Some xenobiotics, e.g. 2,6-dichloroisonicotinic acid and its methyl ester, not only induce the three to four step detoxification system, but also PR proteins (pathogenesis related proteins), e.g. chitinases and /?-glucanases (see Chap., serving to combat pathogens (although their mode of action is frequently not known). Such xenobiotics are called immunochemicals. They are often formed to such a low extent that detection in cell extracts is hardly possible. When screening for a potential immunogenic effect of a xenobiotic a trick is generally used: Instead of the protein, the activation of the gene encoding for it is examined: Since several of these genes are known, activity of their promoter as induced by a potential immunochemical can be investigated. Fusion of the promoter with a reporter gene - e.g. the GUS-(glucuronidase-)gene of E. coli - provides a DNA probe. This probe can then be introduced, via the Agrobacterium system or the particle gun, into the genome of the test plant. After spraying a plant with the potential inducer, induction of the PR promoter by the herbicide can be seen as a stained product from the action of the glucuronidase.

The development of crop plants which respond to immunochemicals is considered as better than development of more herbicides (Box 1.9.3), because immunochemicals do not kill the weed, but rather increase the resistance of the crop. Thus, protective treatment with fungicides and insecticides could be strongly reduced. Treatment with a chemical would, however, still be required, but would induce the nat ural defence of the plant in the battle against detrimental organisms.

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