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 United States, 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.
"In evolutionary terms, widespread and persistent glyphosate use without diversity in weed control practices is a strong selection pressure for weeds able to survive glyphosate," Professor Powles wrote.
Plant genes that endow glyphosate resistance are very rare, but the huge volumes of the chemical sprayed on the world's crops is helping those genes come to the fore.
"Glyphosate resistance evolution is a major adverse development because glyphosate is a one in a 100-year discovery that is as important for reliable global food production as penicillin is for battling disease."
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 gene mutation that restricts movement of glyphosate in the plant, preventing it from reaching EPSPS in toxic levels.
A scientific team led by Dr Todd Gaines, formerly of Colorado State University but now working on the Western Australia Herbicide Resistance Initiative (WAHRI), has now 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," Dr Gaines said.
"What's really interesting is the capacity of these plants to evolve extra gene copies. It would certainly be reasonable to expect other plants to exhibit this same behaviour."
Professor 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," Professor 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 signiÞcant areas of world cropping is a threat to global food production. To avert this situation requires that glyphosate be used more judiciously and with more diversity than is currently the case."
The International Service for the Acquisition of Agri-Biotech Applications (ISAAA) reported that in 2008, "...herbicide tolerance deployed in soybean, maize, canola, cotton and alfalfa [lucerne] occupied 63 per cent or 79 million hectares of the global biotech area of 125 million hectares".
Dr Gaines said most of that inbuilt resistance was to glyphosate.
ISAAA also noted that by 2008, the world had planted a cumulative 800 million hectares, or two billion acres, of biotech crops.
Dr Gaines paper on gene amplification appeared with Professor Powles's commentary in the January 2010 edition of PNAS.
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