Considering the adverse effects of herbicides on crops, Times Food Processing Journal explores the opportunities in herbicide-resistant transgenic crops Herbicide-resistant transgenic crops (HRCs), crops genetically engineered to be resistant to certain herbicides, constituted around 85 per cent of the total area under transgenic crops in India in 2008. With HRCs, farmers can use herbicides to selectively control weeds while the crop remains intact. Since the advent of large-scale cultivation of transgenic crops in 1996, HRCs have maintained their dominance in terms of area coverage. HRCs were readily accepted in developed countries, where weed-control strategies almost always rely on the use of herbicides. The relevance of HRCs for small and fragmented farm holdings of India is, however, contentious. The proponents advocate that this new technology is scale-neutral and its benefits should be made available to Indian farmers. In contrast, the other section firmly believes that HRCs are not suitable to our conditions and pose serious threat to the employment and livelihood opportunities of the poor and marginal farmers and farm labourers, and will adversely impact environment, ecology and biodiversity. Caught between these extremes, the Government of India is approaching the issue cautiously and thus has not placed HRCs on its priority list of traits to be modified through genetic engineering approaches. The article examines various issues surrounding the commercialisation of HRCs and highlights a few technical aspects that have not received the deserved attention yet.  Weed problem in Indian agriculture Weeds are a major problem in Indian agriculture; they compete with crops for moisture and nutrients. Some parasitic weeds draw water and nutrients from crop plants and can inflict severe damage. Further, weeds serve as alternate hosts to pathogens and also harbour pests. Loss of yield due to weed infestation is variable and is more pronounced in crops grown under rain-fed conditions. Control of weeds during early stages of crop growth is crucial to capture the yield potential. For this reason, labour demand for weeding operations is high during the early phases of the crop cycle and manual weed control over large areas is not feasible from the point of labour supply and monetary costs. Some weeds that are wild relatives of crop plants are difficult to distinguish from crop plants at the early stages and pose challenge for manual weeding. Under such conditions, chemical weed control is relevant to realise higher productivity and production. Herbicides: Types and usage In India, around 7,200 tonnes of herbicides was used for weed control in 2008, mainly in irrigated crops and plantations. However, herbicides currently form only 12–14 per cent of the pesticides used on crops in India. A wide variety of weeds, both perennial and annual, are generally encountered in crop fields. However, specific weeds dominate different cropping systems and zones, while both broad spectrum, non-selective and selective herbicides are used. The continuous use of some herbicides has developed resistance in weeds and has exacerbated weed problems. For example, in the rice-wheat cropping system of Punjab and Haryana, Phalaris minor has developed resistance against isoproturon. Herbicides that kill plants by inhibiting specific vital functions do not distinguish between crop plants and weeds. Such non-selective herbicides are generally applied before sowing or emergence of crop plants and their residual effects may affect crop performance. There is limited flexibility in the schedule of their application and their use requires caution. However, some crop plants enjoy naturally endowed resistance to specific herbicides. For example, 2, 4- dichlorophenoxyacetic acid (2, 4-D) kills only broad-leaved weeds and can be used as a selective herbicide in monocot crops like rice, wheat and maize. Similarly, maize is resistant to atrazine and simazine. It is important to recall that although a large number of chemicals have been approved for weed control, their widespread and continuous use is not desirable owing to their toxicity and long-term effects on the environment.  Genetic engineering of herbicide toleranceBefore the emergence of plant genetic engineering, options for selective crop protection against herbicides were limited. Specific herbicides could be used in the crops that were naturally resistant to the herbicide. In rare cases, resistance could be induced in crop varieties through mutations. For example, monocots are naturally resistant to triazine and hence triazine could be used as selective herbicide in monocot crops to control dicot weeds. Developments in plant genetic engineering and knowledge of biochemical action of herbicides on plants spurred innovative approaches to engineer crops to withstand herbicides. These strategies usually involve isolation and introduction of a gene from other organisms, (mostly bacteria) which is able to overcome the herbicide-induced metabolic blockage. For example, tolerance to the herbicide glufosinate (BastaÒ) is conferred by the bacterial gene bar, which metabolises the herbicide into a non-toxic compound. Glyphosate (another popular herbicide) resistance is achieved by the introduction of either Agrobacterium gene CP4 that codes for a glyphosate-insensitive version of the plant enzyme, EPSP-synthase, or gox gene from Achromobacter, which codes for glyphosate oxidoreductase in the breakdown of glyphosate. A number of other genes have been identified that can alleviate the herbicide action through various ways (such as detoxification, sequestration) and thus confer resistance to the plants carrying them. Thus, genetic engineering technology has made it possible to tailor crop varieties to resist specific herbicides by introducing relevant genes. Consequently, the range of selective herbicides has now greatly expanded, and specific genotypes and varieties of crops can be conferred with resistance, instead of generic crops displaying resistance to specific herbicides. These developments have provided herbicide companies new opportunities to promote their herbicides through development and marketing of genetically engineered HRCs. Technical issues with HRCs The effects of the introduction of HRCs on biodiversity have been widely discussed. It is now shown that introduction of herbicide-resistant (HR) soybean has not adversely impacted the genetic diversity of soybean varieties. HRCs have promoted conservation of tillage in the US and thus may have reduced soil erosion. Because of the effective control of weeds, the population of bees and butterflies was lower in fields with HRCs. It should be noted that good weed management practices would check weeds and thus reduce the population of insects and animals that are dependent on weeds. The soybean story The introduction of HRCs may lead to a paradigm shift in the approach to breeding crop varieties. In this connection, it is instructive to examine the story of HR soybean in the US. Monsanto introduced a transgenic soybean resistant to the herbicide glyphosate in the US in 1996. Just after the approval for the commercial release, companies refused to enter their glyphosate-resistant soybean in common variety trials, where conventional herbicides were employed for weed control. Their contention was that since the transgenic varieties were not resistant to conventional herbicides, the benefits of the transgenic HR soybean would be realised only when glyphosate was used for weed control. This led to the introduction of separate trials for HR transgenic soybeans. These did not allow proper comparison between transgenic and conventionally bred cultivars that was the basis of choice of farmers for glyphosate-resistant soybean. Following the widespread adoption of HR soybean by farmers, most soybean breeders had to incorporate this trait into their varieties to remain in business. That meant obtaining sublicenses from Monsanto for the use of its patented CP4 gene to breed glyphosate-resistant varieties. The cotton connection A similar situation is currently witnessed in India with respect to Bt-cotton. The demand for Bt-cotton hybrids has forced almost all major seed companies marketing seeds of cotton into the development of Bt-transgenic cotton hybrids. For a quick entry into the transgenic arena and to remain in competition, companies have entered into sublicensing agreements with Mahyco/Monsanto to use the MON 531 transgenic events, approved by the regulatory authorities, into their own genotypes through backcross breeding. Public-sector breeders have also geared up to produce Bt-cotton hybrids and varieties through their own initiatives and efforts.  Many lessons we can learn from the above with respect to HRCs? There is little doubt about the effectiveness of the herbicides (glyphosate and glufosinate, for which transgenic crops are currently available) in controlling weeds when used in conjunction with HRCs. Therefore, in crops where herbicides are currently being employed in weed control (and where transgenic HRCs will be primarily targeted) HRCs will be quickly accepted. Patents and private players Currently, the patent rights for the relevant genes rest with specific companies, and these would be keen to capture the market for HR varieties. Soon, breeders in both public and private sectors may be compelled to incorporate the resistant trait into their varieties lest they are left out of competition. Unlike Bt-cotton, where many Bt genes can be sourced to bypass patent rights, finding new genes and approaches to overcome patent protection for developing HRCs will be difficult in the short run. The consequence of this will be similar to what has happened in the US for HR soybean. The availability of such HRCs will, in all likelihood, promote herbicide use even in areas and situations where herbicides are not currently used and may adversely impact employment opportunities for the rural poor. It is still unclear how private companies will realise profit if HRCs are to be introduced in self-pollinated crops like rice, wheat and soybean. The Seed Act allows the use of farmer saved seeds and farmer-to-farmer exchange of seeds. Hence, once introduced, HRC varieties of self-pollinated crops will continue to spread unchecked. Large-scale cultivation of HRCs and indiscriminate use of herbicides may lead to the development of herbicide-resistant weeds, especially if wild relatives of crop plants are growing in the vicinity. There are already numerous examples of the development of HR weeds where herbicides are used continuously. The rights and wrongs of resistance
