Prof. Dr. Muhammad Sarwar Khan
Pro-Vice Chancellor/Former Vice Chancellor University of Agriculture, Faisalabad – Pakistan.
E-mail: sarwarkhan_40@hotmail.com
Engineered herbicide tolerance in plants provides a superior mechanism of weed control in crop husbandry using broad spectrum herbicides. Nevertheless, resistance development in plants particularly in weeds, and the carcinogenic effects of glyphosate on human beings are reported as implications of herbicide applications. The significance of these adverse effects depends upon the likelihood that these herbicides directly cause cancer in human being or not, and explore new herbicides to be used as alternative herbicides like glufosinate and atrazine etc. This article precisely looks into the outcome of experiments and perceptions.
Weeds are unwanted plants, growing in the field, competing for resources; including nutrients, water and sunlight that otherwise would be available to the crop. Hence, weeds are a burden on resources and affect farmers’ economics by compromising the yield. The losses caused by weeds in our major crops are estimated to be about Rs. 65 billion. These losses can be reduced by means of weed prevention, eradication and chemical control. Of these three, chemically controlling weeds is most effective method in the developed world. It constitutes about 55 % of the total pesticides used in these countries.
In Pakistan, the herbicides are specifically imported from USA, Europe, Japan, or China and their usage is increasing rapidly. These herbicides are selective in nature and kill weeds with limited damage to crop plants. This also releases thousands of people from the drudgery of hand weeding. Being selective in nature, the use of these herbicides is species-dependent where dose of the chemical is determined, keeping in view the genetic makeup, age, growth rate, morphology, physiology and biochemistry of the plant. Hence, these are used as pre- or post-emergence and specific to broad or narrow leaf pants.
Specificity of the herbicides necessitates the development of non-selective broad spectrum herbicides that do not discriminate plants based on their age, growth rate, morphology, physiology and biochemistry. However, use of non-selective herbicides demands a novel technology that allows desired plants to survive while leaving others susceptible.
The technology that develops genetically modified organisms (GMOs) including herbicide resistant crops is called ‘transgenic technology’, that also known as recombinant DNA (rDNA) technology. And even approach called ‘CRISPR-case technology’ falls under same category to introduce a new trait. To precisely describe the technology, a DNA fragment (a gene) encoding a desirable trait is incorporated into one of the genomes of a plant that normally lacks the trait. Now, question arises what is the source of DNA and which genome of the plant is to be engineered? A gene can be taken from any plant, (cultivated or wild; related or otherwise) an animal or microorganism. As far as genome of the plant is concerned, it could be nucleus or chloroplast.
Nuclear genome is routinely engineered for agronomic traits; whereas chloroplast genome called ‘plastome’ is engineered for pharmaceuticals or vaccine production, because it produces foreign proteins as 70% of the total cellular proteins and is maternally inherited in most of the cultivated plants, leaving transgenics eco-friendly.
Weed management using transgenic technology is of agricultural importance improving crop performance due to a phenomenon of ‘resistance development’ against non-selective herbicides. The availability of a dominant weed killing system, which indiscriminately kills non-GM plants, is greatly facilitating effective weed management. With the introduction of herbicide resistant GM plants, their importance has more been realized owing to their critical role in enhanced crop productivity. To date, two such systems are available in the market. One; where glyphosate is used to kill plants by blocking EPSPS (5-enolpyruvylshikimate 3-phosphate synthase) enzyme, involved in the biosynthesis of aromatic amino acids, vitamins and other secondary metabolites. Second; where glufosinate is used to eradicate plants by inhibiting an enzyme called Glutamine synthetase (GS).
The GS plays an essential role in the metabolism of nitrogen by catalyzing the condensation of glutamate and ammonia to form glutamine. Inhibition of GS causes a rapid build-up of intracellular ammonia levels. The high ammonia levels disrupt chloroplast structure, leading to inhibition of photosynthesis and consequently, death of plant cells.
Glyphosate is the world leading herbicide as it is affordable and GM crops have been developed with a trait which can tolerate the herbicide dosage. About 80 % of the GM crops have resistance to glyphosate. Therefore, it is feasible for farmers to apply it at any time during the crop development. However due to the over-reliance on glyphosate and glyphosate herbicide resistant crops, weeds are rapidly getting resistant to this class of chemicals, contrary to the pre-roundup ready arena when nearly 400 herbicides with 17 different modes of action were used. Further, global consensus is building due to its health related issues. It calls for further research and efforts to develop a new trait which should also have a competitive herbicide in the market, to meet this global need.
In this regard, glufosinate has been introduced in the market as an alternative to glyphosate. Glufosinate kills weeds as a contact herbicide and it must be applied to smaller plants. Development of glufosinate-resistant traits has been reported worldwide in corn, soybean and cotton until now. Although glyphosate beats it in cost and more restrictive application timing relative to weed size, but it is found to be more advantageous as it kills all weeds and plants that have developed resistance against glyphosate. To add more in this account, no weed has been formally reported as glufosinate-resistant yet.
