Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome

Plants are bombarded by a myriad of signals, not just from their physical environment, but from friend and foe alike. As a consequence, they have evolved a remarkably sophisticated system of receptors and signal transduction pathways that generate appropri- ate responses. That light plays a major signaling role in plant development is not surprising. A plant's ability to maximize its photosynthetic productivity depends on its capacity to sense, evaluate, and re- spond to light quality, quantity, and direction. Like- wise, the timing of developmental phenomena, such as flowering or entrance into dormancy, depends on a system of measuring and responding to changes in daylength. This article briefly explores how plant biologists have identified the various photoreceptors and how they have elucidated some of the early events in the transduction of light signals to ultimate plant responses. A red, far-red-reversible chromoprotein, phyto- chrome, was the first photoreceptor identified. It is now known that multiple phytochromes exist and sometimes act independently of one another, sometimes redundantly, sometimes antagonisti- cally, sometimes at the same time in development, and sometimes at different times. The first blue-light receptors to be identified were the two crypto- chromes, chromoproteins that mediate several re- sponses. More recently, another blue-light-absorbing chromoprotein, phototropin, has been identified as a photoreceptor mediating phototropism. A chimeric photoreceptor, phytochrome 3 (phy3), has been identified that contains both phytochrome and pho- totropin sequence motifs. For each of these photo- receptors, gene sequences are known, and plant biologists are working toward a greater understand- ing of their roles in plant development. Let us take a brief look at the events leading to our present knowl- edge of higher plant photoreceptors. PHYTOCHROMES