plant surface microbiology.pdf

384
Uwe Nehls
the utilization of alternative nitrogen sources under nitrogen limitation (e.g.,
Tazebay 1997). Nitrogen-dependent gene repression is presumably regulated
by the internal nitrogen status of cells, and not the external nitrogen availability.
Either, the intracellular ammonium concentration (Ter Schure et. al.
2000) and/or the activity of the glutamine synthetase (Sophianopoulou and
Diallinas 1995) are supposed to sense the endogenous nitrogen status.
In ectomycorrhizal fungi, nitrogen importer gene expression is presumably
also regulated by the internal nitrogen status of the hyphae (Nehls et. al. 1999;
Javelle et al. 2001; Wipf et al. 2002). This could indicate how nitrogen uptake by
soil-growing hyphae and nitrogen export by hyphae of the Hartig net might
be managed (Fig. 3). Since the nitrogen content of forest soil is quite low and
part of the nitrogen is transported to other parts of the growing fungal colony
(e.g., mycorrhizas), soil-growing hyphae are presumably nitrogen-limited,
resulting in a low endogenous nitrogen status and a strong expression of
nitrogen importer genes. On the other hand, mycorrhizas are well supplied
with nitrogen by soil-growing hyphae, thus revealing a high nitrogen status
and a strongly reduced nitrogen importer gene expression. This nitrogen-
Fig. 3. Regulation of fungal nitrogen uptake from soil and nitrogen excretion at the
plant/fungus interface: a model. Nitrogen export to other parts of the fungal colony
together with a low nitrogen content in soil results in a low endogenous nitrogen state in
soil growing hyphae. In consequence, nitrogen importer genes are highly expressed and
nitrogen uptake capacity is high. Nitrogen import from soil growing hyphae causes a
high endogenous nitrogen status in hyphae of the Hartig net. This results in a repression
of nitrogen importer gene expression and, together with posttranslational inactivation
processes, in a low nitrogen uptake capacity. Together with export mechanisms, this
leads to a net export of nitrogen at the plant/fungus interface