Contributions of Biotechnology to Agriculture
Prior to the biological understanding and the invention of the scientific tools we now have at our disposal, for the past 10,000 years any developments in crop yield or quality was led by the farmer in the field, witnessing a crop with a desired phenotype and maintaining storing their seed, which was an enterprise purely reliant on spontaneous mutation within the chosen species. This human interaction in the selective process although considerably accelerating natural evolution still lacked the first critical piece of the puzzle needed for crop breeding to become a truly scientific operation. This was initiated by the application of Mendelian principles in plant breeding in the 1900s enabling breeders to identify desired phenotypes and engineer to some degree their evolution, resulting in crops with greater vigour and yield in previously impractical conditions. Novel variations created by cross species and in some cases cross genus hybridisation led to such improvements as improved nitrogen utilisation, crop vigour and decreased stature (resulting in higher energy investment in grains) to name but a few. These new traits coupled with modernised irrigation, polyploidy, adoption of synthetic fertilisers and pest management programs led to the start of the green revolution in1960s.This culminated in the worlds crop yield doubling by 1985 alleviating widespread food poverty developing areas such as Mexico and India. The term ‘Genetically modified' crop although generally thought of as a relatively new term has been somewhat misconceived as any cross between plants could be considered a massive genetic modification. It was only after our understanding of the transfer of genetic information and the link between genotype and phenotype could we develop targeted and precise ‘Genetic Modification' involving usually only a few genes being manipulated at a time rather than many thousands. Over time and considerable investment our scientific understanding of the underlying mechanisms and metabolic processes enabled work to begin on the first GM crop line in 1983 and due to relatively fast progression led to the first commercialisation of a GM crop in 1996 by Monsanto with a GM soybean. Although initially the technology had been conceived by developed countries, it was the developing world who gained the largest benefit, with 23 countries cultivating GM crops and 90% of the farmers being resource poor. Due to the exponential growth of the human population resources and agriculturally viable land is becoming a precious commodity demanding the highest levels of efficient use that most importantly, is sustainable. When discussing genetically modified traits of biotech crop lines and to a certain extent any form of selective breeding, they can be considered in two distinct groups, those which benefit the consumer and those that benefit the producer.
Output traits- Producer benefit
For a long time the use of pesticides has been the main weapon in the farmers' arsenal for combating crop losses due to pests with one of the most prevalent being the Pyrethrins (derived from chrysanthemums) organophosphates and carbamates to name a few, with 2.3 million tonnes of industrial pesticide used each year (Miller 2002). This has not come without its cost and is by no means a solution to the problem. The ever increasing pesticide usage throughout the world has lead to the development of pest resistance and damage to fragile ecosystems due to the relative unspecific action of these chemicals. For example, as documented in the much publicised “Silent spring” book by Rachel Carson she identified that the use of DDT was being biologically magnified in the food chain of fish eating birds preventing them from reproducing which posed a major threat to biodiversity. As a result the use of DDT has been banned with the exception of a few isolated applications to fight the malarial vector mosquito. After years of research it has become apparent to the scientific world that the damage to non-target organisms such as predators and need for constant costly applications requires an alternative with greater specificity. With GM technology constantly improving scientists set about inserting genes for specific insecticidal proteins from other organisms into crop plants. One of the most commercially viable contenders was the gene for the active toxin from the highly specific Biopesticide Bacillus thuringiensis (Bt) sub-species that had been used for 40 years already and been proven safe.
As I mentioned above, what drew scientists to the Cry proteins was the inherent specificity of a particular Cry gene to its target pest (largely Cry 1 and less so Cry 2,3 and 9). The ability of Cry toxins to target pests whilst leaving predators some of which in the same genus as the pest unharmed is due to their specificity to the receptors in the target insects gut epithelium. In the Biopesticide form the Cry protein is initially excreted as a prototoxin which is proteolytically cleaved by the proteases in the gut and forms lytic spores in the gut wall. This results in colloid osmotic lysis and ultimately the death of the pest. After significant modification of the bacterial gene to enable proper expression in the host plant, the prototoxin stage was removed so therefore could no longer be given the same safety assurances and possibly reducing specificity (Ferry et al 2003). After Bt crops being commercially available for over a decade now we can clearly see the economic and agronomic benefits of their introduction both lowering production cost due to greatly reduced pesticide application and increasing yield due to decreased levels of fusarium infestation hence significantly lower levels of mycotoxin contamination (Ferry et al 2003).
