The

circadian clock acts as the time-keeping mechanism in photoperiodism. Two myb-ralated proteins, LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) are essential for circadian clock function in Arabidopsis. Double loss-of-function of lhy cca1 shows a photoperiod-insensitive phenotype and a shortened generation time under short-days (SD) (Mizoguchi et al. 2002). A circadian-clock controlled flowering pathway comprising the genes GIGANTEA (GI), CONSTANS (CO) and FLOWERING LOCUS T (FT) promotes flowering specifically under long days (LD) in Arabidopsis. Within this pathway, GI regulates circadian rhythms and flowering and acts earlier in the hierarchy than CO and FT (Suarez-Lopez et al. 2001), suggesting that GI might regulate flowerin