COP1B, AN ISOFORM OF COP1 GENERATED BY ALTERNATIVE SPLICING, HAS A NEGATIVE EFFECT ON COP1 FUNCTION IN REGULATING LIGHT-DEPENDENT SEEDLING DEVELOPMENT IN ARABIDOPSIS

COP1 is a negative regulator of Arabidopsis light-dependent developmen t. Mutation of the COP1 locus causes constitutive photomorphogenesis i n the dark. Here, we report the identification of an isoform of the CO P1 protein, named COP1b, which is generated by alternative splicing. C OP1b has a 60-amino acid deletion in the WD-40 repeat domain relative to the full-length COP1. This splicing step is light-independent and t akes place mostly in mature seeds and in germinating seedlings. Transg enic Arabidopsis plants that overexpress COP1b show a de-etiolated phe notype in the dark, with a short hypocotyl, open and developed cotyled ons. The transgenic seedlings are adult-lethal. These phenotypes close ly resemble that of severe cop-1 mutants, indicating that COP1b has a dominant negative effect on COP1 function.