plant surface microbiology.pdf

21 Carbohydrates and Nitrogen: Nutrients and Signals in Ectomycorrhiza 381
1986; Finlay et al. 1988; Chalot and Brun 1998). In accordance with the predominant
occurrence of ammonium as inorganic nitrogen source in the soil,
most ectomycorrhizal fungi grow better on ammonium than on nitrate in
pure culture (France and Reid 1984; Finlay et al. 1992). Nevertheless, even in
mature forests nitrate could be present in large amounts as a result of bacterial
activity (e.g., open forest areas) or as a result of fertilization in areas with
extensive agriculture (Gessler et al. 1998).
In many forest ecosystems, rates of nitrogen mineralization of litter are low
and consequently, the supply of inorganic nitrogen is often limited (Read
1991). In addition, nitrification is usually slow and the poorly mobile ammonium
ion (Keeney 1980) predominates together with organic nitrogen (e.g.,
amino acids or protein). Important for the establishment of forest ecosystems
is thus, the capability of ectomycorrhizal fungi to exploit (in collaboration
with other soil organisms) organic debris (e.g., litter) as a nutrient source
(Nasholm and Persson 2001).
8 Utilization of Inorganic Nitrogen
For a number of ectomycorrhizal fungi growth on nitrate as sole nitrogen
source (France and Reid 1984; Littke et al. 1984; Plassard et al. 1986) as well
as the presence of nitrate reductase activity (Wagner et al. 1989; Sarjala
1990) have been shown. In saprophytic ascomycetes, the expression of
nitrate reductase is repressed in the presence of a reduced nitrogen source
(e.g., ammonium) and induced only the presence of nitrate. In contrast,
nitrate reductase activity of the ectomycorrhizal fungus Hebeloma cylindrosporum
was similar for nitrate and ammonium-fed hyphae (Scheromm et
al. 1990), indicating a different type of regulation. A nitrate transporter gene
has been identified so far only from H. cylindrosporum (Marmeisse, pers.
comm.).
Two ammonium transporter genes of H. cylindrosporum (HcAMT2 and
HcAMT3) were isolated and functionally characterized in yeast (Javelle et al.
2001). HcAMT2 revealed a K M value of 58 mM and HcAMT3A of 260 mM.
When the fungus was grown under optimal nitrogen conditions (ammonium
concentration >2 mM) the expression of both transporter genes was only
barely detectable while gene expression strongly increases under nitrogen
starvation. Similar results were found in Paxillus involutus, where N starvation
triggered a fourfold increase in methylamine transport after 2 h incubation
in nitrogen-free media (Javelle et al. 1999).
In addition, one ammonium transporter gene (TbAMT1) was isolated from
the ascomycete Tuber borchii (Montanini et al. 2002). Heterologous expression
in yeast revealed a K M value of 2 mM. When exposed to ammonium or nitrate,
the gene was expressed at a basal level while nitrogen depletion resulted in a
slow and only slight increase in gene expression. This expression profile is