Paraquat-resistant biotype, herbicide

Intensive use of the herbicide paraquat has resulted in the evolution of resistance in various weed species. Intensive research on the resistance mechanisms was mainly carried out with resistant biotypes from Hordeum spp. and Conyza spp., and altered distribution of the herbicide in the resistant weeds was suggested as the cause - or at least the partial cause - of resistance. In resistant Conyza canadensis it was supposed that a paraquat inducible protein may function by carrying paraquat to a metabolically inactive compartment, either the cell wall or the vacuole. This sequestration process would prevent the herbicide from getting in sufficient amounts into the chloroplasts as the cellular site of paraquat action. Inhibitors of membrane transport systems, e.g., N,N-dicyclohexylcarbodii-mide (DCCD), caused a delay in the recovery of photosynthetic functions of the paraquat-resistant biotype, when given after the herbicide. These transport inhibitor experiments supported the involvement of a membrane transporter in paraquat resistance [75]. 

Translocation studies with two paraquat-resistant biotypes of Hordeum lepori-num revealed that the basipetal transport of paraquat in resistant H. leporinum was much reduced compared with susceptible plants. It was concluded that the resistance to paraquat was the result of the reduced herbicide translocation out of the treated leaves [76]. One can suppose that also in this species herbicide sequestration may have been the primary cause for the altered long-distance transport. 

Until the mid-1990s, multiple-resistance (i.e., resistance to more than one herbicide mode of action within the same biotype) had not been reported within North America. However, Foes et al. (1996) found a kochia biotype from western Illinois resistant to atrazine and several ALS-inhibiting herbicides. Lopez-Martinez et al. (1996) reported that a triazine-resistant Echinochloa species found in atrazine-treated com also showed cross-resistance to quinclorac. Clay and Underwood (1989) and Clay (1989) reported that one triazine-resistant biotype of American willowherb was also resistant to paraquat from a hop garden in the United Kingdom treated annually for 25 years with simazine and paraquat. 

Clay, D.V. and C. Underwood (1989). The identification of triazine- and paraquat-resistant weed biotypes and their response to other herbicides. In Cavalloro, R., and Noye, G., eds, Proc. E. C. Experts, Grays, Tollose, Denmark, November 15-17, 1988. Luxembourg Office for Official Publications of the European Communities, pp. 47-55. 

Surprises or unpredicted events can happen. The early dogma, also based largely on our experience of triazine resistance, suggested that soil residual activity of herbicides was an important, if not essential, contributor to the selection of resistant biotypes. The subsequent development of multiple instances of paraquat resistance, followed by resistance to acetyl CoA carboxylase (ACCase) inhibitors, demonstrated that soil activity was not necessary for resistance to evolve. 

While most plants are susceptible to paraquat, some paraquat-resistant horseweed (Erigeron sp. and Conyza sp.) biotypes are apparently insensitive to the herbicide due either to elevated levels of superoxide dismutase and other enzymes in a pathway detoxifying oxygen radicals or to differential sequestration of paraquat in the weed (8, 9). Data on the mechanism of most other types of herbicide resistance in weeds are still not complete. 

Resistance was not initially expected to evolve to paraquat, as this herbicide has no biological persistence it is only a contact herbicide, and is immediately bound to soil colloids. Farmer persistence made up for this resistant weed populations evolved where paraquat was applied 6-10 times annually in a variety of weeds. It was clearly shown that PSI was totally suppressed in isolated thylakoids [cf. 11] in paraquat resistant Conyza, yet extreme membrane damage appeared only in the paraquat treated, intact chloroplasts of the sensitive biotype [37]. Photosynthesis in intact leaves was immediately inhibited in both biotypes, but the resistant biotype recovered in a few hours [37]. 

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