Some regulatory aspects of [14C]methylamine influx intoPisum sativum L. cv. Feltham First seedlings

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Abstract
[14C]Methylamine influx intoPisum sativum L. cv. Feltham First seedlings showed Michaelis-Menten-type kinetics with apparentV max=49.2 μmol·g-1 FW·h-1 and apparentK m=0.51 mM. The competitive interactions between ammonium and methylamine were most obvious when biphasic kinetics were assumed with saturation of the first phase at ∼0.05 mM. The inhibitor constant for ammonium (K i)=0.027 mM. When [14C]methylamine was used in trace amounts with ammonium added as substrate, the influx of tracer showed Michaelis-Menten-type kinetics with apparentV max=3.46 μmol·g-1 FW·h-1 and apparentK m=0.15 mM. The initial rate of net ammonium uptake corresponded with that found when [14C]methylamine was used to ‘trace’ ammonium influx. The latter was also stimulated by high pHo and inhibited by nitrate. Ammonium pretreatment±methionine sulphoximine or glutamine pretreatment of the seedlings inhibited subsequent [14C]methylamine influx, while methylamine or asparagine pretreatment stimulated [14C]methylamine influx. There was also a stimulatory effect of prior inoculation withRhizobium. The results are discussed in terms of current models for the regulation of ammonium uptake in plants.