Cell biology and genetics of root hair formation in Arabidopsis thaliana

In this review we integrate the information available on the cell biology o f root hair formation with recent findings from the analysis of root hair m utants of Arabidopsis thaliana. The mature Arabidopsis root epidermis consi sts of root-hair-producing cells and non-root-hair-producing cells. Root ha ir growth begins with a swelling of the outer epidermal wall. It has been p ostulated that this is due to a pH-mediated localised cell wall loosening. From the bulge a single root hair emerges which grows by tip growth. The ro ot hair tip consists of a vesicle-rich zone and an organelle-rich subapical zone. The vesicles supply new plasma membrane and cell wall material for e longation. The cytoskeleton and its associated regulatory proteins such as profilin and spectrin are proposed to be involved in the targeting of vesic les. Ca2+ influxes and gradients are present in hair tips, but their functi on is still unclear. Mutants have been isolated with lesions in various par ts of the root hair developmental pathway from bulge identity and initiatio n, to control of tip diameter and extent and polarity of elongation.