Mutations in the mitochondrial orf108 render Moricandia arvensis restorer ineffective in restoring male fertility to Brassica oxyrrhina-based cytoplasmic male sterile line of B. juncea

We reported earlier an evolutionarily conserved mitochondrial orf108 causing cytoplasmic male sterility (CMS) in four alloplasmic lines of Brassica juncea. B. oxyrrhina also carries orf108, but male sterility of B. oxyrrhina-based B. juncea CMS lines is not restored by the Moricandia arvensis restorer that rescues other orf108-containing B. juncea CMS lines. To understand this discrepancy, we characterized B. oxyrrhina-based B. juncea CMS line (oxy-cms) to identify the mitochondrial gene associated with male sterility. Examination of expression patterns of 26 mitochondrial genes in CMS and male fertile (oxy-camp, i.e. amphidiploid B. oxyrrhina × B. rapa, and euplasmic B. juncea) lines revealed polymorphic transcript patterns for six genes: atp1, atp4, ccmfn2, cox3, nad4L and orf108. Detailed analysis showed that orf108 is linked and co-transcribed with atp1. Comparison of orf108-atp1 transcripts in male sterile and male fertile flowers revealed that B. oxyrrhina restorer cleaves orf108-atp1 at a different location within the orf108 coding region as compared with the M. arvensis restorer. A total of eleven SNPs were detected within the orf108 coding region. One of these SNPs was located at the site where M. arvensis restorer processes the orf108-atp1 transcript suggesting that it could be critical for cleavage of orf108-atp1 transcript by the M. arvensis restorer.