Plants constantly monitor their light environment in order to grow and develop optimally, in part through use of the phytochromes, which sense red/far-red light. A phytochrome binding protein, PKS1 (phytochrome kinase substrate 1), was identified that is a substrate for light-regulated phytochrome kinase activity in vitro. In vivo experiments suggest that PKS1 is phosphorylated in a phytochrome-dependent manner and negatively regulates phytochrome signaling. The data suggest that phytochromes signal by serine-threonine phosphorylation.