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Modified tomato plants silence the genes of invaders
  
By
Edward R. Winstead


Scientists have created transgenic tomato plants that are resistant to crown gall disease. The plants were engineered to contain versions of two bacterial genes required to cause the disease. In plants, the extra genes recognize and effectively shut down the expression of the corresponding bacterial genes during infection, preventing the development of disease.


Tomato plants expressing oncogene-silencing transgenes are resistant to crown gall disease development. Stems of tomato seedlings were infected with the plant pathogenic bacterium Agrobacterium tumefaciens and assayed for disease development five weeks after inoculation. On the left, a wild-type plant displaying characteristic massive gall development and stunted growth. On the right, a transgenic plant displaying normal growth and no visible gall development (two inoculation sites are visible on the central stem).

"This study is the first example in the literature of a major bacterial disease that has been controlled through gene silencing," says Abhaya M. Dandekar, of the University of California, Davis, who led the research. Plants have an endogenous mechanism for fighting viruses, and the researchers exploited this mechanism by adding the modified genes. In the study, the plants were still infected but they did not develop galls.

"The incoming bacteria cannot make the hormones they need to make the tumors," says Dandekar. "The tumors disrupt the plant's architecture, so that the plants decline and become less productive."

Crown gall disease is caused by the bacterium Agrobacterium tumefaciens, and it affects hundreds of species. The illness is a perennial problem for growers of ornamental plants, such as roses, and fruit and nut crops. Recently, it caused severe and expensive damage among walnut trees in California.

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Escobar, M.A. et al. 42 RNAi-mediated oncogene silencing confers resistance to crown gall tumorigenesis. Proc Natl Acad Sci USA 98, 13437-13442 (November 6, 2001).
 

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