Catalytic subunit of protein phosphatase 6 (PP6) (Probable). Dephosphorylates phosphorylated phytochromes, with a preference toward Pfr forms. Plays a major role in the photoperiodic control of flowering time in long days by modulating phytochrome signals in flowering time control. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 zinc ions per subunit. Interacts with PHYA and PHYB, mostly when they are phosphorylated and in Pfr forms. Mostly expressed in flowers and stems. Belongs to the PPP phosphatase family. PP-6 (PP-V) subfamily.