Herbicide-resistant genes in weeds

Darmency, H. and J. Gasquez (1990b). Fate of herbicide resistance genes in weeds. In Green, M.B., LeBaron, H.M., and Moberg, W.K., eds, Fundamental and Practical Approaches to Combating Resistance. ACS Symposium Series 421. Washington, DC American Chemical Society, pp. 353-363. 

DARMENCY AND GASQUEZ Fate of Herbicide Resistance Genes in Weeds 355... 

However, each new resistance that develops will produce a different genetic situation. The solutions found by weeds to escape herbicide selection pressures may be varied. Due to the high selective value conferred by the resistance genes in herbicide treated areas, mutation events at very low frequencies have high probability to lead to the appearance of resistant plants. In addition, weeds will certainly display after a short delay the bacterial genes transferred to crops for herbicide resistance, and some wild plants could be expected to become new weeds because of resistant genes. 

A distinction is made between non-specific and specific herbicides. The former inhibit the growth of both cultivated plants and weeds. For this reason, they can only be used before sowing. The introduction of resistance genes in soybean, corn and rape seed, among others, allows their weeds to be controlled by non-specific herbicides even during growth. 

Padgette, S. R., Re, D. B., Barry, G. F. el al. (1994). New weed control opportunities development of soybeans with a Roundup Ready gene. In Herbicide-resistant Crops Agricultural, Economics, Environmental, Regulatory, and Technologycal Aspects, ed. S. O. Duke. Boca Raton, FL CRC Press. 

Radosevich, S.R., B.D. Maxwell, and M.L. Roush (1991). Managing herbicide resistance through fitness and gene flow. In J.C. Caseley, G.W. Cussans, and R.K. Atkin, eds., Herbicide Resistance in Weeds and Crops. Oxford, UK Butterworth-Heinemann Ltd, pp. 129-143. 

The risk of passing resistance genes from a crop to a wild related species is not inconceivable. A number of crops, (e.g., rice, millets, sorghum, oats, rapeseed, sugar beets, sunflower, alfalfa, peas, and potatoes) could be involved in introgression. Therefore, as herbicide resistant crops are engineered, a genetic barrier between crops and weeds must be devised. 

The very low frequencies of resistant individuals in the field must compete with the crop, with resistant members of other weed species, and (when the herbicide is not present) with susceptible members of the same and other species. If triazine resistance evolved only in populations with plastome mutator genes (see above), there is the strong possibility of multiple mutations in these plastid genomes, including deleterious mutations giving unfit alleles of other genes. These may explain much of the unfitness of resistant plants, as well as the published variabilities of plastid fitness. Deleterious nuclear... 

Many of the characteristics which combine to make ALS an excellent target for engineering beneficial herbicide resistance in crop plants may also lead to the proliferation of herbicide-resistant weeds. These characteristics include the following sulfonylurea herbicide resistance is a semi-dominant trait that is carried on a nuclear gene(s) ALS is the single primary site of action there are multiple positions in ALS that can be mutated to confer herbicide resistance mutant ALS enzymes can possess full catalytic activity. The latter property results in engineered crop plants that are fit, but can equally well result in weed biotypes that are fit. 

FIGURE 4 Results from a field test of herbicide-resistant tobacco lines, conducted in North Carolina in 1987 in conjunction with Northrup King Co. The row of plants to the right of center contains elite tobacco lines that were transformed with the mutant tobacco ALS gene from the herbicide-resistant Hra line, while the row to the left of center has non-transformed control plants. After treatments with the sulfonylurea herbicide chlorimuron ethyl at 4X normal field application rate, the weeds were killed, the non-transformed controls were severely injured and remained stunted, and the transformed plants remained as vigorous as the unsprayed controls. 

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