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ROOT HAIR DEFECTIVE4 Encodes a Phosphatidylinositol-4-Phosphate Phosphatase Required for Proper Root Hair Development in Arabidopsis thaliana^[W]

^a Department of Biology, Washington University, St. Louis, Missouri 63130
^b Section of Plant Physiology, Swammerdam Institute for Life Sciences, University of Amsterdam, 1098 SM Amsterdam, The Netherlands
^c Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109
^d Section of Molecular Cytology, Swammerdam Institute for Life Sciences, University of Amsterdam, 1098 SM Amsterdam, The Netherlands

¹ Address correspondence nielsene{at}umich.edu.

Polarized expansion of root hair cells in Arabidopsis thaliana is improperly controlled in root hair–defective rhd4-1 mutant plants, resulting in root hairs that are shorter and randomly form bulges along their length. Using time-lapse fluorescence microscopy in rhd4-1 root hairs, we analyzed membrane dynamics after labeling with RabA4b, a marker for polarized membrane trafficking in root hairs. This revealed stochastic loss and recovery of the RabA4b compartment in the tips of growing root hairs, consistent with a role for the RHD4 protein in regulation of polarized membrane trafficking in these cells. The wild-type RHD4 gene was identified by map-based cloning and was found to encode a Sac1p-like phosphoinositide phosphatase. RHD4 displayed a preference for phosphatidylinositol-4-phosphate [PI(4)P] in vitro, and rhd4-1 roots accumulated higher levels of PI(4)P in vivo. In wild-type root hairs, PI(4)P accumulated primarily in a tip-localized plasma membrane domain, but in rhd4-1 mutants, significant levels of PI(4)P were detected associated with internal membranes. A fluorescent RHD4 fusion protein localized to membranes at the tips of growing root hairs. We propose that RHD4 is selectively recruited to RabA4b-labeled membranes that are involved in polarized expansion of root hair cells and that, in conjunction with the phosphoinositide kinase PI-4Kβ1, RHD4 regulates the accumulation of PI(4)P on membrane compartments at the tips of growing root hairs.