By Sean Henahan, Access Excellence

REHOVOT, Israel - Israeli scientists have come up with an elegant biotech solution to the problem of controlling the parasitic weeds that literally choke the life from the world's food crops.

Parasitic weed, especially broomrape (Orobanche) and witchweed (Striga), attach to the roots of commercially important crops, significantly reducing harvest. Broomrape poses a particular risk to vegetable, legume and sunflower crops, while witchweed has a devastating effect on grain crops in sub-Saharan Africa.

Up until now, the problem has been that the herbicides that are capable of killing these weeds also kill the crops. The Israeli researchers used genetic engineering techniques to confer herbicide resistance on selected crops. In field tests, the genetically altered plants were sprayed with common herbicides. The novel plants remained healthy, while the parasitic weeds were eradicated.

The researchers inserted genes associated with herbicide resistance into tobacco and rapeseed plants, conferring target-site resistance upon them. The enzyme normally affected by the herbicide was modified to protect it from herbicidal inhibition. Such modification allowed the herbicide applied to the leaves to go through the plant and penetrate into the parasites attached to its roots without harming the crop.

This approach offers a novel -- and currently the best -- solution to fighting broomrape and witchweed, which are among the worst agricultural pests in many parts of the world, noted Prof. Jonathan Gressel of the Weizmann Institute's Department of Plant Genetics.

In one study, genes conferring resistance to the herbicide chlorsulfuron were inserted into tobacco plants. The altered plants grew and flowered normally after a single application of the herbicide while the broomrape was fully controlled. Tobacco plants engineered to include a resistance gene for the herbicide asulam also were unaffected by subsequent application of the herbicide. In another study, the insertion of a resistance gene to the herbicide glyphosate completely prevented broomrape infestation but caused no damage to transgenic rapeseed plants.

Control plants that were not provided with the resistance genes and not treated by herbicides were heavily infested with the parasitic weeds. These plants achieved less than half the normal height, did not flower and died soon, while the weeds flowered and produced seeds.

"The doubled yields afforded by control of the parasites will more than offset the added cost of both the transgenic seed and the small amount of herbicide, even in developing countries," notes Dr. Daniel M. Joel, of the Department of Weed Research at the Newe-Ya'ar Research Center

The next step will be to engineer resistance genes into other important crops and to help make the seeds commercially available.

This research appears in the March 16, 1995 issue of Nature (Joel et al.)

Transmitted: 95-03-15 18:31:01 EST

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