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Current Status and Future Prospects of Microbial Insecticide
During the last century, the overuse of synthetic chemical insecticides led to problems associated with natural environment such as harmful effects on nontarget organisms and development of resistance. Therefore, the application of insect pathogenic microorganisms including Bacillus thuringiensis (Bt), baculoviruses, and entomopathogenic fungi has been an environmentally favorable methods for insect control. However, their use has been limited by several factors such as narrow host spectrum and slow speed of insecticidal activities. A number of approaches have been taken to overcome these defects of microbial insecticides.
A novel recombinant baculovirus, NeuroBactrus, was constructed to develop an improved baculovirus insecticide with additional beneficial properties, such as a higher insecticidal activity and improved recovery, compared to wild-type baculovirus. For the construction of the NeuroBactrus, the Bt cry1-5 crystal protein gene was introduced into the Autographa californica nucleopolyhedrovirus (AcMNPV) genome by fusion of polyhedrin-cry1-5-polyhedrin under the control of the polyhedrin promoter. In the opposite direction, an insect-specific neurotoxin gene, AaIT, from Androctonus australis was introduced under the control of an early promoter from Cotesia plutellae bracovirus by fusion of a partial fragment of orf603. The Polyhedrin-Cry1-5-Polyhedrin fusion protein expressed by the NeuroBactrus was not only occluded into the polyhedra, but also activated by treatment with trypsin, resulting in an approximately 65-kDa active toxin. The NeuroBactrus showed a high level of insecticidal activity against Plutella xylostella larvae and a significant reduction in the median lethal time (LT50) against Spodoptera exigua larvae compared to those of wild-type AcMNPV. Expression of the foreign proteins (Bt toxin and AaIT) was continuously reduced during the serial passage of the NeuroBactrus. These results suggested that the NeuroBactrus could be recovered to wild-type AcMNPV through serial passaging.
The cry gene from Bt, encoding the Cry protein, has been recently introduced into crops to generate transgenic plants that are resistant to pest insects. Through the 3D structure prediction and accompanying mutagenesis study for the Mod-Cry1Ac, 7 and 16 amino acid residues from domain I and II, respectively, responsible for its insecticidal activity against larvae of S. exigua and Ostrinia furnacalis were identified. We used site-directed mutagenesis to improve the insecticidal activity of Mod-Cry1Ac, resulted 34 mutant cry genes. These mutant cry genes were expressed, as a polyhedrin fusion form, using a baculovirus expression system. The expressed proteins were 95 kDa and SDS-PAGE analysis of the recombinant polyhedra revealed that expressed Cry proteins was occluded into polyhedra and activated stably to 65 kDa by trypsin. When the insecticidal activities of these mutant Cry proteins against to larvae of P. xylostella, S. exigua and O. furnacalis were assayed, they showed higher or similar insecticidal activity compared to those of Cry1Ac and Cry1C. Especially, Mutant-N16 is considered to have the potential for the efficacious biological insecticide since it showed the highest insecticidal activity.
