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22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 397<br />

where the fungal structures, arbuscules and vesicles, were concentrated.<br />

Northern analyses indicated that the expression of the transporter was higher<br />

in mycorrhized tomato roots than in noncolonized controls. In addition, mycorrhization<br />

caused a significant expression of a nitrate reductase gene of Glomus<br />

intraradices. According to the authors mentioned above, the results sug-<br />

AAT ALAT<br />

2-oxo Glu 2-oxo<br />

2-oxo<br />

NO 3 -<br />

Nrt2<br />

Asp oaa pyr Ala<br />

NR<br />

NO 3 -<br />

Amt1<br />

C metabolism<br />

Glu<br />

Glu<br />

GDH GS<br />

NIR<br />

NO 2 -<br />

NH 4 +<br />

Amt2<br />

NH 4 +<br />

2-oxo<br />

GOGAT<br />

Inhibition (?)<br />

-<br />

Amt3<br />

Gln<br />

Gap1<br />

Gln<br />

-<br />

transcription<br />

AMT1<br />

AMT2<br />

AMT3<br />

GDHA<br />

NAR1<br />

GLNA<br />

mRNA<br />

Fig. 1. A model describing the regulation of nitrogen transport and assimilation in<br />

Hebeloma cylindrosporum. This ectomycorrhizal fungus is able to use nitrate, ammonium<br />

and amino acids as nitrogen sources. Under low ammonium status, AMT1, AMT2,<br />

AMT3, GDHA and GLNA are transcribed, which results in elevated ammonium uptake<br />

and metabolism capacities. Under ammonium excess, AMT1, AMT2 and GDHA are efficiently<br />

repressed, which results in reduced ammonium assimilatory capacities. Under<br />

these conditions, AMT3 and GLNA would ensure the maintenance of a basal level of<br />

ammonium assimilation. AMT1 and AMT2 transcript levels are controlled through the<br />

effect of intracellular glutamine, whereas the GDHA and NAR1 mRNA level is controlled<br />

by ammonium (bold dotted lines). Ammonium uptake activity may be controlled by<br />

intracellular NH 4 + through a direct effect (dotted lines). 2-oxo 2-oxoglutarate, oaa<br />

oxaloacetate, pyr pyruvate, GOGAT glutamate synthase, Aat aspartate aminotransferase,<br />

Alat alanine aminotransferase, NR nitrate reductase, NIR nitrite reductase, Nrt2 nitrate<br />

transporter, GAP1 general amino acid transporter

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