Protein kinase that regulates the auxin transport activity of PIN auxin efflux facilitators by direct phosphorylation. D6PK-mediated PIN phosphorylation promotes auxin transport in the hypocotyl and this is a prerequisite for PHOT1-dependent hypocotyl bending. Phosphorylates PIN1, PIN2, PIN3, PIN4 and PIN7 in vitro and PIN1 in vivo. Acts as a lipid domain-dependent mediator of root epidermal planar polarity via the binding to phosphatidylinositol phosphates (e.g. PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), PtdIns(4,5)P(2), PtdIns(3,4,5)P(3) and phosphatidic acid) (PubMed:27251533). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts (via C-terminal PIF domain) with PDPK1. Colocalizes with PIN1 to the basal (lower) membrane of root stele, cortex, epidermis and lateral root cap cells (PubMed:19168677). Undergoes a phosphatidylinositol-4,5-bisphosphate- and sterol-dependent basal-to-planar polarity switching into the polar, lipid-enriched domain of root hair-forming cells just before hair formation; this process is mediated by DRP1A and PIP5K3 (PubMed:27251533). The activation loop within the kinase domain is the target of phosphorylation. No visible phenotype under normal growth conditions, but the quadruple d6pk, d6pkl1, d6pkl2 and d6pkl3 mutants are deficient in lateral root formation and mildly agravitropic, have fused or single cotyledons and narrow and twisted leaves, form few axillary shoots, exhibits basal shift in root hairs positioning, are almost infertile and impaired in phototropic hypocotyl bending when exposed to lateral white light. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.