Selection and validation of reference genes for real-time qRT-PCR normalization in different tissues of Eucalyptus tereticornis

Reference genes are generally used as endogenous normalization factor for relative quantification of target genes in quantitative real-time PCR (qRT-PCR). The present work aimed at identifying suitable reference genes for normalization of qRT-PCR data in tissues of Eucalyptus tereticornis. The expression levels of housekeeping genes like Actin (EtAct2), Isocitrate dehydrogenase (EtIDH), ribosomal RNA (Et18s rRNA), SAND family protein (EtSAND), Histone protein (EtH2B), alpha-Tubulin (EtTUB), and eukaryotic initiation factor (EteIF4B) were studied to characterize their normalization stability in different tissues including young leaves, internodes, developing and mature xylem. The expression level of these genes was analyzed using different algorithms like geNorm, NormFinder and Best-Keeper. Among the seven reference genes analyzed, EtAct2 was expressed with less variance and was found to be the most stable reference gene across different tissues using all the three programs, while the least stable gene identified was EtH2B. Further, the normalization efficiency of the reference genes were assessed to predict the expression levels of three primary cell wall specific cellulose synthase transcripts (EtCesAs) in E. tereticornis tissues. The relative expression of EtCesA4, EtCesA5 and EtCesA6 was determined to be 3-19 fold higher in leaf and internode tissues when compared to developing and mature xylem tissues. This study will allow accurate normalization of qRT-PCR experiments across different tissues in E. tereticornis for future genomic research in this tropical eucalypt species.