The resistance against isoproturon in P. minor is a burning example. Resistance development is widely discussed with respect to Bt-transgenic crops. However, Bt-resistance and herbicide resistance are qualitatively different. If insects develop resistance against a particular Bt-toxin, alternative Bt-toxins with different modes of action or target sites can be deployed. Various strategies such as pyramiding of different Bt-genes and maintaining refugia have been suggested to delay the development of resistant insects. Similarly, strategies have been defined to delay the development of herbicide-resistant weeds in the case of conventional crop varieties. These include the combined or sequential use of herbicides with different modes of action, crop rotation and integrated weed control. These strategies are less relevant in the case of genetically engineered HR varieties. For example, when the herbicide could be applied at various stages of crop growth, farmers may not opt for integrated weed-control measures. Similarly, when different crops carry engineered resistance to the same herbicide, the use of different herbicides may not be an option. Once the weeds develop resistance, either through acquisition of the gene from the HR variety or by mutation, they will remain resistant to that herbicide. Replacement of the herbicide is the only option in such a scenario.  Since development of new and safer herbicides is time-consuming and resource-intensive, the development of new herbicides is not likely to keep pace with the emergence of HR weeds. At present, a major concern with the introduction of glyphosate- or glufosinate-resistant transgenic crops is that if the weeds develop resistance, these environmentally benign herbicides will become ineffective and will force the use of other less-desirable herbicides for weed control. Glyphosate-resistant Lolium populations have already emerged in Australia, the US and South Africa, defying the low probability of such a scenario predicted by Radshaw et al. Further, glyphosate resistance has also been recorded in Eleusine indica in Malaysia, Lolium multiflorum (Italian rye grass) in Chile and Conyza canadensis (Horseweed). In some of these cases novel mechanisms of resistance—not envisaged earlier—were discovered. The ongoing debate about HRCs is largely focused on their effect on the environment and on rural employment opportunities. Some of the opposition to HRCs also arises from the prevailing perception that the multinational companies which hold the rights over herbicide and/or HR technology are profit-motivated and disregard social issues. We need to recognise the fact that the public sector has minimal presence in the pesticide business and it is inevitable that HRCs and the herbicide business will go hand-in-hand. This situation should not be the rationale for shunning these products. If they are indeed relevant to us, we should consider adopting them. In the modern technology-driven and dominated world, private enterprise is assuming a major role in developing and disseminating technologies. The government is gradually shifting its focus to social sectors. However, while an aversion to private enterprise is not prudent, decisions should be made after an objective assessment of the pros and cons of the choice of technology in relation to relevant broader issues—the socio-economic, ecological and environmental dimensions. Thus, as a technology, HRCs offer some opportunities for the efficient control of weeds. However, there are still doubts about the long-term viability of this strategy, especially considering the emergence of herbicide-resistant weeds following the widespread cultivation of HRCs. In that case, the best herbicides may not be effective even for conventional weed control. From the above discussion, it is clear that many aspects of HRCs need to be seriously considered before widespread introduction of HRCs is accepted in India. References 1. James, C, Preview Global status of commercialised biotech/GM crops: 2004. ISAAA Briefs, ISAAA: Ithaca, NY, 2004, 32, p. 12 2. Biotechhttp://dbtindia.nic.in/biotechstrategy/ Strategy.pdf 3. Malik, RK and Singh, S, Littleseed canarygrass (Phalaris minor) resistance to isoproturon in India. Weed Technol., 1995, 9, 419–425 4. Yaduraju, NT and Ahuja, KN, Response of herbicide resistant Phalaris minor to pre- and post-emergence herbicide mixtures and adjuvants. In Proceedings of the 1995 Brighton Crop Protection Conference, Weeds, Brighton, UK, 1995, pp. 225–230 5. Walia, SS, Brar, LS and Dhaliwal, BK, Resistance to isoproturon in Phalaris minor Retz. in Punjab. Plant Prot. Q., 1997, 12, 138–140 6. Bowler, C., Van Montagu, M. and InŸ e, D., Superoxide dismutase and stress tolerance. Annu. Rev. Plant Physiol., Plant Mol. Biol., 1992, 43, 83–116 7. Anderson, PC and Georgeson, M, Herbicide-tolerant mutants of corn. Genome, 1989, 31, 994–999
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