Metabolic Diversification of Benzylisoquinoline Alkaloid Biosynthesis Through the Introduction of a Branch Pathway in Eschscholzia californica

Abstract

Higher plants produce a diverse array of secondary metabolites. These chemicals are synthesized from simple precursors through multistep reactions. To understand how plant cells developed such a complicated metabolism, we examined the plasticity of benzyl isoquinoline alkaloid biosynthesis in transgenic Eschscholzia californica cells with the ectopic expression of Coptis japonica scoulerine-9-O-methyltransferase (CjSMT). CjSMT catalyzes the O-methylation of scoulerine to produce tetrahydrocolumbamine (THC) in berberine biosynthesis and is not involved in benzophenanthridine alkaloid biosynthesis in E. californica. While a preliminary characterization confirmed that columbamine (oxidized product of THC) was produced in transgenic E. californica cells, many newly found peaks were not identified. Here, we report the identification of novel products, including allocryptopine and 10-hydroxychelerythrine. This result indicates that CjSMT reaction products were further converted by endogenous enzymes to produce double O-methylated compounds instead of a methylenedioxy ring at the 7,8-position of the original benzophenanthridine alkaloids. Further metabolite profiling revealed the enhanced diversification of the alkaloid profile in transgenic cells. Metabolic plasticity and the enzymes involved in metabolic diversity are discussed.