Uptake of 14C-α-aminoisobutyric acid by Phaseolus vulgaris

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Specific uptake (S.U.) of α-aminoisobutyric acid ([1- 14C]AIB), a non-metabolizable neutral amino acid analog, by dwarf bush bean plants ( Phaseolus vulgaris cv Top Crop) demonstrated wide differences in active transport between various plant organs. The kinetic and timed uptake data reported were expressed as S.U. because this corrects for the diffusion of AIB which is part of the total AIB uptake process. Roots accumulated AIB to concentrations up to 18 times and leaf disks to twice those of the incubation medium. Stem tissue showed very little uptake, if any, that could not be accounted for by simple diffusion or water free space. Although initial rate kinetic studies demonstrated the presence of a normal transport system, timed uptake studies revealed greatly decreased transport by etiolated plants, suggesting a relationship between active transport and the lack of photosynthate. The reproducibility of the AIB uptake pattern by mature roots strongly supports the concept that the transport of neutral amino acids is biphasic and suggested one or more carrier systems are inducible by either low intracellular concentrations or repressed by high intracellular concentrations of the amino acid.