Prevalence of a Novel Resistance Mechanism to PPO-Inhibiting Herbicides in Waterhemp (Amaranthus tuberculatus)

Resistance to protoporphyrinogen oxidase (PPO)-inhibiting herbicides in waterhemp has been shown previously to be the result of a unique mechanism. Specifically, a three–base-pair (3-bp) deletion in the PPX2L gene, a gene encoding both plastid- and mitochondria-targeted PPO enzymes, confers herbicide resistance in this species. Furthermore, when this unique mechanism was initially characterized it was presumed that waterhemp contained three PPX genes, PPX1 , PPX2S , and PPX2L , and that the resistant biotypes were missing PPX2S . Here, allele testing and examination of genetic sequence data demonstrate that there are likely only two PPX genes in waterhemp, PPX1 and PPX2L . Next, to determine the prevalence of this mechanism of resistance in Illinois waterhemp, we developed an allele-specific polymerase chain reaction (PCR) marker that amplifies only the 3-bp deletion allele, ΔG210, of PPX2L . By utilizing this marker, we show that the ΔG210 PPX2L allele correlated with whole-plant resistance to PPO inhibitors in each of four other waterhemp populations evaluated from Illinois.