Phenylurea, triazine and diazine represent economically very important compounds since they are used in chemical, pharmaceutical and agricultural industries. Although new herbicides are now available, they still represent the basic products for weed control. All the compounds of these classes constitute about 40 % of all herbicides used at present in agriculture, amounting to thousands of tons all over the world. These compounds are absorbed through the roots and then translocated via the xylem to the leaves. Some of these herbicides are directly absorbed by the leaves. They act by inhibiting photosynthesis at the level of the photosystem II-dependent electron transfer and further block production of ATP and NADPH. In soils, these compounds are quite persistent and they are adsorbed on soil colloids and on organic substances in proportion to the cation exchange capacity of these soil constituents. Bioremediation by photosynthetic plants (phytoremediation) is a well-established method to recover herbicide-polluted soils. The ideal plant for phytoremediation should exhibit the following characteristics: (ⅰ) be resistant to the herbicides, (ⅱ) to allow herbicide translocation to the leaf: (ⅲ) to be able to degrade the herbicide. We can distinguish naturally resistant plants and mutants of sensitive species. In naturally resistant plants the mechanisms of resistance include a slow translocation into the chloroplast, high protein repair turnover, immobilization and detoxication by endogenous enzymes through conjugation and/or degradation. Glutathione S-transferases are involved in the detoxication of herbicides. Often, more than one mechanism is active at the same time. Moreover, prolonged agronomic use of PSII inhibitors, specifically triazine herbicides, has resulted in the evolution of mutants of sensitive species. In this case, the predominant basis for resistance is a single nucleotide substitution in the chloroplast psbA gene encoding the D1 protein, which precludes the binding of herbicide to the protein. However, resistance of mutated biotypes has also been attributed to modifications in the activities of the glutathione enzyme.
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