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Unintended Effects of Genetic Manipulation
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Potatoes genetically modified to store more starch stored less starch.

Manipulated Organism: Potato (Solanum tuberosum).

Inserted Transgenes: Glucokinase gene, derived from a bacterium (Zymomonas mobilis), was transferred into a transgenic line of potatoes that had been modified with the invertase (suc2) gene derived from yeast (Saccharomyces cerevisiae). The patatin B33 promotor was part of both gene constructs so that the foreign genes would be expressed specifically in potato tubers.

Goal: Increase the amount of starch that potato tubers accumulate by inserting genes that would enhance the tubers' ability to metabolize sucrose and glucose (sugars), out of which starch is formed.

Intended Effect: Did not occur.

Unintended Effects: The opposite occurred from the intended effect: the transgenic plants made significantly less starch. Although the plants expressed higher amounts of the sugar-degrading enzymes invertase and glucokinase, instead of forming starch the sugar was metabolized to carbon dioxide and water. In the authors' words, the manipulation led to "an increased flux through the glycolytic pathway at the expense of starch synthesis" (p. 109). In detail the researchers found:
  • Significant decrease in the amount of starch in the tubers: between 19% and 35% less than in unmanipulated potatoes. "The reduction in starch content found in the transgenic lines remains somewhat perplexing" (p. 116).
  • Reduction in the tuber fresh weight per plant by 25%.
  • Significant accumulations of hexose phosphates, which normally are intermediate substances that are used to form starch.
  • About a two-fold increase in the amounts of organic acids and amino acids, but there was not an increase in proteins, which are formed out of amino acids.
  • Two- to threefold increase in the activity of enzymes needed to break down sugar to water and carbon dioxide (respiratory pathway). "The reason for the increase in activity of certain enzymes in the respiratory pathway remains a mystery" (p. 115).
  • Three- to fivefold increase in the amount of carbon dioxide produced by the transgenic plants as a result of the increased respiration.
  • 50% increase in the amount of energy-storing compounds (ATP and UTP) that form as a result of the respiration of sugar.

Additional Comments: The authors conclude that their results demonstrate "an alteration of partitioning of carbohydrate between the two major pathways of respiration and starch biosynthesis in potato tubers, in favour of respiration. . . . The mechanisms that cause this change in partitioning and the reasons for many of the metabolic phenomena observed remain obscure" (p. 116).

Source: Tretheway, R. N., P. Geigenbauer, K. Riedel, M.-R. Hajirezaei et al. (1998). "Combined Expression of Glucokinase and Invertase in Potato Tubers Leads to a Dramatic Reduction in Starch Accumulation and a Stimulation of Glycolysis," The Plant Journal vol. 15, pp. 109-118.

Author Affiliations: Max-Planck-Institut für Molekulare Pflanzenphysiologie, Golm, Germany; Botanisches Institut, University of Heidelberg, Germany; Institut für Pflanzengenetik und Kulturpflanzengenetik, Gatersleben, Germany.

Funding: Fellowships from the European Community and the Max Planck Gesellschaft (partial).

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

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