A. Chitinases and Beta-1,3-glucanases-These enzymes have been found in plants and microbes, and they are capable of degrading the cell walls of fungi. In some instances, the antifungal activity of chitinase in vitro is enhanced when applied in combination with Beta-1,3-glucanase. Genes encoding different chitinases and Beta-1,3-glucanases have been cloned and used to transform tobacco. The transgenic tobacco plants showed increased survival relative to control plants in the presence of pathogenic fungi, such as Rhizoctonia solani (for reviews, see Dixon and Lamb 1990; Broglie et al. 1993; Collinge et al. 1993).
Rice has been transformed using different chitinase genes (Anuratha et al. 1994; Lamb et al. 1994). The R`0` and R`1` generations of transgenic rice plants have been challenged by a fungal pathogen, such as R. solani, and the plants showed some resistance to the pathogen (Anuratha et al. 1994). B. Ribosome-inactivating proteins (RIPs)-Several genes encoding different RIPs have been cloned. A barley RIP gene was used to transform tobacco; the transgenic tobacco plants showed increased protection against the fungal pathogen R. solani (Logemann et al. 1992).
Rice has been transformed with a maize RIP gene, and relatively high levels of RIP were found in a dozen transgenic plants (Kim, J. K., D. Xu, X. Duan, and R. Wu, unpublished results). The effectiveness of these transgenic plants against rice fungal pathogens, including R. solani, will be tested.
C. Thionins-Thionins are highly abundant polypeptides with antifungal activities. These polypeptides are located in cell walls of leaf cells, and the synthesis of thionine mRNA was increased after fungal attack (Bohlmann et al. 1988). Expression of the a-thionin gene from barley in transgenic tobacco has been shown to confer enhanced resistance to bacterial pathogens (Carmona et al. 1993). The usefulness of producing transgenic rice plants harboring a thionin gene remains to be tested.
D. Antifungal peptides-Several antifungal polypeptides have been reported. The gene encoding a 47-amino acid polypeptide, LCI, has been cloned and used to transform tobacco. The transgenic tobacco plants were shown to be more resistant to fungal infection than the control plants (Chen et al. 1994).
In principle, transgenic rice plants harboring one or several antifungal disease gene(s) can be produced to control fungal diseases, thus minimizing the use of chemical fungicides. These transgenic rice plants are expected to show a higher degree of resistance and to maintain a longer duration of resistance if two or three different antifungal disease genes are integrated into the transgenic plants.
The currently available genes to be used for producing insect-resistant and fungal disease-resistant transgenic rice plants are summarized in Table 1. Additional useful genes are expected to be cloned in the near future, and they will certainly enhance the power and flexibility of rice biotechnology.