Examples of herbicides derived from natural products

Duke et al. (2000) and Wakabayashi and Boger (2004) refer to several case studies where chemical leads for novel sites of action and herbicidal activity have been generated from natural products. Both papers are fully referenced with many examples from the literature. The reader is recommended to mine these valuable seams of information collated by these experts in the field. This section aims to highlight some of the most promising structures that have emerged in recent years (Figure 14.3).

Cinmethylin is a pro-herbicide that resembles the plant monoterpene cineoles and has been developed for the control of monocotyledonous weeds. Following the cleavage of the benzyl ether side chain, it inhibits asparagine synthetase, an activity shared by the natural monoterpene.

Hydantocidin is a nucleoside analogue from Streptomyces hygroscopicus. It is also a pro-herbicide that when phosphorylated in vivo can inhibit adenylsuccinate synthetase.

CH3-C-CH3

1,4-cineole CH3

OO leptospermone O O Cl

CH3-C-CH3

1,4-cineole CH3

cinmethylin

CH3 C CH3 H

OO leptospermone O O Cl sulcotrione

NH2 H O bialaphos

COOH

cinmethylin

SO2Me

CH3 C CH3 H

sulcotrione

"*wNH

HO O hydantocidin

H3C-O

phosphinothricin NH2 "CH2OH

sphinganine OH NH2

phytosphingosine

HO O hydantocidin

H3C-O

phytosphingosine

CH3 OH CH

plastoquinone O NH2

CH3 OH CH

OH OH

OH OH

HO2C =O

CO2H

Figure 14.3 Structures of the herbicides mentioned in the text and the natural products from which they were derived.

Phosphinothricin or glufosinate is the breakdown product of bialaphos produced by Streptomyces viridochromogenes and S. hygroscopicus and inhibits glutamine synthase in a non-selective manner.

Sorgoleone is a natural phytotoxin isolated from root extracts of Sorghum spp., which has considerable allelopathic activity towards other plants. It structurally resembles plastoquinone and competes for the QB binding site at the D1 protein.

Trikeones, such as sulcotrione, are derivatives of leptospermone found in the bottle-brush plant (Callistemon spp.), which inhibit HPPD activity.

The AAL-toxin is a natural herbicide that inhibits ceramide synthase, causing the accumulation of phytosphingosine and sphinganine.

14.4.3.1 Strigolactones (Figure 14.4)

In September 2008 a new group of natural plant hormones was announced, the strigolactones, thought to be involved in the branching process. Mutant pea seedlings that exhibited uncontrolled branching were found not to contain these hormones and that the effect could be overcome by the addition of strigolactones (Gomez-Roldan et al., 2008; Umehara et al., 2008).

These terpenoid lactones are biosynthetically derived from carotenoids (see Figure 14.4 for representative examples of chemical structures). Inhibition of shoot branching can have a profound effect on plant architecture and apical dominance. Thus, cut flowers or ornamental potted plants with either more or less branching may have ornamental appeal, while crop yield might be manipulated.

The strigolactones appear to be synthesised in the roots and move throughout the plant. They are also thought to be involved in the mycorrhzal symbiosis by promoting the growth of mycorrhizal fungi and stimulating the germination of seeds of the parasitic weeds Striga and Orobanche. It follows that a synthetic analogue which stimulates the germination of these parasites would find a valuable role in the crop protection armoury.

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