Microtubule stabilization leads to growth reorientation in Arabidopsis trichomes

The single-cell trichomes in wild-type Arabidopsis are either unbranched or have two to five branches. Using transgenic Arabidopsis plants expressing a green fluorescent protein-microtubule-associated protein4 fusion protein, which decorates the microtubular cytoskeleton, we observed that during tri chome branching, microtubules reorient with respect to the longitudinal gro wth axis. Considering branching to be a localized microtubule-dependent gro wth reorientation event, we investigated the effects of microtubule-interac ting drugs on branch induction in trichomes. In unbranched trichomes of the mutant stichel, a change in growth directionality, closely simulating bran ch initiation, could be elicited by a short treatment with paclitaxel, a mi crotubule-stabilizing drug, but not with microtubule-disrupting drugs. The growth reorientation appeared to be linked to increased microtubule stabili zation and to aster formation in the treated trichomes. Taxol-induced micro tubule stabilization also led to the initiation of new branch points in the zwichel mutant of Arabidopsis, which is defective in a kinesin-like microt ubule motor protein and possesses trichomes that are less branched. Our obs ervations suggest that trichome cell branching in Arabidopsis might be medi ated by transiently stabilized microtubular structures, which may form a co mponent of a multiprotein complex required to reorient freshly polymerizing microtubules into new growth directions.