Engineered glyphosate resistance is the most widely adopted genetically modified trait in agriculture, gaining widespread acceptance by providing a simple robust weed control system. However, extensive and sustained use of glyphosate as a sole weed control mechanism has led to field selection for glyphosate-resistant weeds and has induced significant population shifts to weeds with inherent tolerance to glyphosate. Additional weed control mechanisms that can complement glyphosate-resistant crops are, therefore, urgently needed. 2,4-dichlorophenoxyacetic acid (2,4-D) is an effective lowcost, broad-spectrum herbicide that controls many of the weeds developing resistance to glyphosate. We investigated the substrate preferences of bacterial aryloxyalkanoate dioxygenase enzymes (AADs) that can effectively degrade 2,4-D and have found that some members of this class can act on other widely used herbicides in addition to their activity on 2,4-D. AAD-1 cleaves the aryloxyphenoxypropionate family of grass-active herbicides, and AAD-12 acts on pyridyloxyacetate auxin herbicides such as triclopyr and fluroxypyr. Maize plants transformed with an AAD-1 gene showed robust crop resistance to aryloxyphenoxypropionate herbicides over four generations and were also not injured by 2,4-D applications at any growth stage. Arabidopsis plants expressing AAD-12 were resistant to 2,4-D as well as triclopyr and fluroxypyr, and transgenic soybean plants expressing AAD-12 maintained field resistance to 2,4-D over five generations. These results show that single AAD transgenes can provide simultaneous resistance to a broad repertoire of agronomically important classes of herbicides, including 2,4-D, with utility in both monocot and dicot crops. These transgenes can help preserve the productivity and environmental benefits of herbicide-resistant crops.herbicide resistance | weed management | genetically modified crops |
Carboxylesterases hydrolyze numerous endogenous and exogenous ester-containing compounds. They play a role in the detoxification of many agrochemicals including pyrethroids, organophosphates, and carbamates. Research on these enzymes is still developing and there are several topics that should be addressed to further investigations in this area. This paper focuses on a number of these issues including enzyme nomenclature, catalytic mechanism, substrate specificity, agrochemical metabolism, role in insecticide resistance and environmental significance. It is expected that carboxylesterase research will increase with specific emphasis on isozyme and substrate identification. Future research directions are discussed and the current state of the field is evaluated.
The pyrethroids are one of the most heavily used insecticide classes in the world. It is important to develop sensitive and rapid analytical techniques for environmental monitoring and assessment of human exposure to these compounds. Because major pyrethroids contain a phenoxybenzyl group and phenoxybenzoic acid (PBA) is a common metabolite form or intermediate, PBA might be used as a biomarker of human exposure to pyrethroids. A sensitive and selective immunoassay for the common pyrethroid metabolite PBA was developed. Rabbits were immunized with 3-[4-(3-carboxyphenoxy)phenoxy] N-thyroglobulin ethylamine. All sera were screened against numerous coating antigens. The assay with the least interference and the best sensitivity was optimized and characterized. The average IC50 for free PBA was 1.65 ng/mL. No cross-reactivity was measured to parent pyrethroids and other metabolites. Urine matrix effects can be eliminated by simple dilution. Results from urine samples from exposed workers suggest that this PBA immunoassay might be suitable as a monitoring tool for human exposure to pyrethroids.
A method for simply and cheaply preparing inorganic phosphor nanoparticles of Eu2O3 as labels in biology has been demonstrated with a simple microwave-assisted surface chemistry. The capping process adds a silane layer to the surface of the particles and provides amine groups that can be used for biological conjugation. The surface layer also protects the particles during conjugation chemistry. The particles retain their desirable optical properties that are typical of europium, that is, a spectrally narrow, red emission and a long fluorescence lifetime. The application of the nanoparticle labels in an immunoassay yields very good sensitivity in an immunoassay for atrazine (sub-parts-per-billion detection limit) without optimization of the detection system. The microwave functionalization technique will permit a broad range of inorganic nanophase phosphors to be used in high-throughput assays for environmental monitoring.
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