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Title: ALS-resistant Smallflower umbrella (Cyperus difformis) in Arkansas rice: physiological and molecular basis of resistance mechanism

Author
item TEHRANCHIAN, PARSA - University Of Arkansas
item RIAR, DILPREET - University Of Arkansas
item NORSWORTHY, JASON - University Of Arkansas
item Nandula, Vijay
item MCELROY, SCOTT - Auburn University
item CHEN, SHU - Auburn University
item SCOTT, ROBERT - University Of Arkansas

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2015
Publication Date: 9/1/2015
Citation: Tehranchian, P., Riar, D.S., Norsworthy, J.K., Nandula, V.K., Mcelroy, S., Chen, S., Scott, R.C. 2015. ALS-resistant Smallflower umbrella (Cyperus difformis) in Arkansas rice: physiological and molecular basis of resistance mechanism. Weed Science. 63(3):561-568.

Interpretive Summary: Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern U.S. It recently has evolved resistance to the acetolactate synthase (ALS)–inhibiting herbicide halosulfuron in Arkansas rice. Scientists from the University of Arkansas, Crop Production Systems Research Unit (USDA-ARS), and Auburn University conducted laboratory and greenhouse studies to understand extent and why smallflower umbrella sedge populations are resistant to one or more ALS-inhibiting herbicides. Whole plant bioassay revealed that halosulfuron-resistant plants were not controlled by bispyribac-sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD50) was >12 fold compared to a susceptible biotype. Both biotypes were controlled >96% with bentazon and propanil and <23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target-site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing using the Illumina HiSeq detected an amino acid substitution of Pro197-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species. The above findings will provide valuable knowledge towards devising alternative smallflower umbrella sedge control strategies for rice growers of the Midsouth.

Technical Abstract: Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern U.S. It recently has evolved resistance to the acetolactate synthase (ALS)–inhibiting herbicide halosulfuron in Arkansas rice. Studies were conducted to (1) determine if the resistant biotype is cross resistant to other ALS-inhibiting herbicides, (2) evaluate alternative herbicide control options, and (3) determine the mechanism of resistance. Whole plant bioassay revealed that halosulfuron-resistant plants were not controlled by bispyribac-sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD50) was >12 fold compared to a susceptible biotype. Both biotypes were controlled >96% with bentazon and propanil and <23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target-site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing using the Illumina HiSeq detected an amino acid substitution of Pro197-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species.