Early flowering in oilseed-type Brassica rapa plants results from nonsense-mediated mRNA decay (NMD) of BrFLC2

Many Brassica species require vernalization (long-term winter-like cooling) for transition to the reproductive stage. In the past several decades, scientific efforts have been made to discern the molecular mechanisms underlying vernalization in many species. Thus, to identify the key regulators required for vernalization in Brassica rapa L., we constructed a linkage map composed of 7,833 single nucleotide polymorphism (SNP) markers using the late-flowering Chinese cabbage (B. rapa L. ssp. pekinensis) inbred line ‘Chiifu’ and the early-flowering yellow sarson (B. rapa L. ssp. trilocularis (Roxb.)) line ‘LP08’ and identified a single major QTL on the upper-arm of the chromosome A02. In addition, we compared the transcriptomes of the lines ‘Chiifu’ and ‘LP08’ at five vernalization time points, including both non-vernalized and post-vernalization conditions. We observed that BrFLC2 was significantly downregulated in the early flowering ‘LP08’ and had two deletion sites around the BrFLC2 genomic region compared with the BrFLC2 genomic region in ‘Chiifu.’ In the present study, we also demonstrate that early flowering in ‘LP08’ line is attributed to the low expression of BrFLC2, which is caused by nonsense-mediated mRNA decay (NMD). Therefore, this study provides a better understanding of the molecular mechanisms underlying floral transition in B. rapa.One sentence summaryNMD-mediated degradation of BrFLC2 mRNA transcripts is the main cause of rapid flowering of oilseed-type B. rapa ‘LP08’ plants.