Are insects resistant to plant proteinase inhibitors?

Six species of Lepidoptera were evaluated for their susceptibility to serine proteinase inhibitors from cabbage. Trypsin and chymotrypsin activity from larval Pieris rapae and Pieris napi were not significantly inhibited (0–18%), in vitro, by cabbage proteinase inhibitors, while the serine proteinase activity in the midguts of larval Plutella xylostella was moderately inhibited (40–50%), and Trichoplusia ni, Lymantria dispar, and Helicoverpa zea were substantially inhibited (55–85%) by cabbage proteinase inhibitors. These results suggest that the growth and development of the latter three species should be reduced following ingestion of these inhibitors. However, chronic ingestion of cabbage proteinase inhibitors only reduced the growth and development of T. ni. This lack of biological activity of the proteinase inhibitors against the other two species was explained by a shift in the relative proportion of digestive enzymes in response to ingestion of proteinase inhibitors. Following ingestion of cabbage proteinase inhibitors, the predominant trypsin-like enzyme(s) in the midgut of larval L. dispar and H. zea were resistant to inhibition by cabbage trypsin inhibitors (13–18% inhibited), while the trypsin(s) in T. ni was moderately susceptible (37% inhibited). These results were confirmed for H. zea and T. ni feeding on proteinase inhibitors in tomato foliage. This is the first demonstration of adaptation to proteinase inhibitors, and has important implications for gene regulation and physiological plasticity.