Microtubules regulate tip growth and orientation in root hairs of Arabidopsis thaliana

The polarized growth of cells as diverse as fungal hyphae, pollen tubes, al gal rhizoids and root hairs is characterized by a highly localized regulati on of cell expansion confined to the growing tip. In apically growing plant cells, a tip-focused [Ca2+](c) gradient and the cytoskeleton have been ass ociated with growth. Although actin has been established to be essential fo r the maintenance of elongation, the role of microtubules remains unclear. To address whether the microtubule cytoskeleton is involved in root hair gr owth and orientation, we applied microtubule antagonists to root hairs of A rabidopsis. In this report, we show that depolymerizing or stabilizing the microtubule cytoskeleton of these apically growing root hairs led to a loss of directionality of growth and the formation of multiple, independent gro wth points in a single root hair. Each growing point contained a tip-focuse d gradient of [Ca2+](c). Experimental generation of a new [Ca2+](c), gradie nt in root hairs pre-treated with microtubule antagonists, using the caged- calcium ionophore Br-A23187, was capable of inducing the formation of a new growth point at the site of elevated calcium influx. These data indicate a role for microtubules in regulating the directionality and stability of ap ical growth in root hairs. In addition, these results suggest that the acti on of the microtubules may be mediated through interactions with the cellul ar machinery that maintains the [Ca2+](c) gradient at the tip.