Caleb Anderson
BIOL 303 Section 501
Term Paper: Transgenic Crops
November 5, 2009
Transgenic crops are quickly rising to prominence in many of the world’s agricultural
countries. These crops generally possess traits such as herbicide and insect resistance that are not
found in wildtype individuals of those species; thus, they are often more efficient to grow. Figure
1 displays the rapid increase in the global area of transgenic crops over six years. It should be
noted that the area of transgenic crops grown in industrial countries is much higher than that of
developing countries because despite being cost-effective, they require a much greater initial
financial investment than regular crops. As a result, the United States grew 63% of the world’s
transgenic crops in 2003 (James 2003). In addition, a 2003 projection of the global worth of GM
(genetically modified) agriculture in 2005 placed the market at a minimum of US$5 billion.
Although many different kinds of crops are being evaluated for potential transgenic
modification, the vast majority of transgenic crops currently being grown consist of four staple
plants: maize, soybeans, canola, and cotton (James 2003). Transgene research has also been
conducted on other common crops such as tobacco and tomatoes. In most studies of transgenic
plants, an herbicide resistance fragment is added to the transgene in order to easily differentiate
between wildtype individuals and those who have successfully integrated the transgene. Despite
similarities among the agricultural plant species used for this research, multiple methods of
transgene introduction are necessary because of cellular differences between monocot and dicot
species. The terms ‘monocot’ and ‘dicot’ refer to the number of cotyledons, or embryonic leaves,
a species has; monocots have one embryonic leaf while dicots have two. Monocot cells are often
transformed via a technique known as microprojectile bombardment, where the transgene is
attached to particles of a nonreactive metal such as tungsten which are then launched at high
velocity through the cell wall (Chen et al. 2008). Both monocots and dicots can be transformed
with the Agrobacterium method. A. tumefaciens is a soil bacterium that induces genetic
alterations in tree root cells; in laboratory applications, it can be used to efficiently contain and
transfer transgenes (CSU 2004). Transgenes are generally introduced into Agrobacterium cells
via electroporation, which is a technique that results in relatively high levels of cell death, much
like microprojectile bombardment; thus, Agrobacterium transformation of dicot cells is often
more efficient in terms of the cell death rates of the target plant cells.
A current example of microprojectile bombardment in transgenic plant research is the
over-expression of yeast phytase in maize, a monocotyledonous species. The phytase enzyme
allows phytate, a common form of phosphorous in the diets of livestock, to be nutritionally
available for monogastric organisms such as fish and pigs. Thus, the intent of this study was to
induce the expression of sufficient quantities of phytase in maize in order to reduce or eliminate
the issues of expensive phosphate supplements and phosphorous pollution that currently plague
monogastric livestock operations (Chen et al. 2008). As previously mentioned, the transgene
contained an herbicide resistance fragment that allowed for identification of transgenic
individuals by herbicide leaf painting after growth in a greenhouse. As seen in Table 1, phytase
levels in the transgenic maize were significantly higher than those in wildtype maize. The
phytase molecules produced by the maize were smaller than those from the origin yeast due to
differences in glycosylation, but their function was not impaired by this change. However, the
phytase expression levels demonstrated in this study were still insufficient for the purposes of
widespread use in livestock operations.
By contrast, an example of Agrobacterium transformation in transgene research is a
recent study that aimed to over-express a spider venom peptide in tobacco for use as a
bioinsecticide (Hernández-Campuzano 2009). The transgene for the peptide, known as Magi 6,
was derived from the little-studied Japanese spider Macrothele gigas. Although similar peptides
had been expressed in previous studies, Table 2 notes that the concentration of venom peptide
expressed in this study was greater than those studies. Insect biotoxicity assays were conducted
with whole plants and leaf samples for 5 developmental stages of Spodoptera frugipera larvae. A
100% mortality rate was observed for the neonate developmental stage with both transgenic and
wildtype plants, likely due to natural insecticidal properties of tobacco. However, higher
developmental stages (3-4 and 4-5 instar) displayed significantly higher mortality rates with the
transgenic tobacco, suggesting that expression of the Magi 6 peptide could be feasible as a
commercial bioinsecticide. However, the toxicity mechanism of the peptide is still unknown,
which will likely prevent commercial usage until it has been studied.
