Rapeseed (Brassica napus L.) is one of the most important oilseed crops worldwide, and the development of rapeseed varieties with high-quality oil is a long-term breeding goal. Reducing the content of palmitic acid, the main saturated fatty acid in rapeseed oil, can greatly improve oil quality. Here, a genome-wide association study (GWAS) and transcriptome-wide association study (TWAS) of seed palmitic acid content (SPAC) were performed using 393 diverse B. napus accessions. Four genes (BnaA08.DAP, BnaA08.PAA1, BnaA08.DUF106, and BnaC03.DAP) were identified by both GWAS and TWAS. The transcripts per million (TPM) values of these candidate genes at 20 and 40 days after flowering (DAF) were significantly correlated with SPAC in this association panel. Based on genetic variation in the candidate genes, four low-SPAC haplotypes were identified by combining candidate gene association analysis and haplotype analysis. Brassica napus accessions carrying low-SPAC haplotypes had lower SPAC than those carrying high-SPAC haplotypes without affecting seed oil content, seed protein content, or seed yield. Based on the functional single-nucleotide polymorphism (SNP) chrA08_9529850 (C/A) in the promoter of BnaA08.DUF106, a molecular marker (Bn_A8_SPAC_Marker) was developed that can be used to facilitate breeding for low SPAC in B. napus. Our findings provide valuable information for studying the genetic control of SPAC in B. napus. Moreover, the candidate genes, favorable haplotypes, and molecular marker identified in this study will be useful for breeding low-SPAC B. napus varieties.

Yellow seed trait is a desirable characteristic with potential for increasing seed quality and commercial value in rapeseed, and anthocyanin and proanthocyanidins (PAs) are major seed-coat pigments. Few transcription factors involved in the regulation of anthocyanin and PAs biosynthesis have been characterized in rapeseed. In this study, we identified a transcription factor gene BnbHLH92a (BnaA06T0441000ZS) in rapeseed. Overexpressing BnbHLH92a both in Arabidopsis and in rapeseed reduced levels of anthocyanin and PAs. Correspondingly, the expression profiles of anthocyanin and PA biosynthesis genes (TT3, BAN, TT8, TT18, and TTG1) were shown by quantitative real-time PCR to be inhibited in BnbHLH92a-overexpressing Arabidopsis seeds, indicating that BnbHLH92a represses the anthocyanin and PA biosynthesis pathway in Arabidopsis. BnbHLH92a physically interacts with the BnTTG1 protein and represses the biosynthesis of anthocyanins and PAs in rapeseed. BnbHLH92a also binds directly to the BnTT18 promoter and represses its expression. These results suggest that BnbHLH92a is a novel upstream regulator of flavonoid biosynthesis in B. napus.