PHYSIOLOGY OF ORGANIC NITROGEN ACQUISITION BY ECTOMYCORRHIZAL FUNGI AND ECTOMYCORRHIZAS

Ectomycorrhizal fungi are symbiotically associated microorganisms whic h ecological importance has been repeatedly demonstrated. There has be en a considerable amount of research aimed at assessing the ability of ectomycorrhizal fungi and ectomycorrhizas to utilize organic nitrogen sources. The fate of soil proteins, peptides and amino acids has been studied from a number of perspectives. Exocellular hydrolytic enzymes have been detected and characterized in a number of ectomycorrhizal a nd ericoid fungi. Studies on amino acid transport through the plasma m embrane have demonstrated the ability of ectomycorrhizal fungi to take up the products of proteolytic activities. Investigations on intracel lular metabolism of amino acids have allowed the identification of the metabolic pathways involved. Possible intracellular compartmentation of amino acids will be examined by immunocytochemistry. Further transl ocation of amino acids in symbiotic tissues has been established by ex periments using isotopic tracers, although the exact nature of the nit rogenous compounds transferred at the symbiotic interface remained unc lear. One of the main future challenges in the physiology of organic n itrogen acquisition is to determine the nature, the regulation and the location of N-compound transporters at the soil-fungus and fungus-pla nt interfaces. The molecular approach which is just emerging in this p articular research area will greatly improve our knowledge. Future res earch should also address the extent of competition between different ectomycorrhizal species and between different microbial populations fo r organic nitrogen. (C) 1998 Federation of European Microbiological So cieties. Published by Elsevier Science B.V.