Ile-1781-Leu Target Mutation and Non-Target-Site Mechanism Confer Resistance to Acetyl-CoA Carboxylase-Inhibiting Herbicides in Digitaria ciliaris var. chrysoblephara

Digitaria ciliaris var. chrysoblephara is a xerophytic weed severely invading rice fields along with the application of rice mechanical direct seeding technology in China. This study identified one resistant population (M5) with an Ile-1781-Leu substitution in ACCase1 showing broad-spectrum resistance to three chemical classes of ACCase-inhibiting herbicides, including metamifop, cyhalofop-butyl, fenoxaprop-p-ethyl, haloxyfop-p-methyl, clethodim, sethoxydim, and pinoxaden. The other two populations, M2 and M4, without any resistance-responsible mutations, only exhibited resistance to aryloxyphenoxypropionate (APP) herbicides cyhalofop-butyl and fenoxaprop-p-ethyl. Pre-treatment with the cytochrome P450 monooxygenase (P450) inhibitor PBO significantly reduced the cyhalofop-butyl resistance by 43% in the M2 population. Pre-emergence weed control with soil-applied herbicides, such as pretilachlor, pendimethalin, and oxadiazon, can effectively inhibit the germination and growth of D. ciliaris var. chrysoblephara. The present study reported a xerophytic weed species invading rice fields featuring broad-spectrum resistance to ACCase-inhibiting herbicides as a result of Ile-1781-Leu mutation of ACCase. Both target- and P450-involved non-target-site mechanisms may be contributing to resistance in D. ciliaris var. chrysoblephara species.

ACCase mutations in Avena sterilis populations and their impact on plant fitness

Avena sterilis (sterile oat) populations originating from wheat-growing regions of Greece, developed resistance to fenoxaprop, clodinafop and other herbicides. The partial ACCase gene sequence revealed six point mutations (Ile-1781-Leu, Trp-1999-Cys, Trp-2027-Cys, Ile-2041-Asn, Asp-2078-Gly, and Cys-2088-Arg) in 24 out of the 26 resistant (R) populations, confirming the molecular mechanism of resistance to ACCase-inhibiting herbicides. However, DNA sequence of two R populations did not reveal any known ACCase mutations, suggesting possible presence of unknown mutation or metabolism-based mechanism of resistance. The Cys-2088-Arg mutation is the first record for ACCase mutant conferring target-site resistance in A. sterilis worldwide. The evaluation of 12 R and 6 susceptible (S) populations under non-competitive field conditions did not indicate consistent mean growth rate differences, whereas the pot evaluation of the same (12 R and 6 S) populations grown in competition with wheat or in pure stands showed significant growth (fresh weight and panicle number) differences between six S populations and between six R populations containing the same ACCase mutation (Ile-2041-Asn). Finally, one S and five R (Trp-1999-Cys, Trp-2027-Cys, Ile-2041-Asn, Asp-2078-Gly, and Cys-2088-Arg) populations grown under field competitive conditions indicated fresh weight and panicle number differences in competition with other populations as compared with pure stands. These findings suggest clearly that the inconsistent fitness differences between R and S A. sterilis populations are not related with the ACCase resistance trait but they may result from other non-resistance fitness traits selected in their different geographical locations.