plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 409
binuclear zinc cluster followed by a spacer region and a coiled-coil motif that
mediates the formation of a homodimer, the form that is responsible for
sequence-specific DNA binding.
In Hebeloma cylindrosporum, supply of nitrate is not necessary for the
transcription of the NR gene (Jeargeat et al. 2000), suggesting that in this fungus
there is no transcription factor such as NIT4 capable of promoting transcription
in the presence of nitrate. In agreement with this hypothesis, no
motifs resembling the binding sites for NIT4 or NIRA were detected in the
promoter regions of the genes cloned in the ectomycorrhizal fungus (Jeargeat
et al. 2000).
In the yeast Hansenula polymorpha the YNT1 gene encoding the nitrate
transporter is clustered with the structural genes which encode nitrate reductase
and nitrite reductase (Perez et al. 1997). Clustering of these three assimilation
genes was previously reported in Aspergillus nidulans (Johnstone et al.
1990), and more recently in the ectomycorrhizal fungus Hebeloma cylindrosporum
(Jargeat, Gay, Debaud and Marmeisse, pers. comm.; gene accession
number: AJ 238664), which might represent a cell strategy to make the regulation
of this important pathway efficient.
The role of arbuscular mycorrhizal fungi in assisting their host plant in
nitrate assimilation was studied in the association Glomus intraradices/Zea
mays by Kaldorf et al. (1998). With PCR technology, part of the gene coding
for the nitrate reductase apoprotein from either the fungus or from the hostplant
was specifically amplified and subsequently cloned and sequenced.
Northern blot analysis with these probes indicated that the mRNA level of the
maize gene was lower in roots and shoots of mycorrhizal plants than in noncolonized
controls, whereas the fungal gene was highly transcribed in roots of
mycorrhizal plants.
In agreement with these data, the specific nitrate reductase activity of
leaves was significantly lower in endomycorrhizal maize than in the controls.
Nitrite formation catalyzed by nitrate reductase was mainly NADPH-dependent
in roots of mycorrhizal plants, but not in those of the controls, which is
consistent with the fact that these enzymes of fungi preferentially utilize
NADPH as reductant. In addition, it has been shown that the fungal nitrate
reductase mRNA is detected in arbuscules, but not in vesicles by in situ RNA
hybridization experiments (Kaldorf et al. 1998). There is obviously a differential
formation of transcripts of a gene coding for the same function in both
symbiotic partners.
6 Assimilation of Ammonium
Once inside the cell, NH 4 + can be incorporated into the key nitrogen donors
Glu and Gln for biosynthetic reactions. Glutamate dehydrogenase (NADP-
GDH, EC 1.4.1.4) catalyses the reductive amination of 2-oxoglutarate to form