Transcriptome de novo assembly and analysis of differentially expressed genes related to cytoplasmic male sterility in cabbage.

Cytoplasmic male sterility (CMS) is a maternally inherited trait producing abnormal pollen during anther development. To identify the critical genes and pathways that are involved in the sterility and to better understand the underlying mechanisms, cabbage anthers at different developmental stages were cytologically examined and the transcriptomes were analyzed in CMS line and its maintainer line using the next-generation sequencing (NGS) technology. Microscopy showed that anther development in the CMS line was abnormal in the tetrad stage and failed to produce fertile pollen. We obtained 55,663,594 and 54,801,384 raw transcriptome reads from the sterile and maintainer lines, respectively, and assembled these reads into 68,851 unigenes with an average size of 1028 bp. By using the fragments assigned per kilobase of target per million mapped reads (FPKM) method, 5592 differentially expressed genes were identified, consisting of 3403 up- and 2089 down-regulated genes. Furthermore, there were 1011 and 45 genes specifically expressed in the maintainer or sterile line, respectively. Gene Ontology (GO) functional annotation and enrichment analysis of metabolic pathways were performed to map and analyze the candidate genes that may be involved in male sterility. Expression of eighteen genes was examined using qRT-PCR and their expression patterns were found to be same as the sequencing data. A clear cytological difference exists between the sterile and maintainer lines. The differentially expressed genes are associated with carbohydrate and energy metabolisms, or encode transcription factors, heat shock proteins and other stress proteins. Identification of these candidate genes provides a comprehensive understanding of the mechanism underlying CMS in cabbage.