Evolution and diversity of resistance mechanisms to ALS inhibiting herbicides in Papaver rhoeas.

Herbicides have become a major threat to global agriculture. Due to the intensive reliance on chemical control, especially on those herbicides with a specific site of action, resistance in weeds has evolved, jeopardizing crop sustainability. In the vast majority of reported cases, resistance to ALS inhibitors in broadleaves is due to the selection of mutant, herbicide-resistant ALS alleles carrying a mutation at one of a few ALS codons. Instead, herbicide resistance not due to mutations at the herbicide target site (non-target site resistance or NTSR) is the most widespread and major cause of resistance in grass weed species. It includes all mechanisms that cause a compensation for herbicide action and/or a reduction in the amount of herbicide molecules reaching their target protein. Up to now, few data concerning the occurrence of NTSR or the factors influencing its evolution are available for dicotyledonous weed species. This study investigates the presence and inheritance of NTSR to ALS inhibitors in the broadleaved weed Papaver rhoeas (corn poppy) through genetic and molecular approaches. P. rhoeas is a troublesome weed infesting winter cereals in Mediterranean Europe that has evolved resistance to ALS, mainly graminicide sulfonylureas used over years. The study will aid in understanding and predicting the selection and spread of P. rhoeas plants carrying NTSR in agricultural ecosystems. Results demonstrate that resistance to ALS inhibitors in the broadleaf P. rhoeas can be due not only to ALS-based resistance, but also to NTSR mechanisms. NTSR is genetically inherited and the segregation patterns are consistent with an overall polygenic control. Furthermore, mutant ALS alleles can also be present in plants containing NTSR loci, thereby "masking" the presence of NTSR and likely causing underestimation of its significance in P. rhoeas. It thus emerges that NTSR could have a more important role in broadleaved species, as in grass weeds, and all data indicate that the evolution of resistance to ALS inhibitors in P. rhoeas is rather complex. The implementation of other genomic approaches such as next-generation sequencing (genomics, transcriptomics) could be a way forward to identify and characterize the genes involved in NTSR.