plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 417
glutamine synthesis. These results are consistent with the existence of two
pools of glutamate in the fungal cells, as previously demonstrated by
[ 15 N]amino acid analysis in Cenococcum geophilum (Martin et al. 1988). It was
thus suggested that newly absorbed glutamate, as well as glutamate synthetized
by NADP-GDH are converted to glutamine, whereas glutamate produced
by the GOGAT enzyme is utilized by the aminotransferases (Martin et
al. 1988; Botton and Chalot 1995).
The glutamine synthetase–glutamate synthase pathway was shown to be
the main assimilatory route in beech ectomycorrhizas and glutamate dehydrogenase
plays only a minor role, if any, in these tissues (Martin et al. 1986).
Glutamine synthetase and glutamate synthase which share immunological
similarities with higher plant enzymes were detected in beech ectomycorrhizas
by means of Western immunoblotting, whereas a fungal glutamate
dehydrogenase could not be detected (Martin, unpubl. results). The absorption
of NH 4 + is associated with glutamine synthesis in beech ectomycorrhizas
so that 60–80 % of the nitrogen absorbed is present as this amide after a few
hours of absorption (Martin et al. 1986). In addition, there is a rapid and very
high 15 N-labelling in alanine over the time course of the experiment performed
with beech (Martin et al. 1986). These data, together with the measurement
of high alanine aminotransferase activity in ectomycorrhizal fungi
(Dell et al. 1989), suggest that glutamine and alanine might be the major forms
of combined nitrogen exported to the root cells.
7 Amino Acid Metabolism
7.1 Utilization of Proteins by Ectomycorrhizal Fungi and
Ectomycorrhizas
As investigated primarily by Lundeberg (1970), it is generally accepted that
most ectomycorrhizal fungal strains are unable to metabolize and use humusbound
nitrogen. Several ectomycorrhizal and ericaceous fungi in pure culture
are, however, able to grow in nutrient media containing proteins as the sole
nitrogen source (Bajwa et al. 1985; Abuzinadah and Read 1986a), and this correlated
with the production of exocellular proteinase activities (Botton and
Chalot 1991; Leake and Read 1991). In the presence of exogenous proteins
(casein, gelatin, albumin, soil proteins), Cenococcum geophilum was able to
secrete active proteases into the nutrient medium, and ammonia strongly
repressed the induction and secretion of these proteases (El-Badaoui and Botton
1989). This capacity of the mycorrhizal fungus to use amino acids as
nitrogen sources is retained in the symbiotic state. Melin and Nilsson (1953)
have shown that 15 N from [ 15 N]glutamate is transferred to Pinus sylvestris
roots and aerial parts through the mycelia of Suillus granulatus. The ability of
several ectomycorrhizal fungi to assimilate proteins and to transfer its nitro-