Prospects for genetically modified crops

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  • 17Ann. appl. Biol. (2004), 145:17-24Printed in UK

    *Author E-mail: nigel.halford@bbsrc.ac.uk

    2004 Association of Applied Biologists

    Prospects for genetically modified crops

    By NIGEL G HALFORD*

    Crop Performance and Improvement Division, Rothamsted Research, Harpenden, Hertfordshire,AL5 2JQ, UK

    (Accepted 4 May 2004; Received 20 February 2004)

    Summary

    Genetically modified (GM) crops have been in use commercially around the world for almost adecade. This review covers the successes and failures of GM crop varieties in that time, the currentstatus of GM crop adoption and the traits that are being used. It also describes some of the GM cropsthat might come on to the market in the next decade. The barriers in the way of GM crop developmentin Europe, including consumer hostility, the difficulty in gaining official approval and discriminatorylabelling laws are discussed.

    Key words: GM crop status, GM crop traits, future applications of GM crops, farm scale evaluations,GM food legislation and labelling

    Introduction

    Genetic modification using transgenesis is nowan established technique in plant breeding. That isnot to say that it is sweeping away every othertechnique, rather that it is an additional tool in theplant breeders toolbox. All plant breeding, ofcourse, involves the alteration (or, if you like,modification) of plant genes, whether it is throughthe crossing of different varieties, the introductionof a novel gene into the gene pool of a crop species,perhaps from a wild relative, or the artificialinduction of random mutations in the DNA of a cropplant through chemical or radiation mutagenesis.Recently, however, the term genetic modification hasbeen applied to the technique of inserting a singlegene or small group of genes into the DNA of anorganism artificially.

    The methods available for the genetic modificationof plants are described in detail elsewhere (e.g.Halford, 2003; Slater et al., 2003) and I will notdescribe them here. The technique has becomeestablished in plant breeding because it has someadvantages over other techniques. These advantagesare: It allows genes to be introduced into a crop plantfrom any source (although it is likely that the use ofanimal genes would not be acceptable to consumers,at least in food crops). It is relatively precise, single genes can betransferred (this is not possible in conventionalbreeding). Genes and their products can be tested extensivelyin isolation before use to ensure their safety. Genes can be cut and pasted in the laboratory to

    change when and where in a plant they are active,and to change the properties of the proteins that theyproduce.

    There is also, however, a significant down-sidefor the plant breeder in using genetic modificationto produce new varieties for the European market.This is that any genetically modified (GM) crop orfood derived from it has to be approved for usewithin the European Union, and approval isextremely difficult to obtain. Furthermore, any foodcontaining GM crop material above a threshold of0.9% has to be labelled, while novel foods producedin any other way do not. This is preventing thedevelopment of new GM traits specifically for theEuropean market. Nevertheless, the use of GM cropsaround the world continues to increase.

    Current Status of GM Crops

    Detailed information on the uptake of GM cropsby farmers around the world has been provided forseveral years by Clive James at the InternationalService for the Acquisition of Agri-biotechApplications (ISAAA) (www.isaaa.org). In 2003,the ISAAA reported that GM crops were beinggrown commercially in 18 countries: Argentina,Brazil, Canada, Colombia, Honduras, Mexico,Uruguay and the USA in the Americas; Bulgaria,Germany, Romania and Spain in Europe; China,India, Indonesia and the Philippines in Asia;Australia and South Africa. Of these, Argentina,Brazil, Canada, China and the USA dominate interms of total area. The global area of land plantedto GM crops in 2003 was approximately 65 millionha, an increase of 15% on 2002.

  • 18 NIGEL G HALFORD

    More than half of this area is accounted for byherbicide-tolerant GM soybean, in fact more thanhalf of the global soybean crop is now GM. Theother major GM crops are maize (corn), cotton andoilseed rape (canola). There are also relatively smallareas planted to GM virus-resistant papaya andsquash and slow-ripening tomatoes.

    GM Traits Being Used Successfully inCommercial Agriculture

    The most successful traits to date (and it is difficultseeing them being overtaken) are those aimed at thefarmer: herbicide tolerance (soybean, oilseed rape,cotton and maize) and insect resistance (cotton andmaize). Traits affecting the quality or the nutritionalvalue of the product have been more difficult todevelop and market, but there are signs that thesesorts of crops will become important in developingcountries. It is fair to say, at present, that the numberof traits that have been commercialised successfullyis small.

