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.