A transposon insertion in the promoter of BcCPR5 impairs trichome formation in wucai (Brassica campestris L.)

Abstract

Abstract Trichomes, specialized epidermal cells on plant leaves, contribute to resistance against both abiotic and biotic stress. Wucai, an important leafy vegetable in China, is predominantly characterized by low trichome density or even hairlessness in commercial cultivars. However, the genetic and molecular mechanisms underlying trichome loss remain largely unknown. In this study, we investigate the genetic basis of trichome loss using a high-trichome-density wucai inbred line, W7-1, and a low-trichome-density cultivar, W7-2. Through a combination of bulked segregant analysis sequencing and map-based cloning, we identified the homolog of the Arabidopsis CPR5 (Constitutive Pathogen Response 5) gene located on chromosome A06 as the candidate locus. A 3365-bp transposon insertion in the promoter of W7-2 disrupts its transcriptional activity, leading to reduced expression of BcCPR5 and impaired trichome formation. Further analysis showed that BcCPR5 expression is positively regulated by BcWRKY44 through a W-box cis-element in the promoter. In W7-2, this transposon insertion physically separates the W-box from the transcription start site. Population genetic analysis revealed that the insertion is prevalent among low-trichome-density or hairless cultivated varieties, reflecting selection during domestication and breeding. Collectively, our findings partially uncover the genetic mechanism underlying trichome loss in wucai and establish a novel BcWRKY44–BcCPR5 regulatory module controlling trichome formation.