as a direct
consequence of insect-mediated damage or mechanical wounding (Green and
Ryan 1972). Proteinase inhibitors are present constitutively in high concentrations
in plant storage organs, and a possible function as protective agents against insects
was discussed at that time (Lipke et al. 1954; Applebaum and Konijn 1966). Green
and Ryan suggested that the expression of proteinase inhibitors may be regulated in
leaves to make the plant less palatable and perhaps lethal to invading insects. The
accumulation of proteinase inhibitors in aerial tissues was proposed to constitute
an inducible defense system, directly affecting the performance of leaf-consuming
insects by starving them of nutrients, thus resulting in enhanced plant resistance
against herbivory (Green and Ryan 1972). It is now clear that the nutritional quality
of the foliage is an important determinant of herbivore growth and development
(Painter 1936; Berenbaum 1995; Schoonhoven et al. 2005) and anti-nutritional defense
as part of the plant’s arsenal for induced resistance is well accepted (Rhoades
and Cates 1976; Felton 2005).