Primer Red light sensing in plants

Introduction Plants depend on light as their sole source of energy. Consequently, plants will achieve optimal fitness only if they are also able to coordinate their growth and metabolism with their light environment. To enable this coordination, plants have developed a series of photoreceptors that allow them to sense light from the UV-B to the near far-red. The red to near farred region of the light spectrum is particularly rich in environmental information that is most important to plants (Table 1). For example, changes in the seasons and the time of day and the shading from other plants are all indicated by changes in the ratio of red to far-red light. Red light also penetrates the ground further than light of shorter wavelengths and thereby gives a seedling an early indication that it is approaching the soil’s surface. Recent work on the red-light sensing system suggests that it comprises a complex and intriguing signaling network in contrast to the linear amplification cascade of the mammalian rhodopsin-based light sensing systems. The photoreceptors that allow plants to monitor the red-to-far-red band of the spectrum are known as phytochromes. Phytochromes were the first plant photoreceptors to be identified (first described in the late 1950s) and are found all across the plant kingdom. Recently a class of bacterial phytochromes has also been identified, further extending the range of organisms that utilize this photoreceptor for light perception. In most plants phytochromes exist as a small multi-gene family. Arabidopsis thaliana, a plant popular with geneticists, has five distinct phytochromes (phyA–phyE), which are differentially expressed in different plant tissues and during different stages of development. Plant phytochromes exist as dimers of a ~125 kDa polypeptide chain. Each monomer can be divided into different functional regions (Figure 1). The 60 kDa aminoterminal domain houses a covalently linked linear tetrapyrrole chromophore (Figure 1a), while the carboxy-terminal region is responsible for the transduction of the light signal. This signal transduction region can itself be separated into two subregions. The 30 kDa region immediately adjacent to the chromophore binding region contains two PAS domains, while the very carboxy-terminal domain has sequence similarity to two-component histidine kinases (Figure 1b). Phytochromes have been studied intensively by a broad range of experimental approaches ever since their first discovery. Yet, to this date no clear and unified picture of phytochrome action has emerged and the experimental observations hint at a baffling complexity of phytochrome signaling.