From Evolution to Function: A CYC/TB1 Transcription Factor Is a Key Negative Regulator of Tillering in Setaria viridis

The TCP transcription-factor family, unique to the plant kingdom, is instrumental in the regulation of plant growth and development. Within this family, the CYC/TB1 subfamily has been shown to be particularly important in the tillering process of grasses. However, the evolutionary trajectory and functional divergence of CYC/TB1 remain unexplored in Setaria. Here, eight high-quality grass genomes were surveyed and 223 TCP genes were retrieved, allocating 105, 78 and 40 members to the PCF, CIN and CYC/TB1 subfamilies, respectively. Many TCP genes, including those of the CYC/TB1 class, have undergone purifying selection during evolution, indicating a high degree of functional conservation. Within S. viridis, three CYC/TB1 genes are identified and they were expressed tissue specifically, with Sevir.9G122200 expressing highly in tiller buds. Heterologous overexpression of Sevir.9G122200 in rice markedly suppresses early axillary shoot formation, demonstrating its conserved repressor activity. This study establishes the first evolutionary framework for the CYC/TB1 clade in Setaria and identifies it as a negative regulator of tillering, providing targets for functional genomics and breeding in related cereals.