Intracellular trafficking of photoreceptors during light-induced signal transduction in plants

Plants monitor changes in the ambient light environment by highly specialis ed photoreceptors, which include the red/far-red photoreversible phytochrom es, the blue-light-absorbing cryptochromes and phototropin and the so-far-u nidentified UVB photoreceptor(s), Light easily penetrates plant organs/tiss ues and reaches even the subcellular compartments of various cell types, Th erefore, it is not surprising that the determination of the intracellular l ocalisation of photoreceptors has been, for many years, a major, and often controversial, subject of plant photobiology and cell biology research. Pho totropin, one of the blue-light photoreceptors of higher plants, controls p hototropism by monitoring the direction of light, and it is localised in or at the plasmalemma. In contrast, the subcellular localisation of phytochro mes changes dynamically and exhibits a very complex pattern, These photorec eptors are localised in the cytosol in dark-grown tissues. Irradiation, how ever, induces import of phytochromes into the nucleus. The import occurs in a light-quality- and light-quantity-dependent fashion and, as such, seems to be unique to higher plants. Light-induced accumulation of phytochromes i n the nuclei correlates well with various physiological responses mediated by these photoreceptors. These observations indicate that light-dependent i ntracellular redistribution of phytochrome photoreceptors is one of the maj or regulatory steps in photomorphogenesis.