Evaluation of suitable reference genes for qRT-PCR gene expression normalization in reproductive, vegetative tissues and during fruit development in oil palm

Gene expression patterns of a target gene in different tissues and at different stages of development can provide clues towards the understanding of its biological function. The accuracy of gene expression via quantitative real-time reverse transcription-PCR (qRT-PCR) analysis strongly depends on transcript normalization using reference genes that demonstrate consistent expression levels in tissues and experimental conditions studied. However, suitable reference genes for qRT-PCR in oil producing fruit have not been well identified. In this study, the expression stability of fourteen potential reference genes for qRT-PCR analysis in diverse sets of biological samples including six distinct oil palm organs, two floral tissues, four vegetative tissues, five stages of fruit development and two differential phenotype fruit tissue was evaluated. The expression stabilities were assessed using three statistical algorithms including geNorm, NormFinder and qBASE plus. The stability rankings from three outputs were consolidated to obtain the consensus stability ranking of all reference genes. Our analysis showed that a Gibberellin-responsive protein (GRAS) and Cyclophilin 2 (Cyp2) were the most stable genes in oil palm tissues tested. A combination of Cyp2, GRAS and SLU7 were identified as most suitable reference genes for all oil palm tissues and vegetative tissues set while in reproductive tissues set, GRAS and Glutaredoxin was recommended. Cyp2 and GRAS were required for normalization in mesocarp tissues at various developmental stages and differential oil yielding mesocarp tissues. In addition, the relative gene expression profile of two fatty acid biosynthesis genes, EgACCase and EgSTEA was conducted to confirm the validity of the reference genes in this study. Both EgACCase and EgSTEA were analyzed in fruit tissues at various developmental stages. This study identified the most suitable reference genes for normalization of gene expression in oil palm. These results serve as a guideline for the selection of suitable reference genes under different experimental conditions and accurate normalization of gene expression studies in a wide variety of tissues in oil palm.