| Summary: | The shape of the leaf is the primary phenotype which determines the commercial value of leaf mustard (<i>Brassica juncea</i>). However, there arefew reports on the lobed-leaf gene of <i>B. juncea</i>, and the molecular regulatory mechanisms underlying leaf margin formation are unknown. In this study, an F<sub>2</sub> population derived from ‘MN001’ and ‘MU056’ was constructed. Genetic analysis revealed that the lobed-leaf trait is controlled by a major gene, and lobed leavesare dominant compared to round leaves. The GradedPool-Seq analysis combined with the re-sequencing results of parents identified a major interval on chromosome 10 of <i>B. juncea</i>’s genome A. The <i>BjLMI1</i> gene (<i>BjuA040054</i>) was confirmed to be a candidate gene by gene ontology (GO) analysis, and it is homologous with <i>LMI1</i> and encodes HD-Zip protein <i>ATHB-51</i>. A base substitution was observed in the conserved domain, and a 63 bp fragment deletion was found in the exon region between the two parents in the CDs region. The expression of <i>BjLMI1</i> was significantly higher in the lobed-leaf parent than in the round-leaf parent. These findings provide insights into the molecular mechanism underlying leaf margin formation and will be valuable in the development of an ideal leaf shape in <i>B. juncea</i>.
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