Genetic Resistance to Acetyl‐Coenzyme A Carboxylase‐Inhibiting Herbicides in Grain Sorghum

ABSTRACT Weed management is one of the most important considerations impacting sorghum [ Sorghum bicolor (L.) Moench] production in the United States today. The objectives of this study were to evaluate the level of resistance, type of inheritance, and causal mutation of wild sorghum accessions that are resistant to acetyl‐coenzyme A carboxylase (ACCase)‐inhibiting herbicides compared to susceptible grain sorghum seed parents. Acetyl‐coenzyme A carboxylase‐inhibiting herbicides used in this study were fluazifop‐P and quizalofop‐P from the aryloxyphenoxypropionate (APP) family and clethodim and sethoxydim from the cyclohexanedione (CHD) family. Dose–response studies indicated the level of resistance was very high for APP herbicides but low to nonexistent for CHD herbicides. Estimates of the resistance factors to APP herbicides based on herbicide rate required for 50% growth reduction (GR 50 ) were 54 to 64 for homozygous plants and 9 to 19 for heterozygous plants. Resistance to CHD herbicides was very low with resistance factors ranging from one to about five. Genetic segregation studies indicated a single gene is the cause of resistance to APP herbicides. Polymerase chain reaction (PCR) amplicons from the carboxyl transferase (CT) domain of the chloroplast directed ACCase gene were sequenced and a single point mutation that results in a cysteine replacing tryptophan (conversion from tryptophan at amino acid position 2027 to cysteine [Trp‐2027‐Cys]) was discovered. Given the high level of herbicide resistance and simple inheritance, these sources of APP resistance may provide useful herbicide resistance traits for use in sorghum.