Genome and evolution of the shade-requiring medicinal herb Panax ginseng.

Panax ginseng C. A. Meyer, reputed as the king of medicinal herbs, has slow growth, long generation time, low seed production and complicated genome structure that hamper its study. Here, we unveil the genomic architecture of tetraploid P. ginseng by de novo genome assembly, representing 2.98 Gbp with 59 352 annotated genes. Resequencing data indicated that diploid Panax species diverged in association with global warming in Southern Asia, and two North American species evolved via two intercontinental migrations. Two whole genome duplications (WGD) occurred in the family Araliaceae (including Panax) after divergence with the Apiaceae, the more recent one contributing to the ability of P. ginseng to overwinter, enabling it to spread broadly through the Northern Hemisphere. Functional and evolutionary analyses suggest that production of pharmacologically important dammarane-type ginsenosides originated in Panax and are produced largely in shoot tissues and transported to roots; that newly evolved P. ginseng fatty acid desaturases increase freezing tolerance; and that unprecedented retention of chlorophyll a/b binding protein genes enables efficient photosynthesis under low light. A genome-scale metabolic network provides a holistic view of Panax ginsenoside biosynthesis. This study provides valuable resources for improving medicinal values of ginseng either through genomics-assisted breeding or metabolic engineering.

Rapid amplification of four retrotransposon families promoted speciation and genome size expansion in the genus Panax.

Genome duplication and repeat multiplication contribute to genome evolution in plants. Our previous work identified a recent allotetraploidization event and five high-copy LTR retrotransposon (LTR-RT) families PgDel, PgTat, PgAthila, PgTork, and PgOryco in Panax ginseng. Here, using whole-genome sequences, we quantified major repeats in five Panax species and investigated their role in genome evolution. The diploids P. japonicus, P. vietnamensis, and P. notoginseng and the tetraploids P. ginseng and P. quinquefolius were analyzed alongside their relative Aralia elata. These species possess 0.8-4.9 Gb haploid genomes. The PgDel, PgTat, PgAthila, and PgTork LTR-RT superfamilies accounted for 39-52% of the Panax species genomes and 17% of the A. elata genome. PgDel included six subfamily members, each with a distinct genome distribution. In particular, the PgDel1 subfamily occupied 23-35% of the Panax genomes and accounted for much of their genome size variation. PgDel1 occupied 22.6% (0.8 Gb of 3.6 Gb) and 34.5% (1.7 Gb of 4.9 Gb) of the P. ginseng and P. quinquefolius genomes, respectively. Our findings indicate that the P. quinquefolius genome may have expanded due to rapid PgDel1 amplification over the last million years as a result of environmental adaptation following migration from Asia to North America.