The Molecular Basis of Triazine-Herbicide Resistance in Higher-Plant Chloroplasts*

Triazine herbicides inhibit photosynthesis by blocking electron transport in photosystem II. The target site of the herbicide was identified as a chloroplast thylakoid polypeptide (the Q B protein) o f 32,000 daltons. Studies o f triazine-resistant weed biotypes suggested that a subtle change in the QB protein caused the resistance. We have cloned the chloroplast gene (psbA) that codes this protein from herbicide-resistant and herbicide-susceptible biotypes o f Solanum nigrum. By DNA sequencing we detected a single base substitution in the psbA gene o f the resistant plants, resulting in an amino acid change (serine to glycine for the susceptible to resistance conversion). This mutation is exactly the same one which we have described in a herbicide-resistant biotype o f Amaranthus hybridus.