The Specificity of the Carrier-Mediated Uptake of ABA by Root Segments ofPhaseolus coccineusL.

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Abstract
Earlier work has established that the saturable component of uptake of RS-[214C]ABA by bean (Phaseolus coccineus L. cv. Prizewinner) root segments can be attributed to the action of a carrier. We now show that the carrier-mediated uptake is unaffected by RS-2-trans-ABA and lunularic acid and the unnatural R-ABA also appears to be ineffective. The specificity for S-ABA requires the halving of the Km value for ABA determined previously (2.6 mmol m-3 for RS-; 1.3 mmol m-3 for S-ABA). The RS-1', 4'-cis-diol and RS-1'-deoxy ABA reduce the uptake of RS-[2-14C]ABA about as strongly as does unlabelled ABA, the K1 for 1'-deoxy ABA was similar to the Km for ABA. The K1 for RS-1', 4'-trans-diol was 15.7 mmol m-3. Consideration of the stereochemistry of these compounds suggests that the face of the ring of ABA away from the 1'-hydroxyl group interacts with the carrier site. Labelled material diffused out of undamaged root surfaces which had absorbed RS-[3H]ABA through an apical cut, suggesting that ABA is present in the apoplast. A simplified hypothesis is presented that can account for polar transport of ABA based on a gradient of a carrier in a tissue but where the carrier is distributed uniformly on the apical and basal ends of each cell.