The Specificity of the Carrier-Mediated Uptake of ABA by Root Segments of Phaseolus coccineus L.

Earlier work has established that the saturable component ofuptake 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 byRS-2-trans-ABA and lunularic acid and the unnatural R-ABA alsoappears to be ineffective. The specificity for S-ABA requiresthe halving of the K_m value for ABA determined previously (2.6mmol m_-3 for RS-; 1.3 mmol m_-3 for S-ABA). The RS-1', 4'-cis-dioland RS-1'-deoxy ABA reduce the uptake of RS-2-₁₄C]ABA aboutas strongly as does unlabelled ABA, the K¹ for 1'-deoxy ABAwas similar to the K_m for ABA. The K₁ for RS-1', 4'-trans-diolwas 15.7 mmol m_-3. Consideration of the stereochemistry of thesecompounds suggests that the face of the ring of ABA away fromthe 1'-hydroxyl group interacts with the carrier site. Labelled material diffused out of undamaged root surfaces whichhad absorbed RS-³H]ABA through an apical cut, suggesting thatABA is present in the apoplast. A simplified hypothesis is presented that can account for polartransport of ABA based on a gradient of a carrier in a tissuebut where the carrier is distributed uniformly on the apicaland basal ends of each cell. Key words: Uptake carrier, Abscisic acid, 1', 4'-Diol, Lunularic acid, Phaseolus coccineus, Polar-transport, Deoxyabscisic acid