UPTAKE OF ABSCISIC-ACID BY SUSPENSION-CULTURED PHASEOLUS-COCCINEUS L CELLS - EVIDENCE FOR CARRIER PARTICIPATION

The uptake of abscisic acid (ABA) by suspension-cultured P. coccineus L. cv. Prizewinner (runner bean) cells has been studied. In addition to non-mediated diffusive uptake of ABAH, a saturable component of uptake occurs which is demonstrable as inhibition of uptake of submicromolar concentrations of [2-14C]ABA by increasing concentrations of nonradioactive ABA to a constant plateau level. Saturation is not due to competition for extracellular binding sites or cytoplasmic acidification, and can only be partially accounted for by saturation of metabolic enzymes. It is considered largely to represent carrier-mediated ABA uptake. The maximum velocity of the carrier is greater at pH 4.0 than at pH 5.0, although the apparent Km are similar (.apprx. 2.0 mmol m-3). No carrier activity was detectable above pH 6.0. The carrier is unaffected by IAA, gibberellin A3, benzyladenine or 2,3,5-triiodobenzoic acid (TIBA). Use of protein-modifying reagents suggested a role of histidine residues in carrier activity. Reagents expected to modify the transmembrane pH (.DELTA. pH) and electrical (.DELTA. E) gradients were used to study the driving forces for ABA uptake. There was no apparent effect of .DELTA. E, indirectly monitored by effects on an electrogenic TIBA-sensitive component of IAA transport, on ABA uptake. Both diffusive and saturable components were modified in proportion by changes in .DELTA. pH, suggesting a common sensitivity to this driving force. The carrier is suggested to act as an ABA-/H+ symport.