PKS1, a substrate phosphorylated by phytochrome that modulates light signaling in Arabidopsis

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