Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2000
Publication Date: N/A
Interpretive Summary: Although the herbicide cinmethylin has been used for decades, its mechanism of action has not been understood. We found that the growth inhibition caused by this compound and its related natural analogue, 1,4-cineole, was reversed by adding the amino acid, asparagine. We also found that the enzyme, asparagine synthetase (AS), was inhibited by the natural product, 1,4-cineole. This indicated that cinmethylin may be metabolized and the active part of the compound was this natural product. This is one of the first cases where a natural product is the active part of a commercial herbicide, since most natural products and synthetic products have different target sites. This is the first report that asparagine synthetase is a suitable herbicide target site. With current legislation banning several types of synthetic pesticides, the discovery of this new mechanism of action is important.
Technical Abstract: The growth inhibition resulting from the presence of cinmethylin and its natural product analog 1,4-cineole was reversed, 55% & 65%, respectively, by providing 100 uM asparagine exogenously. Dose-response experiments revealed that reversion was dependent on the concentration of asparagine and that the presence of either 1,4-cineole or cinmethylin caused an increase in uptake of [14C]asn. The greatest interference of the biosynthesis of asn by these compounds was further supported by the marked reduction in the pool of free asn in the plants tested with the inhibitors, relative to controls. Finally, biochemical analysis of AS activity showed that the natural monoterpene was a potent inhibitor (I50= ca.5uM) of the enzyme asparagine synthetase, whereas the commercial product was not inhibitory up to levels of 10 mM. This is the first report that asparagine synthetase is a suitable herbicide target site, and that cinmethylin is a proherbicide that requires metabolic bioactivation to affect that site by cleaving the benzyl-ether side chain.