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Canola plants over-expressing a bacterial phytoene synthase gene also had a reduced level of chlorophyll, changed structure of plastids, changed composition of fatty acids, and delayed germination.

Manipulated Organism: Canola (Brassica napus ssp. Oleifera), also known as oilseed rape.

Inserted Transgenes: Phytoene synthase gene crtB from the bacterium Erwinia uredovora fused to the promoter, napin, from Brassica. The promoter is seed-specific to ensure that the transgene is expressed in the seeds.

Goal: Increase the amounts of carotenoids in canola seeds so that canola could eventually be used as commercial source of carotenoids. Normally, canola seeds do not accumulate carotenoids.

Intended Effect: The transgenic mature canola seeds contained up to a 50-fold increase in carotenoids, making the embryo visibly orange. The predominant carotenoids were alpha- and beta-carotene.

Unintended Effects:
  • The level of chlorophyll was reduced in developing seeds.
  • Level of gamma tocopherol (a form of vitamin E) was decreased in transgenic seeds.
  • The electron microscopic structure of some plastids in the embryos was altered: the membrane structure was changed and thread-like bodies of unknown nature appeared.
  • The amount of phytoene was increased compared with control plants. Phytoene is normally an intermediate product of metabolism and does not accumulate.
  • The amount of lutein and xanthophylls was not higher, although they also are end products of the carotenoid pathway.
  • The composition of fatty acids was altered: there was more oleic acid and less linoleic and linolenic acid. This "alteration in fatty acid composition was unexpected" (p. 408). The link between fatty acid synthesis and the carotenoid pathway is not known.
  • Germination in transgenic seeds was delayed by one to two days compared to control plants.

Additional Comments: As the authors note, this study shows the remarkable ability of the canola plant to accumulate large amounts of carotenoids in its seeds where normally carotenoids are not stored. But this channeling of the plant's flexibility via genetic modification does not occur in isolation; it is accompanied by the other significant biochemical, physiological, and morphological changes in the seeds described above.

Source: Shewmaker, C., J. A. Sheehy, M. Daley, S. Colburn et al. (1999). "Seed-specific Overexpression of Phytoene Synthase: Increase in Carotenoids and Other Metabolic Effects," The Plant Journal vol. 20, pp. 401-12.

Author Affiliations: Calgene LLC, Davis, California; Monsanto Company, St. Louis, Missouri.

Funding: Not mentioned, but see author affiliations.

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

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