Identification of suitable reference genes for studies of Syringa pinnatifolia Hemsl

Syringa pinnatifolia Hemsl. (Oleaceae) is a species of shrub with a limited distribution in China. Several chemical compounds with pharmacological effects have been isolated from S. pinnatifolia, including new lignans and sesquiterpenes. Studies of gene expression in this species require the identification of suitable reference genes that are stably expressed under different conditions and in different tissues. To identify candidate reference genes, here we used the geNorm, NormFinder, and BestKeeper algorithms to analyze the stability of 12 candidate genes. The geometric mean of the rankings generated with these algorithms was used to obtain a comprehensive ranking. TBP and PP2A were found to be appropriate reference genes for calli treated with different external stimuli, and TIP41 and TBP were found to be appropriate reference genes in differentiated tissues. When calli and differentiated tissues were considered together, TBP and TIP41 were found to be the most reliable reference genes. The selected genes were validated by analysis of HMGR expression in calli and differentiated tissues. This study is the first to screen candidate reference genes in the genus Syringa and could help guide future molecular studies in this genus.