| In 2001, a population of waterhemp was identified in Adams County, Illinois that survived following treatment of several acetolactate synthase (ALS) inhibitors, and a POST application of lactofen, a protoporphyrinogen oxidase (PPO)-inhibiting herbicide. Greenhouse studies were conducted with this waterhemp biotype, and revealed that plants were also resistant to atrazine, a photosystem II (PSII)-inhibiting herbicide. A waterhemp line selected for multiple herbicide resistance, designated R biotype, was compared in all experiments to a herbicide-susceptible (S) biotype in order to characterize resistance to ALS, PSII, and PPO inhibitors. Resistance to ALS inhibitors was conferred by an altered site of action, specifically a tryptophan to leucine substitution at position 574 of ALS. Resistance to atrazine was inherited as a single nuclear encoded gene, but was not due to an altered target site (D1 protein). The R biotype was susceptible to ametryne, suggesting a role of enhanced degradation of atrazine by conjugation of glutathione via glutathione S-transferases (GSTs). In response to PPO inhibitors, the R waterhemp biotype was resistant to all PPO inhibitor chemical families tested. R plants were susceptible to PRE applied PPO inhibitors at normal use rates, but could be differentiated from the S biotype when approximately 0.01X use rates were applied. Resistance to PPO inhibitors was inherited as a single, nuclear encoded, incompletely dominant gene. A gene encoding two different PPO isoforms, PPX2L, which is predicted to function in both plastids and mitochondria, was correlated with PPO inhibitor-resistant phenotypes. Comparison of inferred cDNA sequences of PPX2L from R and S plants revealed a 3-base-pair in frame deletion in R plants that encodes a glycine residue at position 178 of the predicted protein. A hemG mutant strain of E. coli complemented with a plasmid borne PPX2L gene carrying this deletion was able to grow lactofen-supplemented medium. In conclusion, resistance to ALS, PSII, and PPO inhibitors in waterhemp is conferred by three independent mechanisms that include two altered herbicide target sites and possibly an enhanced detoxification mechanism. |
