A naturally occurring promoter variation of BrGSL-OHa contributes to the conversion of gluconapin to progoitrin in Brassica rapa L. leaves

Glucosinolates (GSLs) are sulfur-rich secondary metabolites that play important roles in human health, plant defenses against pathogens and insects, and flavor. The genetic architecture of GSL biosynthesis in <i>Brassica rapa</i> L. remains poorly understood despite several mapping and gene prediction studies. This study conducted a conventional quantitative trait locus (QTL) analysis to identify putative genes regulating GSL biosynthesis in <i>B. rapa</i> in two field trials. Four consensus QTL clusters were identified for various GSL compounds. Six QTLs exhibited effects of QTL-environment interactions (Q×E), reflecting the genetic variation underlying phenotypic plasticity. QTL-Cluster2 and QTL-Cluster3 on chromosome A03 represented two genetically stable regions for major aliphatic GSLs (Ali-GSLs) without Q×E effects. Interestingly, variation in the expression of <i>BrGSL-OHa</i>, rather than gene sequence variation, explained the association between QTL-Cluster2 and gluconapin and progoitrin accumulation in <i>B. rapa</i>. Further function analysis indicated that the lack of an MYB binding site in the oil-type <i>B. rapa BrGSL-OHa</i> promoter region represented a rare non-functional allele among <i>B. rapa</i> genotypes, which prevented binding with the MYB transcription factor BrMYB29b, thereby repressing <i>BrGSL-OHa</i> transcription and inhibiting progoitrin biosynthesis. This study provides new insights regarding the molecular regulatory mechanism of GSL biosynthesis in <i>B. rapa</i>.