Innovative research has uncovered the secret to how plants make limonoids, a family of valuable organic chemicals that includes bee-friendly insecticides that have potential as anti-cancer drugs.
The research team, a collaboration between the John Innes Center and Stanford University, used groundbreaking methods to reveal the biosynthetic pathway of these helpful molecules, which are made by certain plant families, including mahogany and citrus fruits.
In the study published in La sciencethe John Innes Center research team used genomic tools to map the genome of Chinaberry (Melia azedarach), a species of mahogany, and combined it with molecular analysis to reveal the enzymes of the biosynthetic pathway.
“By finding the enzymes needed to make limonoids, we have opened the door to another source of production for these valuable chemicals,” explained Dr. Hannah Hodgson, co-first author of the paper and postdoctoral researcher at The John Innes Centre.
Until now, limonoids, a type of triterpene, might only be produced by extraction from plant material.
Dr. Hodgson explains: “Their structures are too complicated to be made efficiently by chemical synthesis. With knowledge of the biosynthetic pathway, it is now possible to use a host organism to produce these compounds. she added.
Armed with the complete 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 might allow more widespread use of azadirachtin, the insect-killing limonoid obtained from neem and used in commercial and traditional crop protection. Azadirachtin is an effective, fast-degrading, and bee-friendly option for crop protection, but it is not widely used due to a limited supply.
The team made two relatively simple limonoids, azadirone from Chinaberry and kihadalactone A from citrus, and believe the methods used here can now be applied as a model for making more complex triterpenes.
Professor Anne Osbourn, group leader at the John Innes Center and corresponding co-author of the study, said: “Plants make a wide variety of specialized metabolites that can be useful to humans. We are just beginning to understand how plants make complex chemicals like limonoids. Prior to 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 might benefit people in many ways.
Another example of a high-value limonoid the team hopes to produce is the cancer drug candidate nimbolide, this work might allow easier access to limonoids like nimbolide to allow further study. In addition to producing known products like nimbolide, the research team says the door might open to understanding new activities for limonoids that have not yet been studied.
The John Innes Center team was funded by Syngenta and BBSRC through an industry partnership award.
Research method in more detail
John Innes’ team used genomic tools to assemble a genome at the chromosomal level for Chinaberry (Melia azedarach), in which they found the genes coding for 10 additional enzymes needed to produce the azadirachtin precursor, azadirone. In parallel, the team working at Stanford was able to find the 12 additional enzymes needed to make khidalactone A.
The expression of these enzymes in N. benthamiana allowed their characterization, using both liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance spectroscopy (NMR), technologies that allow the analysis of samples at the molecular level.
