A convergence of research effort in a number of scientific disciplines in the early 1980s resulted in a rapid expansion of knowledge of the structure and function of the photosynthetic reaction center in bacteria and higher plants. The structure of the reaction center from photosynthetic bacteria was determined by X-ray analysis. The herbicide binding protein (the D1 protein) was identified by photoaffinity labelling and found to be an integral part of the photosynthetic reaction center complex in higher plants. Studies using herbicide-resistant mutants enabled the location of the herbicide binding niche on D1 to be determined. Quantitative Structure Activity Relationships (QSAR) of families of inhibitors and their effect on photosynthetic electron transport helped elucidate the nature of the interaction between inhibitors and receptor. Binding appeared to be predominantly hydrophobic with hydrogen bonding also having an important role. Studies with a series of highly potent inhibitors, the 2-cyanoacrylates, identified certain steric constraints in the interaction of these molecules with the binding site. The activity of these inhibitors was particularly sensitive to minor structural change and they proved to be useful probes of receptor topography. The results of structure-activity studies of the 2-cyanoacrylates combined with a refined knowledge of the three-dimensional structure of the inhibitor binding site has enabled computer-based molecular modelling of interactions of these inhibitors with the receptor. The spatial arrangement of the inhibitor functional groups within the binding domain was shown to be a critical factor in determining binding affinity.
Plants resistant to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) were produced through the genetic engineering of a novel detoxification pathway into the cells of a species normally sensitive to 2,4-D. We cloned the gene for 2,4-D monooxygenase, the first enzyme in the plasmid-encoded 2,4-D degradative pathway of the bacterium Alcaligenes eutrophus, into a cauliflower mosaic virus 35S promoter expression vector and introduced it into tobacco plants by Agrobacterium-mediated transformation. Transgenic tobacco plants expressing the highest levels of the monooxygenase enzyme exhibited increased tolerance to 2,4-D in leaf disc and seed germination assays, and young plants survived spraying with levels of herbicide up to eight times the usual field application rate. The introduction of the gene for 2,4-D monooxygenase into broad-leaved crop plants, such as cotton, should eventually allow 2,4-D to be used as an inexpensive post-emergence herbicide on economically important dicot crops.
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