Update on Red-Light Sensing The Red Side of Photomorphogenesis

The importance of light to normal plant growth and development cannot be overstated. As sessile photoautotrophs, plants depend on efficient light capture to compete and reproduce successfully within a relatively restricted geographical realm. For this purpose, these organisms have evolved very sophisticated sensory networks for monitoring the status of several important features of their illuminated surroundings including light intensity, duration, quality, and direction (Kendrick and Kronenberg, 1994). Response to these light signals in the form of altered plant growth and development is termed photomorphogenesis, a process that is distinct from that of photosynthesis, where far greater quantities of light serve as a source of energy for the fixation of carbon. The entire photomorphogenic process rests upon a set of specialized photochromic sensory receptors falling into at least three distinct known classes: phytochromes, cryptochromes, and phototropins. Structural properties of these photoreceptors essentially restrict spectrally important regions for photomorphogenesis to the UV-A and -B, blue, and red portions of the electromagnetic spectrum. Historically, most progress toward understanding the molecular and cellular processes that underlie photomorphogenesis has come from studies of redlight sensing, which spans a relatively broad spectral region (approximately 600–750 nm) to include both red and far-red (approximately 700–750 nm) light. In terms of photomorphogenesis, this region is unique because phytochrome is the only known photoreceptor that absorbs light here exclusively for photosensory purposes. And as a consequence, this characteristic of red-light sensing provides photobiologists with a unique capacity to probe the mechanics of phytochrome-regulated development in isolation from other photosensory systems. However, as an Update on red-light sensing, the intent here is not to provide a comprehensive review of phytochrome because several excellent and quite recent articles are already available to serve this purpose beautifully (Neff et al., 2000; Smith, 2000; Fankhauser, 2001b; Nagy and Schäfer, 2002; Quail, 2002; Wang and Deng, 2003). Rather, this Update is intended to provide new students and educators with a primer to photomorphogenic research in plants. Specifically, it will use both historical and more recent contexts of phytochrome research to describe how our understanding of red-light sensing has evolved over time as well as the challenges that must be successfully addressed for continued progress in the future.