Resistance to herbicides inhibiting the biosynthesis of very‐long‐chain fatty acids

Abstract

Herbicides that act by inhibiting the biosynthesis of very-long-chain fatty acids (VLCFAs) have been used to control grass weeds in major crops throughout the world for the past 60 years. VLCFA-inhibiting herbicides are generally highly selective in crops, induce similar symptoms in susceptible grasses and can be found within the herbicide groups classified by the HRAC as K3 and N. Even after many years of continuous use, only 12 grass weed species have evolved resistance to VLCFA-inhibiting herbicides. Here, the cases of resistance that have evolved in major grass weed species belonging to the Avena, Echinochloa and Lolium genera in three different agricultural systems are reviewed. In particular we explore the possible reasons why VLCFA herbicides have been slow to select resistant weeds, outline the herbicide mode of action and discuss the resistance mechanisms that are most likely to have been selected.

Phorate can reverse P450 metabolism-based herbicide resistance inLolium rigidum

“…A large geographical survey conducted in 2010 across the Western Australian grain belt reported >95% frequency of multiple resistance to ACCase‐ and ALS‐inhibiting herbicides in L. rigidum . One response to resistance and failures of post‐emergent herbicides has been the adoption of pre‐emergent herbicides, especially prosulfocarb, pyroxasulfone and trifluralin . An increasingly common practice has been the use of herbicide mixtures of trifluralin and pyroxasulfone, trifluralin and prosulfocarb or trifluralin and triallate.…”

“…However, as we found partial synergistic interaction between phorate and pyroxasulfone, we suggest (1) that phorate was partially effective in inhibiting putative P450 enzymes mediating pyroxasulfone metabolism or (2) that pyroxasulfone resistance, evolved in MR by low‐dose recurrent pyroxasulfone selection, was probably only partially mediated by enhanced cytochrome P450 metabolic activity on pyroxasulfone. The involvement of GSTs mediating herbicide detoxification via glutathione conjugation is well documented for chloroacetamide and thiocarbamate herbicides (reviewed elsewhere). One study reported evidence of high levels of GST‐mediated pyroxasulfone metabolism in wheat versus low levels in L. rigidum plants to explain the herbicide selectivity in crops versus efficacy on weeds respectively …”

“…For example, cotton plants can be protected by the application of the organophosphate insecticides phorate and disulfoton, which inhibit the P450‐mediated bioactivation via oxidation of the herbicide clomazone and thus prevent the subsequent production of phytotoxic clomazone metabolites . Similar mechanisms of bioactivation are known for thiocarbamate pre‐emergent herbicides (reviewed by Busi). When organophosphate insecticides interact with P450 enzymes that confer herbicide resistance in weeds, the resulting increased toxicity of the mixture of herbicide and insecticide could potentially be exploited to overcome metabolic herbicide resistance.…”

“…Thus far, studies have established the non‐target‐site enhanced herbicide metabolism basis of diclofop‐methyl and chlorsulfuron resistance in this MR L. rigidum population, but there is limited evidence of the mechanistic basis of resistance to pre‐emergent herbicides such as prosulfocarb, pyroxasulfone, S ‐metolachlor, trifluralin and triallate. These studies report that the broad‐spectrum cross‐resistance profile observed in the MR population is likely non‐target‐site‐based enhanced metabolism . As organophosphate insecticides can mediate the inhibition of P450 enzymes involved in metabolic resistance to specific herbicides, in this study we have investigated the effects of the organophosphate insecticide phorate with five different herbicides belonging to four different chemical classes and with three different modes of action.…”

Why was resistance to shorter‐acting pre‐emergence herbicides slower to evolve?

“…However, since the first report of an Australian weed population resistant to diclofop, ACCase post‐emergence herbicide resistance has increased widely across the Australian grainbelt. In contrast, the evolution of resistance to pre‐emergence herbicides has been slower, and some pre‐emergence herbicides have remained largely effective, despite, in some cases, decades of frequent use across extensive areas …”