Phytochromes are red-light photoreceptors that perceive light signals via reversible photoconversion between a red-light-absorbing (Pr) state and a far-red-absorbing (Pfr) state. They play important roles in mediating light responses in plants, bacteria, and fungi. Phytochromes utilize linear tetrapyrroles (also known as bilins) as chromophores to capture photons in the red region (wavelength range 600-800 nm) of the solar spectrum.
Phytochromes are signaling proteins with a modular domain architecture, in which three N-terminal domains (denoted PAS, Per-ARNT-Sim; GAF, cGMP phosphodiesterase/adenyl cylase/FhlA; and PHY, phytochrome) are responsible for light perception and the C-terminal effector domains convert light-induced structural or chemical signals into biological signals (Rockwell et al. 2006). In plant phytochromes, the C-terminal domains include two tandem PAS domains followed by a histidine-kinase-related