Evaluation of suitable reference genes in Brassica juncea and its wild relative Camelina sativa for qRT-PCR analysis under various stress conditions

Quantitative real-time PCR (qRT-PCR) is an efficient method to estimate the gene expression levels but the accuracy of its result largely depends on the stability of the reference gene. Many studies have reported considerable variation in the expression of reference genes (RGs) in different tissue and conditions. Therefore, screening for appropriate RGs with stable expression is crucial for functional analysis of the target gene. Two closely related crucifers Brassica juncea (cultivated) and Camelina sativa (wild) respond differently towards various abiotic and biotic stress where C. sativa exhibits higher tolerance to various stress. Comparative gene expression analysis of the target genes between two such species is the key approach to understand the mechanism of a plant's response to stress. However, using an unsuitable RG can lead to misinterpretation of expression levels of the target gene in such studies. In this investigation, the stability of seven candidate RGs including traditional housekeeping genes (HKGs) and novel candidate RGs were identified across diverse sample sets of B. juncea and C. sativa representing- hormone treated, wounded, Alternaria brassicae inoculated and combination treated samples (exogenous hormone treatment followed by A. brassicae inoculation). In this investigation, we identified stable RGs in both the species and the most suitable RGs to perform an unbiased comparative gene expression analysis between B. juncea and C. sativa. Results revealed that TIPS41 and PP2A were identified as the overall best performing RGs in both the species. However, the most suitable RG for each sample subset representing different condition must be individually selected. In Hormone treated and wounded samples TIPS41 expressed stably in both the species and in A. brassicae inoculated and combination treatment performance of PP2A was the best. In this study, for the first time, we have identified and validated stable reference gene in C. sativa for accurate normalization of gene expression data.