Regulation of directional expansion by the cortical microtubule array in roots of Lemna minor

We reported previously that uniconazole, an inhibitor of gibberellin biosynthesis, does not induce lateral swelling of root in Lemna minor L. This is inconsistent with previous reports that gibberellin inhibitors induce significant lateral swelling of organs in other plants. The present study aims to elucidate the reason for this inconsistency. We assayed cells of the cortex, which occupy a large part of root volume. Treatment with propyzamide, a microtubule-disrupting agent, induced increases in root diameter due to increases in radial and tangential diameters of cells in all tissues, particularly the cortex. Additionally, cortical microtubules of cortical cells were almost removed, and the orientation of cellulose microfibrils became random. Thus, in the root cortex, cortical microtubules regulate orientation of cellulose microfibrils. When plants were cultured in the presence of uniconazole the diameter of roots decreased, mainly due to decreases in cell width in the middle layer of cortex. Arrangement of cellulose microfibrils of cortical cells remained transverse after uniconazole treatment. Although cortical microtubules were fragmented by uniconazole treatment, they also remained arranged in the transverse direction. It seems that fragmented but transversely arranged cortical microtubules can orient cellulose microfibrils transversely.