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Transgenic Plants -- Summary
Gradually working our way into this topic, we note that toxins produced by Bacillus thuringiensis (Bt) supplied to crops via foliar application have been used as a means of combating crop pests for well over half a century. The effectiveness of this management technique depends primarily upon the amount of Bt-produced toxins that are ingested by targeted insects. Hence, it is only natural to wonder how atmospheric CO2 enrichment might impact this phenomenon.

One possibility derives from the fact that if soil nitrogen levels are low, foliar nitrogen concentrations are generally reduced from what they are at the current atmospheric CO2 concentration, which suggests that insects would have to eat more foliage to get their normal requirement of nitrogen for proper growth and development in CO2-enriched air. But by eating more foliage, the insects would also ingest more Bt-produced toxins; and they would thus be more severely impacted by those substances.

To test this hypothesis, Coviella and Trumble (2000) grew cotton plants in each of six Teflon-film chambers in a temperature-controlled greenhouse, where three of the chambers were maintained at an atmospheric CO2 concentration of 370 ppm, and three were maintained at 900 ppm CO2. In addition, half of the plants in each chamber received high levels of nitrogen (N) fertilization, while half received low levels (30 vs. 130 mg N/kg soil/week). After 45 days of growth under these conditions, leaves were removed from the plants and dipped in a Bt solution, after which known amounts of treated leaf material were fed to Spodoptera exigua larvae and their responses measured and analyzed.

By these means, the two researchers determined that the plants grown in the elevated CO2 chambers did indeed have significantly lower foliar nitrogen concentrations than the plants grown in the ambient CO2 chambers under the low N fertilization regime; but this was not the case under the high N regime. They also discovered that older larvae fed with foliage grown in elevated CO2 with low N fertilization consumed significantly more plant material than insects fed with foliage grown in ambient CO2; but, again, no differences were observed with high N fertilization. Last of all, and "consistent with the effect of higher Bt toxin intake due to enhanced consumption," they found that "insects fed on low N plants had significantly higher mortality in elevated CO2." Yet, again, no such effect was evident in the high N treatment. Consequently, with respect to pest management using Bt-produced toxins supplied to crops via foliar application, Coviella and Trumble concluded that "increasing atmospheric CO2 is making the foliar applications more efficacious."

But what happens in the case of transgenic plants into which the Bt gene for producing the toxin has been artificially inserted?

This question was addressed by Coviella et al. (2000), who in an analogous experiment to that of Coviella and Trumble grew cotton plants in twelve Teflon-film chambers in a temperature-controlled greenhouse, where six chambers were maintained at an atmospheric CO2 concentration of 370 ppm and six were maintained at 900 ppm CO2. Half of the cotton plants in each of these chambers were of a transgenic line containing the Bt gene for the production of the Cry1Ac toxin, which is mildly toxic for Spodoptera exigua, while the other half were of a near isogenic line without the Bt gene. In addition, and as before, half of the plants in each chamber received the same low and high levels of N fertilization; and between 40 and 45 days after emergence, leaves were removed from the plants and fed to the S. exigua larvae, after which a number of larval responses were measured and analyzed, along with various leaf properties.

This work revealed that the low-N plants in the elevated CO2 treatment had lower foliar N concentrations than did the low-N plants in the ambient CO2 treatment, and that the transgenic plants from the low-N, high CO2 treatment produced lower levels of Bt toxin than did the transgenic plants from the low-N, ambient CO2 treatment. In addition, the high level of N fertilization only partially compensated for this latter high-CO2 effect. In the ambient CO2 treatment there was also a significant increase in days to pupation for insects fed transgenic plants; but this difference was not evident in elevated CO2. In addition, pupal weight in ambient CO2 was significantly higher in non-transgenic plants; and, again, this difference was not observed in elevated CO2.

In discussing their findings, the three researchers wrote that "these results support the hypothesis that the lower N content per unit of plant tissue caused by the elevated CO2 will result in lower toxin production by transgenic plants when nitrogen supply to the plants is a limiting factor." They also made note of the fact that "elevated CO2 appears to eliminate differences between transgenic and non-transgenic plants for some key insect developmental/fitness variables including length of the larval stage and pupal weight."

These findings suggest that in the case of inadvertent Bt gene transference to wild relatives of transgenic crop lines, elevated levels of atmospheric CO2 will tend to negate certain of the negative effects the wayward genes might otherwise inflict on the natural world. Hence, the ongoing rise in the air's CO2 content could be said to constitute an "insurance policy" against this potential outcome. Indeed, it could even be construed to be a natural manifestation of the Precautionary Principle.

