Multi-layered Regulation of SPL15 and Cooperation with SOC1 Integrate Endogenous Flowering Pathways at the Arabidopsis Shoot Meristem

Flowering is initiated in response to environmental and internal cues that are integrated at the shoot apical meristem (SAM). We show that SPL15 coordinates the basal floral promotion pathways required for flowering of Arabidopsis in non-inductive environments. SPL15 directly activates transcription of the floral regulators FUL and miR172b in the SAM during floral induction, whereas its paralog SPL9 is expressed later on the flanks of the SAM. The capacity of SPL15 to promote flowering is regulated by age through miR156, which targets SPL15 mRNA, and gibberellin (GA), which releases SPL15 from DELLAs. Furthermore, SPL15 and the MADS-box protein SOC1 cooperate to promote transcription of their target genes. SPL15 recruits RNAPII and MED18, a Mediator complex component, in a GA-dependent manner, while SOC1 facilitates active chromatin formation with the histone demethylase REF6. Thus, we present a molecular basis for assimilation of flowering signals and transcriptional control at the SAM during flowering.