Although Bt toxin expressing transgenics have gained high commercial status, in the case of corn and cotton for example total Bt plantations make up 11.1% and 33.6% of the worldwide crop (Brookes 2006) there are many other GM strategies under investigation to combat pest damage as due to comparatively rapid evolution of resistance in pest populations. This is due to the genetic plasticity of the insect design and high reproductive rate therefore we must always be thinking several steps ahead. One of the shortcomings of Bt crops is that there is no effective Cry toxin against sap sucking Homoptera such as aphids leafhoppers and whiteflies which are major pests of economically important crops. Homoptera not only damage the crop and ‘steal' vital nutrients from the phloem by inserting their stylet between cells thus bypassing the plants endogenous responses but are important vectors for disease such as barley yellow dwarf virus (BYDV). One of the emerging transgenic proteins that geneticists have engineered into crops such as the potato is the Snowdrop lectin GNA (Galanthus nivalis agglutinin). The GNA gene was bound to the constitutive promoter ubiquitin and controlled by a promoter specific to the phloem Rss1 (rice sucrolose synthase) resulting in 50% mortality or rice brown and green leaf hopper (Rao et al., 1998; Foissac et al., 2000). There is some debate as to the mechanism of action of GNA but feeding studies on the brown plant hopper have shown binding to the gut epithelial mannose receptors and a reduction in food intake with GNA being sequestered in eggs and adipose tissues. In further studies significant deleterious effects were also noted on chewing insects (Gatehouse et al., 1997; Fitches et al., 1997; Bell et al., 2001). Possible concerns for mammalian toxicity with some lectins, GNA does not bind to any receptors in the mammalian gut so although GNA containing transgenic food is not approved for human consumption yet, it is currently under testing and GNA rice is likely to be the next GM foodstuff on the approval list. In the constant tug of war between pest resistance and crop resistance just like in many aspects of science, the beneficial output of a technology cannot always be measured as the sum of its parts. This is true when we consider gene stacking, or gene pyramiding. This is where the binding capacity of a toxic protein is enhanced by coupling it with another fusion protein capable of binding to a different gut receptor. An example of this would be where Bt was covalently fused to the non-toxic B-chain of ricin (RB) (Mehlo et al 2005). Because of this coupling the protein can express toxicity from binding at multiple receptor glycoproteins thus in theory increasing the chances of toxicity and reducing the likelihood of chance mutations establishing resistant variants of the target pest. Assays carried out to test this theory in both fusion protein variants and Cry1Ac alone in rice and maize showed decreased survival of Chilo suppressalis feeding on the transgenics by more than 50% (N.Ferry et al 2006).
Much crop losses can be attributed to damage caused by pest species, but another significant contributor to low yield is the negative interactions of other plants, what would commonly be referred to as weeds. In days gone by weeds would have had to have been removed by hand or a bit more recently with the aid of targeted weed-killer, but now with genetic modification at our disposal we can infer resistance to our crop from chemicals which would otherwise kill it, thus wiping out any nearby weeds competing for sunlight, water, space etc. Common herbicides used in combination with resistant crop lines include glyphosate, gluphosinate and bromoxinyl. These compounds are used as they do not interrupt the native fauna the genes inferring resistance to them express non toxic proteins in the plant tissues. I have detailed what I feel to be key milestones of the output traits but is by no means a definitive account of technologies in development. Genetic manipulation has enabled crops that can cope with much greater ranges of salinity, drought or poor soil condition, even encode RNAi to silence genes which may be detrimental to the plant in some conditions, to name but a few.
Input traits- Consumer benefit
In the beginning biotechnological advances regarding agriculture could be said to have been mainly focused on production, with emphasis on high yield and low expenditure. Increasingly however there has been more and more interest in the functional aspect that GM foods can provide such as production of foods with higher nutritional value, increased synthesis of essential vitamins and other nutrients beneficial to health. An excellent example of such work can be seen in the ‘Golden rice' program which set out to enable poorer people living off a limited diet of mainly rice to be able to get enough vitamin A in their diet. This was achieved by the addition of three genes for enzymes in the phytoene synthase (Ye,X et al 2000) pathway to the rice genome allowing synthesis of Beta-carotene (vitamin A precursor) which gave the rice a golden colour. This strain was then crossed with another high iron variety so that the final product supplemented both vitamin A and iron (Gura T.) 1999). The hope in creating this variant was to combat malnutrition and vitamin A deficiency blindness. In theory this sounds like a great idea, but limitations in absorption in the gut and the dietary habits of the proposed beneficiary could pose problematic (Miller GT 2002). These principles have been applied to the over expression other phytonutrients that have been reported to have positive effects on the consumers health such as phytoestrogens and phytosterols (Beecher GR.1999). With such a diverse genetic tool at our disposal, with endless possible genomic combinations it is very hard to predict the limitations of genetic modification.
Although this emerging new technology has been heralded with the potential to provide substantial economic growth and agricultural stability to our constantly expanding global population, it has been met with considerable scepticism and fear of the unknown by some parties. This general ignorance towards GM is commonly shared by a large proportion of the public and as a result has impeded its progress into areas of the market place, whereas in other areas they are commonly unaware of GMs existence such as fruit and vegetables (Priest, S. H. 2000). There have been some concerns voiced from the scientific community over fears of genetic drift from organism to organism risking upsetting the balance of nature and a certain weighted paper about the monarch butterfly intending to prove a link to toxicity in a key species. But these are generally improvable. In my opinion biotechnology has contributed several highly valuable benefits to agriculture and the world we live in today, with no measureable drawbacks outweighing positives. The western world needs to snap out of the organic loving culture that we have perpetuated and see the world for what it is. One of scientific progress.
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