May be responsible for light-regulated cytoplasmic sequestration of phytochromes or may be a negative regulator of phytochrome B signaling. Component of the network that modulates the very low-fluence response (VLFR) branch of phyA signaling. Acts positively in PHOT1 signaling. Regulates phytochrome-mediated photomorphogenesis and hypocotyl phototropism. Involved in the control of leaf flattening and leaf positioning. Promotes negative root phototropism and negatively regulates root gravitropism. May act by controlling auxin homeostasis. Interacts with PKS2, RPT3, PHOT1, PHOT2 and the C-termini of both phytochromes A (phyA) and B (phyB). Binds both spectral forms of phytochrome, Pr and Pfr. Expressed in young seedlings in both darkness and light. Moderate in leaves and very low in roots and flowers. Expressed in the elongation zone of the root and hypocotyl. Decreases with development. Up-regulated by white, red, far-red and blue light. Phosphorylated on Ser and to a lower extent on Thr by phytochromes. Phosphorylation is stimulated twofold by red light. Increased hypocotyl growth inhibition and cotyledon unfolding responses in the very low fluence response (VLFR) mode. Reduced phototropic response. Reduced hyponasty when grown under blue light. Loss of negative root phototropism. Auxin accumulation in protoplasts. 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.