GIGANTEA is a nuclear protein involved in phytochrome signaling in Arabidopsis

In a genetic screen of available T-DNA-mutagenized Arabidopsis populations for loci potentially involved in phytochrome (phy) signaling, we identified a mutant that displayed reduced seedling deetiolation under continuous red light, but little if any change in responsiveness to continuous far-red li ght. This behavior suggests disruption of phyB, but not phyA signaling. We have cloned the mutant locus by using the T-DNA insertion and found that th e disrupted gene is identical to the recently described GIGANTEA (GI) gene identified as being involved in control of flowering time. The encoded GI p olypeptide has no sequence similarity to any known proteins in the database . However, by using beta-glucuronidase-GI and green fluorescent protein-GI fusion constructs, we have shown that GI is constitutively targeted to the nucleus in transient transfection assays. Optical sectioning by using the g reen fluorescent protein-GI fusion protein showed green fluorescence throug hout the nucleoplasm. Thus, contrary to previous computer-based predictions that GI would be an integral plasmamembrane-localized polypeptide, the dat a here indicate that it is a nucleoplasmically localized protein. This resu lt is consistent with the proposed role in phyB signaling, given recent evi dence that early phy signaling events are nuclear localized.