Phytochrome - Isoforms or States

Isoforms or States

Phytochromes are characterised by a red/far-red photochromicity. Photochromic pigments change their "colour" (spectral absorbance properties) upon light absorption. In the case of phytochrome the ground state is P_r, the _r indicating that it absorbs red light particularly strongly. The absorbance maximum is a sharp peak 650–670 nm, so concentrated phytochrome solutions look turquoise-blue to the human eye. But once a red photon has been absorbed, the pigment undergoes a rapid conformational change to form the P_fr state. Here _fr indicates that now not red but far-red (also called "near infra-red"; 705–740 nm) is preferentially absorbed. This shift in absorbance is apparent to the human eye as a slightly more greenish colour. When P_fr absorbs far-red light it is converted back to P_r. Hence, red light makes P_fr, far-red light makes P_r. In plants at least P_fr is the physiologically active or "signalling" state.