Comparison of reference genes for quantitative real-time polymerase chain reaction analysis of gene expression

A protocol for reverse transcription followed by real-time quantitative PCR (RT-qPCR) analysis of tissue-specific and genotype-variable gene expression in sugarcane (Saccharum sp.) was developed. A key requirement for this analysis was the identification of a housekeeping gene with transcript levels that were relatively stable across tissues and genotypes, suitable for use as a reference. Primers for beta-actin, beta-tubulin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes and 25S ribosomal RNA were designed and tested by RT-qPCR, and formation of product in the reactions was measured with the SYBR green I dye system. Ribosomal RNA was the most sensitive and consistent as a reference gene. Determination of the expression levels of beta-actin, beta-tubulin, and GAPDH transcripts relative to that of 25S rRNA showed that GAPDH had the most consistent mRNA expression of protein-coding genes across different tissues. GAPDH also showed low variation in expression in maturing stem internodes when compared across 2 cultivars and 3 other Saccharum species. GAPDH therefore appears to be a suitable "housekeeping gene" in addition to 25S rRNA as a reference for measuring the relative expression of other genes in sugarcane. With use of GAPDH as a reference, the relative expression of the sugarcane sugar transporter gene Pst2a was assessed in a range of tissues. The result obtained was similar to our previously published Northern blot analysis. The protocol described here, using GAPDH as a reference gene, is recommended for studying the expression of other genes of interest in diverse tissues and genotypes of sugarcane.