ABSTRACT

Vip3A is a member of the recently discovered family of vegetative insecticidal proteins isolated from Bacillus thuringiensis. The protein possesses insecticidal activity against the major lepidopteran pest insects including black cutworm (Agrotis ipsilon), fall armyworm (Spodoptera frugiperda), beet armyworm (S. exigua), tobacco budworm (Heliothis virescens), corn earworm (Helicoverpa zea), soybean looper (Pseudoplusia includens) and cotton leaf perforator (Bucculatrix thurberiella). Vip3-insecticidal proteins have been found in ~75% of Bacillus strains analyzed. Unlike the d-endotoxins, Vip3 insecticidal proteins are expressed during the vegetative stage of development as well as during sporulation and are secreted proteins that do not undergo N-terminal processing. The Vip3A protein has no homology with any known proteins and work is being conducted to better understand this protein's mode of action. Vip3A requires an activation step by the insect gut enzymes for receptor-specific binding. Competition binding assays have shown Vip3A and Cry1Ab do not share the same receptor. Characterization of voltage-clamping of larval midguts and synthetic planar lipid bilayer membrane assays have demonstrated that activated Vip3A forms a unique ion channel that has properties very different from that of Cry1Ab protein. Histological studies have shown that the Vip3A protein targets midgut epithelium cells of susceptible insects initiating a series of cytological changes comprising profuse vacuolization and swelling prior to cell lysis and larval death. Vip3A has been transformed into several crop plants including maize, rice and cotton in an effort to provide protection against the major lepidopteran pests of these crops. Our transgenic maize events offer excellent protection against black cutworm, armyworm species and corn earworms. In cotton, Vip3A offers excellent control of the bollworm complex, beet armyworm, fall armyworm and loopers. Field trials have provided data suggesting that our Vip cotton is very competitive with bollgard cotton. We show here that Vip cotton has a novel mode of action, which is different to the Bt cotton on the market now, and therefore promises to offer growers a valuable alternative for use in resistance management strategies.