Assembly, Closing, and Finishing of the Sorghum bicolor A1 Cytoplasmic-Male Sterility (CMS) Mitochondrial Genome: Identification of Novel Fusion ORFs

Cytoplasmic Male Sterility (CMS) in flowering plants is typically associated with alterations in the mitochondrial genome resulting from rearrangements, insertions or deletions. Nuclear restoration of fertility (Rf) genes are defined by their ability to suppress the CMS phenotype, and frequently encode pentatricopeptide repeat (PPR) proteins. To identify the molecular basis of A₁-CMS in Sorghum bicolor, we have assembled the complete mitochondrial genome of cultivar A₁Tx623. Due to the complexity of this mitochondrial genome, we performed assembly with a combination of fragment libraries, 2kb-40kb mate pair libraries, and complete fosmid insert sequences. The finished genome is composed of 6 subgenomes consisting of various combinations of 15 different genomic segments. A master circular genome could not be assembled for the A₁ CMS mitochondria based on the present data. A unique fusion ORF has been detected in one of the subgenomes, and RNASeq data supports expression of this ORF in panicles of A₁cytoplasm. The effect of fertility restoration genes on the expression of the chimeric ORF will be discussed.