Tuesday, February 24, 2015

Kochia scoparia's Mechanism for Resistance to Glyphosate Discovered

Glyphosate resistant kochia (Kochia scoparia) was first confirmed in Kansas in 2007 (http://www.weedscience.org/).  Since then, it has been found in South Dakota, North Dakota, Montana, Colorado, Oklahoma, Nebraska, Oregon, Idaho, Wyoming, Alberta (Can), Manitoba (Can), and Saskatchewan (Can).  In fact, in Montana and the three Canadian provinces, there have been populations of kochia with multiple herbicide resistance (resistance to more than one site of action) found; kochia resistant to glyphosate and another type of herbicide.  Discovering how a species evolves resistance to a herbicide can aid researchers in not only learning about the plants, their genetic make-up, and how they are able to withstand stressful conditions, but also may help the researchers to figure out ways to work around the mechanism causing the resistance.

Mostly, instances of glyphosate resistance in other species have been via:

  • Reduced translocation - i.e. herbicide is absorbed by the plant, but not moved within the plant to where it has an effect (target site), or
  • Mutations in the target site - change within the plant at the target site which prevents the herbicide from interacting or binding (ex: the plant changes the "locks" (target site) in its "house" (entire plant) and so the "key" (herbicide) can't get in.)

A recent paper by Jugulam et al. (Tandem amplification of a chromosomal segment harboring 5-enolpyruvylshikimate-3-phosphate synthase locus confers glyphosate resistance in Kochia scoparia, 2014) evaluated how some kochia populations are now resistant to glyphosate.  They found that the type of resistance displayed by the kochia populations they observed was amplification of the genes which produce EPSPS (the target for glyphosate).  Resistant populations were found to have 9-16 copies of the gene, whereas sensitive populations have only 2 copies.  Having more copies of the gene means that more of the target (EPSP) can be produced than the recommended amount of glyphosate will be able to bind up and so the plants can survive when sprayed.  This is the first reported instance of gene amplification on a single chromosome conferring field-evolved herbicide resistance in weeds.

Kochia scoparia. Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

For more on the discovery: Invasive weed Kochia's resistance to well-known herbicide stems from increase in gene copies