Selection of reliable reference genes for quantitative real-time polymerase chain reaction studies in maize grains

The stability of candidate reference genes was evaluated in maize landrace varieties and during multiple grain developmental stages to evaluate the expression of carotenoid-related genes by RT-qPCR for application to maize biofortification. Vitamin A deficiency affects millions of children worldwide; therefore, increasing the content of vitamin A precursors in maize grains is of interest. The study of the expression of genes involved in the carotenoid biosynthetic pathway in maize grains has provided useful information for metabolic engineering approaches. However, reliable results using real-time quantitative polymerase chain reaction (RT-qPCR) experiments are dependent on the use of the appropriate reference genes. In this study, we utilized geNorm and NormFinder softwares to identify the most stably expressed candidate reference genes in samples from seven stages of grain development and from eight landrace varieties. The results of the analysis performed using geNorm indicated that tubulin (TUB) and actin (ACT) were the most suitable reference genes among all experimental conditions, while glyceraldehyde-3-phosphate dehydrogenase gene (GAPDH) showed the least stability. The same result was obtained with the NormFinder software. The minimum number of genes required in each experimental condition to normalize the gene expression data was also determined by geNorm. The expression of phytoene synthase gene (PSY1), the first enzyme in the carotenoid biosynthetic pathway, was overestimated when the least stable candidate gene (GAPDH) was used as the internal control instead of the most stable gene pair (ACT + TUB), thus highlighting the importance of validating reference genes before conducting a RT-qPCR experiment to obtain accurate results. This study is the first survey of the stability of genes for use as reference genes to normalize RT-qPCR data from maize landraces during multiple stages of grain development.