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. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Colocalizes with PIN1 to the basal (lower) membrane of root cells. 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, 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.