Integrated hormone and transcriptome analyses of Chinese cabbage mutant lic86 reveal the involvement of auxin in leaf curling

Leaf curling is a key agronomic trait that promotes the formation of leafy heads in heading vegetable crops. However, the role of auxin in regulating leaf curling in Chinese cabbage remains largely unknown. In this study, we identified a Chinese cabbage mutant, lic86, which exhibited inward leaf curling and displayed significantly increased transverse curvature indices compared to the wild type. Additionally, among the measured hormones the concentration of indole-3-acetic acid (IAA) was significantly reduced in lic86. Exogenous application of IAA caused outward leaf curling, whereas applying the auxin transport inhibitor, TIBA, resulted in inward leaf curling in both wild type and lic86 plants. Transcriptomic analysis revealed that the differentially expressed genes between WT and lic86 were predominantly associated with hormone signal transduction pathways. Notably, the expression levels of two Brassica rapa asymmetric leaves1 (BrAS1) homologs were significantly elevated in lic86, while the expression of B. rapa LIKE-AUX1 (BrLAX1), the auxin influx carrier gene, was markedly downregulated compared to WT plants. Virus-induced gene silencing of BrLAX1 in WT plants resulted in leaf inward curvature, whereas silencing of BrAS1s in lic86 seedlings led to flattened leaf morphology. These findings offer insights into the molecular mechanism underlying leaf curling and leafy head development in Chinese cabbage.