Glyphosate-resistant Weeds

Roundup® (glyphosate) has been used to control weeds since 1974 [How Roundup® Works]. In all those years, the number of reported cases of resistant plants has been far below predictions. Only in the past ten years have Roundup®-resistant plants been identified and there are only 11 species of resistant weeds known at last count (Perez-Jones et al. (2007).

We now know from studies of the mechanism of resistance of the C4 EPSP synthase that resistance to glyphosate requires very special circumstances; namely, an enzyme active site that can exclude glyphosate while still allowing phosphoenolpyruvate to bind efficiently [The Molecular Basis of Roundup® Resistance]. Thus, with hindsight, it is perhaps not surprising that so few resistant plants have turned up.

One of the first resistance mechanisms to be discovered was one that evolved in a population of goosegrass from Malaysia (Baerson et al. 2002). The glyphosate-resistant biotype (strain) was from a region that had been continuously sprayed for 10 years.

Baerson et al. (2007), working out of the Monsanto Labs in St. Louis MO (USA), discovered that the resistant strain of goosegrass was resistant to about five times the normal level of glyphosate. All of this resistance was apparently due to a single amino acid change in the active site of EPSP synthase. The substitution of a proline for a serine residue at position 106 decreased glyphosate binding without affecting phophoenolpyruvate binding.

Taken together, these studies suggest that an altered EPSPS provides a significant component of the glyphosate resistance mechanism in goosegrass, and represents the first example for target-based resistance to glyphosate occurring in any plant species.

The authors cannot explain why this P106S substitution confers resistance in the goosegrass enzyme, since similar substitutions in other plant enzymes affect substrate binding and render the enzyme ineffective. They conclude with,

It is possible that goosegrass may be predisposed to this type of mechanism due to species-specific genetic or physiological characteristics that remain obscure at present.

This has important implications for our understanding of evolution. Taken at face value, it suggests that in some species an evolutionary path is simply not available—there may not be a route to the so-called "top of the fitness peak." On the other hand, in other species a path can open up with a single mutation because that species, by chance, has the right kind of background. Evolution by accident.

Glyphosate-resistance has independently evolved in two strains of Italian ryegrass (Lolium multiflorum) from Oregon and Chile. The mechanism of resistance was studied by Perez-Jones et al. (2007). In this case there are two different mechanisms of resistance.

The strain from Chile had the same EPSP synthase mutation as that found in goosegrass (proline for serine at position 106). The Orgeon strain was defective in absorbing glyphosate in the roots suggesting a defect of some sort in absorption and/or transport. This is a new kind of resistance and it's not well understood at this time.

There have been rumors of Roundup® resistant coca plants in Bolivia—the ones whose leaves are used to produce cocaine. The rumors were so persistent that the magic crop was tested to see if it had been genetically engineered in a secret lab sponsored by the drug lords [The Mystery of the Coca Plant That Wouldn't Die]. The article reports that tests for C4 EPSP synthase were negative suggesting that the plants have acquired a natural resistance.

The implication is that the farmers' decentralized system of disseminating coca cuttings has been amazingly effective - more so than genetic engineering could hope to be. When one plant somewhere in the country demonstrated tolerance to glyphosate, cuttings were made and passed on to dealers and farmers, who could sell them quickly to farmers hoping to withstand the spraying. The best of the next generation was once again used for cuttings and distributed.

This technique - applied over four years - is now the most likely explanation for the arrival of Boliviana negra. By spraying so much territory, the US significantly increased the odds of generating beneficial mutations. There are numerous species of coca, further increasing the diversity of possible mutations. And in the Amazonian region, nature is particularly adaptive and resilient.

"I thought [genetic engineering] was unlikely," says Gressel, the plant scientist at the Weizmann Institute. "But farmers aren't dumb. They obviously spotted a lucky mutation and propagated the hell out of it."

The effects of this are far-reaching for American policymakers: A new herbicide would work only for a limited time against such a simple but effective ad hoc network. The coca-growing community is clearly primed to take advantage of any mutations.

From what we know of glyphosate resistance it seem unlikely that these Bolivian plants are actually resistant to Roundup®. It's probably just over-active imagination.

Baerson, S.R., Rodriguez, D.J., Tran, M., Feng, Y., Biest, N.A., Dill, G.M. (2002) Glyphosate-resistant goosegrass. Identification of a mutation in the target enzyme 5-enolpyruvylshikimate-3-phosphate synthase. Plant Physiol. 129:1265-1275. [PubMed]

Perez-Jones, A., Park K.W., Polge, N., Colquhoun, J., and Mallory-Smith, C.A. (2007) Investigating the mechanisms of glyphosate resistance in Lolium multiflorum. Planta. 2007 Feb 24 (electronic publication, ahead of print).