The Mechanism of Herbicide Resistance in Tobacco Cells with a New Mutation in the QB Protein

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Abstract
A new mutant of the psbA gene conferring resistance to 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) was obtained by selection of photomixotrophic tobacco (Nicotiana tabacum cv. Samsun NN) cells. The 264th codon AGT (serine) in the wild psbA gene was changed to ACT (threonine) in these mutant tobacco cells. All other higher plants resistant to atrazine exhibit a change to GGT (glycine) in this codon. Measurements of Hill reaction activity and chlorophyll fluorescence showed that the threonine 264-containing plastoquinone serving as secondary stable electron acceptor of PSII (QB protein) had not only strong resistance to triazine-type herbicides but also moderate resistance to substituted urea-type herbicides. Threonine-type QB protein showed especially strong resistance to methoxylamino derivatives of the substituted urea herbicides. The projected secondary structures of the mutant QB proteins indicate that the cross-resistance of threonine 264 QB protein to triazine and urea herbicides is mainly due to a conformational change of the binding site for the herbicides. However, the glycine 264 QB protein is resistant to only triazine herbicides because of the absence of an hydroxyl group and not because of a conformational change.