Control of inflorescence architecture in tomato by BTB/POZ transcriptional regulators

Plant productivity depends on inflorescences, flower-bearing shoots that originate from the stem cell populations of shoot meristems. Inflorescence architecture determines flower production, which can vary dramatically both between and within species. In tomato plants, formation of multiflowered inflorescences depends on a precisely timed process of meristem maturation mediated by the transcription factor gene TERMINATING FLOWER (TMF), but the underlying mechanism is unknown. We show that TMF protein acts together with homologs of the Arabidopsis BLADE-ON-PETIOLE (BOP) transcriptional cofactors, defined by the conserved BTB (Broad complex, Tramtrack, and Bric-a-brac)/POZ (POX virus and zinc finger) domain. TMF and three tomato BOPs (S1BOPs) interact with themselves and each other, and TMF recruits S1BOPs to the nucleus, suggesting formation of a transcriptional complex. Like TMF, S1BOP gene expression is highest during vegetative and transitional stages of meristem maturation, and CRISPR/Cas9 elimination of S1BOP function causes pleiotropic defects, most notably simplification of inflorescences into single flowers, resembling tmf mutants. Flowering defects are enhanced in higher-order s1bop tmf mutants, suggesting that S1BOPs function with additional factors. In support of this, S1BOPs interact with TMF homologs, mutations in which cause phenotypes like sibop mutants. Our findings reveal a new flowering module defined by S1BOP TMF family interactions that ensures a progressive meristem maturation to promote inflorescence complexity.