Diversified resistance mechanisms in multi-resistant Lolium spp. in three European countries

Annual ryegrass species (Lolium spp.) infest cereal crops worldwide. Ryegrasspopulations with multiple resistance to the acetyl coenzyme A carboxylase (ACCase)and acetolactate synthase (ALS) inhibitors are an increasing problem in severalEuropean countries. We investigated the resistance pattern and level of resistance inryegrass populations collected in Denmark, Greece and Italy and studied the diversityof mechanisms endowing resistance, both target-site and metabolism based. Allpopulations showed high resistance indexes (RI) to the ALS inhibitors, iodosufuronmethyl-sodium + mesosulfuron-methyl (RI from 8 to 70), whereas the responses tothe two ACCase inhibitors, clodinafop-propargyl and pinoxaden, differed. The Greekand Italian populations were moderately to highly resistant to clodinafop (RI > 8) andshowed low to moderate resistance to pinoxaden (RI ranged from 3 to 13) exceptfor one Italian population. In contrast, the Danish Lolium populations showed low tomoderate resistance to clodinafop (RI ranged from 2 to 7) and only one populationwas resistant to pinoxaden. Different mutant ACCase alleles (Leu1781, Cys2027, Asn2041,Val2041, Gly2078, Arg2088, Ala2096) and ALS alleles (Gly122, Ala197, Gln197, Leu197,Ser197, Thr197, Val205, Asn376, Glu376, Leu574) endowing resistance were detected inthe Greek and Italian populations. In several plants, no mutated ALS and ACCasealleles were found showing a great heterogeneity within and among the Greek andItalian populations. Conversely, no mutant ACCase alleles were identified in the fourDanish populations and only one mutant ALS allele (Leu574) was detected in two Danishpopulations. The expression level of nitronate monooxygenase (NMO), glutathioneS-transferase (GST) and cytochrome P450s (CYP72A1 and CYP72A2) varied broadlyamong populations and individual plants within the populations. Constitutive upregulation of GST, CYP72A1 and CYP72A2 was detected in resistant plants respectto susceptible plants in one Danish and one Italian population. It appears that the mechanisms underlying resistance are rather complex and diversified among Loliumspp. populations from the three countries, coevolution of both target-site resistanceand metabolic based herbicide resistance appears to be a common feature in Denmarkand Italy. This must be considered and carefully evaluated in adopting resistancemanagement strategies to control Lolium spp. in cereal crops