The uptake of gibberellin A1 by suspension-cultured Spinacia oleracea cells has a carrier-mediated component

The kinetics of the uptake of ³H]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 ³H]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 K_m is approx. 0.3 mol dm^-3 at pH 4.4 in light- and dark-grown cells. The V_max 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.Abbreviations and symbols ABA abscisic acid - DMO 5,5-dimethyloxazolidine-2,4-dione - GA gibberellin - GA₃ gibberellic acid - IAA indole-3-acetic acid - P permeability coefficient