Traits related to flowering time are the most promising agronomic traits that directly impact the seed yield and oil quality of rapeseed (Brassica napus L.). Developing early flowering and maturity rapeseed varieties is an important breeding objective in B. napus. Many studies have reported on days to flowering, but few have reported on budding, bolting, and the interval between bolting and DTF. Therefore, elucidating the genetic architecture of QTLs and genes regulating flowering time, we presented an integrated investigation on SNP and haplotype-based genome-wide association study of 373 diverse B. napus germplasm, which were genotyped by the 60K SNP array and were phenotyped in the four environments. The results showed that a total of 15 and 37 QTLs were detected from SNP and haplotype-based GWAS, respectively. Among them, seven QTL clusters were identified by haplotype-based GWAS. Moreover, three and eight environmentally stable QTLs were detected by SNP-GWAS and haplotype-based GWAS, respectively. By integrating the above two approaches and by co-localizing the four traits, ten (10) genomic regions were under selection on chromosomes A03, A07, A08, A10, C06, C07, and C08. Interestingly, the genomic regions FT.A07.1, FT.A08, FT.C06, and FT.C07 were identified as novel. In these ten regions, a total of 197 genes controlling FT were detected, of which 14 highly expressed DEGs were orthologous to 13 Arabidopsis thaliana genes after integration with transcriptome results. In a nutshell, the above results uncovered the genetic architecture of important agronomic traits related to flowering time and provided a basis for multiple molecular marker-trait associations in B. napus.

Plant height is a crucial element related to plant architecture that influences the seed yield of oilseed rape (Brassica napus L.). In this study, we isolated a natural B. napus mutant, namely a semi-dwarf mutant (sdw-e), which exhibits a 30% reduction in plant height compared with Zhongshuang 11-HP (ZS11-HP). Quantitative trait locus sequencing (QTL-seq) was conducted using two extreme DNA bulks in F₂ populations in Wuchang-2017 derived from ZS11-HP × sdw-e to identify QTLs associated with plant height. The result suggested that two QTL intervals were located on chromosome A10. The F₂ population consisting of 200 individuals in Yangluo-2018 derived from ZS11-HP × sdw-e was used to construct a high-density linkage map using whole-genome resequencing. The high-density linkage map harbored 4323 bin markers and covered a total distance of 2026.52 cM with an average marker interval of 0.47 cM. The major QTL for plant height named qPHA10 was identified on linkage group A10 by interval mapping and composite interval mapping methods. The major QTL qPHA10 was highly consistent with the QTL-seq results. And then, we integrated the variation sites and expression levels of genes in the major QTL interval to predict the candidate genes. Thus, the identified QTL and candidate genes could be used in marker-assisted selection for B. napus breeding in the future.

Chlorophyll biosynthesis and chloroplast development are crucial to photosynthesis and plant growth, but their regulatory mechanism remains elusive in many crop species. We isolated a Brassica napus yellow-virescent leaf (yvl) mutant, which exhibited yellow-younger-leaf and virescent-older-leaf with decreased chlorophyll accumulation and delayed chloroplast development. We mapped yvl locus to a 70-kb interval between molecular markers yvl-O10 and InDel-O6 on chromosome A03 in BC₂F₂ population using whole genome re-sequencing and bulked segregant analysis. The mutant had a 9C9 to 9T9 substitution in the coding sequence of BnaA03.CHLH, which encodes putative H subunit of Mg-protoporphyrin IX chelatase (CHLH). The mutation resulted in an imperfect protein structure and reduced activity of CHLH. It also hampered the plastid encoded RNA polymerase which transcribes regulatory genes of photosystem II and I. Consequently, the chlorophyll a/b and carotenoid contents were reduced and the chloroplast ultrastructure was degraded in yvl mutant. These results explain that a single nucleotide mutation in BnaA03.CHLH impairs PEP activity to disrupt chloroplast development and chlorophyll biosynthesis in Brassica napus.

Rapeseed, an allotetraploid oil crop, provides vegetable oil for human consumption. The growing demand for oilseeds has necessitated the development of rapeseed varieties with improved quality. Therefore, a clear understanding of the genetic basis underlying the seed oil content (SOC) is required. In this study, a natural population comprising 204 diverse accessions and recombinant inbred lines (RILs) derived from Brassica napus and Sinapis alba via distant hybridization were collected for genome-wide association analysis (GWAS) and quantitative trait loci (QTL) mapping of the SOC trait, respectively. The variable coefficient of the RIL and natural populations ranged from 7.43 to 10.43% and 8.40 to 10.91%. Then, a high-density linkage map was constructed based on whole genome re-sequencing (WGS); the map harbored 2,799 bin markers and covered a total distance of 1,835.21 cM, with an average marker interval of 0.66 cM. The QTLs for SOC on chromosome A07 were stably detected in both single and multiple environments. Finally, a novel locus qA07.SOC was identified as the major QTL for SOC based on the GWAS and RIL populations. In addition, the RNA-seq results showed that photosynthesis, lipid biosynthesis proteins, fatty acid metabolism, and unsaturated fatty acid biosynthesis were significantly different between the developed seeds of the two parents of the RIL population. By comparing the variation information and expression levels of the syntenic genes within qA07.SOC and its syntenic genomic regions, as well as through haplotype analysis via GWAS, BnaA07.STR18, BnaA07.NRT1, and BnaA07g12880D were predicted as candidate genes in the qA07.SOC interval. These stable QTLs containing candidate genes and haplotypes can potentially provide a reliable basis for marker-assisted selection in B. napus breeding for SOC.