Integration of metabolomics, transcriptomica and DNA barcoding data to describe the diversity of iridoids within the genus Nepeta (Nepetoidae, fam. Lamiaceae)

Abstract

Aim: The chemical richness of iridoids in the genus Nepeta, the exclusive representative of the subfamily Nepetoidae (Lamiaceae) that produces iridoids, has complex roles in nature, facilitating interactions with insects, phytopathogens, and other co-occurring plants. On the other hand, Nepeta species are used widely by humans as medicines, flavour additives, fragrances, as well as toys for cats, and their bioactivities are usually assigned to nepetalactones. Data on the distribution and diversity of iridoids across the genus Nepeta are quite fragmental and only rarely, the two subgroups, iridoid glycosides (IGs) and iridoid aglycones (IAs), are simultaneously analysed. Our aim was to develop and adopt non-targeted metabolomics methods to comprehensively acquire, analyse and interpret the overall diversity of iridoids in selected phylodiverse Nepeta taxa, and thus facilitate reconstruction of this diversity at the molecular level. Material and methods: Metabolomics data were acquired adopting UHPLC/Orbitrap MS, UHPLC/QToF MS, and GC/MS. Leaves of phylodiverse/chemodiverse Nepeta taxa were subjected to RNA-Seq. DNA barcoding analysis was performed using chloroplast markers (matK, rbcL, trnL-trnF) and nuclear ribosomal ITS marker. Main results: Nepeta taxa were robustly grouped into three major chemotypes: (1) producers of both IGs and IAs; (2) taxa exclusively producing IGs; and (3) taxa lacking iridoids. Chemotype A clade was further roughly classified into several sub-chemotypes based on the stereochemistry of the major compound  nepetalactone. Phylogenetic relations between the analysed taxa were reconstructed adopting a selected set of DNA barcoding markers. Transcriptomes of leaves of the distinguished Nepeta chemotypes were searched for transcripts of iridoid-related biosynthetic genes to facilitate the assessment of factors responsible for the presence/absence of IAs and IGs in these plants. Conclusion: Integration of metabolomics, transcriptomics and DNA barcoding data revealed no clear regularities between chemotype affiliation and phylogenetic relatedness in analysed Nepeta taxa and pointed to several key genes in the iridoid’s biosynthetic pathway as significant determinants of the their diversity within the genus.