plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 407
et al. 2000). The authors have put forward the hypothesis that the nitrate
reductase enzyme of the fungus might be extremely stable in vivo and progressively
accumulates in the cells growing on ammonium. In addition, such
results indicate that in Hebeloma cylindrosporum, expression of the nitrate
reductase gene is regulated primarily by the availability of ammonium, but
not by the presence of nitrate in the medium. This regulation pattern clearly
distinguishes this fungus from the other saprophytic and pathogenic species
previously studied.
Assimilatory nitrate reductase of higher plants is subjected to a complex regulation
of its expression and catalytic properties (Kaiser and Huber 2001).The
NR protein is inactivated by phosphorylation combined with a link with a
dimeric protein,which may cause a change in NR conformation that interrupts
electron transport between the heme and the molybdenum-cofactor domains
(Kaiser and Huber 2001).It is known that light as well as CO 2 and oxygen availability
are the major external triggers for a rapid and reversible modulation of
NR activity, and that sugars and/or sugar phosphates are the internal signals
which regulate the protein kinase(s) and phosphatase. In ectomycorrhizal
fungi, there is no evidence, so far, for a specific post-translational inactivation
of the NR protein. In Hebeloma cylindrosporum, the main NR protein named
NAR1,like all other fungal NR polypeptides,lacks the short motifs found in the
N-terminal and hinge 1 domains of plant NRs,which are both necessary for the
post-translational inactivation of these enzymes in response to changes in
light or CO 2 status (Su et al. 1996; Jargeat et al. 2000).
Indeed, in Neurospora crassa the structural genes that encode nitrogen
catabolic enzymes are subject to nitrogen metabolite repression, mediated by
the positive-acting NIT2 protein and by the negative-acting NMR protein (for
“nitrogen metabolite repression”; Pan et al. 1997). NIT2, a globally acting factor,
(or AREA in Aspergillus nidulans, or GLN3 in Saccharomyces cerevisiae) is
a member of the GATA family of regulatory proteins and has a single
Cys 2/Cys 2 zinc finger DNA-binding domain. Deletions or certain amino acid
substitutions within this zinc finger and the carboxy-terminal tail resulted in
a loss of nitrogen metabolite repression (Marzluf 1997). Those mutated forms
of NIT2 that were insensitive to nitrogen repression had also lost one of the
NIT2-NMR protein–protein interactions. These results provide compelling
evidence that the specific NIT2–NMR interactions have a regulatory function
and play a central role in establishing nitrogen metabolite repression (Pan et
al. 1997).
The different genes involved in nitrate assimilation, as well as putative
nitrate transport systems, have been cloned from various saprophytic and
pathogenic filamentous ascomycetes; all of these genes are single-copy genes
and their transcription is subject to ammonium/glutamine repression and
nitrate induction (Kinghorn and Unkles 1994). In the yeast Hansenula polymorpha,
the genes YNT1, YNR1 and YNI1, encoding respectively nitrate transport,
nitrate reductase and nitrite reductase (NiR), have been cloned, as well