Resistance of plants to herbivorous insects: Can this resistance fail?

Serine proteinase inhibitors (PIs) are common natural products in plants that have been extensively studied as phytochemical resistance factors against herbivorous insects. Recently we discovered that herbivorous insects can overcome the activity of these inhibitors by secreting ''inhibitor-resistant'' enzymes. The insect's midgut contains a number of different proteins with trypsin-like activity. Some of these trypsin(s) are susceptible to inhibition by the PI, while other trypsin(s) are not susceptible to inhibition. When ''inhibitor-resistant'' insects ingest PI, the level of activity of ''inhibitor-resistant'' trypsin(s) is enhanced in the midgut, thus allowing the insect to digest dietary protein in the presence of PI. This information suggests that a suite of PIs may be required to inhibit the majority of proteolytic activity in the midgut of the target organism, and thus reduce insect growth and/or development. Once these PIs have been identified, their genes can be transgenically inserted into plants to enhance phytochemical resistance against herbivorous insects.