Light signalling in Arabidopsis

Of the numerous environmental factors that regulate the growth and development of plants, light is one of the most important. Plants employ a series of discrete photoreceptors, absorbing in different regions of the light spectrum, in order to monitor the presence, direction, intensity, quality and duration of light. The principal signal-transducing photoreceptor families are the red/far-red light-absorbing phytochromes and the blue/UV-A light-absorbing photoreceptors, which include the cryptochromes. The application of genetic techniques, particularly using Arabidopsis, is leading to elucidation of the roles of, and interactions between, the various photoreceptors. Genetic screens have also been used to dissect the signal transduction pathways that are triggered by photoreceptor activation. The selection of mutants which, when grown in the dark, resemble light-grown seedlings, has led to the identification of a series of nuclear-localised negative regulators: the products of the COPI/DET/FUS genes. These repressors appear to act downstream of multiple photoreceptors, as well as being involved in other signalling pathways. Other COP and DET genes are involved in regulating cellular levels of cytokinins and brassinosteroids, and these regulators have also been implicated in light signalling. In addition, several mutants that define positive regulators, which appear to act in pathways specific to individual photoreceptors, have also been identified.