A premature stop codon in BrFLC2 transcript results in early flowering in oilseed-type Brassica rapa plants

Nonsense-mediated mRNA decay (NMD)-mediated degradation of BrFLC2 transcripts is the main cause of rapid flowering of oilseed-type B. rapa 'LP08' plants. 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 7833 single nucleotide polymorphism 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) 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 (one at 4th exon and the other at 3' downstream region) around the BrFLC2 genomic region compared with the BrFLC2 genomic region in 'Chiifu'. Large deletion at 3' downstream region did not significantly affect transcription of both sense BrFLC2 transcript and antisense transcript, BrFLC2as along vernalization time course. However, the other deletion at 4th exon of BrFLC2 resulted in the generation of premature stop codon in BrFLC2 transcript in LP08 line. Cycloheximide treatment of LP08 line showed the de-repressed level of BrFLC2 in LP08, suggesting that low transcript level of BrFLC2 in LP08 might be caused by nonsense-mediated mRNA decay removing the nonsense transcript of BrFLC2. Collectively, this study provides a better understanding of the molecular mechanisms underlying floral transition in B. rapa.