Callisto: a very successful maize herbicide inspired by allelochemistry

Derek Cornes

Syngenta AG, CH4002 Basel, Switzerland; E mail: derek.cornes@syngenta.com

Abstract

Mesotrione (trade name Callisto) is a new herbicide developed by Syngenta AG for control of broadleaved weeds in maize crops. Its discovery results from the study of the allelochemical leptospermone, which is produced by the roots of the “bottle brush” plant Callistemon citrinus. Characterisation of leptospermone confirmed its herbicidal properties but also that it was unsuitable for development as a commercial herbicide. Production of synthetic analogues using the structure of leptospermone as a lead, allowed the development of more potent compounds which included mesotrione. An overview of the development process is given and the properties of mesotrione are described.

Media Summary

Mesotrione is a new herbicide for the control of broadleaved weeds in maize crops. Its discovery results from the study of the allelochemical leptospermone.

Key Words

Mesotrione, leptospermone, Callisto, Callistemon citrinus, bottle-brush

Introduction

The search for new herbicide chemistry is an ongoing task for R&D-based agrochemical companies. Lead compounds that deliver novel properties, especially new modes of action, are highly sought after, and new sources of material are always being explored. The development process for new herbicides usually starts with the discovery of a relatively weak compound with interesting properties and, through the production of new analogues and guided optimisation, the aim is to produce a highly active compound that provides benefits to the farmer and can be successfully marketed.

Methods

The “red bottle brush plant”, Callistemon citrinus, is a member of the Myrtaceae family. It originates from Australia but is now spread throughout the world as an ornamental plant in countries with mild climates. It is the most widely cultivated member of the genus both in Australia and overseas due to its very attractive appearance. The common name “red bottlebrush” perfectly describes the bright red flower spikes which make the plant so desirable as an ornamental.

The climate of California, USA is well suited to growing Callistemon citrinus and it was during 1977 that a biologist working at the California, Western Research Centre of the Stauffer chemical company (now Syngenta) noticed that very few plants were growing under the Callistemon citrinus bushes in his garden. The extent to which weeds were suppressed was out of proportion to what he expected from shading or leaf litter. Fascinated by this, he took samples of soil from the area immediately beneath the plants and used a series of extractions in an attempt to isolate potential allelopathic agents.

Results and Further Development

Literature searches revealed that leptospermone was first discovered in 1921 and that a number of researchers had since shown that it could be extracted from a number of plants. However, it had never been associated with Callistemon citrinus and had never been cited as an allelochemical.

Conclusion

The development and commercial success of mesotrione illustrates the potential for the use of allelochemicals as leads for the development of new herbicides. It is unlikely that many allelochemicals will have the potential to be developed as commercial herbicides per se, but in the increasingly difficult search for new chemistry and new modes of action, allelochemicals can provide the base structures for further chemical modification and the development of commercially viable herbicides.

References

1. Bick IRC, Blackman AJ, Hellyer RO, Horn DHS (1965). The isolation and structure of flavesone. Journal of the Chemical Society 3690.
2. Hellyer RO (1968). The occurrence of some beta-triketones in the steam-volatile oils of some myrtaceous Australian plants Australian Journal of Chemistry, 21: 2825-2828.
3. Knudsen CG, Lee DL, Michaelay WJ, Chin H, Nguyen NH, Rusay RJ, Cromartie TH, Gray R, Fraser T, Cartwright D (2000). Discovery of the triketone class of HPPD inhibiting herbicides and their relationship to naturally occurring beta-triketones. In S.S. Narwal et al. Allelopathy in ecological agriculture and forestry. Kluwer academic publishers.