Those who study the history of science and technology understand the important symbiotic relationships that exist between the various disciplines, and how a breakthrough in one area can trigger a chain reaction that leads to even more important progress in another area of science. This has certainly occurred in the discovery of triazine-resistant weeds in the late 1960's, followed in 1973 when Stanley Cohen of Stanford University and Herbert Boyer of the University of California at San Francisco found a simple way to combine DNA from two different organisms and then to clone identical copies of these recombined DNA molecules in bacteria. In 1975, Steven Radosevich made the important observation that isolated chloroplasts from triazine-resistant weeds were not inhibited in their rate of photosynthesis when treated with atrazine. With the use of triazine-resistant weeds and their chloroplasts, our knowledge of the structure of chloroplasts (herbicide binding genes that control the various peptides and amino acid sequences) and many other aspects of photosynthesis has expanded greatly during the past 10 years, with major contributions by Charles Arntzen and many other scientists.

Herbicides marketed for selected crops were chosen on the basis that major cultivars of the specific crop were tolerant. Our work with soybeans [Glycine max (L.) Merr.] has demonstrated that variability exists with regard to sensitivity or tolerance to several herbicides. As a part of our breeding program to develop improved soybean cultivars, we have evaluated advanced breeding lines and germplasm for reaction to herbicides on the market as well as experimental herbicides. A breeding line was identified and released for production which showed very little injury from two over-the-top applications of a double rate of 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid]. Other strains showed severe injury and depressed seed yield. Variation in reaction to glyphosate [N-(phosphonomethyl)glycine] has been observed. Lines tolerant to 0.56 kg ai/ha in one growing season were severely damaged the following season. When 200 germplasm lines from eastern Asia were treated with glyphosate, 6% showed less than 15% injury while 21% showed over 80% injury. The cultivar ‘Tracy′, selected for tolerance to 2,4-DB and a high level of resistance to the soil-borne disease causing phytophthora rot, was found to be sensitive to metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one]. Tracy has two major genes controlling resistance to phytophthora rot caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Erwin. One of these genes is closely linked with a gene controlling reaction to metribuzin. A crossover type was identified. The cultivar ‘Tracy-M’ retains the high level of resistance to phytophthora rot and is tolerant to metribuzin. A small percentage of germplasm lines evaluated were found to be sensitive to bentazon. Reaction to bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] is simply inherited.

The appearance of triazine resistance in a number of weed species has provided some opportunities for plant breeders to develop triazine-resistant crop varieties through classical breeding techniques. The sources of genes utilized by plant breeders for desirable characteristics are usually limited by the normal reproductive barriers that distinguish species. The occurrence of a triazine-resistant biotype of birdsrape mustard (Brassica campestris L. # BRSRA) has allowed for the development of commercially useful triazine-resistant varieties of canola (low erucic acid, low glucosinolate oilseed rape, B. napus L.). The cytoplasm of triazine-resistant birdsrape mustard was transferred interspecifically to oilseed rape, by a technique commonly known as back-crossing, in conjunction with selection for chromosome number. ‘OAC Triton′, the first triazine-resistant canola variety, appears to be finding commercial acceptance in Canada, in spite of some limitations in agronomic performance. Although it should be possible to transfer this triazine-resistant cytoplasm to other economic Brassica species including the cole crops and mustards, conventional plant breeding techniques cannot separate the gene conferring triazine resistance from other genes in the resistant weed biotype's cytoplasm. This problem may limit the utility of triazine-resistant cytoplasms from weed biotypes in classical crop breeding.

The soil bacterium Alcaligenes eutrophus is able to degrade phenoxy herbicides such as 2,4-D [(2,4-dichlorophenoxy)acetic acid]. It has been shown that several degradative genes are located on a large plasmid harbored by this organism. We have demonstrated by HPLC and gas chromatography that 2,4-dichlorophenol is a major metabolite resulting from 2,4-D degradation by A. eutrophus. A portion of the large plasmid carrying the 2,4-D catabolic genes has been cloned and mapped by restriction endonuclease digestion. Overlapping subclones have been produced and were used in conjugation experiments in order to identify the first gene involved in 2,4-D detoxification. Once sequenced, this gene will be transferred to dicotyledonous plant cells using available vector systems.

The target site of glyphosate [N-(phosphonomethyl)glycine] inhibition in plants and bacteria is 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase. Our strategy for developing glyphosate-resistant crops has been to genetically engineer plants with a gene that codes for EPSP synthase with low sensitivity in glyphosate. We cloned such a gene from the aroA locus of a glyphosate-resistant mutagenized strain of Salmonella typhimurium. The enzyme encoded by this gene has a single amino acid change resulting in lower affinity for glyphosate and higher affinity for substrates than either plant or wild-type bacterial counterpart. A chimaeric gene containing the mutant aroA gene behind the octopine synthase promoter was constructed and integrated into Agrobacterium T-DNA vectors. Analysis of gall tissue from Brassica campestris L. (turnip rape) infected with A. tumefaciens K12 containing this chimaera showed mRNA and protein expressed from the bacterial gene; 50% of the total EPSP synthase activity present had kinetic properties of the mutant bacterial enzyme. Tobacco (Nicotiana tabacum L. ‘Xanthi′) plants have been regenerated from cocultivation with A. rhizogenes containing the same construct; analysis indicates expression of the gene and enhanced tolerance to glyphosate.

Combinations of antagonistic herbicides can be helpful in the search for seed-applied chemical safeners to protect crop plants from herbicide injury. If a particular herbicide combination is selectively antagonistic so that the crop is not injured but weed control efficacy is not reduced, it should be possible to develop a new, more selective formulation of the herbicide which includes the antagonist or antidote. A promising new approach involves the use of early pretreatments of crop plants with subtoxic levels of a particular herbicide to increase crop tolerance to later, higher rates of that herbicide. When there are different mechanisms for herbicide detoxification in different plant species, it should also be possible to develop selective herbicide synergists that would provide equal efficacy at lower rates with greater crop tolerance. As our knowledge of herbicide metabolism and mode of action develops, it will be increasingly possible to use other chemicals to selectively synergize or safen herbicides to solve problems in important crop-weed situations.

Genetic engineering is a powerful new technology that makes possible rapid development of herbicide tolerances in superior crop varieties. Use of the technology will have a major impact on future agricultural practices and the agrichemical industry. Possible risks as well as significant likely benefits – to agricultural users, corporate suppliers, and the environment – are discussed. Increased product lifetimes, decreased costs, and increased usefulness of broad-spectrum, high-potency, environmentally safe herbicides and herbicide combinations are predicted.