Physiology of organic nitrogen acquisition by ectomycorrhizal fungi and ectomycorrhizas

Abstract

Ectomycorrhizal fungi are symbiotically associated microorganisms which ecological importance has been repeatedly demonstrated. There has been 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 and ericoid fungi. Studies on amino acid transport through the plasma membrane have demonstrated the ability of ectomycorrhizal fungi to take up the products of proteolytic activities. Investigations on intracellular 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 translocation of amino acids in symbiotic tissues has been established by experiments using isotopic tracers, although the exact nature of the nitrogenous compounds transferred at the symbiotic interface remained unclear. One of the main future challenges in the physiology of organic nitrogen acquisition is to determine the nature, the regulation and the location of N-compound transporters at the soil-fungus and fungus-plant interfaces. The molecular approach which is just emerging in this particular research area will greatly improve our knowledge. Future research should also address the extent of competition between different ectomycorrhizal species and between different microbial populations for organic nitrogen.

Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

“…Our current understanding of both flux patterns among fungal amino acids and associated isotopic fractionation is insufficient to determine the ultimate causes of isotopic patterns in foliage and fruiting bodies. To distinguish the relative importance of these mechanisms would require an estimate of the fluxes of different forms of N from EMF to the host and soil, and a more detailed understanding of the mechanisms of N transfer between EMF and their hosts (Smith & Smith, 1990;Chalot & Brun, 1998). The use of 15 N tracers in fungal-plant laboratory and field studies combined with laboratory studies at natural abundance levels (Emmerton et al, 2001a) is needed to elucidate these mechanisms.…”

Isolation of bacteria from ectomycorrhizae of Tuber aestivum Vittad

“…The soil space directly affected by ectomycorrhizae is called mycorrhizosphere (Linderman 1988) and is characterized by high biological activity (Chalot, Brun 1998). Presence of some bacteria may enhance the formation of ectomycorrhizal structures on the roots and was shown to change gene expression of the ectomycorrhizal fungal hyphae (Deveau et al 2007).…”

Mycorrhizal fungi supply nitrogen to host plants in Arctic tundra and boreal forests: 15N is the key signalThis article is one of a selection of papers in the Special Issue on Polar and Arctic Microbiology.

“…Arbuscular mycorrhizal fungi lack the proteolytic capabilities (Chalot and Brun 1998) present in many ECM and ericoid mycorrhizal fungi (Read and Perez-Moreno 2003). Although direct measures of carbon allocation to arbuscular mycorrhizal fungi are few (Johnson et al 2002;Gavito and Olsson 2003), the available evidence and the much smaller spatial extent of arbuscular mycorrhizal fungal hyphae relative to ECM fungal hyphae indicate that allocation to arbuscular mycorrhizal fungi is less than to ECM fungi.…”