While a great deal of transgenic crop research deals with immediately beneficial
modifications such as pest resistance and improved nutritional content, unexpected applications
of transgene technology occasionally emerge. For example, a 1995 study used the
Agrobacterium method to induce tomatoes to express the glycoprotein that coats the surface of
the rabies virus. Figure 2 displays the results of immunogold labeling of the glycoprotein in the
transgenic tomatoes as well as in wildtype tomatoes for comparison. Previous studies of this
glycoprotein indicated that it elicited rabies immunity in mammals such as raccoons, so the
intent of the study was to advance the idea of using transgenic crops as edible oral vaccines that
could be produced more cheaply and efficiently than current vaccines (McGarvey et al. 1995).
Although GM research is subject to constant ethical debate, in the case of transgenic
crops, there are a number of potential ecological problems that could arise as a result of their
global presence. Table 3 summarizes many of these problems as well as providing examples of
each. For example, the Magi 6 peptide over-expressed in tobacco plants could potentially be
released into the soil by the plant, causing unintended mortality of beneficial soil organisms.
Although enzymes like phytase are not known to be highly toxic, increased concentrations of
these molecules in soil and bodies of water near the transgenic crop planting site could cause
indirect ecological problems as well. This indirect chemical pollution effect is debated; a 2002
study of a common commercial type of transgenic cotton tested soil concentrations of an
insecticidal protein produced by the cotton and found no significant biological effects (Head). It
ought to be noted that the aforementioned study was sponsored by The Monsanto Company,
which sells the majority of the world’s GM seeds, so the results may suffer from bias. Herbicideresistant crops are considered to be useful because they can be treated with broad-spectrum
insecticides that kill all other organisms in the field; however, frequent use of such chemicals is
highly likely to result in the target pest organisms developing resistances that will be difficult to
combat. This is one of many examples of cost-effectiveness in agriculture being at odds with
ecological preservation.
Many of the potential detrimental environmental impacts of transgenic crops are related
to the agricultural techniques that would be used to grow them. However, the issue of transgene
“leakage” into wild populations is one that is unlikely to be resolved through improvements in
cultivation technology. Instead, it is generally accepted that transgene containment is best
achieved through modifying the propagation and dispersal characteristics of crops, as seen in
Table 4. For example, there are multiple versions of sterility that have been demonstrated in
important crops; transgenic individuals with one of these sterility traits could only be propagated
manually. Other properties, such as incompatible genomes and transgenic mitigation, allow for
pollination to take place, but the fertilization will be unsuccessful. It is important to note that out
of eight proposed transgene containment methods, less than half have been demonstrated in
transgenic plants, and only male sterility has seen widespread commercial use. A great deal of
testing is required for any of the other methods to be considered viable. However, the potential
benefits of transgenic crops are attractive enough that research into applications of transgenes
will likely take precedence over studies of detrimental effects until major problems are directly
observed.
Figures and Tables:
Figure 1. Global area of all transgenic crops grown by 18 countries from 1996 to 2003.
Table 1. Results of yeast phytase overexpression in maize. The terms ‘phytic acid’ and ‘phytate’
are interchangeable in this context (Chen et al. 2008).
Table 2. Comparison of Magi 6 spider peptide concentrations in tobacco to peptide toxin
concentrations in previous studies of transgenic plants (Hernández-Campuzano et al.
2008).
Figure 2. Results of immunogold labeling of the rabies glycoprotein in transgenic and wildtype
tomatoes (McGarvey et al. 1995).
Table 3. Overview of potential negative environmental impacts of GM crops.
Table 4. Overview of potential techniques for the management and containment of transgenes.
References:
Chen, R. et al. 2008. Transgenic maize plants expressing a fungal phytase gene. Transgenic Res
17, 633–643.
Colorado State University Department of Soil and Crop Sciences. 2004. Transgenic Crops: An
Introduction and Resource Guide. http://cls.casa.colostate.edu/transgeniccrops/index.html
Dale, P.J. et al. 2002. Potential for the environmental impact of transgenic crops. Nature
Biotechnology 20, 567-574.
Daniell, H. 2002. Molecular strategies for gene containment in transgenic crops. Nature
Biotechnology 20, 581-586.
Head, et al. 2002. No Detection of Cry1Ac Protein in Soil after Multiple Years of Transgenic Bt
Cotton (Bollgard) Use. Environmental Entomology 31(1), 30-36.
Hernández-Campuzano, B., et al. 2009. Expression of a spider venom peptide in transgenic
tobacco confers insect resistance. Toxicon 53, 122–128.
James, C. 2003. Global Status of Commercialized Transgenic Crops: 2003. ISAAA Briefs 30, 138.
McGarvey, P.B. et al. 1995. Expression of the Rabies Virus Glycoprotein in Transgenic
Tomatoes. Biotechnology 13, 1484-1487.