Photoreception: what are Phytochromes?

Plant photoreceptors are able to respond to visible light. There are three main classes of photoreceptors; phytochromes are able to measure red to far-red light ratios; cryptochromes respond to blue light; the third class are phototropins.

The capability to to detect light helps the plant to control developmental processes. The ability to measure red to far red ratios enables the plant to access its growing conditions and determine if light is significant for photosynthesis and/or a shade avoidance response should be carried out. All known flowering plants have phytochromes.

In addition to flowering plants phytochromes are found in many other organisms these include cryptophytes, cyanobacteria, bacteria and fungus. Therefore even though phytochromes are known to have a role in light detection, they are present in many light insensitive organisms; this suggests that phytochromes have a role in addition to that of being a photoreceptor. It is known that their are at least 120 phytochrome related proteins. In non-photosynthetic organisms the role of phytochromes ranges from the regulation of pigmentation to sexual development.

Phytochromes are reversible photo conversion switches

There are two main forms of plant phytochromes P_r and P_fr.If plants are grown in dark light then subjected to red light the P_r form will absorb the light and will convert to the far red light absorbing P_fr form and trigger photomorphogenesis. Vice versa if the P_fr form of phytochrome receive far red light then it will convert to the red light absorbing P_r form of phytochrome. The reaction of phytochromes to light is therefore reversible. A large amount of light is required to maintain a phytochrome P_r/P_frratio in plants as reversion of phytochromes from P_fr to P_r occurs in the dark; this suggests that organisms such as fungi which make use of Phytochromes in sexual activity do not use phytochromes in the same way as flowering plants. Additionally the natural conversion of P_fr to P_r occuring in plants when grown in the dark proceeds much slower than that seen in non-photosynthetic organisms.