A role for the RabA4b effector protein PI-4Kβ1 in polarized expansion of root hair cells in Arabidopsis thaliana

The RabA4b GTPase labels a novel, trans-Golgi network compartment displaying a developmentally regulated polar distribution in growing Arabidopsis thaliana root hair cells. GTP bound RabA4b selectively recruits the plant phosphatidylinositol 4-OH kinase, PI-4Kβ1, but not members of other PI-4K families. PI-4Kβ1 colocalizes with RabA4b on tip-localized membranes in growing root hairs, and mutant plants in which both the PI-4Kβ1 and -4Kβ2 genes are disrupted display aberrant root hair morphologies. PI-4Kβ1 interacts with RabA4b through a novel homology domain, specific to eukaryotic type IIIβ PI-4Ks, and PI-4Kβ1 also interacts with a Ca²⁺ sensor, AtCBL1, through its NH₂ terminus. We propose that RabA4b recruitment of PI-4Kβ1 results in Ca²⁺-dependent generation of PI-4P on this compartment, providing a link between Ca²⁺ and PI-4,5P₂–dependent signals during the polarized secretion of cell wall components in tip-growing root hair cells.