Integration of floral inductive signals in Arabidopsis

Flowering of Arabidopsis is regulated by a daylength-dependent pathway that accelerates flowering in long days and a daylength-independent pathway that ensures flowering in the absence of inductive conditions(1-3). These pathways are genetically separable, as there are mutations that delay flowering in long but not short days(4). Conversely, mutations that block synthesis of the hormone gibberellin abolish flowering in short days, but have on their own only a minor effect in long days(5). A third pathway, the autonomous pathway, probably acts by modulating the other two pathways(3). Understanding where and how these pathways are integrated is a prerequisite for understanding why similar environmental or endogenous cues can elicit opposite flowering responses in different plants(6,7). In Arabidopsis, floral induction leads ultimately to the upregulation of floral meristem-identity genes such as LEAFY(8-13), indicating that floral inductive signals are integrated upstream of LEAFY. Here we show that gibberellins activate the LEAFY promoter through cis elements that are different from those that are sufficient for the daylength response, demonstrating that the LEAFY promoter integrates environmental and endogenous signals controlling flowering time.