Light-Independent Phytochrome Signaling Mediated by Dominant GAF Domain Tyrosine Mutants ofArabidopsisPhytochromes in Transgenic Plants

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Abstract
The photoreversibility of plant phytochromes enables continuous surveillance of the ambient light environment. Through expression of profluorescent, photoinsensitive Tyr-to-His mutant alleles of Arabidopsis thaliana phytochrome B (PHYBY276H) and Arabidopsis phytochrome A (PHYAY242H) in transgenic Arabidopsis plants, we demonstrate that photoconversion is not a prerequisite for phytochrome signaling. PHYBY276H-expressing plants exhibit chromophore-dependent constitutive photomorphogenesis, light-independent phyBY276H nuclear localization, constitutive activation of genes normally repressed in darkness, and light-insensitive seed germination. Fluence rate analyses of transgenic plants expressing PHYBY276H, PHYAY242H, and other YGAF mutant alleles of PHYB demonstrate that a range of altered light-signaling activities are associated with mutation of this residue. We conclude that the universally conserved GAF domain Tyr residue, with which the bilin chromophore is intimately associated, performs a critical role in coupling light perception to signal transduction by plant phytochromes.