Herbicide Resistance and Supersensitivity in Ala250 or Ala251 Mutants of the D1 Protein in Chlamydomonas reinhardtii,

Various site-directed mutants at position Ala250 or Ala251 within the herbicide-binding niche of the D1 subunit of photosystem II were generated by transforming a Chlamydomonas reinhardtii strain specifically engineered to lack a psbA gene fragment coding for the binding region. The following mutants were obtained: Asp250, Phe250, His250, Ile250, Asn250, Arg250, Tyr250, Cys251, Gly251, and the double mutant Ser250/Ile255. A variety of herbicides and inhibitors, such as s-triazines, phenolic herbicides, p-benzo- and naphthoquinones, acridones, a NH-thiazol, and phenmedipham, were assayed for their inhibitory activity in these mutants. In addition, in the Ser250/Ile255 mutant 32 different triazinones were tested for biological activity. In all mutants, either resistance and/or supersensitivity against the herbicides and inhibitors was observed. The resistance and/or supersensitivity against inhibitors in the Ala250 mutants demonstrated for the first time the involvement of this amino acid in herbicide binding.