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Quantitative Effects of 2,4-Dichlorophenoxyacetic Acid on Growth of Suspension-cultured Acer pseudoplatanus Cells

II. Influence of 2,4-D Metabolism and Intracellular pH on the Control of Cell Division by Intracellular 2,4-D Concentration ^1,2

^a Université Pierre et Marie Curie, Service des Hormones végétales, T.53.43.E.3., 75230 Paris Cedex 05, France

The utilization of 2,4-dichlorophenoxyacetic acid (2,4-D) molecules by Acer pseudoplatanus cells is governed mainly by a glucosylation process. Evidence that 2,4-D glucoside molecules are biologically inactive is presented. 2,3,5-Triiodobenzoic acid (TIBA), by inhibiting 2,4-D glucosylation, has a sparing effect on 2,4-D molecules; thus TIBA treatments increase growth yield (expressed as the ratio of the maximum number of cells produced to the initial concentration of 2,4-D in the culture medium).

Significant amounts of intact 2,4-D molecules remain outside and inside the cells when cell division stops at the onset of the stationary phase. This result and the previous demonstration that, at the onset of the stationary phase, 2,4-D is the specific limiting factor of cell division (Leguay JJ, J Guern 1975 Plant Physiol 56: 356-359) suggest that a threshold concentration of auxin is needed for cell division to proceed.

The distribution of 2,4-D molecules between the cells and the culture medium is dependent on the population density at the stationary phase. The extracellular 2,4-D concentration at that time is a linear function of the population density whereas intracellular amounts of 2,4-D and 2,4-D metabolites are constant. By using a modified 2-¹⁴C,-5,5-dimethyloxazolidine-2,4-dione technique, it has been shown that the intracellular pH is markedly lowered as the population density at the plateau is increased. This intracellular pH modification is likely to be responsible for a large modification of the ratio between intracellular and extracellular auxin concentrations.

The intracellular auxin concentration reaches a constant value (about 3 x 10^–7M), independent of population density when cell division stops at the onset of the stationary phase suggesting that it represents the threshold value of the control for cell division.

² Part I was ref. 14.