Whole genomic sequencing of RT98 mitochondria derived from Oryza rufipogon and northern blot analysis to uncover a cytoplasmic male sterility-associated gene

Cytoplasmic male sterility (CMS) is a maternally inherited trait resulting in the failure to produce functional pollen and is often observed when an alien cytoplasm is transferred into a cultivated species. An RT98A CMS line and an RT98C fertility restorer line were obtained by successive backcrossing between Oryza rufipogon W1109 and Oryza sativa cultivar Taichung 65. To uncover the CMS-associated mitochondrial genes, we determined the complete sequence of the RT98-CMS mitochondrial genome using next-generation pyrosequencing, and searched new open reading frames (orfs) absent in a reported mitochondrial genome of O. sativa Nipponbare. Then, six candidates were selected for the CMS-associated genes based on the criteria in which they were chimeric in structure or encoded a peptide with transmembrane domains. One of the candidates, orf113, showed different transcript sizes between RT98A and RT98C on Northern blot analysis. The orf113 gene was shown to be co-transcribed with atp4 and cox3 encoding ATP synthase F0 subunit 4 and Cyt c oxidase subunit 3, respectively, and their transcripts were distinctly processed in the presence of a fertility restorer gene. Our results indicate that orf113 is a CMS-associated gene of RT98-CMS.