The present invention refers to a method for controlling undesired vegetation at a plant cultivation site, the method comprising the steps of providing, at said site, a plant that comprises at least one nucleic acid comprising a nucleotide sequence encoding a mutated protoporphyrinogen oxidase (PPO) which is resistant or tolerant to a PPO-inhibiting herbicide by applying to said site an effective amount of said herbicide. The invention further refers to plants comprising mutated PPO enzymes having a substitution at a position corresponding to position Leu397 of SEQ ID NO:2 and an amino acid substitution at a position corresponding to position Phe420 of SEQ ID NO:2, and methods of obtaining such plants.