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RESEARCH PRODUCT

Mutant ACCase alleles endowing herbicide resistance have a direct effect on seed germination

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Germination and emergence dynamics and herbicide resistance are adaptive traits crucial for weed persistence in arable fields. Herbicide resistance alleles can have pleiotropic effects on other traits. We investigated the pleiotropic effects of acetyl-coenzyme A carboxylase (ACCase) alleles L1781, N2041 or G2078 on seed germination and seedling emergence in the grass weed Alopecurus myosuroides (black-grass). We used black-grass populations with homogenised genetic backgrounds that segregated for L1781, N2041 or G2078 ACCase alleles. In two series of experiments, germination dynamics and seedling growth were compared among seeds containing embryos carrying no, one or two copies of a given mutant ACCase allele. No significant effects were observed for N2041 ACCase. A codominant acceleration in seed germination (1.2 and 1.1-fold decrease in the time to reach 50% germination [t50] for homozygous and heterozygous mutant embryos, respectively) was observed for G2078 ACCase. This effect was associated with a codominant increase in fatal germination in one series of experiments and to a segregation distortion against homozygous mutant embryos in the other. A codominant delay in seed germination (1.4 and 1.2-fold increase in t50 for homozygous and heterozygous mutant embryos, respectively) associated with a substantial codominant decrease in fatal germination was observed for L1781 ACCase. Under current agricultural systems, the fitness advantage conferred by herbicide resistance is enhanced in plants carrying L1781 ACCase, and counterbalanced in plants carrying G2078 ACCase, by direct pleiotropic effects of the mutant ACCase alleles on plant phenology. Superficial soil cultivation promoting fatal germination and delayed crop sowing to eradicate early-germinating seedlings should be effective against G2078 ACCase. Maximising crop competition prior to seedling emergence should be effective against late-germinating L1781 seedlings. N2041 ACCase appears trickier to manage. Whatever the allele, a diversification of weed control practices is the best way to manage resistance and reduce the risk for resistance selection.