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.

Summary Maize is one of the world’s most widely cultivated crops. As future demands for maize will continue to rise, fields will face ever more frequent and extreme weather patterns that directly affect crop productivity. Development of environmentally resilient crops with improved standability in the field, like wheat and rice, was enabled by shifting the architecture of plants to a short stature ideotype. However, such architectural change has not been implemented in maize due to the unique interactions between gibberellin (GA) and floral morphology which limited the use of the same type of mutations as in rice and wheat. Here, we report the development of a short stature maize ideotype in commercial hybrid germplasm, which was generated by targeted suppression of the biosynthetic pathway for GA. To accomplish this, we utilized a dominant, miRNA‐based construct expressed in a hemizygous state to selectively reduce expression of the ZmGA20ox3 and ZmGA20ox5 genes that control GA biosynthesis primarily in vegetative tissues. Suppression of both genes resulted in the reduction of GA levels leading to inhibition of cell elongation in internodal tissues, which reduced plant height. Expression of the miRNA did not alter GA levels in reproductive tissues, and thus, the reproductive potential of the plants remained unchanged. As a result, we developed a dominant, short‐stature maize ideotype that is conducive for the commercial production of hybrid maize. We expect that the new maize ideotype would enable more efficient and more sustainable maize farming for a growing world population.

BACKGROUND Lack of fitness costs has been reported for multiple herbicide resistance traits, but the underlying evolutionary mechanisms are not well understood. Compensatory evolution that ameliorates resistance costs, has been documented in bacteria and insects but rarely studied in weeds. Dicamba resistant IAA16 (G73N) mutated kochia was previously found to have high fecundity in the absence of competition, regardless of significant vegetative growth defects. To understand if costs of dicamba resistance can be compensated through traits promoting reproductive success in kochia, we thoroughly characterized the reproductive growth and development of different G73N kochia biotypes. Flowering phenology, seed production and reproductive allocation were quantified through greenhouse studies, floral (stigma‐anthers distance) and seed morphology, as well as resulting mating and seed dispersal systems were studied through time‐course microcopy images. RESULTS G73N covaried with multiple phenological, morphological and ecological traits that improve reproductive fitness: (i) 16–60% higher reproductive allocation; (ii) longer reproduction phase through early flowering (2–7 days); (iii) smaller stigma‐anthers separation (up to 60% reduction of herkogamy and dichogamy) that can potentially promote selfing and reproductive assurance; (iv) ‘winged’ seeds with 30–70% longer sepals that facilitate long‐distance seed dispersal. CONCLUSION The current study demonstrates that costs of herbicide resistance can be ameliorated through coevolution of other fitness penalty alleviating traits. As illustrated in a hypothetical model, the evolution of herbicide resistance is an ongoing fitness maximization process, which poses challenges to contain the spread of resistance. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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