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An Arabidopsis Root Hair Mutant with Altered Growth Directionality Displays Defective Cytoskeletal Dynamics in vivo.    E.B. Blancaflor and C.-M. Yoo,    Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401

   Because of their rapid growth and accessibility, root hairs have been widely used as a model system for studies of plant cell biology, particularly with regard to the mechanisms underlying tip growth. We screened a population of activation-tagged Arabidopsis seedlings for defects in root development and have identified a number of root hair mutants with altered morphology and growth directionality. Phenotypes resulting from this screen included root hairs with multiple tips, short root hairs, root hairs with swollen bases, root hair branching and exploding tips. Some of these mutants appear to be disrupted in genes that have not previously been implicated in root hair growth. The root hairs of one mutant displayed a wavy and/or spiral growth habit with no apparent impact on the diameter or the length of the root hair. It appeared that the wavy growth pattern in this mutant is likely caused by differential deposition of the cell membrane and wall materials to the tip. Since the cytoskeleton plays a major role in controlling vesicle trafficking important for transporting cell wall materials to the expanding cell, we crossed this mutant and several of the other root hair morphology mutants with transgenic plants expressing green fluorescent protein (GFP) reporters for microtubules, F-actin, and Golgi. The behavior of the GFP reporters in the wavy root hair mutant was different from that of wild-type. In particular, F-actin bundles, microtubules and Golgi occasionally protruded into the growing tip of the wavy root hair mutant. This could result in differential transport of vesicles carrying cell wall precursors to the tip, which could explain the wavy/spiral growth habit of the mutant (Supported by the Noble Foundation and NSF DBI-0400580).