A method for identifying a herbicidally-effective composition that damages or kills a cell of a target plant, the method comprising: (a) providing a first plant and a second plant from the same plant species; (b) identifying a regulatory pathway that is capable of damaging or killing a member of the plant species when activity of the regulatory pathway is increased in the member of the plant species; (c) conditionally increasing the activity of the regulatory pathway to a level that is sub-lethal in the first plant by transformation with a recombinant polynucleotide encoding a toxic regulatory polypeptide under the control of an inducible promoter that can be activated by a chemical or environmental treatment, and wherein in the absence of activation by the chemical or environmental treatment, the toxic regulatory polypeptide does not kill the first plant; (d) identifying a first compound that is more effective at damaging or killing the first plant than the second plant in which the activity of the regulatory pathway is not increased when the first compound is contacted with the first and second plants; and (e) identifying a second compound that, when combined with the first compound in an active composition, is capable of damaging or killing the second plant when the second plant is contacted with the active composition; and when contacted with the second plant in a composition lacking the first compound is less effective at damaging or killing the second plant than the active composition. (a) providing a first plant and a second plant from the same plant species; (b) identifying a regulatory pathway that is capable of damaging or killing a member of the plant species when activity of the regulatory pathway is increased in the member of the plant species; (c) conditionally increasing the activity of the regulatory pathway to a level that is sub-lethal in the first plant by transformation with a recombinant polynucleotide encoding a toxic regulatory polypeptide under the control of an inducible promoter that can be activated by a chemical or environmental treatment, and wherein in the absence of activation by the chemical or environmental treatment, the toxic regulatory polypeptide does not kill the first plant; (d) identifying a first compound that is more effective at damaging or killing the first plant than the second plant in which the activity of the regulatory pathway is not increased when the first compound is contacted with the first and second plants; and (e) identifying a second compound that, when combined with the first compound in an active composition, is capable of damaging or killing the second plant when the second plant is contacted with the active composition; and when contacted with the second plant in a composition lacking the first compound is less effective at damaging or killing the second plant than the active composition. when combined with the first compound in an active composition, is capable of damaging or killing the second plant when the second plant is contacted with the active composition; and when contacted with the second plant in a composition lacking the first compound is less effective at damaging or killing the second plant than the active composition.