THE MOLECULAR BASIS OF RESISTANCE OF PHOTOSYSTEM II HERBICIDES

Abstract

Two groups of compounds with quite different essential chemical elements inhibit photosystem II: the urea/triazine and the phenol-type families. Both groups displace plastoquinone and bind to a site on the D-1 protein subunit of the reaction centre of photosystem II. The first examples of resistance to these herbicides - triazine in Amaranthus hybridus and diuron in Chlamydomonas reinhardtii - were found to be due to a change in the properties of the binding niche and eventually to a mutation in the psbA gene that is responsible for an amino acid substitution in the sequence of the D-1 protein. Further mutations in the psbA gene, both by site selected screening or site directed mutagenesis, have been described leading to amino acid substitutions in the D-1 protein at positions: phe 211, val 219, ala 251, phe 255, gly 256, ser 264, asn 266 and leu 275. Significantly, mutations in triazine/urea resistance lead to an increased sensitivity to phenol-type inhibitors. These amino acid changes in herbicide resistant mutations have been used to model the binding niche for the plastoquinone and herbicides on the D-1 protein. The modelling is based on the homology of photosystem II to the reaction centre of purple bacteria. The cross tolerance of various herbicides in the mutants can be used to identify interactions of side chains with specific amino acid residues. Thus, the molecular basis of herbicide tolerance in photosystem II can be well described.