Abstract:
Protease inhibitors have been proposed as potential defense molecules for increased insect resistance in crop
plants. Compensatory over-production of insensitive proteases in the insect, however, has limited suitability
of these proteins in plant protection, with very high levels of inhibitor required for increased plant resistance. In this study we have examined whether combined used of two inhibitors is effective to prevent this
compensatory response. We show that leaf-specific over-expression of the potato PI-II and carboxypeptidase inhibitors (PCI) results in increased resistance to Heliothis obsoleta and Liriomyza trifolii larvae in
homozygote tomato lines expressing high levels (>1% the total soluble proteins) of the transgenes. Leaf
damage in hemizygous lines for these transformants was, however, more severe than in the controls, thus
evidencing a compensation response of the larvae to the lower PI concentrations in these plants. Development of comparable adaptive responses in both insects suggests that insect adaptation does not entail
specific recognition of the transgene, but rather represents a general adaptive mechanism triggered in
response to the nutritional stress imposed by sub-lethal concentrations of the inhibitors. Combined
expression of defense genes with different mechanisms of action rather than combinations of inhibitors may
then offer a better strategy in pest management as it should be more effective in overcoming this general
adaptive response in the insect.
