Abstract
Knowledge about the mechanisms of herbicide resistance provide valuable insights into evolving weed populations in response to selection pressure and should be used as a basis for designing management strategies for herbicide-resistant weeds. The selection pressure associated with reactive management against glyphosate-resistant Lolium spp. populations would have favored the herbicide resistance to ACCase- and ALS-inhibitors. This work was aimed to determine the sensitivity of 80 Argentinean Lolium spp. populations to ALS- and ACCase-inhibitor herbicides for use in wheat or barley and to study the mechanisms of resistance involved. Sensitivity to pinoxaden and iodosulfuron-mesosulfuron were positively correlated (r = 0.84), even though both affect different target sites. Inhibitors of cytochrome P450 monooxygenases (P450s) increased the sensitivity to pinoxaden and iodosulfuron-mesosulfuron in 94% of herbicide-resistant populations and target-site ACCase resistance mutations were detected only in two cases. Polymorphic variants were obtained with a pair primer designed on P450 sequences, cluster analysis discriminated around 80% of susceptible and P450-metabolic resistant plants sampled from a single population or different populations. Five markers corresponding to herbicide sensitivity were identified to be significantly associated with phenotypic variance in plants. Resistance to ALS- and ACCase-inhibitor herbicides were closely related, challenging the rotation of herbicides of both sites of action as a practice against resistance. In that sense, the use of pinoxaden and iodosulfuron-mesosulfuron would have provoked a selection on P450 genes that conduced a convergence of P450-metabolism based resistant Lolium spp. populations, which was detected by markers in a contribution to elucidate the molecular basis of this type of resistance.
Highlights
The weeds of farm land are a response to the management applied in the last years (Neve et al, 2009) because farming practices impose selective process on the weed community, conducing shifts in species composition or populations (Darmency, 2019)
At a frequency of 40% of resistant plants, a farmer or a technical adviser can detect a failure in chemical control (Burgos et al, 2013) and this threshold was used to compare the herbicide-sensitivity of Lolium spp. populations
P450 monooxygenases (P450s)-mediated herbicide metabolism seems to be the main mechanism of resistance detected among populations with a high percentage of resistant plants (Figure 1)
Summary
The weeds of farm land are a response to the management applied in the last years (Neve et al, 2009) because farming practices impose selective process on the weed community, conducing shifts in species composition or populations (Darmency, 2019). Herbicide resistance is considered an evolutionary process, where the least herbicide-sensitive plants show an advantage in an environment with herbicide use (Délye et al, 2013). These select plants can involve generalist and/or specialist herbicide adaptations associated to the mechanisms of resistance. P450s catalyze hydroxylation or dealkylation reactions related to the metabolism of non-related herbicides, such as ACCaseand ALS-inhibitors and glutathione S-transferases, that have been involved in reactions of conjugation to glutathione, directly processing the active herbicide or after the activity of other enzymes, such as P450s (Gaines et al, 2020). CYP81A genes involved in the detoxification of ACCase-inhibitors have been detected in multiple herbicide resistant E. phyllopogon (Iwakami et al, 2019). P450 genes and others linked to the metabolism of herbicides have been associated with the increased activity of herbicide detoxification in Lolium spp. (Gaines et al, 2014; Duhoux et al, 2017)
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