Abstract
Key enzymes of the urea cycle and (15)N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. (15)NH(4)(+) was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of L-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. (15)N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 microg Arg mg(-1) fresh weight h(-1). The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium.
Highlights
Key enzymes of the urea cycle and 15N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi
A high-affinity NH41 transporter was recently characterized in Glomus intraradices (Lopez-Pedrosa et al, 2006) and AM hyphae possess the enzymes required for uptake of NO32 and NH41 and their assimilation into amino acids (Kaldorf et al, 1994; Bago et al, 1996; Johansen et al, 1996; Toussaint et al, 2004)
Specific activity of GS in the extraradical mycelium (ERM) showed no response over the time course of the experiment, whereas the activity in AM roots increased in response to the supply of nitrogen to the RFC (Fig. 1)
Summary
Key enzymes of the urea cycle and 15N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium. Soil-to-plant nitrogen transport by the ERM of AM fungi was first demonstrated using compartmented pots where 15N-labeled nitrogen sources were applied to soil containing the ERM, but no roots (Ames et al, 1983; Johansen et al, 1992; Frey and Schuepp, 1993). Arg levels were reported in the range of 50 to 200 nmol mg dry weight, depending on the development stage, and this represented more than
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