A dicamba resistance‐endowing IAA16 mutation leads to significant vegetative growth defects and impaired competitiveness in kochia (Bassia scoparia)

BACKGROUND Precise quantification of the fitness cost of synthetic auxin resistance has been impeded by lack of knowledge about the genetic basis of resistance in weeds. Recent elucidation of a resistance‐endowing IAA16 mutation (G73N) in the key weed species kochia (Bassia scoparia), allows detailed characterization of the contribution of resistance alleles to weed fitness, both in the presence and absence of herbicides. Different G73N genotypes from a segregating resistant parental line (9425) were characterized for cross‐resistance to dicamba, 2,4‐d and fluroxypyr, and changes on stem/leaf morphology and plant architecture. Plant competitiveness and dominance of the fitness effects was quantified through measuring biomass and seed production of three F2 lines in two runs of glasshouse replacement series studies. RESULTS G73N confers robust resistance to dicamba but only moderate to weak resistance to 2,4‐D and fluroxypyr. G73N mutant plants displayed significant vegetative growth defects: (i) they were 30–50% shorter, with a more tumbling style plant architecture, and (ii) they had thicker and more ovate (versus lanceolate and linear) leaf blades with lower photosynthesis efficiency, and 40–60% smaller stems with less‐developed vascular bundle systems. F2 mutant plants had impaired plant competitiveness, which can lead to 80‐90% less biomass and seed production in the replacement series study. The pleiotropic effects of G73N were mostly semidominant (0.5) and fluctuated with the environments and traits measured. CONCLUSION G73N is associated with significant vegetative growth defects and reduced competitiveness in synthetic auxin‐resistant kochia. Management practices should target resistant kochia's high vulnerability to competition in order to effectively contain the spread of resistance. An IAA16 mutation (G73N) confers robust resistance to dicamba but moderate to low resistance to 2,4‐D and fluroxypyr in key weed species kochia. The mutation also leads to significant leaf and stem morphological changes and diminished plant size, resulting in plants that are highly vulnerable to competition. Vegetative growth defects of G73N mutant plants suggest that weed management practices which subject resistant plants to substantial competition (e.g. cover crops) will potetially be effective. © 2020 Society of Chemical Industry.