Chromosome-scale genome assembly of Rhododendron molle provides insights into its evolution and terpenoid biosynthesis.

BACKGROUND

Rhododendron molle (Ericaceae) is a traditional Chinese medicine, which has been used to treat rheumatism and relieve pain since ancient times. The characteristic grayanoids of this plant have been demonstrated to be the chemical basis for the analgesic activity. Moreover, unlike morphine, these diterpenoids are non-addictive. Grayanoids mainly distribute in the leaves, flowers, roots, and fruits of R. molle, with low content. Currently the research on the biosynthesis of grayanoids is hindered, partially due to lack of the genomic information.

RESULTS

In the present study, a total of 744 Mb sequences were generated and assembled into 13 chromosomes. An ancient whole-genome duplication event (Ad-β) was discovered that occurred around 70 million years ago. Tandem and segmental gene duplications led to specific gene expansions in the terpene synthase and cytochrome P450 (CYP450) gene families. Two diterpene synthases were demonstrated to be responsible for the biosynthesis of 16α-hydroxy-ent-kaurane, the key precursor for grayanoids. Phylogenetic analysis revealed a species-specific bloom of the CYP71AU subfamily, which may involve the candidate CYP450s responsible for the biosynthesis of grayanoids. Additionally, three putative terpene biosynthetic gene clusters were found.

CONCLUSIONS

We reported the first genome assembly of R. molle and investigated the molecular basis underpinning terpenoids biosynthesis. Our work provides a foundation for elucidating the complete biosynthetic pathway of grayanoids and studying the terpenoids diversity in R. molle.