Making iridoids/secoiridoids and monoterpenoid indole alkaloids: progress on pathway elucidation

Members of the Acanthaceae, Apocynaceae, Bignoniaceae, Caprifoliaceae, Gentianaceae, Labiatae, Lamiaceae, Loasaceae, Loganiaceae, Oleaceae, Plantaginaceae, Rubiaceae, Saxifragaceae, Scrophulariaceae, Valerianaceae, and Verbenaceae plant families are well known to accumulate thousands of bioactive iridoids/secoiridoids while the Apocynaceae, Loganiaceae and Rubiaceae families also accumulate thousands of bioactive monoterpenoid indole alkaloids (MIAs), mostly derived from the secologanin and tryptamine precursors. Several large-scale RNA-sequencing projects have greatly advanced the tools available for identifying candidate genes whose gene products are involved in the biosynthesis of iridoids/MIAs. This has led to the rapid comparative bioinformatics guided elucidation of several key remaining steps in secologanin biosynthesis as well as other steps in MIA biosynthesis. The availability of these tools will permit broad scale biochemical and molecular description of the reactions required for making thousands of iridoid/MIAs. This information will advance our understanding of the evolutionary and ecological roles played by these metabolites in Nature and the genes will be used for biotechnological production of useful iridoids/MIAs.