Manduca chitinase-mediated resistance to tobacco budworm (Heliothis virescens) and tobacco hornworm (Manduca sexta) larvae in transgenic tobacco plants

Food production demands other strategies to control insect pests instead of using chemical pesticides. Plant genetic engineering is a promising alternative approach, but few resistance genes are available.; One strategy has been to derive resistance from a gene of the organism to be controlled. Expression of the foreign gene in a plant at the appropriate time and level could thus disrupt the pest or pathogen. An attractive target for an insecticidal protein is the polysaccharide chitin, a major component of the insect gut peritrophic membrane. Thus, constitutive expression of an insect chitinase in a transformed plant might disrupt the growth and development of insects consuming that plant. We tested this hypothesis by transformation of tobacco via Agrobacterium tumefaciens containing a chimeric vector encoding a chitinase cDNA from the tobacco hornworm, Manduca sexta.; Transgenic tobacco plants expressing the insect chitinase were examined both for their ability to support the growth of Heliothis virescens (tobacco budworm) larvae and for their ability to resist damage from their feeding. Both of these parameters were significantly reduced in the transgenic tobacco. In addition, when incorporated into a wheat germ diet, purified Manduca chitinase from the transgenic tobacco was toxic to the merchant grain beetle, Oryzaephilus mercator, while chitinases from two bacteria and barley were not.; Although the Manduca chitinase transgene enhanced resistance to H. virescens, protection was incomplete. If the Manduca chitinase enhanced resistance via peritrophic membrane breakdown, its use might further enhance resistance from Bacillus thuringiensis (B.t.) toxins. This was tested using foliarly applied B.t. var. karstaki toxin solutions. Indeed, tobacco expressing the chitinase transgene showed increased resistance to neonate larvae of both the tobacco budworm and the tobacco hornworm at concentrations of the B.t. toxin that caused little or no effect on the growth of these insects on control plants. Both larval growth and leaf damage were reduced. Transgenic plants without toxin treatment but expressing the chitinase gene afforded partial resistance to the budworm but not the hornworm. Co-introduction of both {dollar}delta{dollar}-endotoxin and M. sexta chitinase genes into transgenics should enhance the effectiveness of either gene alone.