| Summary: | Plant structure has a large influence on crop yield formation, with branching and plant height being the important factors that make it up. We identified a gene, <i>MtTCP18</i>, encoding a TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor highly conserved with <i>Arabidopsis</i> gene <i>BRC1</i> (BRANCHED1) in <i>Medicago truncatula</i>. Sequence analysis revealed that <i>MtTCP18</i> included a conserved basic helix–loop–helix (BHLH) motif and R domain. Expression analysis showed that <i>MtTCP18</i> was expressed in all organs examined, with relatively higher expression in pods and axillary buds. Subcellular localization analysis showed that MtTCP18 was localized in the nucleus and exhibited transcriptional activation activity. These results supported its role as a transcription factor. Meanwhile, we identified a homozygous mutant line (NF14875) with a mutation caused by Tnt1 insertion into <i>MtTCP18</i>. Mutant analysis showed that the mutation of <i>MtTCP18</i> altered plant structure, with increased plant height and branch number. Moreover, we found that the expression of auxin early response genes was modulated in the mutant. Therefore, <i>MtTCP18</i> may be a promising candidate gene for breeders to optimize plant structure for crop improvement.
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