The trait has also been developed in sugarcane in Pakistan, and the technology has been declared safe for ecosystem including environment, microflora, microfauna, and vertebrates including human. This technology is second in the world after Brazil, and the GM variety developed is saving 1.52 billion US dollars annually. China has adopted this technology for commercial cultivation 2024. Sugar derived from GM cane is equally safe as from organic cane.
The journey that was started from mechanical methods to chemical control; and then to genetically modified crops with post-emergence application of non-selective herbicides has paid off in the form of controlled losses. However, even today there are losses due to weeds, part of them being due to the evolution of herbicide-resistant weeds and reliance on a single commercial herbicide and limited number of genetically modified crops with diverse herbicide- resistant traits. Further, apart from resistance development in weeds against glyphosate there is an evolving story that glyphosate causes health risks in human beings. So, the scenario can become worse if new herbicides are not developed readily.
A number of multinational companies; including DuPont, Dow AgroSciences, Monsanto, Bayer and Syngenta are in a process of developing new herbicide-resistant traits and new herbicides, but none is commercially available yet. We, as a team, are also in a process to develop a new herbicide- resistant trait for an herbicide as an alternative to both the above-mentioned traits. There is a question, has anyone calculated the level of toxic residues in the non-GM crops where multiple sprays are used to control different types of weeds?
A number of articles and technical reports about glyphosate, in support or otherwise, causing health risks in animals and humans have been published in different leading journals. To precisely describe the status, two separate reports have been published in two world leading magazines; namely, Nature Translational Medicine and Science. In Nature, Daniel Cressey- an eminent writer – in his article entitled, “Debate rages over herbicide’s cancer risk” explains that European and UN bodies reach at very different conclusions about safety of ubiquitous herbicide ‘glyphosate’. A report released by the International Agency for Research on Cancer (IARC) explains that glyphosate probably does cause cancer in humans. Nearly after eight months of this report the European Food Safety Authority (EFSA) released its report and expressed that the world’s most widely-used herbicide, glyphosate, is unlikely to pose a cancer risk to humans.
Both bodies reached on to two divergent conclusions because the two bodies were not assessing the same thing. EFSA looked at glyphosate only, whereas IARC looked at both the chemical itself and the chemical-containing products. As per EFSA report, some studies have shown genotoxic effects from glyphosate-containing products and some don’t. Hence, it is highly likely such effects may be because of other compounds in these products, rather than glyphosate itself. The aspect calls for further investigation of the herbicide, either taking active ingredient alone into consideration or the whole product by investigative teams, under potentially similar conditions.
Looking into details both agencies used different approaches to risk assessment. IARC has assessed the cancer hazard, considering the likelihood that a chemical might, at least in some circumstances, cause cancer in humans. Whereas, EFSA is principally concerned with whether there is sufficient confidence that a pesticide will not pose an unacceptable risk to human health or the environment when used following the conditions of its approval. EFSA assessed some studies conducted by industry groups that were excluded from the IARC analysis. The IARC team looked only at evidence that is in the public domain and available to independent scientists to review (For details; an article by Daniel Cressey in Nature issue November 13, 2015).
Whereas Derek Lowe in his article “Glyphosate And Cancer” expresses that “Glyphosate (Roundup) is the most widely used agricultural chemical in the world. What, if anything, is it doing to people?” Based on different reports Derek has explained so that’s the state of the art: there is, from what I can see, nothing very clearly linking glyphosate to human cancer. There’s certainly room for more evidence to come in, though, and it looks like we’re going to need it, because this is a topic that’s never going to go away until we have more data (For details –an article by Derek Lowe in Science Translation Medicine issue May 18, 2016).
Despite all debatable stories published in different journals and newspapers, Environment Protection Agency (EPA) has approved Enlist Duo™, an herbicide containing 2,4-D and glyphosate, for use in 34 states on Enlist™ cotton, soybeans and corn. The herbicide was primarily registered in 2014. Initially, it was for use on genetically engineered (GE) corn and soybean crops in 6 states. Later on, 9 more states were included. Currently, EPA is amending the registration to allow its usage not only in on GE cotton in those 15 states but also extending it on GE corn, soybean and cotton to an additional 19 states. Cutting long story short, in spite of the risk factors, EPA is trying to increase the usage at a much broader and wider level by adding more and more states. This is described in the final registration decision document that can be found at EPA docket # EPA HQ OPP 2016 0594 at www.regulations.gov.
Herbicide resistant crops have a great potential in the simplification of weed management and safeguard the health. Handled judiciously, these crops may be beneficial to the environment but these should be carefully evaluated prior to releasing into a cropping system, especially when GM crop possess weedy characters or may outcross to related weeds.
Further, extensive and continuous use of a single herbicide should be avoided to exclude the possibilities of resistance development in plants and precautionary measures should be taken to safeguard human health. Nevertheless, GM technology is the well accepted technology for sustainable agriculture and food sufficiency and security in Pakistan, and in the developing world. A leading example of GM technology-based weed control in crops is the GM sugarcane, which has recently been approved by the National Biosafety Committee (NBC) for commercial cultivation in Pakistan.