These authors made equal contributions to this study. SummaryThe¯oral transition in Arabidopsis is regulated by at least four¯owering pathways: the long-day, autonomous, vernalization, and gibberellin (GA)-dependent pathways. Previously, we reported that the MADSbox transcription factor SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) integrates the long-day and vernalization/autonomous pathways. Here, we present evidences that SOC1 also integrates signaling from the GA-dependent pathway, a major¯owering pathway under non-inductive short days. Under short days, the¯owering time of GA-biosynthetic and -signaling mutants was well correlated with the level of SOC1 expression; overexpression of SOC1 rescued the non-¯owering phenotype of ga1-3, and the soc1 null mutant showed reduced sensitivity to GA for¯owering. In addition, we show that vernalization-induced repression of FLOWERING LOCUS C (FLC ), an upstream negative regulator of SOC1, is not suf®cient to activate SOC1; positive factors are also required. Under short days, the GA pathway provides a positive factor for SOC1 activation. In contrast to SOC1, the GA pathway does not regulate expression of other owering integrators FLC and FT. Our results explain why the GA pathway has a strong effect on¯owering under short days and how vernalization and GA interact at the molecular level.

Cold induces expression of a number of genes that encode proteins that enhance tolerance to freezing temperatures in plants 1,2 . A cis-acting element responsive to cold and drought, the C-repeat/dehydration-responsive element (C/DRE), was identified in the Arabidopsis thaliana stress-inducible genes

Flowering is regulated by an integrated network of several genetic pathways in Arabidopsis. The key genes integrating multiple flowering pathways are FT, SOC1 and LFY. To elucidate the interactions among these integrators, genetic analyses were performed. FT and SOC1 share the common upstream regulators CO, a key component in the long day pathway, and FLC, a flowering repressor integrating autonomous and vernalization pathways. However, the soc1 mutation further delayed the flowering time of long day pathway mutants including ft, demonstrating that SOC1 acts partially independently of FT. Although soc1 did not show an obvious defect in flower meristem determination on its own, it dramatically increased the number of coflorescences in a lfy mutant, which is indicative of a defect in floral initiation. Therefore, double mutant analysis shows that the three integrators have both overlapping and independent functions in the determination of flowering time and floral initiation. The expression analysis showed that FT regulates SOC1 expression, and SOC1 regulates LFY expression, but not vice versa, which is consistent with the fact that FT and LFY have the least overlapping functions among the three integrators. The triple mutation ft soc1 lfy did not block flowering completely under long days, indicating the presence of other integrators. Finally, vernalization accelerated flowering of flc ft soc1 and ft soc1 lfy triple mutants, which shows that the vernalization pathway also has targets other than FLC, FT, SOC1 and LFY. Our genetic analysis reveals the intricate nature of genetic networks for flowering.

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