Fit as a weed

September 2014

 

Wild rice. CC photo by Denrdoica cerulea on Flickr

The vast majority of GM crops now being grown commercially have had a gene inserted to make them resistant to glyphosate herbicide.

 

After spraying with glyphosate, the yield of the GM crop is protected because the weeds competing for nutrients are killed.

 

Gene escape from glyphosate-tolerant crops into wild relatives has never been considered an important problem because unless the wild GM derivatives are sprayed with the herbicide, they will have no special fitness advantage and no reason to run riot. But, this 'wisdom' has been challenged by a team of Chinese scientists.

 

Glyphosate kills plants by inactivating an enzyme, 'EPSPS'*. EPSPS is vital to a number of key metabolic processes because it's responsible for generating a class of essential amino acids (the building block of proteins). These amino acids are vital to the formation of, for example, the plant's supportive material (lignin), plant growth hormone, and a huge range of immune-system substances, which together can account for as much as 35% of a plant's biomass. 

*5-enolpyruvylshikimate-3-phosphate synthase

 

The trick inserted into today's GM plants is a bacterial gene which generates a bacterial look-alike EPSPS which is unaffected by glyphosate. This novel gene expression is driven by a powerful viral promoter which the plant can't switch off.

 

Monsanto has commercialised one GM maize line containing a modified maize gene for a glyphosate-resistant EPSPS driven by a rice promoter. Presumable this tactic has proved less effective than the standard bacterial version with its viral switch.

 

Another technique tried has been to insert extra copies of the plant's own gene for the vital enzyme, so as to boost ESPS production and out-play the toxic glyphosate. However, such over-expression of a natural gene was found to be inadequate and was abandoned.

 

The Chinese team constructed an ingenious experiment using GM rice which they made glyphosate resistant by inserting an extra copy of its own gene for EPSPS, with a maize promoter. This GM rice was bred together with four related, weedy rice strains. Their progeny were bred together to produce a GM rice with a pair of the extra genes, and a sibling non-GM rice with no extra genes but the same genetic background.

 

This allowed the four GM and non-GM weedy rice varieties to be directly compared for physiological characteristics and field performance.

 

To their surprise, the scientists found that the GM weeds had “major increases” in photosynthetic rate and grown rate. It seems the extra gene, known to enable protein synthesis and growth hormone production, ramped up the GM weeds' physiology. The novel weeds on speed also exhibited “dramatic changes” in reproduction: larger numbers of seeds (up to 125% more) and increased germination of their seeds (up to 55% greater).

 

 

OUR COMMENT

 

 

Suddenly, the notion that gene flow from a GM crop into a wild relative will never do anything beyond simmering safety and silently at insignificant levels in the background, becomes untenable. Glyphosate-resistant crop-weed hybrids could come to dominate natural ecosystems. All that's needed is for a few individuals to arise there in the first place.

 

The authors suggested that the technique of giving a crop plant a boost from extra copies of its own EPSPS gene might be a way to increase yields. However, this effect on yield certainly hasn't been observed in current GM crops: GM soya with its bacterial EPSPS has suppressed yields, and other glyphosate-tolerant GM crops have had no direct yield benefit. The suggestion would be for science to explore because weeds may have a greater unused physiological reserve for the extra gene to act on compared with highly bred crop plants.

 

Food plants subject to this kind of artificial physiological acceleration should be very closely examined for the production of toxic by-products from the unnatural excess of substances arising. For example, one of the protein building blocks whose production is promoted by EPSPS is tryphotphan. In the 1980s an attempt to manufacture tryptophan supplement using GM bacteria led to the unexpected generation of a toxin which killed at least 37 people and disabled thousands more in the US alone.

 

The recent discovery of the role of pollinator insects in gene flow from 'self'-pollinating rice [1] makes the Chinese research and suggested commercial potential all the more worrying.

 

Closer to home, an application to import seeds of a GM glyphosate-tolerant oilseed rape into Europe for processing is under consideration. Transport of GM oilseed rape seed has resulted in feral GM plants growing along roadsides in all areas of the world [2,3]. Europe is a centre of biological diversity for oilseed rape and has many wild relatives for the GM escapees to interbreed with. The possibility that GM oilseed rape weedy relatives could become a dominant invasive species now seems quite realistic. Ask your MEPs to fight the importation of viable GM oilseed rape seed (you can contact them through www.writetothem.com).

 

 

[1] BEES DO WHAT? - News, August 2014

 

[2] WHY 'CO-EXISTENCE' IS IMPOSSIBLE - January 2013

 

[3] GENE POLLUTION UPDATE 2013 - News, November 2013

 

SOURCES

• Wei Wang, et al., 2013,  A novel 5-enolpyruvoylshikimate-30phosaphate (EPSPS) synthase transgene for glyphosate resistance stimulates growth the fecundity in weedy rice (Oryza sativa) without herbicide, New Phytologist
• Rice research shed new light on GM traits in 'superweeds', Farming Online 24.09.13
• Allison Snow,  Modifying Rice Crops to Resist Herbicide Prompts Weedy Neighbors' Growth Spurt, Ohio State University Research and Innovation Communications 23.09.13
• European Food Safety Authority plays down risks of Monsanto';s genetically engineered oilseed rape, Testbiotech 25.07.14
• A. M. Mayeno and G. J. Gleich, 1994, Eosinophisia-myalgia syndrome and tryptophan production: a cautionary tale, TIBTECH 12 September