Modulates phytochrome-mediated control of hypocotyl growth orientation. Involved in PHYA and PHYB signaling. Acts as an inhibitor of asymmetric growth. Not involved in the control of leaf flattening. Interacts in vitro with PHYA and PHYB. Expressed in the hypocotyl elongation zone. Not found in the root elongation zone. Down-regulated by light. Reduced phototropic response. Altered light-induced deviation from vertical growth and decreased phytochrome-mediated inhibition of hypocotyl elongation and cotyledon opening. No visible phenotype at the level of leaf flattening and leaf positioning. No effect on negative root phototropism. PKS1, PKS2 and/or PKS4 are essential for phototropism but not for inhibition of gravitropism under long-term blue light irradiation. Belongs to the PKS family.