New Ways To Kill Plants

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Finally, Is This Really “The Last Roundup®”?

It’s been a pretty horrible year or so for herbicides.

Last month there was a lot of bad news regarding the commonly-used herbicide Roundup® (glyphosate).

A jury ordered Monsanto to pay $289 million to a cancer patient in a Roundup® lawsuit. (This led General Mills to remove the “natural” label from its granola bars.)

And also last month, a report was published that showed trace amounts of glyphosate in breakfast cereals marketed to children.

Indeed, the following headline sort of sums up what’s been happening recently with the world’s most widely-used herbicide: “Glyphosate under fire from San Francisco to Sri Lanka”. (By the way, a thoughtful commentary regarding the current controversies involving glyphosate can be found here.)

And glyphosate isn’t the only herbicide on the receiving end of recent bad news.

The auxin-based herbicide Dicamba, which has gained a resurgence in agricultural use due to increased glyphosate-tolerant “superweeeds” and the advent of Dicamba-insensitive GMO crop plants, has also been the subject of negative reporting in recent years, including a story of murder.

Somewhat analogous to the crisis regarding antibiotic-resistant bacteria, weed scientists are running out of conventional options to combat the ever-increasing numbers of herbicide-tolerant weed species. (For example, please see Ref. 1 below.)

Are there alternative approaches?

Discovering New Ways To Kill Plants

It’s somewhat amazing to read that “…no new commercial herbicide modes of action (MOAs) have been discovered in 30 years.” (from Ref. 1 below).

There are several reasons for this, including the huge success of glyphosate-resistant, GMO crops (most prominently “Roundup-Ready®” crop plants) and the costs involved in developing new, commercially-viable herbicides.

A so-called “emerging approach” to developing new strategies for weed control involves investigating natural compounds toxic to plants, typically produced by microbes, as new leads for herbicides. This seems a somewhat obvious strategy since there are many well-known examples of bacteria and fungi waging chemical warfare on plants.

“The structural diversity and evolved biological activity of natural phytotoxins offer opportunities for the development of both directly used natural compounds and synthetic herbicides with new target sites based on the structures of natural phytotoxins. Natural phytotoxins are also a source for the discovery of new herbicide target sites that can serve as the focus of traditional herbicide discovery efforts. There are many examples of strong natural phytotoxins with MOAs other than those used by commercial herbicides, which indicates that there are molecular targets of herbicides that can be added to the current repertoire of commercial herbicide MOAs.” (from Ref. 2 below).

However obvious this approach may seem in theory, it has been technically difficult to achieve in practice….until now.

A recent report in Nature magazine (see Ref. 3 below) has strongly supported the strategy of using genomic techniques to identify natural herbicides, produced by microbes, with new and effective modes of action.

Briefly, a team of investigators from the U.S. and China chose a critical metabolic pathway found in plants, but not in animals, and identified a key enzyme in this pathway as a potential target for a new herbicide. They then used a bioinformatics approach to discover a new herbicide targeting this enzyme.

For more information regarding this work, I’ll refer you to an excellent description in a press release from UCLA.

Who knows if the latest troubles with glyphosate mark the beginning of the end for this ubiquitous herbicide. But the apparent validation of a bioinformatics approach for the discovery of new herbicides (Ref. 3 below) may reinvigorate weed management.

UPDATE (11/7/2018)

Do It Again – Dicamba Use Re-Extended By EPA

Despite serious concerns regarding damage from herbicide drift, the EPA has extended use the of Dicamba through 2020. (See more about this here.)

Despite all the issues involved with resurrecting this old herbicide, as mentioned in previous posts (here and here), weed scientists are currently faced with few choices (at least until 2050 – see Ref. 4 below).

References

1. Davis, A. S. and G. B. Frisvold (2017) “Are herbicides a once in a century method of weed control?” Pest Management Science, Vol. 73, pp. 2209-2220. DOI: 10.1002/ps.4643 (Abstract)

2. Dayan, F. E. and S. O. Duke (2014) “Natural Compounds as Next-Generation Herbicides.” Plant Physiology, Vol. 166, pp. 1090-1105. DOI: 10.1104/pp.114.239061 (Full Text)

3. Yan, Y., et al. (2018) “Resistance-gene-directed discovery of a natural-product herbicide with a new mode of action.” Nature, Vol. 559, pp. 415–418. DOI: 10.1038/s41586-018-0319-4 (Abstract)

4. Westwood, J. H., et al. (2018) “Weed Management in 2050: Perspectives on the Future of Weed Science.” Weed Science, Vol. 66, pp. 275-285. DOI: 10.1017/wsc.2017.78 (Full Text)

“Do It Again”- The Kinks

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