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Background: Brassica nigra (BB), also called black mustard, is grown as a condiment crop in India. B. nigra represents the B genome of U's triangle and is one of the progenitor species of B. juncea (AABB), an important oilseed crop of the Indian subcontinent. We report the genome assembly of B. nigra variety Sangam. Results: The genome assembly was carried out using Oxford Nanopore long-read sequencing and optical mapping. A total of 1549 contigs were assembled, which covered ~ 515.4 Mb of the estimated ~ 522 Mb of the genome. The final assembly consisted of 15 scaffolds that were assigned to eight pseudochromosomes using a high-density genetic map of B. nigra. Around 246 Mb of the genome consisted of the repeat elements; LTR/Gypsy types of retrotransposons being the most predominant. The B genome-specific repeats were identified in the centromeric regions of the B. nigra pseudochromosomes. A total of 57,249 protein-coding genes were identified of which 42,444 genes were found to be expressed in the transcriptome analysis. A comparison of the B genomes of B. nigra and B. juncea revealed high gene colinearity and similar gene block arrangements. A comparison of the structure of the A, B, and C genomes of U's triangle showed the B genome to be divergent from the A and C genomes for gene block arrangements and centromeric regions. Conclusions: A highly contiguous genome assembly of the B. nigra genome reported here is an improvement over the previous short-read assemblies and has allowed a comparative structural analysis of the A, B, and C genomes of the species belonging to the U's triangle. Based on the comparison, we propose a new nomenclature for B. nigra pseudochromosomes, taking the B. rapa pseudochromosome nomenclature as the reference. [ABSTRACT FROM AUTHOR]

Key message: BjuWRR1, a CNL-type R gene, was identified from an east European gene pool line of Brassica juncea and validated for conferring resistance to white rust by genetic transformation. White rust caused by the oomycete pathogen Albugo candida is a significant disease of crucifer crops including Brassica juncea (mustard), a major oilseed crop of the Indian subcontinent. Earlier, a resistance-conferring locus named AcB1-A5.1 was mapped in an east European gene pool line of B. juncea—Donskaja-IV. This line was tested along with some other lines of B. juncea (AABB), B. rapa (AA) and B. nigra (BB) for resistance to six isolates of A. candida collected from different mustard growing regions of India. Donskaja-IV was found to be completely resistant to all the tested isolates. Sequencing of a BAC spanning the locus AcB1-A5.1 showed the presence of a single CC-NB-LRR protein encoding R gene. The genomic sequence of the putative R gene with its native promoter and terminator was used for the genetic transformation of a susceptible Indian gene pool line Varuna and was found to confer complete resistance to all the isolates. This is the first white rust resistance-conferring gene described from Brassica species and has been named BjuWRR1. Allelic variants of the gene in B. juncea germplasm and orthologues in the Brassicaceae genomes were studied to understand the evolutionary dynamics of the BjuWRR1 gene. [ABSTRACT FROM AUTHOR]