What are the two forms of phytochrome and how are they switched?

Question

What are the two forms of phytochrome and how are they switched?

Answer 1

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 Pr, 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 Pfr 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 more greenish colour. This image of Cph1 phytochrome shows the Pr ground state on the left and predominantly Pfr on the right In plants at least Pfr is the physiologically active or "signalling" state. When Pfrabsorbs far-red light it is converted back to Pr. Hence, red light makes Pfr, far-red light makes Pr. In plants at least Pfr is the physiologically active or "signalling" state. Since daylight contains a lot of red light, during the day phytochrome is mostly converted to Pfr. At night, phytochrome will slowly convert back to the Pr form. Treatment with far-red light will also convert Pfr back to Pr. Since plants use red light for photosynthesis, and reflect and transmit far-red light, the shade of other plants also can make Pfr into Pr, triggering a response called shade avoidance. In most plants, a suitable concentration of Pfr stimulates or inhibits physiological processes, such as those mentioned in these examples.

Since both the ground state Pr and excited state Pfr are unusually stable (Pfr has a half-life of hours or days) the quantum nature of this transition was not immediately recognized. These two forms are therefore commonly (though technically incorrectly) referred to as isoforms.