Cytokinin regulates flowering time in Arabidopsis thaliana through the gibberellin pathway

Cytokinin and gibberellin are two plant hormones promoting flowering in Arabidopsis thaliana grown under short-day conditions. The activities of both hormones depend on similar signaling components, including several elements of the age pathway. However, to date, the interaction between cytokinin and gibberellin in regulating the transition to flowering remains unknown. In this work, we show that exogenous treatment of late-flowering or non-flowering cytokinin mutants by gibberellin restores their ability to flower. In contrast, the promotion of flowering by cytokinin is suppressed by mutations in gibberellin biosynthesis genes, and cytokinin-deficient plants show reduced expression of these genes. Consistently, cytokinin-deficient plants show an altered gibberellin homeostasis in leaves and shoot apices. Simultaneous mutation of the DELLA genes GAI and RGA, or of RGL1 and RGL3, caused earlier flowering in cytokinin receptor mutants. This is consistent with their downstream activity in response to cytokinin and reveals a previously unrecognized role for these genes in regulating flowering time. Together, the results indicate that cytokinin acts upstream of a redundant network of gibberellin metabolism and signaling genes and suggest that hormonal crosstalk occurs in both leaves and the shoot apical meristem. Cytokinin action through different elements of the gibberellin system underscores the complexity of hormonal interactions in controlling the floral transition.