Arg-128-Leu target-site mutation inPPO2evolves in wild poinsettia (Euphorbia heterophylla) with cross-resistance to PPO-inhibiting herbicides

Wild poinsettia (Euphorbia heterophyllaL.) is a troublesome broadleaf weed in grain production areas in South America. Herbicide resistance to multiple sites of action has been documented in this species, including protoporphyrinogen oxidase (PPO) inhibitors. We investigated the physiological and molecular bases for PPO-inhibitor resistance in aE. heterophyllapopulation (R-PPO) from Southern Brazil. Whole-plant dose-response experiments revealed a cross-resistance profile to three different chemical groups of PPO inhibitors. Based on dose-response parameters, R(PPO)was resistant to lactofen (47.7-fold), saflufenacil (8.6-fold), and pyraflufen-ethyl (3.5-fold). Twenty-four hours after lactofen treatment (120 g ha(-1)) POST, R(PPO)accumulated 27 times less protoporphyrin than the susceptible population (S-PPO). In addition, R(PPO)generated 5 and 4.5 times less hydrogen peroxide and superoxide than S-PPO, respectively. The chloroplast PPO (PPO1) sequences were identical between the two populations, whereas 35 single-nucleotide polymorphisms were found for the mitochondrial PPO (PPO2). Based on protein homology modeling, the Arg-128-Leu (homologous to Arg-98-Leu in common ragweed [Ambrosia artemisiifoliaL.] was the only one located near the catalytic site, also in a conserved region ofPPO2. The cytochrome P450 monooxygenase inhibitor malathion did not reverse resistance to lactofen in R-PPO, and both populations showed similar levels ofPPO1andPPO2expression, suggesting that metabolic resistance andPPOoverexpression are unlikely. This is the first report of an Arg-128-Leu mutation inPPO2conferring cross-resistance to PPO inhibitors inE. heterophylla.