INDUCED NEW MUTATION OF D1 SERINE-268 IN SOYBEAN PHOTOSYNTHETIC CELL-CULTURES PRODUCED ATRAZINE RESISTANCE, INCREASED STABILITY OF S(2)Q(B)(-) AND S(3)Q(B)(-) STATES, AND INCREASED SENSITIVITY TO LIGHT STRESS

We have isolated several herbicide-resistant cell lines from photosynt hetic cell suspensions of soybean (Glycine max) that possessed differe nt levels of herbicide resistance, photosystem II activity, and chloro phyll a/b ratio. We have further studied the STR7 mutant, which showed the highest level of resistance to atrazine as well as a cross-resist ance to 3-(3,4-dichlorophenyl)1,1-dimethylurea (50- and 3-fold, respec tively, compared with the wild type). Sequencing of the psbA gene (cod ing for the D1 polypeptide of photosystem II) from this mutant reveale d a single change, serine-268 to proline, in the D1 protein. To our kn owledge, this substitution has not previously been described in any ph otosynthetic organism. In addition to affecting atrazine resistance, t his single amino acid change caused a decrease in the electron transfe r rate between the secondary accepters Q(A) and Q(B) and a stabilizati on of the S(2)Q(B)(-) and S(3)Q(B)(-) states. The mutant also showed a larger antenna size, an increase in non-Q(B)-reducing centers, and a higher sensitivity to light stress. The unusual stability of the S(2)Q (B)(-) and S(3)Q(B)(-) states indicates that STR7 belongs to a new cla ss of Q(B)-site mutants.