title: The uptake of gibberellin A1 by suspension-cultured Spinacia oleracea cells has a carrier-mediated component.
abstract: The kinetics of the uptake of [(3)H]gibberellin A1 (GA1) by light- and dark-grown suspension-cultured cells of Spinacia oleracea (spinach) have been studied. Use of nonradioactive GA1 and gibberellic acid (GA3) show that the uptake has a saturable and a nonsaturable component. The nonsaturable component increases as the pH is lowered at a fixed concentration of [(3)H]GA1 and is probably caused by non-mediated diffusion of the uncharged protonated species of GA1. The saturable component is not the result of metabolic transformation or to GA1 binding to the cell wall and is suggested to represent the operation of a transport carrier for which GA1 and GA3 are substrates. Auxin, abscisic acid and a cytokinin did not alter the GA1 uptake. The Km is approx. 0.3 μmol dm(-3) at pH 4.4 in light- and dark-grown cells. The Vmax of the carrier is higher in the light-grown cells. The optimum pH for the carrier at a physiological GA1 concentration (3 nmol dm(-3)) was pH 4.0, with no activity detectable at pH 7.0. Both saturable and nonsaturable components were decreased by protonophores indicating that the pH gradient between the cells and the medium may be a component of the driving forces for both types of transport. Both the permeability coefficient for the undissociated GA1 and the ratio V max/K m for the carrier are lower than the corresponding values for the indole-3-acetic acid and abscisic acid carriers studied in other species.
authors.affiliation: Department of Biochemistry, University of Cambridge, Tennis Court Road, CB2 1QW, Cambridge, UK.
authors.forename: J M
authors.initials: JM
authors.lastname: Nour
authors.forename: P H
authors.initials: PH
authors.lastname: Rubery