Properties and transduction chains of the UV and blue light photoreceptors

Abstract

Blue light (B) acts on a very wide range of processes and organisms (Chapter 5.1). Although action spectra do not always faithfully reflect the absorption spectra of the active pigments, it is still almost certain that a single chromophore cannot account for the varied array of action spectra that have been determined. Some organisms respond to most of the B and UV-A region, often following the spectral pattern that defines hypothetical B/UV-A photoreceptor(s), referred to as cryptochrome (Senger 1987), while the active wavelengths for others are confined to a single narrow peak (Chapter 5.1). Whereas many responses are apparent, the chemical identity of B perceiving chromophores is still only guessed at, despite years of study: flavins, pterins, haems, carotenoids or others. At first glance, there is considerable difficulty in discussing the transduction pathways of photo-receptors that have not yet been chemically characterized. However, detailed knowledge of the phytochrome chromophore and apoproteins has not yet led to knowledge of the steps in its transduction chain, so similar questions remain unanswered for photoresponses at both the red (R) and B ends of the visible spectrum. The problem of B transduction must be considered within the framework of the more general problem of signal transduction in plant cells. Stimuli other than B include: R, nutrition, gravity, stress, pathogens, etc. These stimuli start transduction chains leading to a variety of responses, some of which may be shared by B. Some of the B-perceiving organisms and responses, including plant and fungal phototropism and growth rate changes, induction of fungal sporulation and modified biosynthesis, are particularly amenable to genetic analysis.