Genetic Mechanisms of Leafy Head Formation in Brassica Rapa

Leafy head formation in Brassica rapa arises from coordinated morphological and physiological changes governed by a complex genetic network. Central to this process are genes that regulate leaf curvature, adaxial–abaxial polarity, and hormone signalling. Expanded gene families resulting from a whole-genome triplication event provide multiple homologues, some of which have diverged in function and expression to shape heading traits. MicroRNAs and their processing factors orchestrate temporal and spatial patterns of leaf incurvature by modulating target transcripts involved in polarity and growth. Key pathways include auxin transport and signalling, which influence cell division asymmetry, and gibberellin biosynthesis, which drives tissue elongation and head tightness. Quantitative trait loci control variation in heading time, head size and shape through loci encoding hormone pathway components and cell-wall modifiers. Purifying selection on polarity genes and differential retention of duplicated genes underlies adaptation to leafy-head morphotypes. Recent advances in multi-omic integration have begun to resolve regulatory modules and epigenetic layers that fine-tune gene expression, while gene-editing approaches offer new opportunities to validate candidate regulators and accelerate breeding of novel varieties with enhanced yield and quality.

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Technical terms

Quantitative trait locus (QTL): A chromosomal region associated with variation in a quantitative trait such as head size or heading time.

MicroRNA (miRNA): Small non-coding RNA molecules that regulate gene expression by guiding the cleavage or translational repression of target mRNAs.

Adaxial-abaxial polarity: The distinction between the upper (adaxial) and lower (abaxial) leaf surfaces, essential for controlled curvature during head formation.

Subgenome dominance: The preferential retention and higher expression of one set of duplicated chromosomal segments following genome duplication.

Gibberellin: A class of plant hormones that promote cell division and elongation, influencing leaf growth and head tightness.

References

1. Exploiting Brassica rapa L. subsp. pekinensis Genome Research. Plants (2024).
2. BcpLH organizes a specific subset of microRNAs to form a leafy head in Chinese cabbage (Brassica rapa ssp. pekinensis). Horticulture Research (2020).
3. Genetic Variation and Divergence of Genes Involved in Leaf Adaxial-Abaxial Polarity Establishment in Brassica rapa. Frontiers in Plant Science (2016).
4. The mutation of ent-kaurene synthase, a key enzyme involved in gibberellin biosynthesis, confers a non-heading phenotype to Chinese cabbage (Brassica rapa L. ssp. pekinensis). Horticulture Research (2020).
5. Genome triplication drove the diversification of Brassica plants. Horticulture Research (2014).
6. QTL Mapping of Leafy Heads by Genome Resequencing in the RIL Population of Brassica rapa. PLOS ONE (2013).
7. Divergence of three BRX homoeologs in Brassica rapa and its effect on leaf morphology. Horticulture Research (2021).