Influence of Application Placement, Resistance Genotype, and PPO-Inhibiting Herbicide on the PPO-Resistance Phenotype in Waterhemp
The PPO inhibitors are a valuable group of herbicides that provide soil-residual and foliar control of glyphosate-resistant Amaranthus species. The ΔG210 mutation in the PPX2 gene confers PPO-inhibitor resistance and has been present in the Midwest for more than a decade. Until recently, PPO-inhibitor resistance in waterhemp was attributable to just the ΔG210 mutation in the PPX2 gene, but recently, several new PPO-resistant biotypes have been discovered in waterhemp and Palmer amaranth. A possible explanation is a change in PPO-inhibitor use patterns and commonly used active ingredients. Research was conducted to directly compare the ΔG210 mutation with the recently discovered R128G mutation to PPO inhibitors applied PRE. The greatest resistance observed was to fomesafen with a 20- to 37-fold resistance ratio. Other herbicides tested had resistance ratios less than 9 for both mutations. Sulfentrazone was the only herbicide for which the R128G mutation conferred greater resistance than did the ΔG210 mutation with a 0.48 ΔG210 to R128G ED50 ratio (4.2 vs 8.8 R/S ratios). Overall, the data do not support our hypothesis that the R128G mutation was selected for by soil-applied PPO inhibitors. We conclude that the R128G mutation in waterhemp is not more robust than the ΔG210 mutation with respect to conferring resistance to PPO inhibitors applied preemergence. Furthermore, there is no evidence that the utility of PPO inhibitors applied preemergence will diminish any further as a result of the R128G mutation increasing in frequency. A set of field trials was conducted to investigate how a new PPO inhibitor, trifludimoxazin, will select for resistant biotypes in the field. Plants that emerged through or survived a PPO-inhibitor application were genotyped for the ΔG210 mutation. Overall, a greater number of resistant plants survived the foliar herbicide applications than emerged through soil applications. Trifludimoxazin did not increase the frequency of PPO-resistant individuals when applied to soil, but when applied to foliage, increased the frequency of PPO-resistant individuals by 2.5- to 2.6- fold, similar to other PPO inhibitors applied to foliage. Several waterhemp populations have been identified that have PPO-inhibitor resistance not completely explained by known target-site mutations. Malathion and NBD-Cl, known inhibitors of cytochrome P450 and GST, were coapplied with fomesafen to partially reverse resistance to fomesafen. While there was some support for the initial hypothesis, that fomesafen detoxification is contributing to overall resistance, the lack of consistency between herbicide rates and the noticeable phytotoxic response to malathion and NBD-Cl renders it impossible to rule out additive effects from the multiple applied xenobiotics. We conclude that our methodology was insufficient to address our hypothesis. Another set of waterhemp populations had a resistance phenotype in the absence of target site mutations. After a purifying screen and confirmed absence of target site resistance mutations, the progeny generation of four waterhemp populations were subjected to dose-response, target site expression, and lipid-peroxidation assays. Models for lethal dose (LD) indicate a less robust R phenotype than ΔG210, with LD50 ratios of only 1.9- to 6.1- vs 19- to 27-fold resistance in the ΔG210 populations. A target site expression experiment and lipid peroxidation experiment were inconclusive, but provided some evidence of increased target-site expression or increased antioxidant capacity as causal mechanisms, although no mechanisms have been fully ruled out. These data add to the body of literature that suggests that many factors contribute to PPO-inhibitor resistance. With greater utilization of PPO-inhibitors in the future, the complexity of resistance will likely increase also.
Johnson, Purdue University.
Genetics|Plant sciences|Agricultural chemistry
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