plant surface microbiology.pdf

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A. Javelle et al.
In Paxillus involutus, the apparent Km derived from the Eadie-Hofstee
plots ranged from 7 mM for alanine to 27 mM for glutamate. Maximal velocities,
expressed as mmol (g dry weight) –1 min –1 , were between 0.24 for alanine
and 0.71 for glutamine. In this fungus, the uptake of amino acids markedly
depended on the pH and was optimal at pH 3.9–4.3 for glutamate and glutamine,
and at pH 3.9–5.0 for alanine and aspartate.
Both pH dependence and inhibition by protonophores, such as 2,4-dinitrophenol
(DNP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP), were
consistent with a proton symport mechanism for amino acid uptake by Paxillus
involutus. Competition studies indicated a broad substrate recognition by
the uptake system, which resembles the general amino acid permease of yeast
(Chalot et al. 1996, 2002).
The impact of birch mycorrhization with Paxillus involutus led to a profound
alteration of the metabolic fate of exogenously supplied amino acids
(Blaudez et al. 2001). Inoculation increased [ 14 C]glutamate and [ 14 C]malate
uptake capacities by up to 8 and 17 times, respectively, especially in the early
stages of mycorrhiza formation. In addition, it was demonstrated that Gln was
the major 14 C-sink in mycorrhizal roots and in the free-living fungus. In contrast,
citrulline and insoluble compounds were the major 14 C compounds in
nonmycorrhizal roots (Blaudez et al. 2001).
In order to study how amino acid transport characteristics were affected by
mycorrhization, Sokolovsky et al. (2002) used an electrophysiological
approach in Calluna vulgaris associated or not with the ericoid fungus
Hymenoscyphus ericae. Both the V max and Km parameters of amino acid
uptake were affected by fungal colonization in a manner consistent with an
increased availability of amino acid to the plant. Indeed, the transport capacity
for asparagine, histidine, ornithine and lysine, in particular, was increased
after colonization. Interestingly, a-aminobutyric acid led to a large depolarization
only in colonized cells. This implies that mycorrhization triggers a
capacity to transport a broader range of substrates, including amino acids
that are not metabolized.
4.2 Molecular Regulation of Amino Acid Transport
In Amanita muscaria, only a low, constitutive expression of the amino acid
transporter was detected in the presence of amino acids and ammonium,
which are both sources of N for the fungus (Nehls et al. 1999). By contrast,
under N starvation, or in the presence of nitrate or phenylalanine, not utilized
by the fungus as N sources, expression of the gene was considerably
enhanced. Therefore, in Amanita muscaria, as in S. cerevisiae or Aspergillus
nidulans (Sophianopoulou and Diallinas 1995), gene expression of amino
acid transporters is regulated at the transcriptional level by N repression. In
addition to amino acid uptake for nutrition, the enhanced expression of the