Bacillus thuringiensis (Bt) crystal protein genes encode insecticidal ␦-endotoxins that are widely used for the development of insectresistant crops. In this article, we describe an alternative transgenic strategy that has the potential to generate broader and more sustainable levels of resistance against insect pests. Our strategy involves engineering plants with a fusion protein combining the ␦-endotoxin Cry1Ac with the galactose-binding domain of the nontoxic ricin B-chain (RB). This fusion, designated BtRB, provides the toxin with additional, binding domains, thus increasing the potential number of interactions at the molecular level in target insects. Transgenic rice and maize plants engineered to express the fusion protein were significantly more toxic in insect bioassays than those containing the Bt gene alone. They were also resistant to a wider range of insects, including important pests that are not normally susceptible to Bt toxins. The potential impact of fusion genes such as BtRB in terms of crop improvement, resistance sustainability, and biosafety is discussed.Bt genes ͉ transgenic plants ͉ transgenic maize ͉ transgenic rice T ransgenic plants expressing Bacillus thuringiensis (Bt) toxins have been used successfully to provide resistance against selected insect pests for several years. Indeed, insect resistance is the second most widely used trait in transgenic crops (after herbicide tolerance) in world agriculture (1-5). One potential problem with Bt genes is that Bt insecticides are very widely used, with up to 90% of microbiological insect control products based on topically applied Bt toxins. For this reason, there is concern that insects might evolve resistance to Bt toxins (6, 7). The diamondback moth (Plutella xylostella) has evolved resistance in some open field populations in response to repeated exposure to foliar sprays containing Bt proteins (8), whereas laboratory selection experiments with other insect pests have shown that recessive mutant alleles can confer resistance to multiple Bt toxins (7-10). However, note that the evolution of resistance in insects against transgenic plants expressing Bt toxins has yet to be seen in the field.Recent strategies to address potential limitations in conventional transgenic insect pest control include the stacking or pyramiding of multiple transgenes in the same transgenic plant (11) and the use of hybrid toxins (e.g., fusions between a synthetic truncated Cry1Ba and domain II from Cry1Ia; ref. 12). We have devised an alternative strategy in which the Bt toxin Cry1Ac is fused to the nontoxic ricin B-chain (RB). The recognition of toxin-binding sites in the insect midgut is an important factor determining the spectrum of Bt toxin activity and the severity of toxemia (13). Several groups investigating the mechanism of toxin recognition have identified N-acetyl galactosamine residues as an important component of Bt toxinbinding receptors (14-16). Therefore, we selected the ricin B subunit as a fusion partner for the Bt toxin because RB is a galactose...

The combined suppression of only two genes, γ kafirin-1 (25 kDa) and γ-kafirin-2 (50 kDa), significantly increases sorghum kafirin in vitro digestibility. Co-suppression of a third gene, α-kafirin A1 (25 kDa), in addition to the two genes increases the digestibility further. The high-digestibility trait has previously only been obtained either through the co-suppression of six kafirin genes (α-A1, 25 kDa; α-B1, 19 kDa; α-B2, 22 kDa; γ-kaf1, 27 kDa; γ-kaf 2, 50 kDa; and δ-kaf 2, 18 kDa) or through random chemical-induced mutations (for example, the high protein digestibility mutant). We present further evidence that suppressing just three of these genes alters kafirin protein cross-linking and protein body microstructure to an irregularly invaginated phenotype. The irregular invaginations are consistent with high pepsin enzyme accessibility and hence high digestibility. The approach we adopted towards increasing sorghum protein digestibility appears to be an effective tool in improving the status of sorghum as a principal supplier of energy and protein in poor communities residing in marginal agro-ecological zones of Africa.

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