioning of th is multitude of symbiotic associations are presented. On selected culture m odels, physiological and molecular investigations have shown that the suppl y of hexoses has a key function in controlling the plant-fungus interaction via partner-specific regulation of gene expression. Environmental factors which affect fungal carbon supply, such as increased nitrogen availability, also affect mycorrhiza formation. Based on such laboratory results, the ad aptative capability of ectomycorrhizas to changing field conditions is disc ussed. The second approach consists of analysing the distribution of mycorr hizas in ecosystem compartments and to relate distribution patterns to vari ations of ecological Factors. Recent :advances in identification of fungal partners in ectomycorrhizas by analysing the internal transcribed spacer of ribosomal DNA are presented, which can help to resolve sampling problems i n field studies. The limits of the laboratory and the field approaches are discussed. Despite some problems, this combined approach is the most promis ing. Direct investigation of gene expression. which has been introduced for soil bacteria, will be difficult in the case of mycorrhizal fungi which co nstitute organisms with functionally varying structures. (C) 2000 Federatio n of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.