Submitted to: Pesticide Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2006
Publication Date: 6/7/2006
Citation: Zhu, Y.C., Abel, C.A., Chen, M.S. 2007. Interaction of Cry1Ac toxin (Bacillus thuringiensis) and proteinase inhibitors on the growth, development, and midgut proteinase activities of the bollworm, Helicoverpa zea. Pesticide Biochem. Physiol. 87:39-46. Interpretive Summary: We investigated the interaction between Bt toxin and proteinase inhibitors within the gut of the bollworm. Information from this study may lead to an understanding of how to enhance Bt toxicity and how to slow down tolerance or resistance development in insects. Currently in the Mid-south area, the primary concern is the increasing pest status of plant bugs due to reduced insecticide applications and resistance development since the broad adoption of Bt cotton varieties. Another major concern is the potential threat to Bt cotton as a result of resistance development in lepidopteran pests. A potential solution is the introduction of proteinase inhibitors into the ingestion and digestion systems of target pests. Excessive proteinase inhibitors directly suppress proteolytic processes of not only lepidopteran insects, but also non-lepidopteran insects, such as plant bugs with sucking mouth part. Proteinase inhibitors also synergize Bt toxicity by stabilizing Bt toxins within insect guts.
Technical Abstract: Potential resistance development to Bt cotton in certain lepidopterans has prompted research to develop strategies that will preserve this environmental-friendly biotechnology. Proteinase inhibitors are potential candidates for enhancing Bt toxicity against lepidopteran pests and for expanding the spectrum of control for other insects. Interactions of Bt toxin from Bacillus thuringiensis and proteinase inhibitors were investigated by monitoring growth, development, and gut proteinase activities of the bollworm, Helicoverpa zea. Several proteinase inhibitors were combined with Bt protoxin Cry1Ac in artificial diet and fed to newly molted 3rd-instar bollworm larvae to determine effects on larval body weight and length, pupation progress, and mortality rate. Major midgut proteinase activities, including caseinase, tryptic, and chymotrypsin activities, were examined after treatment. A concentration of Bt at a level causing minimal mortality (<10%), was mixed with the following proteinase inhibitors: benzamidine, phenylmethylsulfonyl fluoride (PMSF), and N-'-tosyl-L-lysine chloromethyl ketone (TLCK). When compared with controls, the synergistic effect of Bt toxin and proteinase inhibitors caused significant decreases in mean larval weight and length over time. Midgut samples tested against the substrates azocasein, '-benzoyl-DL-arginine-p-nitroanilide (BApNA), and N-succinyl-alanine-alanine-proline-phenylalanine-p-nitroanilide (SAAPFpNA) showed significant decreases in the protease activity of larvae fed Bt plus inhibitor versus control. Interaction of Bt and proteinase inhibitors significantly retarded larval growth and resulted in developmental delay and up to 20% mortality.