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Tomato plants overexpressing phytoene synthase gene were stunted in growth.

Manipulated Organism: Tomato (Lycopersicon esculentum mill.).

Inserted Transgenes: Phytoene synthase gene (Psy1) from the tomato fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the target gene would be expressed in all parts of the plant.

Phytoene synthase is an enzyme that catalyzes the transformation of geranylgeranyl diphosphate (GGDP) to phytoene, the first step in the formation of carotenoids (essential plant pigments). In tomatoes the enzyme is usually only expressed in fruits; carotenoids are responsible for the red color of tomatoes.

Goal: Transgenic tomato plants that produce more carotenoids.

Intended Effect: The transgenic tomato plants produced high levels of phytoene, and also more zeta-carotene and lycopene, both of which are precursors to beta-carotene in the carotenoid synthesis pathway. Carotenoids were produced and accumulated in parts of the plant that normally do not produce and accumulate them, such as the seed coat, the seed leaves, abscission zones (where leaves and fruits break off the stem), and the immature fruit.

Unintended Effects: The plants with the highest levels of phytoene synthase expression and carotenoid accumulation showed marked anomalies:
  • they remained small (dwarfing): controls were 25 cm high after four weeks, while the high-expression transgenic plants were only 5 cm high
  • no fruit unless cross-pollinated with wild-type pollen
  • "severely stunted plants lacked chlorophyll in young leaves" (p. 700)
  • they showed up to a 30-fold reduction in the production of the hormone gibberellin A1

Additional Comments: The authors hypothesize that the over-production of phytoene synthase in the transgenic plants may have caused the nontarget effects because GGDP is also a substrate both for the formation of gibberellin A1, which is known to affect plant height, and phytol, which is required for chlorophyll formation. In other words, the plant may have compensated for the over-production of carotenoids by reducing gibberellin and chlorophyll production.

Source: Fray, R., A. Wallace, P. Fraser, D. Valero et al (1995). "Constitutive Expression of a Fruit Phytoene Synthase Gene in Transgenic Tomatoes Causes Dwarfism by Redirecting Metabolites from Gibberellin Pathway," The Plant Journal vol. 8, pp. 693-701.

Author Affiliations: Nottingham University, University of London, University of Bristol, UK.

Funding: Not mentioned.

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

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