plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 399
of plant growth (Burgstaller 1997). At a neutral pH typical of cell cytosol,
approximately 99 % of ammonium is present as the cation NH 4 + . By definition,
a decrease of one pH unit is accompanied by a tenfold increase of the
ratio NH 4 + :NH3 . Therefore, in spite of the general acceptance that NH 3 can
readily diffuse across natural membranes, it was postulated that ammonium
uptake in cells could also be mediated by other mechanisms.
3.2 Physiology of Ammonium Transport in Ectomycorrhizas
The first evidence that a specific ammonium transport system acts in fungi
came from the works of Hackette et al. (1970). They used the ammonium-analogue
tracer [ 14 C]methylammonium and suggested that an ammonium transporter
acts in the fungus Penicillium chrysogenum. The radioactive ammonium
analogue [ 14 C]methylammonium has been widely used to assay uptake.
Roon et al. (1975) measured an uptake in Saccharomyces cerevisiae which
resulted in a 1000-fold accumulation. In a further study, Dubois and Grenson
(1979) showed that the uptake of ammonium/methylammonium in S. cerevisiae
is mediated by at least two functionally distinct systems, but this study
was hampered by the lack of molecular characterization of the transport systems.
The first ammonium transporter genes characterized were MEP1
cloned in S. cerevisiae (Marini et al. 1994), and AMT1 cloned in Arabidopsis
thaliana (Ninnemann et al. 1994). They belong to a multigenic family, the socalled
Mep/Amt family.
Ammonium mobilization by mycelium from soil sources is directly linked
to hyphal uptake capacities. Using [ 14 C]methylamine, kinetics of ammonium/methylammonium
transport in ectomycorrhizal fungi have been characterized
(Jongbloed et al. 1991; Javelle et al. 1999). A saturable mediated
uptake was obtained, which conformed to simple Michaelis-Menten kinetics,
and was consistent with a carrier-mediated transport. Both pH dependence
and inhibition by protonophores indicate that methylamine transport in P.
involutus is dependent on the electrochemical H + -gradient (Javelle et al.
1999). These results suggest that ammonium uptake is an active (energyrequiring)
process. Comparing the ammonium uptake capacity of the two
partners separately or in symbiosis, it was found that mycelia have much
higher capacities for ammonium uptake than nonmycorrhizal roots and ectomycorrhizal
fungi increase ammonium uptake capacities of their host roots
(Plassard et al. 1997; Javelle et al. 1999).
Nitrogen starvation increased methylamine transport in P. involutus
(Javelle et al. 1999) and similarly, N-starved plants usually showed a faster
NH 4 + net uptake than N-fed plants (Howitt and Udvardi 2000). However, these
studies were hampered by the lack of molecular characterization of the transport
systems involved and their regulation at the molecular level remains to
be clarified.