Modes of action of phytochromes

Abstract

through largely unknown pathways into molecular and physiological changes that modulate growth and developPhytochrome is a family of photoreceptors whose ment. Phytochromes posses two photo-interconvertible apoproteins are encoded by divergent genes. Three forms, the R-absorbing form (Pr) and the FR-absorbing modes of action of phytochromes, very-low-fluence form (Pfr) and are synthesised in the Pr form. In recent responses (VLFR), low-fluence responses (LFR) and years it has been established that phytochromes are a high-irradiance responses (HIR), have been considered small family of photoreceptors whose apoproteins are in the literature to define the quantitative relationship encoded by diVerent genes (Clack et al., 1994; Hauser between response and predicted levels of the far-red et al., 1995). Traditionally, three ‘modes of action’ of light absorbing form of phytochrome. The results of phytochromes have been described: very-low-fluence recent experiments with phytochrome mutant and responses ( VLFR); low-fluence responses (LFR) and transgenic seedlings are consistent with the notion high-irradiance responses (HIR). Assigning particular that phytochrome B, and in some cases other stable responses to particular molecular species of phytochromes phytochromes, mediate LFR whereas phytochrome A is one of the important areas of research in the field mediates both VLFR and HIR. The mechanisms of photomorphogenesis. This review seeks to bring involved in phytochrome A inductive responses (VLFR) together data on the modes of action of phytochrome versus HIR appear to be different in some ways. with information on diVerent phytochrome molecular However, VLFR, LFR and HIR can converge to control species. the transcriptional activity of a single gene promoter. The analysis of phytochrome transduction chains is The implications of these findings for the under- currently concentrating the eVorts of many laboratories. standing of phytochrome phototransduction are Experimental approaches include the characterization of discussed. the photoreceptor molecule, in particular the biological significance of diVerent domains (Cherry et al., 1993;