Point mutations including a novel Pro-197-Phe mutation confer cross resistance to acetolactate synthase (ALS) inhibiting herbicides in Lactuca serriola in Australia.

Abstract

Control of prickly lettuce has become increasingly difficult for lentil growers in southern Australia due to widespread resistance to common herbicides, a lack of alternative herbicide options, and prolific production of highly mobile seed. This study aimed to quantify ALS-inhibiting herbicide resistance in the Mid North (MN) and Yorke Peninsula (YP) of South Australia, characterise the resistance mutations present, and investigate population structure and gene flow in this species. Resistance was identified in all populations tested, with average survival of 92% to chlorsulfuron and 95% to imazamox + imazapyr. Five different amino acid substitutions were identified at Proline-197 of the ALS gene. There was no significant difference in the LD50 between plants with these five different substitutions when treated with metsulfuron-methyl, however, imidazolinone resistance level was higher in plants with a phenylalanine substitution and lower in plants with a serine. Population structure based on 701 SNPs and 271 individuals provided evidence for both the independent evolution of the same mutation in different populations, as well as frequent short to medium distance dispersal accompanied by occasional long-distance dispersal events. The overall inbreeding coefficient (FIS ) was calculated at 0.5174, indicating an intermediate level of outcrossing despite the cross-pollination experiment showing only low outcrossing. In the structure analyses, most individuals from YP were assigned to a single cluster, whereas most individuals from MN were assigned 50% to each of two clusters, indicating some genetic differences between these two regions, but also evidence for dispersal between them. Use of imidazolinone herbicides has selected for mutations conferring higher levels of resistance, such as the Pro-197-Phe mutation, and resulted in further spread of resistance in this species. This article is protected by copyright. All rights reserved.