Mechanism of Resistance in Common Ragweed to PPO-inhibiting Herbicides

Synthetic herbicides have been reliable tools to help growers produce a healthy and profitable crop for several decades but their efficacy is under attack from the evolution of herbicide-resistant weeds. Almost all commercially available herbicidal modes of action have documented cases of herbicide-resistant weeds. One particular herbicidal class is known as the protoporphyrinogen oxidase (PPO, Protox) inhibiting herbicides. Only four weed species have been documented with resistance to this herbicide class. Waterhemp was the first weed species documented with PPO-resistance, which was due to a single codon deletion in a dual-targeting gene known as PPX2L. Common ragweed, from Delaware, was the third documented weed species that evolved PPO-inhibiting herbicide resistance. This thesis provides information on the resistance mechanism of this common ragweed biotype to PPO-inhibiting herbicides. Chi-square goodness-of-fit analysis on a segregating F2 population of common ragweed sprayed with a PPO-inhibitor, fomesafen, revealed that data were consistent with resistance conferred by a single, dominant gene. Sequences of the target-site genes were obtained through the screening of a sensitive common ragweed cDNA library and 5???-RACE. Analysis of sequence from the target-site genes revealed multiple polymorphisms between the parental alleles of an F1 plant that could be responsible for resistance. Molecular marker analysis of the F2 common ragweed population revealed that the PPX2 gene co-segregated with PPO-inhibiting herbicide resistance. One of the polymorphisms, R98L, was suspected of being responsible for conferring resistance since it was at a conserved location and functionally involved in substrate coordination within the active site. Complementation of a mutant bacterial system with a construct containing a R98L mutation conferred resistance while in the presence of a PPO-inhibiting herbicide, lactofen. Growth curves were performed with the same complemented mutant bacterial system while in the presence of lactofen to quantitatively confirm the role of the mutation in resistance. Currently, the R98L mutation of common ragweed is the second documented resistance mechanism to PPO-inhibiting herbicides. Both of the resistance mechanisms of waterhemp and common ragweed are due to mutations present in the PPX2 gene. Resistance in waterhemp can be obtained by a deletion, but for common ragweed only a single base pair change is sufficient to confer resistance to PPO-inhibiting herbicides