Comparative transcriptome analysis between embryogenic and nonembryogenic callus of Kalopanax septemlobus

Kalopanax septemlobus is a deciduous medicinal tree species belonging to the family Araliaceae. It contains a number of triterpenoid saponins primarily constituted of hederagenin aglycones. Here, we presented de novo comparative transcriptome analysis using 454 pyrosequencing technology between embryogenic (EC) and nonembryogenic callus (NEC) of K. septemlobus. A total of 372,229 high-quality reads were generated by combining 168,065 EC reads and 204,164 NEC reads, which were then assembled into 80,280 and 63,128 unigenes, respectively. A comparative analysis of transcription profiles showed that 13,766 genes were differentially expressed between EC and NEC. Moreover, functional enrichment analysis revealed that a number of genes involved in the proteolysis and embryogenesis were highly expressed in EC, whereas oxidative stress-responsible genes were overpresented in NEC. Interestingly, the expression levels of genes involved in saponin biosynthesis, such as squalene epoxidase, cytochrome P450s, and glycosyl transferase, were also higher in NEC than in EC, suggesting that their expression is stress-inducible. In this study, we characterized the K. septemlobus transcriptome profiles involved in somatic embryogenesis (SE), which might enhance knowledge of SE process and provide a useful molecular resource for further study of K. septemlobus.