De novo transcriptome assembly based on RNA-seq and dynamic expression of key enzyme genes in loganin biosynthetic pathway of Cornus officinalis

Cornus officinalis Sieb. et Zucc. is a small tree that is recognized with notable medicinal, economic, and ecological values. It has been used as traditional Chinese medicine (TCM) for thousands of years in China. Modern pharmacological research has revealed that cornel iridoid glycosides (CIGs, e.g., loganin and morroniside) in dried pericarp of C. officinalis have significant medicinal activities for strengthening immune functions. However, little is known on the molecular processes responsible for the production of these compounds. This is partly due to the absence of genomic resources, such as sequences of key enzyme genes in the biosynthetic pathways. In the present study, the transcriptome of C. officinalis was analyzed by the RNA sequencing. A total of 54,827 unigenes were yielded by de novo assembly, of which 31,780 unigenes were successfully annotated. As potential molecular markers, 121, 118, 96, 89, and 82 transcription factors belonged to bHLH, MYB, PHD, WRKY, and AP2-ERF were obtained, respectively. Moreover, the results showing that geraniol 10-hydroxylase (G10H) and secologanin synthase (SLS) were differentially expressed in fruits and leaves during different growing stages were confirmed by qRT-PCR. Furthermore, we identified two distinct expression patterns of G10H and SLS in loganin synthesis of C. officinalis fruits. Collectively, the genomic information and gene expression results presented in this study will be helpful for future studies on gene discovery and molecular process of loganin synthesis in C. officinalis.