Insecticidal components Produced by Bacillus thuringiensis

Cry proteins Cyt proteins Spores

Vegetative insecticidal proteins (VIPs)

ß-exotoxin Zwittermicin Phospholipases Chitinases

FIGuRE 3.2 Larvae of the navel orangeworm (Amyelois transitella), killed during a natural epizootic of Bacillus thuringiensis subsp. aizawai in wheat grain. Top, dead larvae. Bottom, a nutrient agar plate on which a small piece of tissue from a dead larva was streaked to assess the reproductive capacity of this strain. Note that the larva is essentially a pure culture of this strain. In general, larvae of grain moths of the lepidopteran family Pyralidae are excellent hosts for Bt reproduction, each larvae being capable of producing millions of spores.

FIGuRE 3.2 Larvae of the navel orangeworm (Amyelois transitella), killed during a natural epizootic of Bacillus thuringiensis subsp. aizawai in wheat grain. Top, dead larvae. Bottom, a nutrient agar plate on which a small piece of tissue from a dead larva was streaked to assess the reproductive capacity of this strain. Note that the larva is essentially a pure culture of this strain. In general, larvae of grain moths of the lepidopteran family Pyralidae are excellent hosts for Bt reproduction, each larvae being capable of producing millions of spores.

Although these other factors are important to Bt's insecticidal activity, regardless of the target insect, Cry proteins are the most important of the insecticidal components found in commercial Bt formulations. Without these, for example, when endotoxin plasmids are eliminated from Bt strains by curing, the resulting spores, which lack a parasporal body containing endotoxins, have few toxic or pathogenic effects on insects.

In an attempt to account for the complexity of the toxicity factors that occur in many Bt isolates, it appears that the various other components besides Cry proteins evolved to optimize the chances that the bacterium could overcome host defenses, kill the insect, and then use the dead insect for reproduction. The evidence suggests that this set of components evolved in grain-feeding and other pyralid insects, specifically in larvae of species such as the southern European sunflower moth (Homoeosoma nebulella, a grain pest); the navel orangeworm (Amyelois transitella, which feeds on rotting fruit and tree nuts); and the Mediterranean flour moth (Ephestia kuehniella, from which the Bt type species, B. thuringiensis subsp. thuringiensis, was isolated by Ernst Berliner in 1911). Larvae of these moths, all members of the family Pyralidae, are the only species of the order Lepidoptera in which natural epizootics of B. thuringiensis, spreading as an infectious disease, are known to occur routinely.17,18 In such species, larval cadavers filled with Bt spores and insecticidal crystals resulting from infection and colonization of the body serve as the source of inoculum for epizootics. The intoxication and infection processes are initiated by Cry proteins, after which vegetative growth and invasion of the hemocoel occur, possibly with the aid of one or more of the other toxicity components noted above.

Other types of lepidopterans, which are not known to be "natural" hosts for Bt subspecies, are sensitive to Bts because they contain the same "receptors" for Cry proteins that occur in the larvae of grain-feeding moths. The degree of sensitivity will depend on the species, specifically on the number and affinity of midgut micro-villi receptors for various Cry proteins. For insect species recalcitrant to Bt, such as most Spodoptera species (family Noctuidae), the components of toxicity other than Cry proteins play an important role in bringing about death, even if the vegetative cells are not successful in colonizing the larva. The importance of these other toxic components (e.g., VIP3, a protein toxin that also targets midgut epithelial cells) has been demonstrated for larvae of Agrotis ipsilon and Spodoptera frugiperda. When the VIP3 gene was deleted from B. thuringiensis, its pathogenicity was reduced markedly against these species.19

Another example of a contributing toxic component is P-exotoxin, which syner-gizes the activity of Cry proteins and other proteins produced as spores germinate. The P-exotoxin is an inhibitor of mRNA polymerase; it appears to act by preventing intoxicated midgut epithelial cells from recovering and regenerative midgut cells from developing. Thus, although Bt apparently evolved in the larvae of grain-feeding moths, the common occurrence of receptors (i.e., docking molecules) for Cry proteins in many lepidopteran species makes them susceptible to many Bts, but mortality in species not highly sensitive to Cry proteins requires other toxic components. Nevertheless, even if eventually killed by Cry proteins in combination with other factors, Bt might not colonize the body of some species, making these species poor hosts for Bt reproduction.

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