Abstract
Phytochromes in plants are dimeric proteins that have a single linear tetrapyrrole molecule, phytochromobilin, as chromophore in each protein molecule. In prokaryotes, phytochrome-like proteins named bacteriophytochromes have a different prosthetic group, biliverdin or phycocyanobilin. The genes and proteins for tetrapyrrole metabolism have been identified in plants by a molecular genetic approach with Arabidopsis photomorphogenic mutants and in algae by comparative genomics. Using mutants and biosynthetic genes of phytochrome chromophores as tools, a genetic system was developed allowing a structure-function assay of phytochrome chromophores in photochromic and physiological responses in plants. The phyA-mediated far-red-high-irradiance response represented by hypocotyl elongation inhibition was impaired in pcyA-expressing plants where phytochrome chromophore was substituted to phycocyanobilin. Further monochromatic irradiations indicated that the response was deficient under far-red light of 730 nm but normal under that of 715 nm in this plant. We discuss the chromophore structure from an evolutionary point of view.
