Many of the pre-requisites necessary for breeding plants with inducible resistance to pests are no different from breeding for constitutive resistance. In both cases it is necessary to have resistance genes giving high enough yield gains from pest protection, efficient selection methods and means of introducing resistance genes into agronomically acceptable plant material. In addition, resistance traits need to be neutral or positive to non-target organisms. In inducible resistance, there is also the need for proper timing and specificity of induction. Tentatively the ideal inducible resistance is triggered by specific cues which rapidly induce specific resistance traits with long duration relative to the sensitive period of the crop and which give a systemic plant response. It takes knowledge to develop appropriate selection methods for resistance and in the case of inducible resistance we also need knowledge about how the inducing factors are operating. Inducing cues may come from insect activities on the plant (e.g. feeding, oviposition), from neighbouring plants or from manmade chemical formulations of elicitors. Plant selections can be based on plant damage levels, insect numbers, insect responses, plant resistance traits and/or molecular characteristics of the plant genome. Breeding for insect-inducible resistance to insects has been applied in the form of rapidly induced highly specific resistance in insect - crop combinations where there are gene-for-gene relationships, such as with the Hessian fly and the Russian wheat aphid in wheat. To my knowledge there are no examples of traditional breeding where less specific types of resistance; induced by insects, by neighbouring plants or by chemical formulations; have been deliberately bred into commercial cultivars. It is likely that the accumulating knowledge about the mechanisms of induced resistance will find applications in cultivars produced by genetic engineering.
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