Triple florigenic signaling in rice promotes stem elongation via transcriptional de-repression

The beginning of the reproductive phase of cereals is marked by inflorescence formation and stem elongation. In rice, florigenic proteins encoded by HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1) coordinate both developmental transitions by inducing expression of floral identity genes at the apical meristem and establishing stem competence to respond to gibberellins. Stem elongation is repressed during the vegetative phase by the activity of PREMATURE INTERNODE ELONGATION 1 (PINE1), and the florigens reduce its transcription, thus relieving growth repression. Here, we first resolve the relationships occurring between regulatory factors at the shoot apical meristem and underlying stem tissues in controlling reproductive phase transitions. We next show that the PINE1 protein forms higher order complexes with TOPLESS-related and HISTONE DEACETYLASE proteins and is required to repress a set of target genes by reducing histone acetylation levels and increasing chromatin accessibility. We propose that PINE1 controls chromatin accessibility through histone deacetylation, preventing stem growth, and that florigenic proteins act upstream of growth-promoting signals.