RESUMO
The evolution of glyphosate-resistant weeds has recently increased dramatically. Six suspected glyphosate-resistant Amaranthus tuberculatus populations were studied to confirm resistance and determine the resistance mechanism. Resistance was confirmed in greenhouse for all six populations with glyphosate resistance factors (R/S) between 5.2 and 7.5. No difference in glyphosate absorption or translocation was observed between resistant and susceptible individuals. No mutation at amino acid positions G101, T102, or P106 was detected in the EPSPS gene coding sequence, the target enzyme of glyphosate. Analysis of EPSPS gene copy number revealed that all glyphosate-resistant populations possessed increased EPSPS gene copy number, and this correlated with increased expression at both RNA and protein levels. EPSPS Vmax and Kcat values were more than doubled in resistant plants, indicating higher levels of catalytically active expressed EPSPS protein. EPSPS gene amplification is the main mechanism contributing to glyphosate resistance in the A. tuberculatus populations analyzed.
Assuntos
3-Fosfoshikimato 1-Carboxiviniltransferase/metabolismo , Amaranthus/efeitos dos fármacos , Amplificação de Genes , Glicina/análogos & derivados , Resistência a Herbicidas , Herbicidas/farmacologia , 3-Fosfoshikimato 1-Carboxiviniltransferase/genética , Absorção Fisiológica , Amaranthus/genética , Amaranthus/crescimento & desenvolvimento , Amaranthus/metabolismo , Transporte Biológico , Relação Dose-Resposta a Droga , Dosagem de Genes , Regulação da Expressão Gênica de Plantas , Glicina/administração & dosagem , Glicina/metabolismo , Glicina/farmacologia , Herbicidas/administração & dosagem , Herbicidas/metabolismo , Illinois , Missouri , Mutação , Componentes Aéreos da Planta/efeitos dos fármacos , Componentes Aéreos da Planta/genética , Componentes Aéreos da Planta/crescimento & desenvolvimento , Componentes Aéreos da Planta/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Plantas Daninhas/efeitos dos fármacos , Plantas Daninhas/genética , Plantas Daninhas/crescimento & desenvolvimento , Plantas Daninhas/metabolismo , RNA Mensageiro/metabolismo , RNA de Plantas/metabolismo , Regulação para Cima , GlifosatoRESUMO
A glyphosate-tolerant population of Canavalia ensiformis was collected in a cover crop in citrus orchards in Veracruz (Mexico), where glyphosate had been used for the first time. A susceptible Amaranthus hybridus L. population was collected from a nearby field that had never been treated with glyphosate. Dose-response experiments indicated a glyphosate tolerance ratio [ED(50)(C. ensiformis)/ED(50) (A. hybridus)] of 7.7. The hypothesis of a high level of glyphosate tolerance was provisionally corroborated on the basis of shikimate accumulation in both species. The susceptible population accumulated 6 times more shikimic acid in leaf tissue 96 h after glyphosate application than the tolerant leguminous crop. Two different physiological factors were involved in the glyphosate tolerance of this C. ensiformis population, which were confirmed by [(14)C]glyphosate, being a lack of penetration of glyphosate through the cuticle of the leguminous plants and an impaired herbicide translocation to the roots and the rest of shoots. This paper reports that two different nontarget site-based mechanisms, limited absorption and reduced translocation, contribute to the glyphosate tolerance found in C. ensiformis.