Soybean Kunitz, C-II and PI-IV inhibitor genes confer different levels of insect resistance to tobacco and potato transgenic plants

In modern, highly intensive agriculture, the control of insect pests is bas ically achieved with the application of chemical pesticides. Heavy reliance on this sole strategy is associated with several drawbacks, and the develo pment of alternative or complementary methods to chemical control is desira ble. In this work, three soybean genes (KTi3, C-II and PI-IV) coding for se rine proteinase inhibitors were isolated by PCR and transferred to Agrobact erium tumefaciens EHA 105, which in turn was used for transforming tobacco leaf and potato tuber discs. Biochemical assays confirmed that transgenic p lants synthesized serine proteinase inhibitors; rates of expression varied among plants. The level of insect resistance (tested with Spodoptera littor alis Boisduval) was particularly high in tobacco, where many plants caused the death of all larvae. In potatoes, larval mortality was much less freque ntly achieved, but the results were still encouraging in that larval weight gain was reduced by 50% in the presence of adequate amounts of inhibitor. When 8-day-old larvae were fed different KTi3-expressing tobacco plants, a highly significant (P<0.01) correlation was observed be tween inhibitor con tent and larval live weight. Larval weight gain was found to be dependent o n midgut proteolytic activity. On the basis of the evidence collected, it i s suggested that further work is required to identify more specific inhibit ors for the main proteinases of the target insect.