Moleculargenetical analysis of phytochrome siggnal transduction

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Phytochrome is a chromoprotein, which is found in all green plants. Phytochrome is know to be act as a major photoreceptor mediating divergent plant responses to light. Although phytochrome has been studied intensively, molecular mechanism by which phytochrome transduce the light remains unknown. We addressed this problem with the aid of molecular genetical techniques. Two topics were pursued in this study. Firstly, activation-tagged Arabidopsis lines were produced and mutants that show deficiencies in light responses are isolated and analyzed. Secondly, nuclear localization of phytochrome B (phyB) was studied by using GFP as a fluorescent tag.Mutants that exhibit partial greening under dim light were isolated from the activation-tagged lines. Among them is a mutant, which resembles brassinolide-less mutants. The molecular analysis revealed that a novel cytochrome P450 gene is over-expressed in this mutant. We further confirmed that the gene is indeed responsible for the phenotypes by producing transgenic Arabidopsis lines over-expressing this gene. Unfortunately, the phenotype appeared to be related not only to phytochrome but also to other photoreceptors. Nevertheless, the gene will be a powerful tool to elucidate relationships between light and brassionsteroid signals.We examined intracellular localization of phyB by expressing a phyB-GEP fusion protein in the phyB deficient mutant of Arabidopsis. The results indicated that the introduced phyB-GFP is biologically active. Microscopic observation of the cells revealed that phyB-GFP localizes to the nucleue under light conditions. In contrast, phyB-GFP is distributed throughout the cell in darkness. Hence, phyB appears to translocate to the nucleus in a light-dependent manner. Although phytochrome has been believed to function in the cytoplasm, oure results suggest that the nucleus is one of the cellular compartments where the phytochrome action takes place