    Herbicide toleranceHerbicides have been used since the 1950s, long

    before the advent of genetic modification, and arean essential part of weed control for farmers indeveloped countries. Most herbicides are selectivein the types of plant that they kill and a farmer hasto select a herbicide or combination of herbicides,applied at different times in the season, that istolerated by the crop that he is growing but kills theproblem weeds. Some of these herbicides have togo into the ground before planting, some pose ahealth risk to farm workers and some are persistentin the soil, making crop rotation difficult. They allrequire equipment and labour to apply and they allcost money.

    Herbicide-tolerant GM crops were produced toovercome or reduce these problems. The first to begrown commercially were soybeans developed byMonsanto that were modified to tolerate the broad-range herbicide, glyphosate (Padgette et al., 1995).Glyphosate is relatively safe to use, does not persistlong in the soil because it is broken down by micro-organisms and is taken up through the foliage of aplant, so it is effective after weeds have becomeestablished. It is also relatively cheap. Its target is5-enolpyruvoylshikimate 3-phosphate synthase(EPSPS), an enzyme in the shikimate pathway thatis required for the synthesis of many aromatic plantmetabolites, including some amino acids. Theshikimate pathway is not present in animals, henceglyphosates low toxicity to animals. The gene thatconfers tolerance of the herbicide is from the soilbacterium Agrobacterium tumefaciens and makes anEPSPS that is not affected by glyphosate.

    Over 150 US seed companies now offer varieties

    carrying the trait and 81% of the US soybean cropin 2003 was glyphosate-tolerant (Benbrook, 2003).This success is due to simple factors: simplified andsafer weed control, reduced costs and moreflexibility in crop rotation. Overall, between 1995and 1998 there was estimated to be a reduction of$380 million in annual herbicide expenditure by USsoybean growers (Gianessi et al., 2002). However,farmers who used glyphosate-tolerant varieties hadto pay a technology fee of $6 per acre. This reducedthe overall cost saving to $220 million. Anotherreport has suggested that although herbicide use fellwith the introduction of these crops it has since risen(Benbrook, 2003). The fact that the GM system hasled to a switch to conservation tillage systems whichinvolve leaving weeds and stubble undisturbed overwinter and then spraying with herbicide in the springcould explain an increase in herbicide use. If this isthe case the consequent benefits of reductions in soilerosion and pollution from run-off would faroutweigh the disadvantage of a modest increase inherbicide use. Nevertheless, it is not clear how thesereports should reach such different conclusions.

    There are two other broad-range herbicide tolerantGM systems in use, involving the herbicidesgluphosinate (or glufosinate) and bromoxynil, bothmarketed by Bayer. The gene used to make plantsresistant to gluphosinate comes from the bacteriumStreptomyces hygroscopicus and encodes an enzymecalled phosphinothricine acetyl transferase (PAT).This enzyme detoxifies gluphosinate. Crop varietiescarrying this trait include varieties of oilseed rape,maize, soybeans, sugar beet, fodder beet, cotton andrice. The oilseed rape variety has been particularlysuccessful in Canada. Bromoxynil tolerance isconferred by a gene isolated from the bacteriumKlebsiella pneumoniae ozanae. This gene encodesan enzyme called nitrilase, which convertsbromoxynil into a non-toxic compound. So far thishas only been used commercially in Canadianoilseed rape.

    Interestingly there is a fourth broad-rangeherbicide-tolerance trait available in commercialoilseed rape varieties in Canada. The herbicide inthis case is imidazolinone and the varieties wereproduced by Pioneer Hi-Bred, now part of DuPont.However, the trait was produced by mutagenesis,not genetic modification.

    Herbicide tolerance has now been engineered intomany crop species and is undoubtedly the mostsuccessful GM trait to be used so far. In the USA in2003, 59% of the upland cotton and 15% of the maizewas herbicide-tolerant (Benbrook, 2003), as well asthe 81% of the soybean crop already discussed.Herbicide-tolerant soybeans have been adopted evenmore enthusiastically in Argentina and now accountfor 95% of the market, while herbicide-tolerantoilseed rape has taken 66% of the market in Canada.

  • 19Prospects for genetically modified crops

    is to use antisense or co-suppression techniques(Grierson et al., 1996) to block the activity of viralgenes when the virus infects a plant. A potato varietycarrying a replicase gene from potato leaf roll virus(PLRV) was marketed by Monsanto in the 1990s,later in combination with the Bt insect-resistancetrait. These GM potato varieties have since beenwithdrawn in the USA because of reluctance to usethem in the important fast-food industry.

