Insights into triterpene synthesis and unsaturated fatty-acid accumulation provided by chromosomallevel genome analysis of Akebia trifoliata subsp . australis

Akebia trifoliata subsp.australis is a well-known medicinal and potential woody oil plant in China.The limited genetic information available for A.trifoliata subsp.australis has hindered its exploitation.Here,a high-quality chromosomelevel genome sequence of A.trifoliata subsp.australis is reported.The de novo genome assembly of 682.14 Mb was generated with a scaffold N50 of 43.11 Mb.The genome includes 25,598 protein-coding genes,and 71.18%(485.55 Mb)of the assembled sequences were identi fi ed as repetitive sequences.An ongoing massive burst of long terminal repeat(LTR)insertions,which occurred~1.0 million years ago,has contributed a large proportion of LTRs in the genome of A.trifoliata subsp.australis.Phylogenetic analysis shows that A.trifoliata subsp.australis is closely related to Aquilegia coerulea and forms a clade with Papaver somniferum and Nelumbo nucifera,which supports the well-established hypothesis of a close relationship between basal eudicot species.The expansion of UDP-glucoronosyl and UDP-glucosyl transferase gene families and fi-amyrin synthase-like genes and the exclusive contraction of terpene synthase gene families may be responsible for the abundant oleanane-type triterpenoids in A.trifoliata subsp.australis.Furthermore,the acyl-ACP desaturase gene family,including 12 stearoyl-acyl-carrier protein desaturase(SAD)genes,has expanded exclusively.A combined transcriptome and fatty-acid analysis of seeds at fi ve developmental stages revealed that homologs of SADs,acyl-lipid desaturase omega fatty acid desaturases(FADs),and oleosins were highly expressed,consistent with the rapid increase in the content of fatty acids,especially unsaturated fatty acids.The genomic sequences of A.trifoliata subsp.australis will be a valuable resource for comparative genomic analyses and molecular breeding.