| Root hairs and pollen tubes are cells that grow by the process of polarized secretion, in which secretory vesicles are targeted to the growing tip of the cell, adding new cell wall material only at the tip. Polarized secretion is known to be regulated by the actin cytoskeleton, a tip-focused Ca 2+ gradient, and small GTPases. The GTPase RabA4b is localized to the tips of growing root hairs, and is thought to regulate trafficking of cell wall materials to the tips of these cells. The phosphoinositide 4-kinase beta1 (PI4Kbeta1) was identified as a RabA4b effector protein, suggesting that its product, phosphoinositide-4-phosphate (PI(4)P), may function in polarized secretion. We have identified a double mutant of PI4K1 and its homolog PI4Kbeta2, and find that the double mutant is a smaller plant with aberrant root hairs. We show that PI4Kbeta1/PI4Kbeta2 function in proper organization of the trans-Golgi network (TGN), as mutants display an aggregated TGN morphology. Full-length, catalytically active PI4Kbeta1 is required for rescue of the double mutant phenotype.To further understand the process of polarized growth, we characterized the root hair defective mutant rhd4-1. Using a map-based cloning approach, we found that RHD4 encodes a PI(4)P phosphatase, further implicating the function of PI(4)P in root hair growth. In wild type root hairs, PI(4)P was shown to accumulate on the plasma membrane at the tips of root hairs, whereas in rhd4 root hairs, PI(4)P accumulated on subcellular membrane compartments. RHD4 localizes to a TGN compartment at the tips of root hairs, in contrast to its yeast and mammalian counterparts, which localize to the ER and/or Golgi. The RHD4 homolog, SAC6, is expressed only in floral tissue. We identified a sac6 T-DNA mutant and found that not only did sac6 display pollen growth defects in vitro, rhd4 and PI4Kbeta1/PI4Kbeta2 double mutants did as well. Taken together, our results suggest that RHD4 may function to regulate pools of PI(4)P produced by PI4Kbeta1/PI4Kbeta2 in root hairs, while RHD4 and SAC6 may function to regulate pools of PI(4)P produced by PI4Kbeta1/PI4Kbeta2 in pollen. |
