plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 415
culture (Brun et al. 1993). In the association Douglas fir-Laccaria laccata,the
fungal GS was uniformly detected over the entire section of the ectomycorrhizas
where the fungal cells were present and no particular accumulation
was detected in the mantle, or in the Hartig net fungal cells (Botton and
Chalot 1995). The similar patterns of GS distribution observed in the free-living
mycelia and in the ectomycorrhizal tissues suggest that the fungal enzyme
plays an active role in the primary assimilation of ammonium in ectomycorrhizas.
The expression level of the GS enzyme was studied by Javelle et al. (2003b)
in the ectomycorrhizal fungus Hebeloma cylindrosporum, where a single
mRNA of about 1.2 kb was detected. Transfer of the organism from ammonium-containing
media to nitrogen-free media resulted in an increase of GS
transcripts, correlating with an increase of GS activity. However, when the culture
media were resupplemented with ammonium, up to the concentration of
10 mM, GS transcripts remained almost unchanged or decreased very slowly,
indicating that GS in this fungus is not highly regulated, although highly
expressed (Javelle et al. 2003b; Fig. 2). Such a regulatory process at the transcriptional
level has also been found in Agaricus bisporus (Kersten et al. 1997),
while in Saccharomyces cerevisiae, the enzyme seems to be highly regulated at
the post-transcriptional level (ter Schure et al. 1995).
6.3 Role and Properties of Glutamate Synthase
Three classes of glutamate synthases (GOGAT) have been defined, based on
their amino acid sequences and the nature of the electron donor (Vanoni and
Curti, 1999). (1) Bacterial NADPH-dependent GOGAT consists of two different
subunits, the a-subunit of about 150 kDa and the b-subunit of about
50 kDa; (2) Ferredoxin-dependent GOGAT found in photosynthetic cells
(higher plants, algae and cyanobacteria) is monomeric and shares considerable
homology throughout its sequence with the a-subunit of bacterial
GOGAT; (3) plants (especially nonphotosynthetic cells) and fungi including
yeasts, as well as lower animals contain a monomeric NAD(P)H-dependent
GOGAT of about 200 kDa which results from the fusion of two fragments similar
to the a and b-subunits of bacterial GOGAT.
In plants, both enzymes (NADH-GOGAT: EC 1.4.1.14. and ferredoxin (Fd)-
GOGAT: EC 1.4.7.1) display different physico-chemical, immunological and
regulatory properties and are encoded by separate genes (Ireland and Lea
1999). Fd-GOGAT is an iron-sulphur monomeric flavoprotein, plastid-located
and represents the predominant molecular form in photosynthetic tissues
although its presence has also been reported in roots and nodules (Temple et
al. 1998). In most plants analysed, Fd-GOGAT is encoded by a single gene,
however, in Arabidopsis, two genes have been characterized (Coschigano et al.
1998). GLU1 is exclusively expressed in the leaf and is light-regulated, whereas