Phenotypic, cytogenetic, and molecular marker analysis of Brassica napus introgressants derived from an intergeneric hybridization with Orychophragmus

The genome of Orychophragmus violaceus provides genomic insights into the evolution of Brassicaceae polyploidization and its distinct traits

Orychophragmus violaceus, referred to as "eryuelan" (February orchid) in China, is an early-flowering ornamental plant. The high oil content and abundance of unsaturated fatty acids in O. violaceus seeds make it a potential high-quality oilseed crop. Here, we generated a whole-genome assembly for O. violaceus using Nanopore and Hi-C sequencing technologies. The assembled genome of O. violaceus was ∼1.3 Gb in size, with 12 pairs of chromosomes. Through investigation of ancestral genome evolution, we determined that the genome of O. violaceus experienced a tetraploidization event from a diploid progenitor with the translocated proto-Calepineae karyotype. Comparisons between the reconstructed subgenomes of O. violaceus identified indicators of subgenome dominance, indicating that subgenomes likely originated via allotetraploidy. O. violaceus was phylogenetically close to the Brassica genus, and tetraploidy in O. violaceus occurred approximately 8.57 million years ago, close in time to the whole-genome triplication of Brassica that likely arose via an intermediate tetraploid lineage. However, the tetraploidization in Orychophragmus was independent of the hexaploidization in Brassica, as evidenced by the results from detailed phylogenetic analyses and comparisons of the break and fusion points of ancestral genomic blocks. Moreover, identification of multi-copy genes regulating the production of high-quality oil highlighted the contributions of both tetraploidization and tandem duplication to functional innovation in O. violaceus. These findings provide novel insights into the polyploidization evolution of plant species and will promote both functional genomic studies and domestication/breeding efforts in O. violaceus.

Fine mapping of genes controlling pigment accumulation in oilseed rape (Brassica napus L.)

Purple/red appearance is one of the common phenotypic variations in leaves, stems, and siliques of oilseed rape (Brassica napus L.) but very rare in flowers. In this study, the causal genes for the purple/red traits in stems and flowers in two accessions of oilseed rape (DH_PR and DH_GC001, respectively) derived from the wide hybridization were fine mapped, and candidate genes were determined by methods combined with bulked segregant analysis (BSA) and RNA-seq analysis. Both traits of purple stem and red flowers were mapped to the locus as AtPAP2 homologous genes (BnaPAP2.C6a and BnaPAP2.A7b, respectively) belonging to the R2R3-MYB family. Sequence comparisons of full-length allelic genes revealed several InDels and SNPs in intron 1 as well as exons, and completely different promoter region of BnaPAP2.C6a and a 211 bp insertion was identified in the promoter region of BnaPAP2.A7b of DH_GC001. Our results not only contribute to a better understanding of anthocyanin inheritance in B. napus, but also provide a useful toolbox for future breeding of cultivars with purple/red traits through the combination of different functional alleles and homologs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01365-5.

Alternatively Spliced BnaPAP2.A7 Isoforms Play Opposing Roles in Anthocyanin Biosynthesis of Brassica napus L

Brassica napus L. (rapeseed, oilseed rape, and canola) and varieties of its two diploid parents, B. oleracea and B. rapa, display a large amount of variation in anthocyanin pigmentation of the leaf, stem, and fruit. Here, we demonstrate that BnaPAP2.A7, an ortholog of the B. oleracea anthocyanin activator BoMYB2 that confers purple traits, positively regulates anthocyanin biosynthesis in leaves of B. napus. Sequencing of BnaPAP2.A7 and transgenic analysis suggests that activation of this gene in purple rapeseed may result from a single nucleotide and/or 2bp insertion in its promoter region. BnaPAP2.A7 gives rise to three splice variants, designated BnaPAP2.A7-744, BnaPAP2.A7-910, and BnaPAP2.A7-395 according to the length of the transcripts. While BnaPAP2.A7-744 encodes a full-length R2R3-MYB, both BnaPAP2.A7-910 and BnaPAP2.A7-395 encode truncated proteins that lack both a partial R3 repeat and the complete C terminal domain, and so in vitro are unable to interact with the Arabidopsis bHLH protein AtTT8. Although expression of either BnaPAP2.A7-910 or BnaPAP2.A7-395 in green rapeseed does not result in purple leaves, both genes do modify genome-wide gene expression, with a strong repression of anthocyanin-related genes. We have demonstrated that BnaPAP.A7 regulates anthocyanin accumulation in leaves of B. napus and propose a potential mechanism for modulation of anthocyanin biosynthesis by alternative splicing.