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Mechanisms of herbicide resistance include (1) modi®ed target site, (2) enhanced detoxi®cation or delayed activation, and (3) alterations in the uptake, translocation, or compartmentalization of a herbicide. The ®rst two mechanisms have mainly been identi®ed in plants. Herbicide resistance genes were isolated for several herbicides of different modes of action. Genes that coded for herbicide target or detoxi®cation enzymes were transferred into crop plants. The transgenic plants expressing these genes were tolerant of the active ingredients of herbicides.Before commercialization, the transgenic plants were tested in the ®eld for risk assessment. In the case of crops with herbicide detoxi®cation enzymes, including cytochrome-P450-species-metabolizing xenobiotics, the substrate speci®city of the enzymes as well as the toxicological properties of the herbicide metabolites and the pattern of secondary metabolites in plants must be evaluated.

Cytochrome P450 monooxygenases play an important role in detoxification and/or activation of xenobiotics including agrochemicals as well as in the modes of action of agrochemicals including fungicides, plant growth regulators, insecticide synergists and herbicide safeners. Current genome sequencing revealed that P450 genes are distributed into the five kingdoms of life: bacteria, protoctista, plants, animals and fungi, and a large number of P450 genes occur in animal and plant species. Particularly, Arabidopsis is estimated to have around 400 genes. Biochemical characterization of these P450 enzymes is important for understanding their physiological functions. In higher plants, Arabidopsis T-DNA mutant lines appear to be useful for analysis of their physiological functions. In addition, the yeast expression system is practically important for analysis of their enzymatic functions. Engineering of transgenic plants expressing certain P450 species is also important for crop breeding with useful traits including selective herbicide tolerance and phytoremediation.

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