GIGANTEA: a circadian clock-controlled gene that regulates photoperiodic flowering in Arabidopsis and encodes a protein with several possible membrane-spanning domains

Flowering of Arabidopsis is promoted by long days and delayed by short days . Mutations in the GIGANTEA (GT) gene delay flowering under long days but h ave little or no effect under short days, We have now isolated the GI gene and show that it encodes a novel, putative membrane protein. By comparing t he sequence of the Arabidopsis gene with that of a likely rice orthologue a nd by sequencing mutant alleles, we identify regions of the GI protein that are likely to be important for its function. We show that GI expression is regulated by the circadian clock with a peak in transcript levels 8-10 h a fter dawn. The timing, height and duration of this peak are influenced by d aylength, We analysed the interactions between GI and the LHY, CCA1 and ELF 3 genes, previously shown to affect daylength responses; we show that the r hythmic pattern of GI expression is altered in the elf3, CCA1-OX and lhy ge notypes, and that CCA1 and LHY expression are reduced by gi mutations. Our results are consistent with the idea that GI plays an important role in reg ulating the expression of flowering time genes during the promotion of flow ering by photoperiod.