LAUSR

BACKGROUND Annual sowthistle is a weed that is difficult to control in lentil crops in southern Australia due to a lack of herbicide options, widespread herbicide resistance and prolific production of highly mobile seed. This study investigates herbicide resistance in annual sowthistle in the Mid North (MN) and Yorke Peninsula (YP) regions of South Australia, identifies and characterises the mechanisms of ALS-inhibitor resistance in this amphidiploid species, and combines this with analyses of population structure and gene flow. RESULTS ALS-inhibitor resistant annual sowthistle was widespread across the YP and MN of South Australia and associated with a variety of Proline-197 mutations of the ALS gene, including leucine, alanine, arginine, serine, threonine, and histidine. These mutations were found in different combinations on either of the two copies of the ALS gene. An additional 200 tissue samples were collected from across a single field on the YP and the ALS gene was sequenced for all these individuals. Different ALS-inhibitor resistance profiles were evident between mutation combinations and within mutation combinations, possibly mediated by assortment of the mutations to the ALS gene copies or altered gene expression. Population genetics analysis showed evidence of long-distance dispersal, resulting in highly mobile resistance genes, and multiple instances of resistance mutation evolution. CONCLUSIONS Continuing selection of S. oleraceus populations with ALS-inhibiting herbicides has resulted in the accumulation of additional mutations within the ALS gene. New practices to control herbicide-resistant S. oleraceus should be examined, and control should focus on reducing seed set and dispersal to prevent spread of emerging cases of resistance.