Modes of action of phytochromes

Phytochrome is a family of photoreceptors whose apoproteins are encoded by divergent genes, Three modes of action of phytochromes, very-low-fluence responses (VLFR), low-fluence responses (LFR) and high-irradiance responses (HIR), have been considered in the literature to define the quantitative relationship between response and predicted levels of the far-red light absorbing form of phytochrome. The results of recent experiments with phytochrome mutant and transgenic seedlings are consistent with the notion that phytochrome B, and in some cases other stable phytochromes, mediate LFR whereas phytochrome A mediates both VLFR and HIR, The mechanisms involved in phytochrome A inductive responses (VLFR) versus HIR appear to be different in some ways, However, VLFR, LFR and HIR can converge to control the transcriptional activity of a single gene promoter, The implications of these findings for the understanding of phytochrome phototransduction are discussed.