Soil waterlogging is a major environmental stress that suppresses the growth and productivity of rapeseed (Brassica napus L.).  Natural genetic variations in waterlogging tolerance (WT) were observed but no QTL mapping has been done for WT related traits in rapeseed. In this study, QTL associated with three WT related traits including relative root length (RRL), relative hypocotyl length (RHL) and relative fresh weight (RFW) were dissected using a set of reciprocal introgression lines (ILs) derived from the cross GH01×ZS9, which showed significant difference in WT.  Genotyping-by-sequencing (GBS) of the populations were performed, totally 1 468 and 1 450 binned SNPs were identified for GIL (GH01 as the recurrent parent) and ZIL (ZS9 as the recurrent parent) population, respectively.  A total of 66 distinct QTLs for WT at the seedling establishment stage including 31 for RRL, 17 for RHL and 18 for RFW were detected.  Among the 66 QTLs, 20 (29.4%) QTLs were detected in both genetic backgrounds and then they were integrated into six QTL clusters, which can be targeted in rapeseed breeding for improvement of WT through marker-assisted selection (MAS).  Based on the physical positions of SNPs and the functional annotation of the Arabidopsis thaliana genome, 56 genes within the six QTL cluster regions were selected as preliminary candidate genes, then the resequencing and transcriptome information about parents were applied to narrow the extent of candidate genes.  Twelve genes were determined as candidates for the six QTL clusters, some of them involved in RNA/protein degradation, most of them involved in oxidation-reduction process.  These findings provided genetic resources, candidate genes to address the urgent demand of improving WT in rapeseed breeding.

Low temperature is one of the most important abiotic factors inhibiting growth, productivity, and distribution of rapeseed (Brassica napus L.).  Therefore, it is important to identify and cultivate cold-tolerant germplasm.  The objective of this study was to figure out the mechanism of chilling (4 and 2°C) and freezing (–2 and –4°C) stresses along with a control (22°C) in B. napus cultivars (1801 and C20) under controlled environment (growth chamber).  The experiment was arranged in a complete randomized design with three replications.  Our results exhibited that under chilling and freezing stresses, the increment of proline accumulation, soluble sugar and protein contents, and antioxidant enzyme activity were enhanced more in 1801 cultivar compared with C20 cultivar.  At –2 and –4°C, the seedlings of C20 cultivar died completely compared with 1801 cultivar.  Hydrogen peroxide (H₂O₂) and malondialdehyde contents (MDA) increased in both cultivars, but when the temperature was decreased up to –2 and –4°C, the MDA and H₂O₂ contents continuously dropped in 1801 cultivar.  Moreover, we found that leaf abscisic acid (ABA) was enhanced in 1801 cultivar under chilling and freezing stresses.  Additionally, the transcriptional regulations of cold-tolerant genes (COLD1, CBF4, COR6.6, COR15, and COR25) were also determined using real-time quantitative PCR (RT-qPCR).  RT-qPCR showed that higher expression of these genes were found in 1801 as compared to C20 under cold stress (chilling and freezing stresses).  Therefore, it is concluded from this experiment that 1801 cultivar has a higher ability to respond to cold stress (chilling and freezing stresses) by maintaining hormonal, antioxidative, and osmotic activity along with gene transcription process than C20.  The result of this study will provide a solid foundation for understanding physiological and molecular mechanisms of cold stress signaling in rapeseed (B. napus).