A reference-grade genome assembly for Astragalus mongholicusand insights into the biosynthesis and high accumulation of triterpenoids and flavonoids in its roots

Astragalus membranaceusvar. mongholicus(AMM), a member of the Leguminosae, is one of the most important medicinal plants worldwide. The dried roots of AMM have a wide range of pharmacological effects and are a traditional Chinese medicine. Here, we report the first chromosome-level reference genome of AMM, comprising nine pseudochromosomes with a total size of 1.47 Gb and 27 868 protein-encoding genes. Comparative genomic analysis reveals that AMM has not experienced an independent whole-genome duplication (WGD) event after the WGD event shared by the Papilionoideae species. Analysis of long terminal repeat retrotransposons suggests a recent burst of these elements at approximately 0.13 million years ago, which may explain the large size of the AMM genome. Multiple gene families involved in the biosynthesis of triterpenoids and flavonoids were expanded, and our data indicate that tandem duplication has been the main driver for expansion of these families. Among the expanded families, the phenylalanine ammonia-lyase gene family was primarily expressed in the roots of AMM, suggesting their roles in the biosynthesis of phenylpropanoid compounds. The functional versatility of 2,3-oxidosqualene cyclase genes in cluster III may play a critical role in the diversification of triterpenoids in AMM. Our findings provide novel insights into triterpenoid and flavonoid biosynthesis and can facilitate future research on the genetics and medical applications of AMM.