Maize as example of model crop species for production of high-value products
Maize is globally the most widely grown food crop and is said to be at the forefront of the next green revolution. The era in which crops were planted to supply only food, feed and commodities is passing as the world’s demand for sustainable and low input-cost production of molecules needed for pharmaceuticals, industrial enzymes, chemical precursors and fuels are rising.
One recent article that provides a thorough review of the different high-value products derived from transgenic maize, as well as regulatory frameworks by which these plants are grown and their products are used and traded, was published in the journal Biotechnology Advances. The article is called High-value products from transgenic maize by Naqvi et al. (2011) and can be freely accessed at: http://www.writescience.com/RMT%20PDFs/Naqvi_2011_BiotechnolAdv.pdf
Naqvi et al. (2011) give the advantages of maize for the production of recombinant proteins as follows:
1. Compared to other crops maize yields the largest annual quantity of grain (8300kg/ha).
2. In comparison to other crops maize also has much lower production costs.
3. Maize has a relatively high seed protein content of 10%. This implies that more protein accumulates in a smaller seed volume and less hectares of land is necessary for the extraction of the target amount of recombinant protein. This not only leads to natural sustainability but economic viability as well.
4. Extraction of recombinant protein starts with a seed grinding step which often leads to a rise in temperature and the subsequent degradation of recombinant proteins. Maize somehow retains recombinant protein activity even after the grinding step.
5. Existing infrastructure such as equipment, agronomic practices and personnel training is already firmly in place for the maize industry.
6. Maize is grown on all continents and has no co-localized weedy relatives except in Central America. This reduces the chances of transgene escape.
This article further touches on advances in gene transfer in maize, from single genes to stacked traits and multiple pathways. Value added products are discussed in sections of primary metabolism, vitamins and minerals as well as recombinant proteins. Lastly the regulatory, economic and political aspects of high-value products from maize are outlined.
Multiple gene transformation
One aspect of the article that I want to focus on is multiple gene transformation (MGT). Single gene transformation strategies were useful in creating herbicide tolerant and pest and disease maize lines of agricultural importance. In terms of metabolic pathway engineering, single gene transformation may be of limited use since these pathways may have multiple branches, multifunctional enzymes and feedback mechanisms that might overrule the function of 1 single transgene. The simultaneous introduction of multiple genes can overcome these challenges by reconstructing metabolic pathways.
Often when explaining plant biotechnology to people unfamiliar with the concept and sometimes people of older generations, I get feedback in the form of eager and very creative input ideas. Overwhelmed by the potential ability of altering traits in species as to create biological products of economic value people jump on a runaway train of creative and emotional brainwaves. On its way through the rabbit hole to a Wonderland of unlimited possibilities this train rushes straight past the station of logic. With a serious tone my grandfather explained to me over the weekend that I should start research on a hardy little bush that he’s been observing along the banks of the Vaal river just outside the small town of Bothaville in the Free Sate. Apparently this cunning little shrub is able to flourish in the most severe of droughts and still boast brilliant green leaves after other plants have been buried under sludge of were carried away in floods. “Why not take out all the hardy genes from this plant and put it in those sickly maize plants of yours?” my grandfather wanted to know. A logical halt to this runaway train would be explaining that transferring a single, a few or even a few hundred such ‘hardy’ genes would be far from successful. Not only is each gene transferred with the possibility of disrupting an endogenous maize gene but the additive effect of having to express foreign proteins as dictated by the transgenes will add such a metabolic burden to the crop plant that yield will be severely disadvantaged and physiology will be completely altered. This is in addition to the amount of time and research input that will be necessary to unravel such a complex genetic pathway such as drought – or flood resistance. A trait is not conferred by a few genes alone but by complicated networks of genes and proteins interacting at a spatial and temporal level. Of course this is often a difficult concept to explain to people lacking a genetics background but I tried and it seems that the little plant I expect belongs to the Datura genus will not safe the world any time soon. However, multiple gene transformation might just be a step closer to the extension or incorporation of novel biosynthetic pathways in plants and soon such advances will not be far from impossible.
As far as methods of MGT is concerned I’ll refer to an extensive review article well worth reading which was published in the journal Trends in Plant Science in 2009. The article called When more is better: multigene engineering in plants was written by Naqvi et al. (http://www.sciencedirect.com/science/article/pii/S1360138509002532 ) and explains how applied research caught on with the genomics revolution which is focused on the investigation of multiple genes and proteins simultaneously. Not only does MGT allow for more ambitious plant phenotypes, but also for the importing of entire metabolic pathways and the expression of entire protein complexes, all of which was previously impossible. These authors also mention the possibility of creating GM crops possessing a spectrum of value added traits. Within this article a summary of different multigene transfer methods are given in terms of conventional stacking methods and cotransformation of linked as well as unlinked genes.
