Date Published: April 15, 2019
Publisher: Public Library of Science
Author(s): Olivia A. Obenland, Rong Ma, Sarah R. O’Brien, Anatoli V. Lygin, Dean E. Riechers, Debalin Sarangi.
To date, the only known mechanism conferring protoporphyrinogen IX oxidase (PPO)-inhibitor resistance in waterhemp (Amaranthus tuberculatus) is a glycine deletion in PPO2 (ΔG210), which results in cross-resistance to foliar PPO-inhibiting herbicides. However, a metabolism-based, HPPD-inhibitor resistant waterhemp population from Illinois (named SIR) was suspected of having a non-target site resistance (NTSR) mechanism due to its resistance to carfentrazone-ethyl (CE) but sensitivity to diphenylethers (DPEs). In greenhouse experiments, SIR sustained less injury than two PPO inhibitor-sensitive populations (WCS and SEN) after applying a field-use rate of CE, and after initial rapid necrosis, regrowth of SIR plants was comparable to a known PPO inhibitor-resistant population (ACR) possessing the ΔG210 mutation. Dose-response analysis determined 50% growth reduction rates in CE-resistant (SIR and ACR) and sensitive (SEN) waterhemp populations, which showed SIR was 30-fold resistant compared to SEN and two-fold more resistant than ACR. Deduced amino acid sequences derived from SIR PPX2 partial cDNAs did not contain the ΔG210 mutation found in ACR or other target-site mutations that confer PPO-inhibitor resistance previously reported in Palmer amaranth (Amaranthus palmeri). Although several SIR cDNAs contained amino acid substitutions, none were uniform among samples. Additionally, SIR plants treated with malathion and CE showed a significant reduction in biomass accumulation compared to CE alone. These results indicate robust CE resistance in SIR is not mediated by amino acid changes in the PPO2 protein, but instead resistance may be conferred through a NTSR mechanism such as enhanced herbicide metabolism.
Protoporphyrinogen IX oxidase (PPO) is an essential enzyme in the biosynthesis of chlorophyll and heme in plants . When PPO is inhibited by herbicides, protoporphyrinogen IX leaks into the cytoplasm, which oxidizes to form highly photodynamic protoporphyrin IX . In the cytosol, protoporphyrin IX generates singlet oxygen, a reactive oxygen species that causes rapid lipid peroxidation . Affected plants display symptoms including wilting, chlorosis, bleaching, desiccation, and necrosis within hours after treatment, and death can occur in a few days .
These experiments demonstrated a novel resistance mechanism that affects CE but not other PPO-inhibiting herbicides examined so far in the SIR population. The precise mechanism has yet to be determined, but it is not due to known PPO2 mutations in Amaranthus [6–9]. Target-site mediated resistance to CE remains possible, such as PPX2 copy number variation, increased PPX2 expression, or a mutation in the PPX1 gene . The results of the CE treatment in combination with malathion suggest that SIR possesses enhanced metabolism of CE via P450s. An NTSR mechanism such as this could potentially affect CE but not DPEs or saflufenacil in SIR. In support of this theory, metabolic resistance to atrazine in the MCR population  did not confer cross-resistance to metribuzin , an asymmetrical triazine.