N2-Fixing Cyanobacteria as BTI Toxin Genes Delivery System—A Biotechnogical Approach to Control Malaria Mosquitoes

Abstract

Rice is one of the most important crops in developing countries. About two thirds of the 143 million hectares of the world’s riceland are used as wetland cultures: water from rainfall and irrigation sources is conserved to maintain shallow flooded conditions for almost the entire growth duration of the crop. Such water body creates ecological conditions favorable to the propagation of diseases such as malaria, schistosomiasis, and Japanese encephalitis. The malaria parasites of humans, which belong to the genus Plasmodium, are the most important in relation to rice field diseases. Malaria is transmitted by anopheline mosquitoes at a wide range of transmission levels as measured by the basic case reproduction rate. The anopheline mosquitoes, which carry malaria, and culicine mosquitoes, which carry many arboviruses asssociated with Japanese encephalitis, dengue and yellow fever, have larval stages that develop in water. In some area, rice fields facilitate anopheline breeding and may greatly increase transmission, also likely to prolong the transmission season (Bradley, 1988). The vectors (mosquitoes and the blackfly) require an aquatic environment at certain stages of their life cycle. Numerous studies have shown the association of wetland rice culture with mosquito-borne diseases in various parts of the tropical area. Most of the world’s riceland farmers in tropical countries, east Asian and Latin American suffer from high incidence of diseases carried by mosquito larvae, which breed in water, as well as from lack of adequate nitrogen fertilizers for the rice plants at reasonable cost (Boussiba 1988, 1990, 1991). With application of a bifunctional transgenic cyanobacterial strain which will be released into the rice field for control of mosquito larvae and for excretion of nitrogen-fixed ammonium as a fertilizer while prevailing in rice fields longer than the commonly used laboratory strains, the life quality of the population in those area will be improved regarding these problems. The idea is realized by introducing the genes involved in the specific mosquitocidal δ-endotoxin formation from Bacillus thuringiensis subsp. israelensis (Bti) into an ammonium releasing mutant which we have derived from a rapidly growing rice field indigenous strain of Anabaena siamensis (Thomas et al., 1990). The high levels of mortality produced by several virulent strains of B.thuringiensis var. israelensis and Bacillus sphaericus in several culicine and anopheline larvae without undue effects on non target organisms, make them viable candidates for use in rice fields (de Barjac 1990). The high larvicidal activity of Bti is specific to water-dwelling dipteran species. The origin of the toxicity resides in 5-endotoxin contained in parasporal crystals produced during sporulation, which is composed of five major polypeptides (Hofte, Whiteley 1989) differing in their immunogenicity (Ibarra, 1986) and mosquito larvicidal potential. Synergism seems to exist between combination of the four (Ibarra, 1986; Chang, 1993).KeywordsRice FieldMosquito LarvaLarvicidal ActivityJapanese EncephalitisHelper PlasmidThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.