A novel amino acid substitution Ala‐122‐Tyr in ALS confers high‐level and broad resistance across ALS‐inhibiting herbicides

BACKGROUND: Wild radish, a problem weed worldwide, is a severe dicotyledonous weed in crops. In Australia, sustained reliance on ALS‐inhibiting herbicides to control this species has led to the evolution of many resistant populations endowed by any of several ALS mutations. The molecular basis of ALS‐inhibiting herbicide resistance in a novel resistant population was studied. RESULTS: ALS gene sequencing revealed a previously unreported substitution of Tyr for Ala at amino acid position 122 in resistant individuals of a wild radish population (WARR30). A purified subpopulation individually homozygous for the Ala‐122‐Tyr mutation was generated and characterised in terms of its response to the different chemical classes of ALS‐inhibiting herbicides. Whole‐plant dose‐response studies showed that the purified subpopulation was highly resistant to chlorsulfuron, metosulam and imazamox, with LD 50 or GR 50 R/S ratio of > 1024, > 512 and > 137 respectively. The resistance to imazypyr was found to be relatively moderate (but still substantial), with LD 50 and GR 50 R/S ratios of > 16 and > 7.8 respectively. In vitro ALS activity assays showed that Ala‐122‐Tyr ALS was highly resistant to all tested ALS‐inhibiting herbicides. CONCLUSION: The molecular basis of ALS‐inhibiting herbicide resistance in wild radish population WARR30 was identified to be due to an Ala‐122‐Tyr mutation in the ALS gene. This is the first report of an amino acid substitution at Ala‐122 in the plant ALS that confers high‐level and broad‐spectrum resistance to ALS‐inhibiting herbicides, a remarkable contrast to the known mutation Ala‐122‐Thr endowing resistance to imidazolinone herbicide. Copyright © 2012 Society of Chemical Industry