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

Precise quantification of the fitness cost of synthetic auxins resistance has been impeded by lack of knowledge for the genetic basis of resistance in weeds. Recent elucidation of a resistance endowing IAA16 mutation (G73N) in a 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. 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: 1) being 30-50% shorter with a more tumbling style plant architecture; 2) 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 produced up to 90% less biomass and seeds in the replacement series study. The pleiotropic effects of G73N was mostly semi-dominant (0.5) and fluctuated with the environments and traits measured. 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 to effectively contain the spread of resistance.