Effects of Ca2+ on Amino Acid Transport and Accumulation in Roots of Phaseolus vulgaris

Ca(2+) stimulates the uptake of alpha-aminoisobutyric acid (AIB) into excised or intact Phaseolus vulgaris L. roots by a factor of two. In roots depleted of Ca(2+) by preincubation with ethylenediaminetetraacetate, ethyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid, or streptomycin, the stimulatory effect is 7- to 10-fold. In the presence of Ca(2+), roots accumulate AIB more than 100-fold; Ca(2+)-depleted roots only equilibrate with AIB. Radioautography shows [(14)C]AIB to be present in all cells after 90 min. Although Ca(2+)-depleted roots lose accumulated [(14)C]AIB about 10 times faster than roots supplied with Ca(2+), this increased efflux is not the main cause for the decrease in net uptake observed. The latter is rather due to a less negative membrane potential Deltapsi in Ca(2+) depleted roots (-120 mV --> -50 mV). The basic feature explaining all the results of Ca(2+) deficiency is an increase in general membrane permeability. No indication of a specific regulatory function of Ca(2+) in membrane transport of roots has been obtained.