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

Chance W Riggins,Patrick J Tranel

doi:10.1155/2012/305764

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.

Highlights

Herbicides that inhibit protoporphyrinogen oxidase (PPO) have been used for many years for control of broadleaf weeds in large-scale crop production systems in the United States
In the case of A. quitensis, seed was obtained from the North Central Regional Plant Introduction Station (NCRPIS) in Ames, Iowa, and represented semi domesticated types from the Pacific side of South America and field weeds collected from Brazil
Sequence comparisons between a PPO-inhibitor-sensitive biotype of A. tuberculatus (GenBank accession DQ394875) and A. hypochondriacus (GenBank accession EU024569) revealed the presence of a single-nucleotide polymorphism (SNP) in the third position of the Gly210 codon in exon 9 of the genomic PPX2 gene. This SNP is a synonymous substitution, its position is significant in being part of a codon that, when deleted, confers resistance to PPO-inhibiting herbicides in A. tuberculatus [7]

Summary

Introduction

Herbicides that inhibit protoporphyrinogen oxidase (PPO) have been used for many years for control of broadleaf weeds in large-scale crop production systems in the United States. Their use began to slowly decline during the late 1990s due to the introduction and subsequent widespread adoption of glyphosate-resistant crop varieties, such as Roundup Ready soybean, corn, and cotton. The continuous broad-scale use of glyphosate over time has triggered the evolution of glyphosate-resistant biotypes among an increasing diversity of weed species [2]. As glyphosate resistance continues to increase in frequency, distribution, and the number of species [2], growers are once again relying on PPO-inhibiting herbicides as an alternative approach to control weeds

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Conclusion