Functional PPO2 mutations: co-occurrence in one plant or the same ppo2 allele of herbicide-resistant Amaranthus palmeri in the US mid-south

Pest Manag Sci. 2021 Feb;77(2):1001-1012. doi: 10.1002/ps.6111. Epub 2020 Oct 28.


Background: Protoporphyrinogen IX oxidase 2 (PPO2) inhibitors are important for the management of glyphosate- and acetolactate synthase-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.]. The evolving resistance to PPO inhibitors is of great concern. We surveyed the evolution of resistance to fomesafen in the US Mid-south and determined its correlation with the known functional PPO2 target-site mutations (TSM).

Results: The 167 accessions analyzed were grouped into five categories, four resistant (147) and one susceptible (20). Arkansas accessions constituted 100% of the susceptible group while the Missouri accessions comprised 60% of the most resistant category. The majority of Mississippi accessions (88%) clustered in the high-survival-high-injury category, manifesting an early-stage resistance evolution. One hundred and fifteen accessions were genotyped for four known TSMs; 74% of accessions carried at least one TSM. The most common single TSM was ΔG210 (18% of accessions) and the predominant double mutation was ΔG210 + G399A (17%). Other mutations are likely less favorable, hence are rare. All TSMs were detected in three accessions. Further examination revealed that 9 and two individuals carried G399A + G210 and G399A + R128G TSM in the same allele, respectively. The existence of these combinations is supported by molecular modeling.

Conclusions: Resistance to PPO inhibitors is widespread across the Mid-southern USA. Highly resistant field populations have plants with multiple mutations. G399A is the most prone to co-occur with other ppo2 mutations in the same allele. Mutation at R128 in the configuration of the PPO2 catalytic domain restrains the co-occurrence of R128G with ΔG210, making ΔG210 + G399A the most plausible, tolerable functional mutation combination to co-occur in the same ppo2 allele.

Keywords: PPO inhibitor resistance; genotyping; herbicide resistance; molecular assay; mutation accumulation; nontarget-site resistance; target-site resistance.

MeSH terms

  • Alleles
  • Amaranthus* / genetics
  • Arkansas
  • Herbicide Resistance / genetics
  • Herbicides* / pharmacology
  • Humans
  • Mississippi
  • Missouri
  • Mutation
  • Protoporphyrinogen Oxidase / genetics


  • Herbicides
  • Protoporphyrinogen Oxidase