Leaf margin morphology is an important quality trait affecting the commodity and environmental adaptability of crops. is an ideal research material for exploring the molecular mechanisms underlying leaf lobe development. Here, we identified to be a promising gene underlying the QTL controlling leaf lobe formation in which was detected in our previous study. Sequence comparison analysis showed that the promoter divergences were the most obvious variations of between parental lines. The higher expression level and promoter activity of in the lobe-leafed parent indicated that promoter variations of were responsible for elevating expression and ultimately causing different allele effects. Histochemical GUS staining indicated that is mainly expressed at the leaf margin, with the highest expression at the tip of each lobe. Subcellular localization results showed that BrrA02.LMI1 was in the nucleus. The ectopic expression of in resulted in a deep leaf lobe in the wild-type plants, and lobed leaf formation was disturbed in -downregulated plants. Our findings revealed that plays a vital role in regulating the formation of lobed leaves, providing a theoretical basis for the selection and breeding of leaf-shape-diverse varieties of .