The role of MGT in metabolic engineering
The best known example of metabolic engineering in plants is probably that of Golden Rice. This example is also discussed by Naqvi et al. (2009). By transforming rice with three different daffodil carotenogenic genes as well as a selectable marker gene, a GM rice line rich in β-carotene was created. As part of the human metabolism β-carotene is converted to vitamin A. The increased accumulation of β-carotene in Golden Rice (GR) as compared to wild-type (WT) rice can be observed in the picture below:
During January this year I was fortunate enough to spend some time in the company of Prof Joseph Hirschberg from the Department of Genetics at the Hebrew University of Jerusalem, Israel. He attended the 38th annual South African Association of Botanists (SAAB) conference hosted by the University of Pretoria. His research group pioneered the molecular analysis of the carotenoid biosynthesis pathway in plant and was the first to clone genes for carotenoid biosynthetic enzymes in cyanobacteria, algae as well as plants. It was also under his lead that the regulation of carotenoid accumulation in fruits and flowers were elucidated. This was imperative for the creation of Golden Rice. Prof Hirschberg is truly a remarkable pioneer in the field of plant metabolic engineering.
One example of achieved MGT that rises above the rest is that of Indian mustard (Brassica juncea) for which the incredible amount of up to 9 transgenes were introduced. This number of transgenes is a significant achievement. The targeted pathway was PUFA synthesis and the transgenes were all fatty acid desaturases and elongases encoded by genes from 5 different microbial species (Naqvi et al., 2009). The Agrobacterium-linked cotransformation strategy was used which implies a series of transformations with increasing numbers of transgenes. Significant results were obtained which include a 25% increase in arachidonic acid and 15% increase in eicosapentaenoic acid, both which possess considerable human health benefits. This stepwise metabolic engineering process is described by Wu et al. (2005) (http://www.nature.com/nbt/journal/v23/n8/abs/nbt1107.html ) in Nature Biotechnology and was truly a benchmark for MGT at that time.
Gene stacking – industrial application
Eventually MGT is a way of achieving ‘gene stacking’. Gene stacking is the incorporation of multiple transgenes into a plant species to create a phenotype/genotype of interest. Usually gene stacking is a term associated with GMOs against an industrial background. No example can be more applicable than that of the stacked maize variety Smartstax as developed by Monsanto in conjunction with Dow AgroScience. Smartstax is the world’s first eight-stacked GM food crop and contains six Bt genes and two genes conferring tolerance to Monsanto’s Round-up and Dow AgroScience’s Liberty Link herbicides respectively.
A paper released by the African Centre for Biosafety (ACB) in 2010 (http://stopogm.net/webfm_send/135 ) summarizes the rationale underpinning the industry’s push for the adoption of stacked GMOs. As the amount of transgenes and the efficiency of incorporation thereof into crops increase, so does the conservational, socio-economic, regulatory and health concerns that has been haunting GMO crops since its first release in 1996. This article by the ACB provides a discussion on all these concerning issues. South Africa is one of the world’s top 10 GMO growing countries and the following table summarizes this by the amount of permits granted for stacked GM maize in South Africa, by company.
Conclusion
It becomes evident that as the upper limit of the amount of transgenes that can be stably transformed into plants quickly shifts upward, the sky may indeed be the limit. As we move closer to the point where whole new metabolic pathways can be introduced into plant species, the question remains whether the world is ready for such a transformation. While international regulatory bodies such as Codex Alimentarius and UN Environment Programme still disagrees on the development and implementation of standards and practices regarding stacked GMOs and while the ACB protests against toxicology, nutritional and allergenicity reports (forming part of GM applicational procedure) which are lined with liberal terms such as ‘substantial equivalence’ and compositional equivalence’ as well as ‘confidential business information’ which prohibits review of some of these applications, one cannot help but wonder whether one day we will find ourselves a bewildered Jack climbing a skyscraper bean stalk that’s monstrous and unstoppable. Or will we discover on top of the bean stalk a different world in which sufficient food security and sustainable fuel sources enables continual existence of an ever increasing world population amidst changing and unpredictable climatic conditions? If applied ethically and fair, just and equality-promoting, the latter can for certain be the case…
No comments:
Post a Comment