Activity of axillary meristems dictates the architecture of both vegetative and reproductive parts of a plant. In Arabidopsis thaliana, a model eudicot species, the transcription factor LFY confers a floral fate to new meristems arising from the periphery of the reproductive shoot apex. Diverse orthologous LFY genes regulate vegetative-to-reproductive phase transition when expressed in Arabidopsis, a property not shared by RFL, the homolog in the agronomically important grass, rice. We have characterized RFL by knockdown of its expression and by its ectopic overexpression in transgenic rice. We find that reduction in RFL expression causes a dramatic delay in transition to flowering, with the extreme phenotype being no flowering. Conversely, RFL overexpression triggers precocious flowering. In these transgenics, the expression levels of known flowering time genes reveal RFL as a regulator of OsSOC1 (OsMADS50), an activator of flowering. Aside from facilitating a transition of the main growth axis to an inflorescence meristem, RFL expression status affects vegetative axillary meristems and therefore regulates tillering. The unique spatially and temporally regulated RFL expression during the development of vegetative axillary bud (tiller) primordia and inflorescence branch primordia is therefore required to produce tillers and panicle branches, respectively. Our data provide mechanistic insights into a unique role for RFL in determining the typical rice plant architecture by regulating distinct downstream pathways. These results offer a means to alter rice flowering time and plant architecture by manipulating RFL-mediated pathways.axillary meristem ͉ inflorescence branching ͉ flowering transition ͉ tillering A rabidopsis thaliana LFY and its homologs encode an evolutionarily conserved land plant-specific transcription factor. Early studies on the expression pattern and phenotypes of loss-of-function mutations in LFY and FLO, homologs in two dicots A. thaliana and Antirrhinum majus, showed them to confer a floral fate to new meristems arising on the flanks of the shoot apex (1, 2). LFY homologs from species as diverse as gymnosperms, primitive land plants, and from many angiosperms retain the ability to at least partially complement Arabidopsis lfy mutants (3). These data show activation of floral meristem fate to be a conserved LFY function. Protein domains recognizable in all LFY homologs are an N-terminal proline-rich domain and a C-terminal domain; substitutions in these largely conserved DNA-binding domains are suggested to contribute to its potentially divergent functions (3). In fact, mutations in some LFY homologs show additional developmental roles (e.g., compound leaf development in pea and cell division in moss) (4, 5).Unlike the simple inflorescence of Arabidopsis, grass inflorescences are striking in the multiple kinds of branch meristems made from the apical inflorescence meristem. In rice upon transition to reproductive phase, the vegetative apical meristem transforms to an inflorescence meristem. The...