Mechanisms and control of waterhemp populations resistant to group 15 herbicides

Herbicides that inhibit very long-chain fatty acids (VLCFAs) have been applied for preemergence (PRE) control of annual grasses and small-seed broadleaf weeds for over sixty years. Waterhemp is a small-seeded, summer annual weed species native to the Midwestern United States. The species can be challenging to control due to important biological characteristics, such as reproductive output, genetic variability, seed dormancy, and an ability to evolve resistance to herbicides from multiple site-of-action groups. These and other characteristics allow waterhemp to effectively interfere with row crops and reduce seed yield potential. Waterhemp is resistant to herbicides from seven site-of-action groups, with resistance to VLCFA-inhibiting herbicides the most recently evolved. Control of two Illinois waterhemp populations (CHR and MCR) with the VLCFA inhibitor S-metolachlor was much less than expected in field research. An alternative VLCFA-inhibiting herbicide, acetochlor, remained relatively effective. Additional research was conducted to investigate VLCFA-inhibitor efficacy on each population in the field and greenhouse. Experiments identified few effective VLCFA-inhibiting herbicides for controlling CHR, and that both populations (CHR and MCR) displayed reduced sensitivity to acetochlor, dimethenamid-P, pyroxasulfone, and S-metolachlor under greenhouse conditions. The research presented herein was initiated in 2018 to identify the resistance mechanism(s) within CHR and MCR/SIR to the VLCFA-inhibitor, S-metolachlor. Chapter 1 includes a literature review describing waterhemp biology, waterhemp resistance mechanisms, resistance to soil-applied herbicides, VLCFA-inhibiting herbicides, and herbicide detoxification in plants. Chapter 2 described my initial laboratory experiments that identified rapid S-metolachlor metabolism was the resistance mechanism within CHR and SIR. CHR and SIR metabolized S-metolachlor twice as fast as sensitive waterhemp, but equally rapid as corn. Enzyme inhibitor studies suggested glutathione S-transferases (GSTs) and cytochrome P450s both might be involved in rapid S-metolachlor metabolism. Chapter 3 describes the experiments that directly investigated GSTs and microsomal P450s along with the S-metolachlor-treated metabolome of resistant waterhemp. Results identified that CHR and SIR possess approximately 2× more GST activity than sensitive waterhemp, but less than corn. Microsomal assays investigating P450 activity revealed CHR and SIR microsomes formed a single O-demethylated S-metolachlor metabolite at a significantly greater rate (>20-fold) than sensitive waterhemp or corn. Finally, mass spectrometry and β-glucosidase enzyme assays revealed GST-and P450-mediated detoxification activities of S-metolachlor with subsequent glucose-conjugates. Chapter 4 included results of three years of field research at multiple locations in Illinois. Two VLCFA-inhibitor resistant and two sensitive locations were included. While VLCFA-inhibiting herbicides provided poor control of resistant waterhemp, most remained efficacious against sensitive waterhemp, regardless of soil type