Functions and Actions of Arabidopsis Phytochromes

Abstract

Environmental cues play a crucial role in regulating plant growth and development. Given the sedentary nature of plants, this close coupling between environment and development is essential for survival under conditions that are ever changing. The light environment is a particularly important determinant of plant development and the periodicity, direction, quantity and quality of incident light are continuously monitored by plants. Regulatory light signals are perceived by a number of specialised photoreceptor systems, each of which displays maximal light absorption in a specific region of the light spectrum. The photoreversible phytochromes absorb predominantly in the red (R) and far-red (FR) region of the spectrum, the cryptochromes and phototropin absorb light in the blue/UV-A region of the spectrum, and other, as yet unknown, photoreceptors are involved in perception of UV-A and UV-B light. The phytochromes are perhaps the best characterised of these photoreceptor systems. In the higher plants, phytochromes comprise a number of closely related photoreceptor proteins, the apoproteins of which are encoded by a small family of genes. In Arabidopsis thaliana there are five apophytochrome-encoding genes, PHYA, PHYB, PHYC, PHYD and PHYE (Sharrock and Quail 1989, Clack et al 1994). In the last few years genetic approaches, mainly focussed on Arabidopsis, have had a major impact in increasing our understanding of the functions of individual members of the phytochrome family and in identifying components of the signal transduction networks that couple light reception to alteration in gene expression and changes in growth and development.