Involved in the regulation of phytochrome signaling. May regulate phytochrome-interacting factor 3 (PIF3) through the dephosphorylation of phytochrome. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 magnesium or manganese ions per subunit. Interacts with phytochromes (via N-terminus). Localizes in the nucleus upon illumination. Abnormal hypocotyl elongation under continuous red light. Belongs to the PP2C family.