A population of common ragweed from Delaware was not controlled in the field by herbicides that inhibit acetolactate synthase (ALS) or protoporphyrinogen oxidase (PPO). Research was conducted to ascertain if this population was resistant to these herbicidal modes of action and, if so, to determine the resistance mechanism(s). Resistance was confirmed by dose-response studies on greenhouse-grown plants with multiple ALS- and PPO-inhibiting herbicides. DNA sequence data revealed that resistance to ALS-inhibiting herbicides was due to the previously reported W574L ALS mutation. To assist in determining the mechanism of resistance to PPO-inhibiting herbicides, an F₂ population was derived from a cross between the resistant biotype (Del-R) and a sensitive biotype (DV1-S). This population segregated in the ratio of three resistant ∶ one sensitive when treated with fomesafen, indicating that resistance to PPO-inhibiting herbicides was conferred by a single, (incompletely) dominant, nuclear gene. Sequences of the target-site genes, PPX1 and PPX2, for PPO-inhibiting herbicides were obtained through the screening of a common ragweed cDNA library and subsequent cDNA extension (5′-RACE). Molecular marker analysis with the F₂ population revealed that the PPX2 gene cosegregated with resistance to PPO-inhibiting herbicides. A mutation substituting an arginine codon for a leucine codon at a conserved location (R98L) of the PPX2 gene was suspected of being responsible for resistance. By using a transgenic Escherichia coli system, it was demonstrated that the R98L mutation was sufficient to confer resistance to PPO-inhibiting herbicides. The level of resistance to acifluorfen conferred by the R98L mutation in the E. coli system was about 31-fold, similar to the level of resistance seen in the whole-plant dose-response study. Last, a DNA-based assay was developed to identify the presence or absence of the common ragweed PPX2 R98L mutation. The R98L PPX2 mutation is the second mechanism identified for evolved resistance to PPO-inhibiting herbicides.

Nomenclature: Common ragweed, Ambrosia artemisiifolia L. AMBEL