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Plant Cell Advance Online Publication Published on February 15, 2008; 10.1105/tpc.107.054304
Received July 17, 2007 ROOT HAIR DEFECTIVE4 Encodes a Phosphatidylinositol-4-Phosphate Phosphatase Required for Proper Root Hair Development in Arabidopsis thaliana
1 Department of Biology, Washington University, St. Louis, Missouri 63130 * To whom correspondence should be addressed. E-mail: 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-4K
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