Is the role of the short-day solely to switch off the CONSTANS in Arabidopsis?

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Plants in the genus Arabidopsis are facultative LD plants that flower much earlier under LD conditions than SD regimens, with the photoperiod (or LD) pathway contributing to floral acceleration. LHY and CCA1 genes, among other factors, have central roles in the circadian clock of Arabidopsis, which plays a key role in measuring day length. GI gene mediates the circadian clock and floral activator genes, CO and FT, to control photoperiodic flowering. GI is required to set the peak phase of CO expression at the end of the light period under LD conditions, so that the CO protein is stabilized and activated by light to increase FT expression. However, recent studies have demonstrated that the role of SDs is not solely to switch off CO activity. For example, GI interacts with SPY, a negative regulator of the GA signal. The flowering times of gi mutants were still significantly later under SD conditions than LD regimes, which suggests that GI has a potential role in accelerating the start of flowering, even under SDs. Over-expression of either FT or TSF genes caused early flowering, and the acceleration of flowering was enhanced under SDs, suggesting that SDs have an additional role to that in the LHY/CCA1-GI-CO-FT pathway. In this short review, we discuss the hidden roles of SDs in controlling flowering based on recent studies of the molecular genetics of flowering time in Arabidopsis.