Blue Light Signaling through the Cryptochromes and Phototropins. So That's What the Blues Is All About

Sunlight is the ultimate energy source for nearly all life on Earth. Yet, its importance to life extends far beyond a source of energy because it is also a critical information carrier. Plants and animals sample the light environment to gain information about local surroundings, time of day, and season of the year. Although animals can use opsin-based visual sys- tems to capture information from their light environ- ments, plants don't have eyes in a metazoan sense. How, then, do plants sample their light environ- ment? In short, plants have evolved a number of diverse, nonopsin photoreceptors capable of perceiv- ing a broad range of light qualities and intensities. Just like children who sort M&Ms into different color groups before eating them, plants utilize various photoreceptors to sort the colors of incident light. However, unlike children who ultimately eat all the M&Ms independent of color and prior sorting, plants have preference for particular colors of light, making them more selective than the child in what they consume. For example, if one considers just the vis- ible and near-infrared regions of the light spectrum (400-850 nm), plants have evolved three major classes of photoreceptors—the cryptochromes (cry), the phototropins (phot), and the phytochromes (phy)—capable of absorbing the blue (400-500 nm) and red/far-red (600-800 nm) portions in particular (Quail, 2002). Thus, like parents who have an appar- ent proclivity for green M&Ms, these plant photore- ceptors have a consumption preference with blue and red being the colors of choice. On the surface, it appears that plants have re- stricted their information gathering capacity relative to the light environment. However, it makes adaptive sense to tune ones morphogenic and developmental program to the same light qualities, blue and red wavelengths, utilized by the photosynthetic appara- tus. In fact, a majority of the photomorphogenesis (light-dependent changes in morphology or develop- ment) occurring in plants is induced by blue or red/ far-red signals through the aforementioned three classes of photoreceptors. Gene duplication and se- lection events have led to the evolution of multiple cry, phot, and phy receptors in higher plants that allow efficient sampling of the blue and red/far-red portions of the spectrum over a wide range of envi- ronmental and developmental conditions. Recent de- velopments in our understanding of red/far-red light-dependent photomorphogenic events induced by the phy class of receptors are the subject of an- other Update. Here, we wish to highlight some of the exciting advances that have occurred over the past couple of years in the area of blue light sensing and response associated with the cry and phot classes of receptors. Although this Update is focused on cry and phot signaling, it is important for the reader to keep in mind that other BL-absorbing pigments, such as xanthophylls (see Talbott et al., 2003 in this issue), although not predominant in their action, can influ- ence cry- and phot-dependent and -independent processes.