Title: Weeds show different resistance mechanisms to glyphosate
[NDSU 11 Aug 2011] -- Most are aware and can name weed biotypes that have become resistant to glyphosate. Weeds that have been found resistant to glyphosate and other herbicides in ND and the U.S. are summarized on pages 102-103 in the ND Weed Control Guide. Determining the mechanism of resistance or by what means plants escape the phytotoxicity of glyphosate has become a lengthy process and the results surprising. The process is not complete but scientists are finding different resistance mechanisms in different species.
In contrast, weed resistance to ALS herbicides are rather simple and involves an altered target site - genetic mutations within a herbicide site of action can prevent complete herbicide interaction with binding sites, allowing the target-site protein to remain functional. The incomplete inhibition of the altered site of action may result in little to no observed plant injury. Where the herbicide has such little inhibitory effect on the site of action, plants may survive greater than 10 times the normal herbicide rate (considered high-level resistance). Modes of action where high-level resistance is most often seen include ACCase, ALS, and photosystem II inhibitors.
How is horseweed (marestail) resistant to glyphosate? Recently a team of scientists from Washington University in St. Louis and Monsanto followed molecules of glyphosate after absorption by tracking the chemical fate of P (phosphorus) contained in the glyphosate molecule. Scientists were able to distinguish the glyphosate signal from those of other phosphorus-bearing plant metabolites. They found a major signal from glyphosate in the plant vacuole, a large water-filled compartment found in all plant cells that can serve as a garbage disposal for chemicals foreign to the plant. Within 24 hours, resistant horseweed had shuttled 85 percent of the glyphosate into the vacuole. Sensitive horseweed transport only 15% of glyphosate to the vacuole. Once glyphosate gets to the vacuole it is trapped so less is available for translocation to susceptible areas.
Does waterhemp and amaranth species have the same mechanism? Some other glyphosate resistant species may use the same mechanism, but it appears there is at least another mechanism of glyphosate resistance. Scientists at Colorado State University, the USDA, and the University of Adelaide in Australia have found certain variants of Palmer amaranth (a type of pigweed). These Palmer amaranths have become resistant by overproducing the EPSPS enzyme to the point that it cannot all be bound by glyphosate. Many of these plants have up to 100 copies or more of the EPSPS gene and produce more target site enzyme than glyphosate can fully inhibit. This case is the only known example of this type of mechanism.
So how is this information helpful to growers and those who depend on glyphosate for weed control? Mother Nature ALWAYS wins. It appears that weed resistance has taken a leap forward in complexity. Different weed species can develop different resistance mechanisms to the same herbicide. This does not change the recommendations to use crop and herbicide rotation to delay resistance and to scout fields, detect lone plants or small patches, and to kill plants possessing that resistant gene, so it doesn’t increase.
Can this information help to increase weed control and herbicide longevity? Probably not with waterhemp/amaranth, but scientists noticed a differential response in horseweed when glyphosate was applied at different temperatures. Horseweed plants maintained at room temperature had much more rapid vacuole sequestration, but cold-acclimated resistant horseweed (50 F) plants were much more susceptible to glyphosate. Mining Monsanto’s weed control database validated this finding. Resistant horseweed sprayed with glyphosate in early spring showed that kill rates correlated with temperature under field conditions was the same as under laboratory conditions. These experiments suggest farmers might be able to improve control of resistant horseweed by spraying in early spring, when the weather is cooler.
Summary: Mother Nature ALWAYS wins - plants are sophisticated, extraordinarily complex, and masters at survival.