Abstract

Leaf shape plays a crucial role in plant growth and development. Among various leaf traits, marginal lobation serves as an ideal morphological marker for breeding programs. However, the genetic mechanism underlying leaf margin lobation in Brassica juncea L. remains unclear. Through RNA sequencing and map-based cloning, we identified an incompletely dominant gene, BjA10.LL , which encodes an HD-ZIP I protein and is responsible for the formation of leaf margin lobation in B. juncea . Sequence analysis of parental alleles revealed no critical variations in the coding region but identified substantial variations in regulatory regions. Heterologous expression of BjA10.LL in Arabidopsis thaliana confirmed its sufficiency to induce lobed leaves. To functionally link the regulatory variations to the phenotype, we analyzed promoter activity and developed a co-dominant molecular marker targeting key indels in a core enhancer. The promoter activity was significantly affected by these sequence variations, and the marker exhibited perfect co-segregation with the lobed-leaf phenotype in an F₂ population, collectively establishing these regulatory polymorphisms as the causal basis for divergent BjA10.LL expression and leaf morphology. These results demonstrate that BjA10.LL positively regulates marginal lobe formation, providing insights into leaf shape regulation in B. juncea and facilitating the genetic improvement of rapeseed.