plant surface microbiology.pdf

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A. Javelle et al.
capabilities of reducing NO 3 – similar to those of most higher plants. However,
nitrate reductase activity varies greatly between mycorrhizal species and isolates.For
example,in Rhizopogon vulgaris,nitrate reductase was 32-fold higher
in the S-251 isolate than in the S-219 isolate (Ho and Trappe 1987).
In the ectomycorrhizal basidiomycete Suillus bovinus, nitrate reductase
proved to be substrate-induced and activity could only be measured after
exposure of the mycelia to exogenous nitrate (Grotjohann et al. 2000). Similar
results were found in Scleroderma verrucosum (Prima Putra et al. 1999), and
Pisolithus tinctorius (Aouadj et al. 2000), where both nitrate reductases were
strongly induced in the presence of nitrate and repressed by ammonium.
5.2 Reduction of Nitrite to Ammonium
Nitrite reductase from the ectomycorrhizal basidiomycete Hebeloma cylindrosporum
is specific for NADPH and was found to be very unstable (Plassard
et al. 1984b). The saturation curve of the enzyme for NO 2 – was biphasic with
two apparent Km values at 13 and 350 mM. This suggests that the enzyme of
Hebeloma cylindrosporum has two types of binding sites for NO 2 – which could
make the reaction continuously responsive to concentration changes over a
wide range. Nitrite reductase activity measured in Hebeloma cylindrosporum
was similar to the nitrate reductase activity, ranging from 10 to 30 mmol h –1 g –1
fresh weight, which is considerably higher than the in vivo NO 3 – uptake capacity
of the mycelium (Plassard et al. 1984b). Consequently, nitrite does not
accumulate in the fungal cells, and this indicates that nitrite reductase is obviously
not a limiting step of NO 3 – assimilation in this ectomycorrhizal fungus.
5.3 Molecular Characterization of Nitrate Reductase and Nitrite
Reductase
Genes encoding proteins involved in nitrate assimilation are usually induced
by nitrate and subjected to nitrogen catabolite repression. Cloning of two
nitrate reductase (NR) genes has been carried out in the ectomycorrhizal fungus
Hebeloma cylindrosporum (Jargeat et al. 2000). One of these genes (nar1)
is transcribed and codes for a 908 amino acid polypeptide, while the other
gene (nar2) for which no mRNA transcripts were detected, is considered to be
an ancestral, nonfunctional duplication of nar1. It is well known that high
nitrate reductase activities are found in mycelia of Hebeloma cylindrosporum
cultivated in ammonium-containing media, sometimes higher than those
exhibited in the presence of nitrate (Plassard et al. 1986). However, Northern
analyses showed that nar1 in Hebeloma cylindrosporum was strongly
repressed by ammonium, while low nitrogen concentrations or high levels of
nitrate, urea, glycine or serine sustained a high level of transcription (Jargeat