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Light-Independent Phytochrome Signaling Mediated by Dominant GAF Domain Tyrosine Mutants of Arabidopsis Phytochromes in Transgenic Plants^[W],[OA]

Section of Molecular and Cellular Biology, University of California, Davis, California 95616

¹ Address correspondence to jclagarias{at}ucdavis.edu.

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 (PHYB^Y276H) and Arabidopsis phytochrome A (PHYA^Y242H) in transgenic Arabidopsis plants, we demonstrate that photoconversion is not a prerequisite for phytochrome signaling. PHYB^Y276H-expressing plants exhibit chromophore-dependent constitutive photomorphogenesis, light-independent phyB^Y276H nuclear localization, constitutive activation of genes normally repressed in darkness, and light-insensitive seed germination. Fluence rate analyses of transgenic plants expressing PHYB^Y276H, PHYA^Y242H, and other Y^GAF 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.

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