Environmental light information such as quality, intensity, and duration in red (approximately 660 nm) and far-red (approximately 730 nm) wavelengths is perceived by phytochrome photoreceptors in plants, critically influencing almost all developmental strategies from germination to flowering. Phytochromes interconvert between red light-absorbing Pr and biologically functional far-red light-absorbing Pfr forms. To ensure optimal photoresponses in plants, the flux of light signal from Pfr-phytochromes should be tightly controlled. Phytochromes are phosphorylated at specific serine residues. We found that a type 5 protein phosphatase (PAPP5) specifically dephosphorylates biologically active Pfr-phytochromes and enhances phytochrome-mediated photoresponses. Depending on the specific serine residues dephosphorylated by PAPP5, phytochrome stability and affinity for a downstream signal transducer, NDPK2, were enhanced. Thus, phytochrome photoreceptors have developed an elaborate biochemical tuning mechanism for modulating the flux of light signal, employing variable phosphorylation states controlled by phosphorylation and PAPP5-mediated dephosphorylation as a mean to control phytochrome stability and affinity for downstream transducers.