Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some bacteria. They mediate light responses as varied as visual perception, phototropism and phototaxis, as well as responses to light-dark cycles such as circadian rhythm and other photoperiodisms including control of flowering times in plants and mating seasons in animals.
Photoreceptor proteins typically consist of a protein moiety and a non-protein photopigment that reacts to light via photoisomerization or photoreduction, thus initiating a change of the receptor protein which triggers a signal transduction cascade. Pigments found in photoreceptors include retinal (retinylidene proteins, for example rhodopsin in animals), flavin (flavoproteins, for example cryptochrome in plants and animals) and bilin (biliproteins, for example phytochrome in plants).
Photoreceptors in animals
(Also see: Photoreceptor cell)
- Melanopsin: in vertebrate retina, mediates pupillary reflex, involved in regulation of circadian rhythms
- Photopsin: reception of various colors of light in the cone cells of vertebrate retina
- Rhodopsin: green-blue light reception in the rod cells of vertebrate retina
- Protein Kinase C: mediates photoreceptor deactivation, and retinal degeneration 
- OPN5: sensitive to UV-light
Photoreceptors in plants
- UVR8: UV-B light reception
- Cryptochrome: blue and UV-A light reception
- Phototropin: blue and UV-A light perception (to mediate phototropism and chloroplast movement)
- Zeitlupe: blue light entrainment of the circadian clock
- Phytochrome: red and far-red light reception
All the photoreceptors listed above allow plants to sense light with wavelengths range from 280 nm (UV-B) to 750 nm (far-red light). Plants can use light with different wavelengths as their environment cue for the initiation of important developmental transitions.
In plant seeds, the photoreceptor phytochrome is responsible for the process termed photomorphogenesis. This occurs when a seed initially situated in an environment of complete darkness is exposed to light. A brief exposure to electromagnetic radiation, particularly that whose wavelength is within the red and far-red lights, results in the activation of the photorecepter phytochrome within the seed. This in turn sends a signal through the signal transduction pathway into the nucleus, and triggers hundreds of genes responsible for growth and development.
Photoreceptors in phototactic flagellates
(Also see: Eyespot apparatus)
- Channelrhodopsin: in unicellular algae, mediates phototaxis
- Chlamyopsin and volvoxopsin
Photoreceptors in archaea and bacteria
Photoreception and signal transduction
Responses to photoreception
- Smith, D. P., Hardy, R. W., & al, e. (1991). Photoreceptor deactivation and retinal degeneration mediated by a photoreceptor-specific protein kinase C. Science, 254(5037), 1478-1478. Retrieved from http://search.proquest.com/docview/213560980
- Kojima, D. Mori S., Torii M., Wada, A., Morishita R., & Fukada, Y. (2011) UV-Sensitive Photoreceptor Protein OPN5 in Humans and Mice. PLoS ONE, 6(10): e26388. DOI: 10.1371/journal.pone.0026388 <http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0026388>
- Galvão, Vinicius Costa; Fankhauser, Christian (October 2015). "Sensing the light environment in plants: photoreceptors and early signaling steps". Current Opinion in Neurobiology. 34: 46–53. doi:10.1016/j.conb.2015.01.013. PMID 25638281.
- Winslow R. Briggs, Margaret A. Olney, Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome, 2003 <http://www.plantphysiol.org/content/125/1/85.full>