Plants having increased tolerance to herbicides

An isolated nucleic acid encoding a herbicide-tolerant mutant protoporphyrinogen oxidase (PPO) enzyme comprising an amino acid substitution at a position corresponding to position R128 of SEQ ID NO:

• 2 and an amino acid substitution at a position corresponding to position F420 of SEQ ID NO:
• 2, wherein the amino acid corresponding to position R128 of SEQ ID NO:
• 2 is substituted with alanine, wherein the amino acid corresponding to position F420 of SEQ ID NO:
• 2 is substituted with methionine, isoleucine, or leucine, and wherein, when expressed in a plant, the mutant-PPO enzyme provides at least 10-fold as much tolerance to a benzoxazinone-derivative herbicide as compared to that provided by a control PPO enzyme not having the substitutions at positions corresponding to positions R128 and F420 of SEQ ID NO:
• 2, wherein the benzoxazinone-derivative herbicide is a benzoxazinone of formula I.a
• wherein R1 is F, R3 is F, and R4 is CH2C≡CH.