Dr. John Brown, Chair, WSU Department of Entomology
Man has never synthesized an insecticide that has a mode of action not found in nature! Most synthetic insecticides have modes of action similar to those found in plants, animals, minerals, and microbes. Certain Chrysanthemum species have defenses against herbivores (pyrethrins). Cryolite is an example of an inorganic element with insecticidal properties. The delta-endotoxin of the Bacillus Thuringiensis (B.t.) microbe causes insects to stop feeding. Cartap" is an example of an animal defensive secretion with insecticidal properties. Insecticides may also be modeled after some physiologically important factor within an insect itself (insect growth regulators). It should not be surprising that insect chitinases are being considered as possible biopesticides.
Chitinases are enzymes that degrade the chitin contained in an insect's shell-like outer covering called the exoskeleton. An insect's exoskeleton and the thin, threadlike filaments of fungi both possess chitin. Chitin and plant cellulose are both composed of repeating sugar units but differ in how these are connected to each other in forming the large complex molecules known as polymers.
Plants have developed chitinases to defend against fungal attack, but these same enzymes could provide resistance to insect attack also. Cereal seeds have substantial (0.01%) amounts of chitinases. These may adequately defend against fungal attack, but they are clearly insufficient to deter an attack from a stored product insect. Plant chitinases may target the chitin in an insect's midgut membrane, rather than the chitin in the insect's skeleton. This membrane can contain as much as 12% chitin. An enzymatic attack on the membrane would result in severe and fatal abrasion of the insect's gut lining; feeding activity would immediately stop. A plant's ability to produce such a feeding deterrent would give it an evolutionary advantage over plants lacking such a mechanism.
Researchers interested in new biopesticides are giving considerable attention to chitinases. Insects are too expensive to be a commercial source, so other sources are being explored.
Drs. Kramer and Muthukrishnan of Kansas State University recently reviewed work on insect chitinases in Insect Biochemistry and Molecular Biology. Chitinase from
multiple sources has been characterized, and the DNA encoding the enzymes has been cloned. The resulting chitinase genes have been inserted into plants. These transgenic plants are created using technology similar to that used to create plants currently marketed with the B.t. delta-endotoxin gene. Transgenic tobacco plants containing chitinase genes from the tobacco hornworm express a chitinase immunologically identical to the hornworm chitinase. Another insect, the tobacco budworm, was fed these chitinase-negative or chitinase-positive tobacco plants. Those budworm larvae feeding and surviving on chitinase-positive leaves weighed only 1/6 as much as the larvae fed chitinase-negative plants (Ding et al. 1997).
Kansas State University researchers incorporated chitinases from Manduca (insect), Serratia (bacteria), Streptomyces (Actinomycete), and Hordeum (plant) into a semi-artificial diet fed to the merchant grain beetle. Beetle larvae survived on diets containing chitinases from microbial or plant origin, but they died within a few days after feeding on the diet containing the Manduca chitinase (Wang et al. 1996). Chitinase from an insect source seems to have better potential as a bioinsecticide than chitinase from plant or microbial sources.
Chitinases can increase the effect of B.t. delta-endotoxin. Chitinases weaken an insect's midgut membrane barrier and enhance the toxicity of B.t. Insect chitinase expressed by the transgenic tobacco plant multiplied the effect of sublethal doses of B.t. toxin (Ding et al. 1997).
You may see chitinase as an additive to the B.t. you purchase in the future. Insect chitinases may, like delta-endotoxin today, eventually be incorporated into transgenic crop plants. Alternatively, Dr. Kramer's laboratory is currently developing a virus expressing an insect chitinase gene for possible direct-spray use in insect control.
References:
Ding. X., B. Gopalakrishnan, L. B. Johnson, F. F. White, X. Wang, T. D. Morgan, K. J. Kramer and S. Muthukrishnan. (1997) Insect resistance of transgenic tobacco expressing an insect chitinase gene. Transgen. Res. In Press.
Kramer, K. J. and S. Muthukrishnan. (1997) Insect chitinases: Molecular biology and potential use as biopesticides. Insect Biochem and Mol. Biol. 27: 887-900.
Shapiro, M., H. K. Preisler, and J. L. Robertson. (1987) Enhancement of baculovirus activity on gypsy moth (Lepidoptera: Lymantriidae) by chitinase. J. Econ. Entomol. 80: 1113-1116.
Wang, X., X. Ding, B. Gopalakrishnan, T. D. Morgan, L. Johnson, F. White, S. Muthukrishnan, and K. J. Kramer. (1996) Characterization of a 46 kDa insect chitinase from transgenic tobacco. Insect Biochem and Mol. Biol. 26: 1055-1064.
Return to Table of Contents for the June 1998 issue
Return to Agrichemical and Environmental News index