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Abstract

Lolium perenne ssp. multiflorum (Italian ryegrass) resistant to ACCase inhibiting herbicides has been reported in many wheat producing counties across Arkansas. Resistance is believed to be the result of point mutations creating amino acid substitutions in the CT domain of the plastidic ACCase gene. This study explores the occurrence of mutations in the ACCase gene of ryegrass populations. Plant material was collected and DNA was extracted from 10 Arkansas ryegrass populations. Six of the populations were known to be resistant to the ACCase inhibitor diclofop-methyl, while the remaining four populations were known to be susceptible to diclofop-methyl. Two highly conserved regions of the plastidic ACCase gene known to contain mutations that confer resistance to ACCase inhibiting herbicides were then amplified and sequenced. Analysis of the sequences revealed that only 41% of the resistant populations expressed a mutation known to confer resistance. Several resistant populations of ryegrass did not contain any of the known mutations in their plastidic ACCase gene. This result means that either a mutation in a different region of the CT domain affects the affinity to ACCase inhibiting herbicides or the plants harbor a different mechanism of resistance. Further, in some resistant populations, not all plants within that population possessed a mutation known to cause resistance to ACCase inhibitors. This suggests that within a population, multiple mechanisms of resistance may exist. Further research is needed to determine the mechanism of resistance in diclofop-resistant plants that do not harbor mutations in the tested ACCase herbicide-binding domains.

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