Mh. Turnbull et al., EVALUATING THE CONTRIBUTION OF GLUTAMATE-DEHYDROGENASE AND THE GLUTAMATE SYNTHASE CYCLE TO AMMONIA ASSIMILATION BY 4 ECTOMYCORRHIZAL FUNGALISOLATES, Australian journal of plant physiology, 23(2), 1996, pp. 151-159
Combined gas chromatography-mass spectrometry were used to evaluate th
e contributions of glutamate dehydrogenase (GDH) and the glutamate syn
thase cycle in N-15-labelled ammonium assimilation by four ectomycorrh
izal fungal isolates. In all four species (Elaphomyces, Amanita, Pisol
ithus and Gautieria), glutamine was the major product accumulated foll
owing transfer of 14-day-old nitrogen-limited cultures to fresh medium
. Label was rapidly assimilated into fungal tissue, with rates of 733
nmol g(-1) FW h(-1) in Pisolithus, 972 nmol g(-1) FW h(-1) in Amanita,
2760 nmol g(-1) FW h(-1) in Gautieria and 6756 nmol g(-1) FW h(-1) in
Elaphomyces sp in the first 4 h of incubation. Incorporation of [N-15
]ammonium was sensitive to the inhibitory effects of both methionine s
ulfoximine (MSX, an inhibitor of glutamine synthetase (GS)) and albizz
iin (an inhibitor of glutamate synthase (GOGAT)) in three species (Ama
nita, Gautieria and Pisolithus) and labelling patterns were consistent
with the action of the glutamate synthase cycle in ammonium assimilat
ion. In all three species glutamine synthesis was almost totally block
ed by MSX and there was no continued incorporation of N-15 into glutam
ate. Elaphomyces displayed high levels of total incorporation of label
led ammonium in mycelium even in the presence of MSX, although incorpo
ration into glutamine was reduced by 88%. This inhibition of GS by MSX
, in addition to its partial inhibition by albizziin suggests strongly
the action of glutamate synthase cycle in ammonium assimilation. The
reduction in label entering glutamate under the influence of albizziin
is direct evidence for the inhibition of GOGAT activity. However, MSX
treatment had the effect of increasing significantly the quantity of
label recovered in both glutamate and alanine. In the absence of GS in
hibition there is clearly competition for ammonium which under normal
physiological conditions results in assimilation through the glutamate
synthase cycle. However, when GS is blocked by MSX label is able to c
ycle through the GDH pathway.