A new type of photoreceptor in algae.

Abstract

Light serves two major purposes in biological systems: the transduction of light to chemical free energy, photosynthesis, and the transduction of light to initiate a signaling pathway, to sense the environment. In animals, retinal pigments underlie signaling by light: vision and photoperiodism. Indeed, the use of retinal, the aldehyde of vitamin A, by visual pigments for their chromophore was the very first example in which the reason that humans require a vitamin was elucidated (1). Plants do more with light than animals and thus have more pigments to transduce it. Besides chlorophyll, used in photosynthesis, at least some plants and algae use other pigments for the processes that underlie photomorphogenesis, photoperiodism, and photomovement. The last category involves both phototropism in sessile organisms and phototaxis in motile organisms, mostly some types of algae. Recently a combination of having the complete genome of a plant, Arabidopsis, and substantial parts for an algae, Chlamydomonas, and being able to manipulate the genes expressed in plants and algae is starting to clarify the role of light in signaling. Several types of photoreceptor molecules that are associated with some well known light responses of plants have been demonstrated. In the model plant Arabidopsis, it seems that there are at least three types of photoreceptor pigment families, each with multiple members: two cryptochromes (each with two chromophores), five phytochromes, and two phototropins (2), none of which use retinal as their chromophore.