REP1,a basic helix-loop-helix protein, is required for a branch pathway ofphytochrome A signaling in Arabidopsis

Phytochromes are primary photoreceptors mediating diverse responses ranging from induction of germination to floral induction in higher plants. We hav e isolated novel recessive rep1 (reduced phytochrome signaling 1) mutants, which exhibit a long-hypocotyl phenotype only under far-red light but not u nder red light. Physiological characterization showed that rep1 mutations g reatly reduced a subset of phytochrome A-regulated responses, including the inhibition of hypocotyl elongation, cotyledon expansion, modulation of gra vitropic growth of hypocotyl, and induction of the CAB (encoding chlorophyl l a/b binding protein) gene, without affecting the accumulation of anthocya nin, far-red-preconditioned blocking of greening, induction of germination, and induction of CHS (encoding chalcone synthase) and FNR (encoding ferred oxin-NADP(+) oxidoreductase) genes. These results suggest that REP1 is a po sitive signaling component, functioning in a branch of the phytochrome A si gnaling pathway. Molecular cloning and characterization of the REP1 gene re vealed that it encodes a light-inducible, putative transcription factor con taining the basic helix-loop-helix motif.