The effects of colchicine and gibberellic acid on growth and microtubules in excised lettuce hypocotyls.

The effect of colchicine on growth and microtubules in H2O-and gibberellic acid (GA3)-treated hypocotyls of lettuce (Lactuca sativa L. cv. Arctic) was examined. Hypocotyls of seedlings from γ-irradiated as well as non-irradiated seedlings were used in order to establish that the drug was affecting cell elongation and not cell division. Although colchicine inhibited elongation of GA3-treated hypocotyl sections at concentrations as low as 1 μM, it did not affect the growth of sections. Rather, colchicine treatment caused a reorientation of the plane of cell expansion. Lumicolchicine had no effect on elongation of hypocotyl sections at a concentration (100 μM) at which colchicine abolished GA3-induced elongation. Colchicine was only effective in reorienting the plane of growth in light; in darkness, it inhibited both radial and longitudinal cell expansion. Analogs of colchicine as well as other antimicrotubular agents were ineffective in changing the polarity of growth in lettuce. Electron microscopy showed that transversely oriented microtubules were numerous in the cortical cytoplasm adjacent to the longitudinal wall of freshly excised hypocotyl sections. Incubation in GA3 did not change the number or orientation of these microtubules, while incubation in H2O caused a reduction in their number and altered their distribution. Colchicine abolished microtubules under all conditions of incubation tested, but did not affect other characteristics of cell ultrastructure.