Will theAmaranthus tuberculatusResistance Mechanism to PPO-Inhibiting Herbicides Evolve in OtherAmaranthusSpecies?

Abstract

Resistance to herbicides that inhibit protoporphyrinogen oxidase (PPO) has been slow to evolve and, to date, is confirmed for only four weed species. Two of these species are members of the genusAmaranthusL. Previous research has demonstrated that PPO-inhibitor resistance inA. tuberculatus(Moq.) Sauer, the first weed to have evolved this type of resistance, involves a unique codon deletion in thePPX2gene. Our hypothesis is thatA. tuberculatusmay have been predisposed to evolving this resistance mechanism due to the presence of a repetitive motif at the mutation site and that lack of this motif in other amaranth species is why PPO-inhibitor resistance has not become more common despite strong herbicide selection pressure. Here we investigate inter- and intraspecific variability of thePPX2gene—specifically exon 9, which includes the mutation site—in ten amaranth species via sequencing and a PCR-RFLP assay. Few polymorphisms were observed in this region of the gene, and intraspecific variation was observed only inA. quitensis. However, sequencing revealed two distinct repeat patterns encompassing the mutation site. Most notably,A. palmeriS. Watson possesses the same repetitive motif found inA. tuberculatus. We thus predict thatA. palmeriwill evolve resistance to PPO inhibitors via the samePPX2codon deletion that evolved inA. tuberculatus.