Journal
WEED SCIENCE
Volume 66, Issue 6, Pages 702-709Publisher
CAMBRIDGE UNIV PRESS
DOI: 10.1017/wsc.2018.60
Keywords
Cytochrome P450; fomesafen; glutathione S-transferase; GST; PPX1; PPX2; protoporphyrinogen oxidase
Categories
Funding
- Arkansas Soybean Promotion Board
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Palmer amaranth (Amaranthus palmeri S. Watson), a dioecious summer annual species, is one of the most troublesome weeds in U.S. cropping systems. The evolution of resistance to protoporphyrinogen oxidase inhibitors in A. palmeri biotypes is a major cause of concern to soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) growers in the midsouthern United States. The objective of this study was to confirm and characterize the non-target site mechanism in a fomesafen-resistant accession from Randolph County, AR (RCA). A dose-response assay was conducted to assess the level of fomesafen resistance, and based on the GR(50) values, the RCA accession was 18-fold more resistant to fomesafen than a susceptible (S) biotype. A TaqMan allelic discrimination assay and sequencing of the target-site genes PPX2 and PPX1 revealed no known or novel target-site mutations. An SYBR Green assay indicated no difference in PPX2 gene expression between the RCA and S biotypes. To test whether fomesafen resistance is metabolic in nature, the RCA and the S biotypes were treated with different cytochrome P450 (amitrole, piperonyl butoxide [PBO], malathion) and glutathione S-transferase (GST) (4-chloro-7-nitrobenzofurazan [NBD-Cl]) inhibitors, either alone or in combination with fomesafen. Malathion followed by (fb) fomesafen in RCA showed the greatest reduction in survival (67%) and biomass (86%) compared with fomesafen alone (45% and 66%, respectively) at 2 wk after treatment. Interestingly, NBD-Cl fb fomesafen also resulted in low survival (35%) compared with the fomesafen-only treatment (55%). Applications of malathion or NBD-Cl preceding fomesafen treatment resulted in reversal of fomesafen resistance, indicating the existence of cytochrome P450- and GST-based non-target site mechanisms in the RCA accession. This study confirms the first case of non-target site resistance to fomesafen in A. palmeri.
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