MORPHOLOGY AND MICROTUBULE ORGANIZATION IN ARABIDOPSIS ROOTS EXPOSED TO ORYZALIN OR TAXOL

In roots of Arabidopsis thaliana, we examined the effects of low conce ntrations of microtubule inhibitors on the polarity of growth and on t he organization of microtubule arrays. Intact 6 d old seedlings were t ransplanted onto plates containing inhibitors, and sampled 12 h, 24 h and 48 h later. Oryzalin, a compound that causes microtubule depolymer ization, stimulates the radial expansion of roots. The amount of radia l swelling is linearly proportional to the logarithm of the oryzalin c oncentration, from the response threshold, 170 nM, to 1 mu M. Cells in the zone of division were slightly more sensitive to oryzalin than we re cells in the zone of pure elongation. Radial swelling is also stimu lated by taxol, a compound that causes microtubule polymerization. Tax ol at 1 mu M causes little swelling, but at 10 mu M causes extensive r adial swelling of cells in the elongation zone, and does not affect ce lls in the division zone. To examine the microtubules in these roots, we used methacrylate sections with immunofluorescence microscopy. At a ll concentrations of oryzalin, cortical arrays are disorganized and de pleted of microtubules, and the microtubules themselves often appear f ragmented. These effects increase in severity with concentration, but are unmistakable at 170 nM. In taxol, cortical arrays appear to be mor e intensely stained than those of controls. At 10 mu M, many cells in growing regions of the stele have longitudinal microtubules, whereas m any cells in the cortex appear to have transversely aligned microtubul es. Taxol affects microtubules in cells of division and elongation zon es to the same extent, despite the observed difference in growth. We c onclude that the precise, spatial pattern of cortical microtubules may not be primarily responsible for controlling growth anisotropy; and t hat control over growth anisotropy may differ between dividing and non -dividing cells.