The mode of action of 2,4-D in counter-acting the elongation of carrot cells grown in culture

The growth regulator 2,4-D (2,4-dichlorophenoxyacetic acid) has been used to investigate the inter-relationship between cell elongation and cell division in carrot suspension cells. Maintained in 1 mg/l. 2,4-D, dividing populations of cells remain spheroidal and in clusters. But when subcultured into lower levels or zero, 2,4-D they increasingly elongate at the expense of division. Over the range of 0 to 1.0 mg/l, 2,4-D, elongation and division are therefore inversely related. However, by suppressing the mitogenic effect with FUdR it can be shown that cells do elongate in 1 mg/l. 2,4-D--a concentration which otherwise produces dividing, spheroidal cells. This indicates that mitogenc levels of 2,4-D do not perturb structures which support cellular elongation. This conclusion is confirmed by immuno- and electron-microscopy which show that development of elaborate arrays of cytoplasmic microtubules is unaffected by 1 mg/l. 2,4-D when FUdR is present. It is concluded that over the time periods under study here, 2,4-D regulates cell size (and shape) by stimulating growing cells to enter the division cycle and not by inhibiting elongation per se.