Weeds Become Resistant To Key Herbicide – Setback For Many Crops

Weeds have evolved a new form of resistance to the world's most important herbicide, glyphosate, and that could have global implications.

The cultivation of food and fibre crops including rice, wheat, soybeans, maize and cotton could be badly impacted, warns Dr Todd Gaines, now of the WA Herbicide Resistance Initiative (WAHRI) in the University of Western Australia's School of Plant Biology. He was associated with the Colorado State University when carrying out the research in Georgia.

In a paper published recently in the US journal Proceedings of the National Academy of Sciences (PNAS) lead author Dr Gaines documents his research on a new resistance mechanism in weeds, discovered in the particularly damaging weed species, Amaranthus palmeri.

This weed infests large areas of US crop land and can devastate yield, Dr Gaines said. In some regions of the US southern states, glyphosate resistant weeds are becoming so rife in cotton crops that mechanical harvesters are damaged and weed control sometimes must be done by hand.  "Many locations are back to full tillage, and even manual weed control," he said.

It is through a process of gene amplification that the pesticide resistance has come about, the researchers concluded and noted, “This occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology.”

Gene amplification is a cellular process characterized by the production of multiple copies of a particular gene or genes to amplify the phenotype that the gene confers on the cell. Drug resistance in cancer cells is linked to amplification of the gene that prevents absorption of the chemotherapeutic agent by the cell. The same could apply in relation to pesticides too.

In a commentary in the same issue of journal PNAS, the West Australian University's Herbicide Initiative  Director Stephen Powles, writes: “A major threat to food production occurs every single growing season, when wild plant species (weeds) infest crop fields. Humans have battled since the dawn of agriculture to control weeds and to minimize their negative influence on food production. Modern herbicides have largely replaced human labor as the primary tool for weed control, and this has contributed significantly to the productivity of world cropping. However, despite the success of herbicides, weeds remain a primary challenge to food production, in part because selection pressure from herbicides has resulted in the evolution of herbicide resistance in weeds. A current and important example is evolved resistance to the world’s most important herbicide, glyphosate (1). 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.

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“In the past decade there has been a revolution in world cropping with the advent and widespread adoption in the Americas of transgenic soybean, maize, cotton, and canola crops (3). In these crops engineered to be glyphosate resistant, this herbicide removes infesting weeds without any damage to the crop. The massive adoption of transgenic glyphosate-resistant crops has meant excessive reliance on glyphosate for weed control across vast areas. 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. Genes endowing glyphosate resistance are initially very rare; however, repeated use of glyphosate without diversity selects for such rare glyphosate resistance genes. This is occurring particularly in areas with transgenic glyphosate-resistant crops, where glyphosate is repeatedly used as the only weed control measure. Inevitably, glyphosate resistance is evolving in many important weed species. In some but not all parts of the world, glyphosate resistance evolution can no longer be prevented, and therefore resistance must be managed. Globally, no weed control tools are as good as glyphosate, and its potential widespread loss because of resistance is a looming threat to global cropping and food production….  

“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. Indeed, the precious herbicide glyphosate is at risk for 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. To avert this situation requires that glyphosate be used more judiciously and with more diversity than is currently the case.”

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Source-Medindia
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