plant surface microbiology.pdf

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A. Javelle et al.
GLU2 is expressed in leaves and roots and is not regulated by light. The
expression pattern of the genes and the physiological characterization of
defective mutants support a role of GS 2 and Fd-GOGAT in the assimilation of
ammonium derived from the reduction of nitrate and from photorespiration
(Coschigano et al. 1998). NADH-GOGAT, also an iron-sulphur monomeric
flavoprotein, is present at a low level in leaves, but is more abundant in nonphotosynthetic
tissues such as roots and nodules, where it is located in
nonchlorophyllous plastids (Temple et al. 1998). The structure of the alfalfa
gene encoding NADH-GOGAT has been reported by these authors, and its
expression is restricted to root nodules where it plays a significant role in the
assimilation of ammonium derived from symbiotic N 2 fixation (Trepp et al.
1999). The localization of GS 1 and NADH-GOGAT proteins in the root vascular
bundles of rice, and very likely in many other plants, supports the possibility
of a co-ordinated function in the assimilation of ammonium in roots
(Ishiyama et al. 1998).
In fungi, NADH-GOGAT was purified and studied in Neurospora crassa
where the enzyme was found as a single polypeptide of 200 kDa (Hummelt
and Mora 1980) and in Saccharomyces cerevisiae where the enzyme is
trimeric, composed of three identical 199-kDa subunits (Cogoni et al. 1995).
In ectomycorrhizal fungi, very little is known about this enzyme. An
NADH-dependent GOGAT was, however, detected in Laccaria bicolor by Vézina
et al. (1989). In Pisolithus tinctorius, the kinetics of 15 N labelling and the
effects of enzyme inhibitors have given evidence that ammonium assimilation
occurs through the GS/GOGAT cycle (Kershaw and Stewart 1992). In
agreement with these data, Botton and Dell (1994) failed to detect NADP-
GDH in this fungus. In Scleroderma verrucosum, glutamine synthetase and
NAD-glutamate synthase activities were clearly detected, while NADP-GDH
was almost undetectable (Prima Putra et al. 1999). This is consistent with the
view that ammonium assimilation occurs through the GS/GOGAT cycle in
this fungus. In Cenococcum geophilum, a number of results based on the use
of enzyme-specific inhibitors, enzyme assays and estimation of the amino
acid pools are also consistent with the operation of the GS/GOGAT cycle (A.
Khalid and B. Botton, unpublished results).
The results obtained by Chalot et al. (1994a) with Paxillus involutus, also
emphasize a GS/GOGAT cycle in this fungus. Indeed, feeding the fungus with
[ 14 C]-glutamine resulted in a significant labelling of glutamate, while addition
of azaserine, an inhibitor of the GOGAT enzyme, led to both an accumulation
of 14 C-glutamine and a reduced pool of labelled glutamate. Interestingly, in
these experiments, 14 C-aspartate and 14 C-alanine did not accumulate under
azaserine treatment where 14 C-glutamine degradation was inhibited, thus
indicating that aspartate and alanine synthesis depends on the carbon skeletons
from glutamine (Chalot et al.1994a). In addition, feeding Paxillus involutus
with 14 C-glutamate resulted in a significant accumulation of 14 C-glutamine
under azaserine treatment, suggesting that the supplied glutamate is used for