Expression of proteinase inhibitors I and II in transgenic tobacco plants: effects on natural defense against Manduca sexta larvae.

Genes containing the cauliflower mosaic virus 35S promoter fused to open reading frames coding for tomato proteinase inhibitor I, tomato inhibitor II, and potato inhibitor II were expressed in transgenic tobacco plants. Inhibitor I and II proteins were identified by immunoblotting and quantified by immunoradial diffusion. Both inhibitors exhibited the molecular weights found for the native proteins in their natural environments. Extracts of leaves from transformed plants contained inhibitory activities against trypsin and chymotrypsin that reflected the levels of inhibitor I or II protein present. The results demonstrate that in tobacco leaves the introns of both inhibitor I and inhibitor II genes were excised correctly and that pre and prepro inhibitor I and II proteins were correctly processed. Growth of Manduca sexta larvae (tobacco hornworms) feeding on leaves of transgenic plants containing inhibitor II, a powerful inhibitor of both trypsin and chymotrypsin, was significantly retarded, compared to growth of larvae fed untransformed leaves. Levels of inhibitor II protein as low as 50 micrograms/g of tissue moderately affected larval growth, whereas levels above 100 micrograms/g severely reduced growth. The presence of tomato inhibitor I protein, a potent inhibitor of chymotrypsin but a weak inhibitor of trypsin, in transgenic tobacco leaves had little effect on the growth of the larvae. These experiments indicated that trypsin inhibitory activity, but not chymotrypsin inhibitory activity, was mainly responsible for the inhibition of larval growth.