Orchestration of Floral Initiation by APETALA1

The MADS-domain transcription factor APETALA1 (AP1) is a key regulator of Arabidopsis flower development. To understand the molecular mechanisms underlying AP1 function, we identified its target genes during floral initiation using a combination of gene expression profiling and genome-wide binding studies. Many of its targets encode transcriptional regulators, including known floral repressors. The latter genes are down-regulated by AP1, suggesting that it initiates floral development by abrogating the inhibitory effects of these genes. While AP1 acts predominantly as a transcriptional repressor during the earliest stages of flower development, regulatory genes known to be required for floral organ formation were found to be activated by AP1 at more advanced stages, indicating a dynamic mode of action. Our results further imply that AP1 orchestrates floral initiation by integrating growth, patterning and hormonal pathways. Overall design: We used the AP1-GR system to conduct chromatin immunoprecipitation experiments with AP1-specific antibodies followed by deep-sequencing (ChIP-Seq) in order to determine AP1 binding sites on a genome-wide scale. Samples were generated from tissue in which the AP1-GR protein was induced for 2h using a single treatment of 1 uM DEX to the shoot apex. As control, we performed ChIP experiments using the same antibody on uninduced tissue. Experiments were done in two biological replicates.