Secret recipe for limonoids opens door to bee-friendly crop protection

New research has uncovered the secret of how plants make limonoids, a family of valuable organic chemicals that contain bee-friendly insecticides and have potential as anti-cancer drugs.

The research team, a collaboration between the John Innes Center and Stanford University, used pioneering methods to uncover the biosynthetic pathway of these beneficial molecules, made by certain plant families including mahogany and citrus.

In the study that appears in ScienceThe John Innes Center research team used genomic tools to map the genome of the cinchona (Melia azedarach), a species of mahogany, and combined this with molecular analysis to reveal the enzymes in the biosynthetic pathway.

“By finding the enzymes needed to make limonoids, we have opened the door to an alternative production source for these valuable chemicals,” said Dr. Hannah Hodgson, co-first author of the publication and postdoctoral fellow at the John Innes Centre.

Until now, limonoids, a type of triterpene, could only be produced by extraction from plant material.

dr Hodgson explained: “Their structures are too complicated to be produced efficiently by chemical synthesis. With knowledge of the biosynthetic pathway, it is now possible to use a host organism to make these compounds.” She added.

Armed with the full biosynthetic pathway, researchers can now produce the chemicals in commonly used host plants such as Nicotiana benthamiana. This method can produce larger amounts of limonoids in a more sustainable way.

Increasing the supply of limonoids could enable wider use of azadirachtin, the anti-insect limonoid derived from the neem tree used in commercial and traditional crop protection. Azadirachtin is an effective, rapidly degrading, bee-friendly crop protection option, but is not widely used due to limited supply.

The team produced two relatively simple limonoids, azadirone from chinaberry and kihadalactone A from citrus, and believe the methods used here can now be used as a template for the production of more complicated triterpenes.

John Innes’ team used genomic tools to assemble a chromosome-level genome for the cinchona (Melia azedarach), in which they found the genes encoding 10 additional enzymes required for the production of the azadirachtin precursor, azadirone. In parallel, the team working at Stanford was able to find the 12 additional enzymes needed to produce khidalactone A.

The expression of these enzymes in N. benthamiana allowed their characterization using liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance spectroscopy (NMR), technologies that allow analysis of samples at the molecular level.

Professor Anne Osbourn, group leader at the John Innes Center and co-author of the study, said: “Plants make a variety of specialized metabolites that may be useful to humans. We’re just beginning to understand how plants make complex chemicals like limonoids. Before this project, their biosynthesis and the enzymes involved were completely unknown; now the door is open for future research to build on this knowledge, which could benefit people in many ways.”

Another example of a high-quality limonoid the team hopes to make is cancer drug candidate Nimbolide; This work could provide easier access to limonoids such as nimbolide to enable further studies. In addition to making well-known products like Nimbolid, the research team say it could open the door to understanding new activities for limonoids that have not yet been studied.