    This technology is being applied to many otherplant virus diseases, just one example of resistancebeing achieved at least under trial conditions beingwith potato tuber necrotic ringspot disease (Racmanet al., 2001). It has tremendous potential fordeveloping countries where losses to viral diseasesare the greatest and have the most severeconsequences.

    Modified oilsOilseed rape was first grown in the UK during the

    second world war to provide industrial oil, high inerucic acid (which is poisonous to humans), andthese varieties are still grown today for that purpose.In the second half of the last century, however,varieties were bred with reduced levels of erucic acidand another group of poisonous compounds calledglucosinolates. When these varieties were passedas acceptable for human consumption (oilseed rapereceived its seal of approval from the Food and DrugAdministration of the USA in 1985), Canadianproducers came up with the name Canola for edibleoilseed rape oil. This name was adopted all overNorth America as the name not only for the edibleoil but also for the crop itself.

    A problem for farmers who grow oilseed rape isthat its oil is one of the cheapest edible oils on themarket. The value of the crop is, therefore, relativelylow and there is a lot of interest in increasing it.This has been achieved through genetic modificationby introducing a gene from the California Bay plantthat causes an accumulation of lauric acid toapproximately 40% of the total oil content, comparedwith 0.1% in unmodified oilseed rape. Lauric acidis a detergent traditionally derived from coconut orpalm oil.

    A different modification has been made to the oilof soybean. In this case, the genetically modifiedvariety accumulates oleic acid to approximately 80%of its total oil content, compared with approximately20% in non-GM varieties (Mazur et al., 1999;Kinney, 1996). This was achieved by co-suppression(Grierson et al., 1996) of a gene that encodes anenzyme that converts oleic acid to linoleic acid.Oleic acid is very stable at high temperatures and atpresent the oil from the GM soybeans is used forindustrial purposes.

    Relatively small amounts of these GM oilseed rapeand soybean varieties are grown to contract, but

    Insect resistanceOrganic and salad farmers have been using a

    pesticide based on a soil bacterium, Bacillusthuringiensis (Bt), for several decades. Thebacterium produces a protein called the Cry proteinthat is toxic to some insects but has no toxicity tomammals, birds or fish. Different strains of thebacterium produce different versions of the proteinthat are effective against different types of insects.Cry1 proteins, for example, are effective against thelarvae of butterflies and moths, while Cry3 proteinsare effective against beetles.

    The Cry1A gene has now been introduced intoseveral crop species (de Maagd et al., 1999) and themodified varieties are generally referred to as Btvarieties. As with herbicide tolerance, the benefitsof using the insect-resistant GM crops depend onmany factors, most obviously the nature of the majorinsect pests in the area (not all are controlled by Bt)and the insect pressure in a given season. However,Bt varieties have been successful in many parts ofthe USA (in 2003, 29% of the maize and 41% of theupland cotton crop was Bt) and Bt cotton in particularis gaining ground in Australia, China, India and thePhilippines. Farmers who use Bt varieties citereduced insecticide use and/or increased yields asthe major benefits. A further, unexpected benefit ofBt maize varieties is that the Bt grain contains loweramounts of fungal toxins (mycotoxins) such asaflatoxin and fumicosin.

    A different Cry gene, Cry3A, has been used tomodify potato to make it resistant to the Coloradobeetle. These GM potato varieties were withdrawnin the USA due to poor sales, farmers preferring touse broad-range insecticides instead. However, theymay have a role to play elsewhere in the world wherethe Colorado beetle is a problem.

    Virus resistanceThere are two methods currently in use to

    genetically modify plants to be resistant to viruses.The first arose from studies into the phenomenon ofcross protection, in which infection by a mild strainof a virus induces resistance to subsequent infectionby a more virulent strain (reviewed by Culver, 2002).Modifying a plant with a gene that encodes the viralcoat protein has been found to mimic thephenomenon.

    An example of the commercialisation of thistechnology comes from the papaya industry in thePuna district of Hawaii (Ferreira et al., 2002;Gonsalves, 1998). After an epidemic of papayaringspot virus (PRSV) in the 1990s almost destroyedthe industry growers switched to a virus-resistantGM variety containing a gene that encodes a PRSVcoat protein. The GM variety was successful andprobably saved the papaya industry in Hawaii.

    The other method used to engineer virus resistance

  • 20 NIGEL G HALFORD

    those farmers who can get in on this business benefitfrom a premium price for their crop.

    Slow-ripening fruit Fruit ripening is a complex process that brings

    abou...