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Unintended Effects of Genetic Manipulation
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Apples over-expressing a fruit-ripening enzyme lacked flowers and had malformed stomata and altered composition of cell walls.

Manipulated Organism: Apple (Malus domestica, Borkh.), cultivar Royal Gala.

Inserted Transgenes: Fruit-specific apple polygalacturanase (PG) gene (MdPG1) derived from the apple cultivar Golden Delicious. PG is known as a fruit-specific enzyme associated with fruit ripening (pectin degradation) and other processes. The gene was fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the gene would be expressed in all parts of the plant.

Goal: Create transgenic apple trees that overexpress the enzyme PG "with the expectation that disruption of cell wall metabolism would occur in ripening fruit" (p. 123), since PG is known to break down pectin, which is an important component of cell walls.

Intended Effect: The trees did over-express the enzyme PG in their leaves.

Unintended Effects: The transgenic trees formed no flowers and fruits in contrast to unmanipulated trees. The leaves of the transgenic trees were abnormal in a variety of ways:
  • The leaves were silver-colored and brittle; they wilted and broke off easily.
  • Microscopically, there were more air spaces between cells.
  • The stomata, which let air pass into and out of leaves, were frequently malformed and did not close at night, which caused higher transpiration (loss of moisture). There were also holes next to the stomata, which is not normally the case.
  • Biochemically, there was less cell wall material, a changed composition of the cell walls, and less pectin on cell wall surfaces.
Additional Comments: The authors conclude that this manipulation "has led to a range of novel phenotypes including disrupted leaf organization, perturbed water relations, malformed and malfunctioning stomata, silvery leaves, and changes in leaf abscission. The modification of apple trees by this single fruit gene, therefore, has offered a new and unexpected perspective on a number of physiological and developmental processes" (p. 126).

Source: Atkinson R. G., R. Schröder, I. Hallett, D. Cohen et al. (2002). "Overexpression of Polygalacturonase in Transgenic Apple Trees Leads to a Range of Novel Phenotypes Involving Changes in Cell Adhesion," Plant Physiology vol. 129, pp. 122-33.

Author Affiliations: The Horticulture and Food Research Institute of New Zealand.

Funding: Grants from the Foundation for Research, Science, and Technology of New Zealand.

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

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