Mechanisms of Resistance to Pyroxsulam in Italian Ryegrass (Lolium Multiflorum Lam.) from China

Italian ryegrass ( Lolium multiflorum Lam.), a heterogeneously pollinated weed, is gradually becoming a predominant grass in Chinese wheat fields and has evolved resistance to many classes of herbicides. In this study, a pyroxsulam-resistant (PR) L. multiflorum population (DH-HZYC-2020-1), collected from Henan Province, is also resistant to ALS and other mode of actions (MoA) herbicides. Based on the resistance index (RI), the resistant population (R) was 116.01-, 34.61-, 12.00-, 12.16-, 13.02-, 42.22- and 230.20-fold resistant to Pyroxsulam (TP), Penoxsulam (TP), Bispyribac-sodium (PTB), Imazethapyr (IMI), Imazamox (IMI), Mesosulfuron-methyl (SU) and Flucarbazone-sodium (SCT), respectively, compared with the sensitive population DH-JNXW-2020-2 (S). Furthermore, the R population also presented multiple resistance to ACCase and PSII inhibiting herbicides. ALS gene sequencing revealed multiple substitutions in 197 position (Pro-197-Thr and Pro-197-Gln) and a Trp-574-Leu substitution in the R plants. Based on the ALS gene sequence analysis, molecular markers were also developed to identify the specific Pro-197-Thr, Pro-197-Gln and Trp-574-Leu mutations. Pretreatment with the CYP450 inhibitors malathion and piperonyl butoxide (PBO) reduced the GR 50 values of pyroxsulam by 30% and 60% in the R population, respectively. This indicated that CYP450-mediated enhanced herbicide metabolism is one likely mechanism for pyroxsulam resistance in L. multiflorum . The PR population of L. multiflorum may possess diverse resistance mechanisms, including target-site and non-target-site resistance.