The Effects of Increased CO2 on Biomass and Exogenous-Toxin Quantity in Transgenic Bt Cotton and Rice Crops

Transgenic crops have become an increasingly important component of modern agroecosystems, ideally providing environmentally friendly, disease resistant crops with combinations of multiple genes that improve productivity and agricultural yield. One of the most common types of transgenic crop is Bacillus thuringiensis (“Bt”), which is produced worldwide and exhibits a strong resistance to lepidopteran pests in multiple cropping environments. With CO2 levels expected to increase in the future, scientists question the ability of Bt crops to adapt to changing atmospheric conditions and some hypothesize that increasing CO2 will pose new ecological risks for Bt crops and possibly reduce their effectiveness against target pests. In this study, a series of open-top chamber (OTC) experiments were conducted to asses whether measured exogenous-toxin quantity is reduced in transgenic Bt cotton and rice due to increased plant biomass under elevated atmospheric CO2. This study also examines the effectiveness of Bt cotton and rice transgenes against H. armigera and C. suppressalislarvae respectively. The study showed that there are significant differences between the exogenous-toxin levels of Bt cotton and rice under increased CO2and both showed differences in toxin quantity among developmental stages. Also, the new properties of Bt crops under elevated CO2 significantly affected the performance of H. armigeraand C. suppressalis larvae, despite the adverse effects of Bt gene expression in elevated CO2conditions.—Taylor Jones
Chen, F., Wu, G., Ge, F., Parajulee, M. N., 2011. Relationships between exogenous-toxin quantity and increased biomass of transgenic Bt crops under elevated carbon dioxide. Ecotoxicology and Environmental Safety 74, 1074–1080.

          Chen and colleagues performed a series of OTC experiments with ambient (375μl/L) and elevated (750μl/L) CO2  conditions that were maintained via a continuous automatic control system. Thirty-six pots of transgenic Bt cotton and twenty-six pots of transgenic Bt rice were planted and their positions were randomized each day to limit positional effects in the OTC chambers. In select plots, cotton bollworm (H. armigera) and rice stem borer C. suppressaliswere added to examine the effects of pest larvae on Bt cotton and rice. Biomass index was used to determine the increased amount of biomass under elevated CO2conditions and plant tissues were tested for exogenous-toxin quantity. Chen et al. recognize the potential “dilution effect” in which percent biomass increase exceeds the percent increase in exogenous-toxin increase, resulting in decreased levels of exogenous-toxin.
          The results show that increasing CO2levels significantly increased leaf, petiole, shoot and total plant biomass production of 45-DAS (“days after seedling”) Bt cotton as well as increases in shoot and total plant biomass production in 90-DAS cotton. Similar trends resulted for Bt rice as root, above-ground, and total stem biomass increased in 50-DAS rice and root tissues increased in 100-DAS rice. Overall, elevated levels of CO2 led to increased biomass in Bt cotton and rice, as predicted by previous studies showing increased photosynthesis and growth rates.
          Elevated CO2 conditions significantly reduced exogenous-toxin content in both Bt cotton and rice tissues. However, the effect of CO2 level on exogenous-toxin amount varied among crops and their respective plant tissues. For example, increased CO2 significantly reduced exogenous-toxin content per plant in 45-DAS and 90-DAS Bt cotton, while simultaneously increasing exogenous-toxin content in the stems of 50-DAS Bt rice. This shows that the responses of transgenic Bt cotton and rice (relating to exogenous-toxin content) to increased ambient CO2 are different. Also, each plant exhibited different responses in different phases of development. Chen and colleagues compared percent changes in biomass and exogenous-toxin levels and concluded that a dilution effect exists in shoot and petiole tissues of 45-DAS Bt cotton and in root, above-ground, and total stem tissues of 50-DAS Bt rice as well as leaf sheaths of 100-DAS rice. This is probably related to increased plant nitrogen-use efficiency and the authors predict that increased plant carbohydrate concentration diluted Bt proteins. For other increases in exogenous-toxin levels, the authors conclude that the dilution effect is only partly responsible and the reduction is due to reduced expression of the Bt gene under increased CO2 conditions.
          The results also suggest that most H. armigera larvae preferred to feed on transgenic Bt cotton squares and bolls, and most C. suppressalis larvae preferred to feed upon leaf sheaths of transgenic Bt rice. These areas of the Bt crops correlate with decreases in exogenous-toxin production, as expected. Although feeding increased in certain areas, the study found overall decreases in larval survival rate and pupal weight of H. armigera and C. suppressalis, suggesting that the Bt cotton and rice in this study to not face serious risks of reduced efficiency against pests in increased CO2 conditions.

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