A. Protease inhibitors-Many different proteases have been detected in insect digestive tracts. Different protease inhibitors are known to inhibit the following classes of proteases: serine proteases, cysteine proteases, zinc proteases, and aspartyl proteases. Several genes encoding different types of protease inhibitors have been cloned (for a review, see Hilder et al. 1993).
B. Bacillus thuringiensis insecticidal proteins-A number of insecticidal crystal proteins have been reported (for a review, see Hofte and Whitely 1989), and a total of over 20 insecticidal crystal protein genes have been cloned (Ely 1993). A codon-modified cry1a (b) gene has been introduced into rice, and the transgenic rice showed more resistance to two major rice insect pests, striped stemborer and leaffolder, than did untransformed control plants (Fujimoto et al. 1993).
C. Lectins-Snowdrop lectin (GNA) added to an artificial diet has been shown to inhibit the growth of brown planthopper (BPH). The gene encoding GNA has been cloned (Boulter et al. 1993; Gatehouse et al. 1994). Once the GNA gene is introduced into transgenic plants, it is expected that the plants will be resistant to BPH and green leaffolder, the two most important rice insect pests among the sucking insects (hemopteran). Since some of these insects are vectors for transmission of viruses, such as the rice tungro virus, control of these insects will also stop or minimize the spread of certain viral disease.
D. Ribosome-inactivating proteins (see Section IIB for details)-This group of proteins has been shown to inhibit the growth of certain species of insects and fungi. However, the proteins have not yet been tested for inhibiting the growth of rice insect pests.
A number of genes encoding individual members of these four groups of proteins have been tested by transformation of tobacco. The transgenic tobacco plants have been shown to confer partial resistance against specific insects. Several of the genes have been tested by transformation of rice or other monocots, and these genes have been shown to confer partial resistance against specific insects that attack monocot plants.
For future work, we strongly recommend the use of two or three different insecticidal-protein genes to transform rice because the transgenic plants contain- ing multiple insecticidal protein genes are expected to show a higher degree of resistance and to maintain a longer duration of resistance.