[Stock & Land, 28 Jan, 2010, p.20] A new form of glyphosate resistance has prompted a warning that the herbicide has become "as important for reliable global food production as penicillin is for battling disease". A vigorous weed of cotton fields in the south-east US, palmer amaranth, has been found capable of resisting the effects of glyphosate through "gene amplification" the third type of resistance mechanism now discovered, and one that has rung alarm bells with researchers.
Winthrop Professor at the University of Western Australia Stephen Powles wrote in an introduction to the latest Proceedings of the National Academy of Science (PNAS) that massive adoption of transgenic glyphosate-resistant crops has meant "excessive reliance" on glyphosate for weed control.
Glyphosate acts by inhibiting a plant enzyme, EPSPS. Researchers had previously found two modes of glyphosate resistance: a mutation of EPSPS that is not affected by the chemical, or more usually, a single geue mutatiou that restricts movemeut of glyphosate in the plant, preventing it from reaching EPSPS in toxic levels. A scieutific team led by Dr Todd Gaines, formerly of Colorado State University but now working on the Western Australia Herbicide Resistance Initiative, has found that palmer amaranth can genetically multiply its output of EPSPS. "It acts like a sponge to absorb the normal rate of glyphosate that we apply to these plants, and so they survive," said Dr Gaines, whose paper on gene amplification appeared with Prof Powles’ commentary in the January 2010 edition of PNAS."
"What’s really interesting is the capacity of these plants to evolve extra gene copies." Prof Powles noted that insects have been known to amplify genes that detoxify insecticides, but in this case palmers amaranth is amplifying the target gene itself to produce something like 20 times the quantity of the enzyme that normal applications of glyphosate shut down. "With this development, we have an even stronger basis to urge world agriculture to use glyphosate-resistant crop technology more wisely than has occurred until now," Prof Powles said in his PNAS commentary. "Indeed, the precious herbicide glyphosate is at risk of being driven into redundancy because of overuse without diversity in weed control practices. "It is not an exaggeration to state that the potential loss of glyphosate to significant areas of world cropping is a threat to global food production." - MATtHEW CAWOOD