In roots of Arabidopsis thaliana, we examined the effects of low concentrations of microtubule inhibitors on the polarity of growth and on the organization of microtubule arrays. Intact 6 d old seedlings were transplanted onto plates containing inhibitors, and sampled 12 h, 24 h and 48 h later. Oryzalin, a compound that causes microtubule depolymerization, stimulates the radial expansion of roots. The amount of radial swelling is linearly proportional to the logarithm of the oryzalin concentration, from the response threshold, 170 nM, to 1 microM. Cells in the zone of division were slightly more sensitive to oryzalin than were cells in the zone of pure elongation. Radial swelling is also stimulated by taxol, a compound that causes microtubule polymerization. Taxol at 1 microM causes little swelling, but at 10 microM causes extensive radial swelling of cells in the elongation zone, and does not affect cells in the division zone. To examine the microtubules in these roots, we used methacrylate sections with immunofluorescence microscopy. At all concentrations of oryzalin, cortical arrays are disorganized and depleted of microtubules, and the microtubules themselves often appear fragmented. These effects increase in severity with concentration, but are unmistakable at 170 nM. In taxol, cortical arrays appear to be more intensely stained than those of controls. At 10 microM, many cells in growing regions of the stele have longitudinal microtubules, whereas many cells in the cortex appear to have transversely aligned microtubules. Taxol affects microtubules in cells of division and elongation zones to the same extent, despite the observed difference in growth.(ABSTRACT TRUNCATED AT 250 WORDS)