disease affecting silkworms (Bombyx mori) in Japan. Obviously, infection by Bt is
detrimental for silkworm production. However, it was later noted that Bt had toxic
effects on caterpillar larvae of most Lepidoptera species (moths and butterflies), which
gives the Bt species great potential as a tool for protecting crop plants. In later years,
additional strains of Bt were identified that are toxic to Coleoptera (beetles), Diptera
(flies and mosquitoes), and even nematodes. The specificity of insecticidal activity of Bt
on a particular insect species is determined by the form(s) of thecrygene(s) carried by
the bacterium. Only certain species of insects are controlled by particular endotoxins.
The crygenes encoding the toxic proteins in Bt take their name from thecrystal
inclusions formed inside the bacterium when it enters into its spore-forming stage. These
crystals often contain more than one specific type ofcrygene product. Before they
become toxic, thecry-encoded Bt proteins exist as protoxins and must be activated
inside the insect digestive tract. Once they are ingested by a susceptible insect, the crystals
break down in the alkaline environment of the insect midgut, generally dissolving at pH
8.0. At that point, the termini of the Bt protoxin proteins are cleaved by specific proteases
inside the gut, yielding the toxic protein. The active protein will then bind to specific protein
receptors on the insect microvillar membrane of the midgut (Fig. 8.3). In most cases, when
Bt proteins are expressed in transgenic plants the entire coding region of the protoxin is not
transferred to the plant. Rather, a shortened version of the gene will typically be expressed
because levels of Bt protein accumulation are higher using this strategy (Barton et al. 1987;
Fischoff et al. 1987).
After binding to a receptor, the active Bt toxin will enter the insect cell membrane,
where multiple copies of the protein will oligomerize and form pores. This results in ion
leakage through the membrane, which causes membrane collapse from osmotic lysis.
Once the membranes on the epithelia of the gut cells are disrupted, the insects effectively
starve and die. In the case of a trueB. thuringiensisinfection, bacterial cells would form
spores during the latter stages of infection and insect collapse, thereby readying themselves
for subsequent infections of other insects. In transgenic plants, susceptible insects usually
stop feeding within a few hours after feeding on the plants, and die a short time later.
It is generally the presence or absence of specific forms of midgut receptors that deter-
mines whether a particular insect species is susceptible to a given Bt protein (Hofmann et al.
Figure 8.3.The Bt toxin binds to very specific receptors on the epithelial membrane of the insect gut.
The toxin then forms channels in the membrane that leads to ion leakage and ultimately, death of the
insect. This mode of action explains the specificity of Bt (from the presence of the necessary recep-
tors) and also shows why the toxin needs to be eaten by the insect to function.
8.3. TRAITS FOR IMPROVED CROP PRODUCTION 201