Herbicide resistant weeds in a GM field

Not the way that you might think or fear, though.

Genetically modified crops face public resentment, especially in Europe, perhaps simply as a figurehead of big corporate agriculture. One concern that often comes up is the possibility that the foreign genes will escape, to non-GM crops nearby or to weed populations. It’s not as unlikely as it might sound: even quite distantly related plants will very occasionally interbreed, so some genes can swap between species.

Since some of the most important GM crops are herbicide resistant (so farmers can use powerful weedkillers that would normally kill the crop), the spectre of herbicide-resistant ‘superweeds’ is invoked. So when herbicide resistant amaranth appeared in fields of GM cotton in the US, it must have been worrying.

Palmer's amaranth, also pleasingly called 'carelessweed'. Image by pompilid (Wikipedia)

As it turns out, the amaranth isn’t using the gene that was engineered into the cotton. They developed their own resistance, a textbook example of evolution by natural selection (if artificial herbicides could be called natural). Of course, that doesn’t make the weed any less of a problem, but it does mean that GM isn’t directly to blame.

The herbicide in question goes by the chemical name glyphosate, while Monsanto sell it under the brand name Roundup (hence the Roundup Ready series of resistant, GM crops). It works by blocking an enzyme called EPSPS. What EPSPS does isn’t important to the discussion, but it is important to the plant. With that enzyme out of action, a key part of their metabolism is blocked, and they die.

Roundup Ready GM crops are given a different form of EPSPS which isn’t affected by glyphosate. But the amaranth hasn’t got that gene, and it hasn’t independently evolved a resistant EPSPS gene, either. It’s used a rather more interesting strategy, a kind of brute force approach.

The amaranth simply produces a lot of the enzyme, and swamps the herbicide. And it’s achieved that by copying the gene for the enzyme, anywhere from five to 160 times, splashing it around its chromosomes. Like photocopying a recipe and giving it to dozens of people, that’s a simple but effective way to get more enzyme (or cake) made. One of the plants they bred in the lab had the enzyme processing 20 times faster than normal.

That’s interesting in its own right as a simple evolutionary case study, but it’s got other implications. Copying genes is a key part of evolution, because you get ‘spares’. The plant with 160 copies of the EPSPS gene could likely survive with 159 of them, leaving one free to evolve into… who knows? An enzyme producing doing something different in a small but significant way, perhaps. And does this massive duplication of genes happen by chance from time to time, or was it somehow caused by the stress of herbicide applications? What would happen to all those extra genes if we stopped using glyphosate?

Finally, I said that GM isn’t directly to blame. You could make a case that it’s indirectly responsible, because it allows the widespread use of a single herbicide, which creates a strong selection pressure on weeds. But, even if they sound evil, powerful herbicides are economically useful (so long as they don’t kill the crop, of course). The herbicide resistant weeds don’t make it any harder for farmers, they just cancel out the benefit of using the herbicide in the first place. In the end, we find ourselves in an arms race against natural selection, which has already equipped amaranth with resistance to several other herbicides. So far, we seem to be keeping up.

Reference:

Gaines, T., Zhang, W., Wang, D., Bukun, B., Chisholm, S., Shaner, D., Nissen, S., Patzoldt, W., Tranel, P., Culpepper, A., Grey, T., Webster, T., Vencill, W., Sammons, R., Jiang, J., Preston, C., Leach, J., & Westra, P. (2009). Gene amplification confers glyphosate resistance in Amaranthus palmeri Proceedings of the National Academy of Sciences, 107 (3), 1029-1034 DOI: 10.1073/pnas.0906649107