Differential activities of the Arabidopsis phyB/D/E phytochromes in complementing phyB mutant phenotypes.

The Arabidopsis phyB, phyD, and phyE phytochromes regulate plant developmental and growth responses to continuous red light and to the ratio of red to far-red light. They are also more highly related in sequence to each other and more recently derived evolutionarily than phyA and phyC. In order to directly compare the signaling activities of these three photoreceptor apoproteins, an assay was developed based upon complementation of the phyB-1 null mutant phenotype with transgenes consisting of the PHYB promoter (PB) driving expression of the PHYB, PHYD, or PHYE coding sequences. Expression analysis indicates that the PB-phyB, PB-phyD, and PB-phyE transgenes are transcriptionally and translationally active. However, whereas the PB-phyB transgene complements the phyB-1 red light hypocotyl growth phenotype completely, the PB-phyD and PB-phyE transgenes are only weakly active in restoring seedling growth regulation. Red light fluence curves indicate that this difference is not likely to be due to differences in dark reversion rates. The PB-phyD and PB-phyE transgenes also both partially restore the rosette leaf morphology phenotype of the phyB-1 mutant. However, the PB-phyD gene complements the early flowering phenotype of phyB-1 very strongly whereas PB-phyE does not. These results demonstrate that Arabidopsis phyB-related apoproteins differ significantly in their capacities to signal in various seedling and adult plant phytochrome responses.