Photoreceptors in Plants

In general, plants glean four types of information from light: quantity, quality, direction, and photocycle. Plants can determine the level of irradiance, the wavelength of light they receive, its point of origin, and daily fluctuations in light using photoreceptors. Phytochromes are red/far-red light photoreceptors that play a role in germination, flowering, shade avoidance, and circadian rhythm entrainment. They are inactive as monomers, but when red light hits the plant, the chromophore of two phytochrome monomers will go through a cis-trans isomerization that leads to dimerization. Phytochromes are an exception among photoreceptors as their activation is reversible. For example, imbibed lettuce seeds can be induced to germinate by a pulse of red light, but when the flash of red light is followed by a flash of far-red light, germination is inhibited. Two other kinds of photoreceptors, both of which respond to blue and UV-A light, are cryptochromes and phototropins. Cryptochromes are responsible for de-etiolation, flowering, and circadian rhythm entrainment. When seeds germinate underground, they are said to undergo etiolation, by which they elongate, inhibit leaf growth, and do not produce chlorophyll. Upon breaching the surface of the ground, cryptochromes will detect blue and UV-A light, causing the seedling to become de-etiolated. That is, it will grow a short stem, produce leaves and internodes, and begin chlorophyll production. Phototropins are aptly named after one of the effects they have, phototropism. If the shoot apical meristem is exposed to blue and UV-A light, the plant will grow towards that light. Phototropins are also responsible for moving chloroplasts away from high irradiance light and moving them towards low irradiance light. Lastly, upon detecting blue light, they increase solute potential in guard cells, which causes the stomata to open. Though these are some of the important players in the light detection mechanisms of a plant, there are many other photoreceptors with functions ranging from UV protection to developmental control.