Characterization of PPO-inhibitor resistance in waterhemp (Amaranthus tuberculatus) and Palmer amaranth (Amaranthus palmeri)

Waterhemp and Palmer amaranth are two of the most troublesome weed species in the Midwest. The overuse of acetolactate synthase (ALS), photosystem II, and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitors to control these species led to the evolution of resistance to these herbicides and, thus, protoporphyrinogen oxidase (PPO) inhibitors were used to help control these resistant weeds. Waterhemp was the first weed species to evolve resistance to PPO inhibitors, and Palmer amaranth evolved resistance ten years later. The first known mechanism for this resistance involves the deletion of a glycine codon corresponding to the 210th amino acid position (dG210) of the PPO enzyme (PPX2). Chapter 1 includes a literature review of the PPO enzyme, the mode of action of PPO-inhibiting herbicides, the biology of waterhemp and Palmer amaranth, and an overview of PPO-inhibitor resistance in these two species. Chapter 2 explores how resistance to PPO inhibitors evolved in a population each of waterhemp and Palmer amaranth growing in the same field. The results indicate that they did not hybridize with one another to pass on resistance, but that they each evolved resistance to PPO inhibitors independently via two different forms of convergent evolution. Chapter 3 further analyzes PPO-inhibitor-resistance evolution in these two species by characterizing the relative level of resistance to PPO inhibitors in Palmer amaranth and waterhemp conferred specifically by the dG210 mutation. The results suggested that Palmer amaranth is naturally more tolerant to PPO inhibitors than waterhemp when treated POST, giving it less of a selective advantage to evolve resistance. This could be why Palmer amaranth took long to evolve resistance to PPO inhibitors. Three different amino acid changes at a homologous site of PPX2 have conferred resistance to PPO-inhibiting herbicides in two weed species. Chapter 4 is aimed at exploring additional amino acid substitutions that could occur at this site to determine whether they can also confer resistance to PPO inhibitors. The effect of these substitutions coupled with the dG210 mutation was also tested. In vitro PPO enzyme assays were carried out to determine the level of resistance conferred by these modifications. Inhibitors used were lactofen, fomesafen, saflufenacil, and trifludimoxazin. Although some modifications rendered the enzyme inactive, most did not. The dG210 mutation and the double mutants conferred the highest levels of resistance to all of the inhibitors, and the single mutants all exhibited varying levels of resistance to the inhibitors. A transgenic E. coli cell line was used to test these enzyme modifications in a living system. It was found that all of the modifications were active when tested in vivo. The fifth and final chapter discusses concluding remarks and future research.