On the other hand, Coviella et al.'s results also suggest that transgenic crops designed to produce Bt-type toxins may become less effective in carrying out the objectives of their design as the air's CO2 content continues to rise. Coupling this possibility with the fact that the foliar application of Bacillus thuringiensis to crops should become even more effective in a higher-CO2 world, as found by Coviella and Trumble, one could argue that the implantation of toxin-producing genes in crops is not the way to go in the face of the ongoing rise in the air's CO2 content, which reduces that technique's effectiveness at the same time that it increases the effectiveness of direct foliar applications.

Although it is difficult to predict which path society will pursue in regard to the genetic modification of crops for pesticidal purposes, it is comforting to know that the ongoing rise in the atmosphere's CO2 concentration will help both nature and agriculture alike, whatever the outcome of the current debate surrounding this topic.

In a study of three different types of rice -- a wild type (WT) and two transgenic varieties, one with 65% wild-type Rubisco (AS-77) and one with 40% wild-type Rubisco (AS-71) -- Makino et al. (2000) grew plants from seed for 70 days in growth chambers maintained at 360 and 1000 ppm CO2, after which they harvested the plants and determined their biomass. In doing so, they found that the mean dry weights of the WT, AS-77 and AS-71 varieties grown in air of 360 ppm were, respectively, 5.75, 3.02 and 0.83 g, while in air of 1000 ppm CO2, corresponding mean dry weights were 7.90, 7.40 and 5.65 g. Consequently, although the growth rates of the genetically-engineered rice plants were far inferior to that of the wild type when grown in normal air of 360 ppm CO2 (with AS-71 producing less than 15% as much biomass as the wild type), when grown in air of 1000 ppm CO2 they experienced far greater CO2-induced increases in growth: a 145% increase in the case of AS-77 and a whopping 581% increase in the case of AS-71. Hence, whereas the transgenic plants were highly disadvantaged in normal air of 360 ppm CO2 (with AS-71 plants attaining a mean dry weight of only 0.83 g while the WT plants attained a mean dry weight of 5.75 g), they were found to be pretty much on an equal footing in highly-CO2-enriched air (with AS-71 plants attaining a mean dry weight of 5.65 g while the WT plants attained a mean dry weight of 7.90 g). This finding thus bodes well for the application of this type of technology to crop improvement in a future world of higher atmospheric CO2 content.

Returning to cotton, Chen et al. (2005) grew well watered and fertilized plants of two varieties -- one expressing Cry1A (c) genes from Bacillus thurigiensis and a non-transgenic cultivar from the same recurrent parent -- in pots placed within open-top chambers maintained at either 375 or 750 ppm CO2 in Sanhe County, Hebei Province, China, from planting in mid-May to harvest in October, throughout which period several immature bolls were collected and analyzed for various chemical characteristics, while others were stored under refrigerated conditions for later feeding to cotton bollworm larvae, whose growth characteristics were closely were monitored. In pursuing this protocol, the five researchers found that the elevated CO2 treatment increased immature boll concentrations of condensed tannins by approximately 22% and 26% in transgenic and non-transgenetic cotton, respectively (see Tannins in our Subject Index for a discussion of the significance of this observation). In addition, they found that elevated CO2 slightly decreased the body biomass of the cotton bollworms and reduced moth fecundity. The Bt treatment was even more effective in this regard; and in the combined Bt-high-CO2 treatment, the negative cotton bollworm responses were expressed most strongly of all. Chen et al. thus concluded that the expected higher atmospheric CO2 concentrations of the future will "either not change or only slightly enhance the efficacy of Bt technology against cotton bollworms."

Two years later, Chen et al. (2007) reported growing the same two cultivars under the same conditions from the time of planting on 10 May 2004 until the plants were harvested in October, after which egg masses of the cotton bollworms were reared in a growth chamber under ambient-CO2 conditions, while three successive generations of them were fed either transgenic or non-transgenic cotton bolls from plants grown in either ambient or twice-ambient atmospheric CO2 concentrations, during which time a number of physiological characteristics of the cotton bollworms were periodically assessed. This work revealed, in the words of Chen et al., that "both elevated CO2 and transgenic Bt cotton increased larval lifespan," but that they decreased "pupal weight, survival rate, fecundity, frass output, relative and mean relative growth rates, and the efficiency of conversion of ingested and digested food." As a result, they say that "transgenic Bt cotton significantly decreased the population-trend index compared to non-transgenic cotton for the three successive bollworm generations, especially at elevated CO2 [our italics]."

