Summary: Pleiotropic effects associated with genes endowing resistance to herbicides are generally predicted to reduce plant fitness. Quantifying these effects is necessary to develop management strategies against herbicide‐resistant weeds. We assessed the pleiotropic effects associated with three mutant alleles of the herbicide target enzyme acetyl‐coenzyme A carboxylase (ACCase) on plant growth and seed production in black‐grass Alopecurus myosuroides. In each of two field experiments, black‐grass populations segregating for Leu‐1781 (five populations), Asn‐2041 (three populations) or Gly‐2078 (two populations) ACCase were produced to obtain several distinct, homogenized genetic backgrounds and to permit reliable comparisons among wild‐type, heterozygous and homozygous mutant ACCase plants grown in competition with a wheat crop. No significant differences from wild‐type plants in vegetative biomass, height and seed production were observed in Leu‐1781 or Asn‐2041 ACCase plants. Over both experiments, homozygous Gly‐2078 ACCase plants displayed a significant reduction in biomass (42%), height (6%) and seed production (36%). Reduction varied with the segregating population and between field experiments. Synthesis and applications. Our work illustrates the variation in fitness cost depending on the resistance gene, the plant genetic background and the environment. This underlines the necessity to identify the resistance gene(s) present in a weed population before designing resistance‐management strategies. Competitive crops should be effective against Gly‐2078 ACCase plants. The effect of resistant ACCase alleles on seed survival and germination needs to be studied in order to develop cultural practices creating or maximizing fitness costs in resistant plants. However, the variation of fitness cost with the environment and the weed population, as well as the likely absence of fitness cost associated with resistance genes such as Leu‐1781 ACCase, renders the success of specific cultural control practices uncertain. A solution could be a weed control programme maximizing the diversity of cultural practices and including anti‐resistance cultural practices to avoid or reduce selection for resistant ACCase alleles.

Selective pressures exerted by agriculture on populations of arable weeds foster the evolution of adaptive traits. Germination and emergence dynamics and herbicide resistance are key adaptive traits. Herbicide resistance alleles can have pleiotropic effects on a weed's life cycle. This study investigated the pleiotropic effects of three acetyl-coenzyme A carboxylase (ACCase) alleles endowing herbicide resistance on the seed-to-plant part of the life cycle of the grass weed Alopecurus myosuroides. In each of two series of experiments, A. myosuroides populations with homogenized genetic backgrounds and segregating for Leu1781, Asn2041 or Gly2078 ACCase mutations which arose independently were used to compare germination dynamics, survival in the soil and seedling pre-emergence growth among seeds containing wild-type, heterozygous and homozygous mutant ACCase embryos. Asn2041 ACCase caused no significant effects. Gly2078 ACCase major effects were a co-dominant acceleration in seed germination (1·25- and 1·10-fold decrease in the time to reach 50 % germination (T₅₀) for homozygous and heterozygous mutant embryos, respectively). Segregation distortion against homozygous mutant embryos or a co-dominant increase in fatal germination was observed in one series of experiments. Leu1781 ACCase major effects were a co-dominant delay in seed germination (1·41- and 1·22-fold increase in T₅₀ for homozygous and heterozygous mutant embryos, respectively) associated with a substantial co-dominant decrease in fatal germination. Under current agricultural systems, plants carrying Leu1781 or Gly2078 ACCase have a fitness advantage conferred by herbicide resistance that is enhanced or counterbalanced, respectively, by direct pleiotropic effects on the plant phenology. Pleiotropic effects associated with mutations endowing herbicide resistance undoubtedly play a significant role in the evolutionary dynamics of herbicide resistance in weed populations. Mutant ACCase alleles should also prove useful to investigate the role played by seed storage lipids in the control of seed dormancy and germination.

We investigated the molecular bases for resistance to several classes of herbicides that bind tubulins in green foxtail (Setaria viridis L. Beauv.). We identified two α- and two β-tubulin genes in green foxtail. Sequence comparison between resistant and sensitive plants revealed two mutations, a leucine-to-phenylalanine change at position 136 and a threonine-to-isoleucine change at position 239, in the gene encoding α2-tubulin. Association of mutation at position 239 with herbicide resistance was demonstrated using near-isogenic lines derived from interspecific pairings between green foxtail and foxtail millet (Setaria italica L. Beauv.), and herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Association of mutation at position 136 with herbicide resistance was demonstrated using herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Both mutations were associated with recessive cross resistance to dinitroanilines and benzoic acids, no change in sensitivity to benzamides, and hypersensitivity to carbamates. Using three-dimensional modeling, we found that the two mutations are adjacent and located into a region involved in tubulin dimer-dimer contact. Comparison of three-dimensional α-tubulin models for organisms with contrasted sensitivity to tubulin-binding herbicides enabled us to propose that residue 253 and the vicinity of the side chain of residue 251 are critical determinants for the differences in herbicide sensitivity observed between organisms, and that positions 16, 24, 136, 239, 252, and 268 are involved in modulating sensitivity to these herbicides.