Rice War Continues
Editor's note: The productivity of rice has been falling along with that of other food grains. Chief among the causes of the fall in productivity are severe water shortages due to over-irrigation and depletion of aquifers, eroded soils from over-application of chemical fertilizers and pesticides, and rising temperatures from global warming.
While innovative farmers have been addressing these problems with a range of effective measures to increase yields through regenerating degraded soils, conserving water and minimizing inputs (see many articles in SiS23), pro-GM scientists in the three major rice-growing countries, China, India and Japan, have all been researching and promoting GM rice with scant regard for safety or sustainability.
We are circulating Professor Joe Cummins' review on GM rice in China, and making available two others, on GM rice in India and Japan respectively on the I-SIS website:
GM Rice in Japan
GM Rice in India
Prof. Joe Cummins reviews recent genetically modified rice research in India.
India is the second largest consumer of rice on the globe after China, and more than twice that of the next country, Indonesia [1]. India produced 93 million tonnes of rice in 2001 and 2002, and the government is targeting 129 million tonnes by 2011 and 2012 [2]. India approved genetically modified (GM) cotton for food, feed and fiber in 2002 and other approvals may soon follow [3]. A lot of technically sophisticated research on GM rice is being done in India, and I shall describe some of it here. Currently, no GM rice variety has been commercially released in India.
An extensive review exists on the production of transgenic rice up to the year 2000 [4]; I shall therefore concentrate on some important studies published since.
Elite Indica transgenic rice containing a synthetic gene from Bacillus thuringiensis (Bt) expressing Cry1Ac toxin had enhanced resistance to stem borer. The cry gene was driven by the ubiquitin promoter from maize and its transcription was terminated by the terminator of the nos gene from Agrobacterium, tnos; and accompanied by an antibiotic (hygromycin) resistance gene and the bacterial gus gene as negative and positive selection markers respectively. The transformed rice lines had up to 0.25% of their total protein as Cry1Ac toxin. The successful lines caused 100% mortality when consumed by yellow stem borer larvae [5]. Production of toxin at high levels (approaching 1% of the cellular protein) is considered important in preventing resistance in the target insect, but that will mean a lot of toxin consumed by humans eating the rice. Cry1Ac is known to be a strong immunogen [6], and has also been found to be sufficiently similar to known allergens to cause concern [7].
Following development of rice with single transgenic traits, it is inevitable that transgenic varieties with multiple traits would appear, produced by gene stacking or multiple transformations of a transgenic line, or by crossing transgenic lines to combine their transgenes. It appears from current research that varieties with multiple transgenes will soon exceed varieties with single transgenic traits.
A stacked combination of Bt toxins Cry1Ab and Cry1Ac along with tolerance to the herbicide glufosinate (bar gene) was created in order to serve as a parental line for generating hybrid rice varieties. The bar gene was driven by the CaMV promoter and terminated by tnos. Both cry genes were synthetic and driven by the maize ubiquitin promoter and terminated with tnos. The two cry gene cassettes, each accompanied by the bar cassette, were transformed from different plasmids and ended up at different sites in the rice genome. The resultant transgenic line was resistant to stem borer insects and to the herbicide glufosinate [6]. An Indica rice variety was stacked with cry1Ab and cry1Ac, plus the snowdrop lectin gene gna conferring multiple resistance to insects and all of these were joined to the bar gene for glufosinate tolerance with the CaMV promoter and tnos terminator. The cry genes were driven by the maize ubiquitin promoter and terminated with tnos while the gna gene was driven by phloem-specific rice-sucrose synthase promoter and terminated by tnos. The cry genes and the gna gene were inserted at different sites in the rice genome [7]. The stacked rice variety was assumed to confer strong insect-killing power and to guard against appearance of insect resistance.
An elite Indica rice was modified for resistance to insects, fungal disease and bacterial disease. One parental strain contained the xa21 gene conferring resistance to bacterial blight. The gene was isolated from wild rice and used to transform rice, accompanied by its original promoter and terminator. Another parental strain had a rice chitinase gene for protection against the sheath blight fungus and a synthetic cry gene consisting of the fused active portions of cry1Ab and cry1Ac to protect against yellow stem borer. The fused cry gene and the chitinase gene were integrated at the same site on the rice genome. The cry fusion gene was driven by the maize ubiquitin promoter and the chitinase gene was driven by the CaMV promoter. The two parental lines were combined by crossing to create the pyramided strain resistant to three kinds of pathogens, insect, fungus and bacteria [8].
Another technique, marker assisted selection, has been combined with pyramiding and stacking to produce rice resistant to fungal blast and bacterial blight. Marker assisted selection is traditional breeding assisted by molecular markers in the rice genome which are used to follow desirable genes in a cross. Two rice genes for blast resistance were identified and pyramided, and both of the pyramided lines were stacked by modification with the rice xa21 gene accompanied by a bacterial hygromycin resistance gene driven by a CaMV promoter. The rice pyramid was resistant to both blast and blight [9]. Pyramiding, stacking and fusing genes are being used with a vengeance against pests, with little consideration for the safety of the rice, which would be consumed as food.
The plant’s natural defense against pests includes an oxidative burst mediated by hydrogen peroxide. Such a burst may mean suicide for the plant cell and a few neighboring cells; but it saves the plant from the invading pest. A fungal glucose oxidase (gox) gene was used to release hydrogen peroxide in the transformed rice, driven by a rice wound inducible promoter. The transformation also included a bacterial hygromycin resistance gene as a selectable marker. A transformant was resistant to both bacterial and fungal pests; and the resulting rice strain had broad-spectrum disease resistance [10].
Flavonoid pigments such as a purple anthocyanin pigment from maize have been found to increase blast resistance. A maize gene for an enzyme triggering purple pigment production was used to transform rice, under the control of a rice actin promoter and accompanied by a bacterial hygromycin resistance gene driven by a CaMV promoter [11]. The purple rice resisted rice blast.
Drought limits crop yields. A barley gene hva1 protects the cell membrane during drought, and when inserted in rice, could reduce drought damage. The rice was transformed with the barley hva1 gene, driven by the rice actin promoter and terminated with a potato protease terminator. The rice transformation also included the bar gene for herbicide tolerance as a selectable marker driven by a CaMV promoter and terminated with tnos. The transgenic crop showed greater growth than the non-transgenic line under drought conditions [12].
Heat tolerance is important in most rice producing countries. It has been found that a heat shock protein (hsp 100) is produced during heat stress. By over expressing the rice hsp100 gene, it was possible to increase growth under high temperatures. Basmati rice was transformed with the Arabidopsis hsp101 gene driven by a maize ubiquitin promoter and first intron and transcription terminated by tnos; it was accompanied by a hygromycin resistance marker driven by a CaMV promoter and terminated by tnos. The transgenic rice performed better than the parent strain in heat and in recovery from heat shock [13].
While the GM rice being researched in India may show some promise in providing pest and disease resistance, or tolerance to abiotic stress, it is clear that research has raced ahead of safety considerations. As rice is consumed as a staple food, any GM rice considered for commercialization must undergo a thorough pre-market safety assessment and have its safety proven beyond reasonable doubt.
Some areas of especial concern include the use of Bt biopesticides that are potentially toxic and allergenic (see "GM rice release in China?", this series); and the complicated, multiple traits that may be especially prone to genetic instability through illegitimate recombination. Furthermore, not only the effects of each of the genes, but also their combined effects must be adequately tested for safety.
Article first published 30/11/04
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