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A. Javelle et al.
gen to plants of Pinus contorta was also clearly demonstrated (Abuzinadah
and Read 1986a, b; Abuzinadah et al.1986). The use of nitrogen sources not
available to nonmycorrhizal plants contributes, therefore, to an increased
uptake of nitrogen by infected roots.
7.2 Amino Acids Used as Nitrogen and Carbon Sources
Utilization of amino acids by ectomycorrhizal symbionts and ectomycorrhizas
may have important implications, not only for their nitrogen metabolism,
but also for the overall carbon economy of the plant. Axenic mycelia of
the ectomycorrhizal basidiomycete Suillus bovinus have been grown in liquid
media in the presence of glucose as the only carbohydrate source and under
such conditions, they produced similar amounts of dry weight with ammonia,
with nitrate or with alanine, 60–80 % more with glutamate or glutamine, but
about 35 % less with urea as the only exogenous nitrogen source (Grotjohann
et al. 2000).
Recently, the fate of carbon derived from alanine, glutamate and glutamine
was investigated in the ectomycorrhizal fungus Paxillus involutus (Chalot et
al. 1994a, b). The result of the 14 C tracer experiments suggested that the carbon
skeletons derived from newly absorbed glutamate were mainly used for
the synthesis of glutamine. The accumulation of [ 14 C]glutamate and the
marked decrease of [ 14 C]glutamine under MSX treatment were consistent
with a rapid utilization of glutamate by the glutamine synthetase (GS)
enzyme. The newly absorbed, as well as the newly synthesized [ 14 C]glutamine
were degraded into [ 14 C]glutamate, suggesting the operation of the glutamate
synthase (GOGAT) enzyme. This was confirmed by the striking accumulation
of [ 14 C]glutamine when the fungus was cultivated in the presence of azaserine,
an inhibitor of GOGAT. In addition, a strong inhibition of glutamine utilization
by aminooxyacetate indicated that glutamine catabolism in Paxillus
involutus might involve a transamination process as an alternative pathway to
GOGAT for glutamine degradation (Chalot et al. 1994a).
The use of 14 C-labelled amino acids also showed a direct involvement of
glutamate and glutamine in the respiration pathways, these amino acids being
obviously channelled through the tricarboxylic acid (TCA) cycle and oxidized
to CO 2. Feeding the fungus with [ 14 C]alanine resulted in a rapid labelling of
pyruvate, citrate, succinate, fumarate and CO 2. Further labelling was detected
in glutamate, glutamine and aspartate. The presence of aminooxyacetate completely
suppressed 14 CO 2 evolution and decreased the flow of carbon to the
Krebs cycle intermediates and amino acids, suggesting that alanine aminotransferase
plays a key role in metabolizing alanine in Paxillus involutus
(Chalot et al. 1994b).
It has been shown by measuring enzyme capacities and metabolite pools
that mycorrhization causes a re-arrangement of the main metabolic pathways