Acts on phosphatidylinositol (PtdIns) in the first committed step in the production of the second messenger inositol-1,4,5-trisphosphate. Necessary for proper organization of the trans-Golgi network (TGN) and post-Golgi secretion in root hairs. Together with PI4KB1, required during polarized root hair expansion and pollen tube elongation. Functions redundantly with PI4KB1 upstream of the cold response phosphoinositide-dependent phospholipase C (PI-PLC) pathway