2,4-Dichlorophenoxyacetic acid (2,4-D)–resistant crops and the potential for evolution of 2,4-D–resistant weeds

In their recent article, we feel that Wright et al. (1) misrepresented the potential for 2,4-dichlorophenoxyacetic acid (2,4-D)–resistant weeds in 2,4-D–resistant cropping systems and exaggerated the sustainability of their approach to addressing glyphosate-resistant weed problems in agriculture.

Wright et al. (1) announced the development by Dow AgroSciences of transgenes in corn, soybean, and cotton that will confer metabolic resistance in these crops to the herbicide 2,4-D. They promoted this technology as a solution to the epidemic of glyphosate-resistant weeds caused by overuse of the herbicide glyphosate in glyphosate-resistant crops. In addressing the obvious criticism that their technology will enable a similar overuse of 2,4-D that will result in new 2,4-D–resistant weeds, Wright et al. (1) stated:

Despite its widespread use, very few 2,4-D–resistant weed species have been identified … The lack of widespread development of 2,4-D–resistant weeds may be because of the genetic redundancy in auxin/2,4-D receptors, the essentiality of auxin perception for plant development, and/or the pleiotropic nature of the downstream auxin effects. These observations suggest that the frequency of 2,4-D–resistant weed appearance may be low.

We were surprised that Wright et al. (1) stated that only “very few” 2,4-D–resistant weed species have evolved without quoting a specific number (1). We checked the database that they used to support this claim (2) and were alarmed to learn that, globally, 28 species across 16 plant families have already evolved resistance to the synthetic auxin herbicides, the mode of action to which 2,4-D belongs. Of these, 16 are known to be resistant to 2,4-D specifically (for comparison, 21 species are resistant to glyphosate globally). Furthermore, the claim that 2,4-D resistance is unlikely to evolve because of the complex and essential functions that auxin plays in plants is unsubstantiated. In many cases where resistance has evolved to synthetic auxins, the biochemical mechanism is unknown. However, in at least two cases (Kochia scoparia and Sinapis arvensis) (3, 4), resistance is conferred by a single dominant allele, indicating that resistance could develop and spread quite rapidly (5).

Historically, 2,4-D and other synthetic auxins have had restricted use in cereals or as preplant applications in broadleaf crops. The transgenes will allow 2,4-D to be applied in crops in the same fields in successive years and across dramatically expanded acreage, creating intense and consistent selection pressure for the evolution of resistance. Taken together, the facts on the current distribution of synthetic auxin resistance suggest that the potential for the evolution of 2,4-D–resistant weeds in transgenic cropping systems is not negligible but is actually quite high.

The global spread of herbicide-resistant weeds is a serious problem requiring a serious rethinking of our approach to weed management. Given that resistant weeds are the direct result of herbicide overuse, the problem will not be resolved simply by adding new herbicide-resistance traits into our crops. Rather, integrated weed management (IWM) approaches that incorporate a diversity of chemical and nonchemical practices and effectively reduce reliance on herbicides offer the promise of a truly robust weed-control system.