Based on these findings, the four researchers concluded that the negative effects of elevated CO2 on cotton bollworm physiology and population dynamics "may intensify through successive generations," in agreement with the findings of Brooks and Whittaker (1998, 1999) and Wu et al. (2006). Hence, they additionally concluded that "both elevated CO2 and transgenic Bt cotton are adverse environmental factors for cotton bollworm long-term population growth," and that the combination of the two factors may intensify their adverse impact on the population performance of the cotton bollworm, which should be especially good news for cotton growers.

Last of all, Fu et al. (2008) note that "heat stress is a major constraint to wheat production and negatively impacts grain quality, causing tremendous economic losses, and may become a more troublesome factor due to global warming." Consequently, as they describe it, they "introduced into wheat the maize gene coding for plastidal EF-Tu [protein synthesis elongation factor]," in order to assess "the expression of the transgene, and its effect on thermal aggregation of leaf proteins in transgenic plants," as well as "the heat stability of photosynthetic membranes (thylakoids) and the rate of CO2 fixation in young transgenic plants following exposure to heat stress." These operations led, in their words, "to improved protection of leaf proteins against thermal aggregation, reduced damage to thylakoid membranes and enhanced photosynthetic capability following exposure to heat stress," which results "support the concept that EF-Tu ameliorates negative effects of heat stress by acting as a molecular chaperone."

Fu et al. describe their work as "the first demonstration that a gene other than HSP [heat shock protein] gene can be used for improvement of heat tolerance," noting it also indicates that the improvement is possible in a species that has a complex genome," such as hexaploid wheat. Hence, they conclude their report by stating their results "strongly suggest that heat tolerance of wheat, and possibly other crop plants, can be improved by modulating expression of plastidal EF-Tu and/or by selection of genotypes with increased endogenous levels of this protein." Consequently, they affirm there is significant reason to believe that humanity's major crops could well be genetically "inoculated" against deleterious consequences of potential future global warming, irrespective of whatever its cause or causes might be.

In summary, it would appear that specific genetic alterations to humanity's crop plants may enable them to (1) better withstand the assaults of insects pests, (2) better bear the consequences of possible future increases in seasonal maximum air temperatures, and (3) take better advantage of the positive effects of atmospheric CO2 enrichment on various plant properties and processes, while the elevated CO2 simultaneously reduces the severity of possible negative effects that could arise from the escape of transplanted genes into the natural environment.

Brooks, G.L. and Whittaker, J.B. 1998. Responses of multiple generations of Gastrophysa viridula, feeding on Rumex obtusifolius, to elevated CO2. Global Change Biology 4: 63-75.

Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change Biology 5: 395-401.

Chen, F., Wu, G., Ge, F., Parajulee, M.N. and Shrestha, R.B. 2005. Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and feeding of an insect herbivore, the cotton bollworm. Entomologia Experimentalis et Applicata 115: 341-350.

Chen, F., Wu, G., Parajulee, M.N. and Ge, F. 2007. Long-term impacts of elevated carbon dioxide and transgenic Bt cotton on performance and feeding of three generations of cotton bollworm. Entomologia Experimentalis et Applicata 124: 27-35.

Coviella, C.E., Morgan, D.J.W. and Trumble, J.T. 2000. Interactions of elevated CO2 and nitrogen fertilization: Effects on production of Bacillus thuringiensis toxins in transgenic plants. Environmental Entomology 29: 781-787.

Coviella, C.E. and Trumble, J.T. 2000. Effect of elevated atmospheric carbon dioxide on the use of foliar application of Bacillus thuringiensis. BioControl 45: 325-336.

Fu, J., Momcilovic, I., Clemente, T.E., Nersesian, N., Trick, H.N. and Ristic, Z. 2008. Heterologous expression of a plastid EF-Tu reduces protein thermal aggregation and enhances CO2 fixation in wheat (Triticum aestivum) following heat stress. Plant Molecular Biology 68: 277-288.

Makino, A., Harada, M., Kaneko, K., Mae, T., Shimada, T. and Yamamoto, N. 2000. Whole-plant growth and N allocation in transgenic rice plants with decreased content of ribulose-1,5-bisphosphate carboxylase under different CO2 partial pressures. Australian Journal of Plant Physiology 27: 1-12.

Wu, G., Chen, J.F. and Ge, F. 2006. Response of multiple generations of cotton bollworm Helicoverpa armigera Hubner, feeding on spring wheat, to elevated CO2. Journal of Applied Entomology 130: 2-9.

Last updated 3 December 2008