Donegan, K. K., and Ramon J. Seidler. 1998. Effects of transgenic cotton expressing the Bacillus thuringiensis var. kurstaki endotoxin on soil microorganisms In: Y.P.S. Baja, editor Biotechnology in Agricultural and Forestry, Vol 42 Cotton. Springer-Verlag, Berlin. pp. 299-312.
The genetic engineering of plants has facilitated the production of agronomically-desirable crops that exhibit increased resistance to pests, herbicides, pathogens and environmental stress and enhancement of qualitative and quantitative crop traits (Gasser and Fraley 1992). Along with these many benefits, however, comes the potential for adverse ecological effects because of the often sustained expression in the genetically engineered (transgenic) plant of the engineered trait(s) and the persistence of the transgenic plant or plant residue in the environment. Consequently, we have undertaken research to evaluate the potential ecological effects of transgenic plants and their products. Some of our research has included microcosm studies with cotton plants that are genetically engineered to produce the Bacillus thuringiensis var. kurstaki (B.t.k.) endotoxin (Perlak et al. 1990). Many agriculturally important plants have been engineered to produce endotoxins from different subspecies of the bacterium Bacillus thuringiensis (B.t.) (Vaeck et al. 1987; Delanney et al. 1989; Perlak et al. 1990; Lundstrum 1992; Koziel et al. 1993). The endotoxin of Bacillus thuringiensis var. kurstaki (Btk) has demonstrated insecticidal activity against lepidopterans (Hofte and Whiteley 1989). Although high specificity has been assumed for most B.t. endotoxins, their effects on non-target organisms have not been fully evaluated. Studies have been performed exposing non-target invertebrates to various Bt-producing bacterial strains and have demonstrated such detrimental effects as mortality and reduced fecundity (Ali et al. 1973; Tolstova et al. 1976; Molloy and Jamnback 1981; Mulla et al. 1982; James et al. 1993; Flexner et al. 1986; Miller 1990). In preliminary experiments where transgenic Btk cotton plants were placed in natural soils and decomposed (Pratt et al. 1993; Palm et al 1994), we discovered that the B.t.k. endotoxin persisted and retained its immunological and biological activity at levels similar to what has been observed with microbially-produced Bt endotoxins. Therefore, we considered it important to determine the impact of the B.t.k. endotoxin in decomposing transgenic plants on soil microorganisms because of the ubiquity of microorganisms in soil and the crucial role they play in soil processes. Most concern about the environmental release of plants containing Bt endotoxins has been for the development of resistance in the target pests (Fox 1991; Johnson and Gould, 1992; USDA 1992) or for gene flow and plant invasiveness (Umbeck et al. 1991; Manasse 1992; Crawley et al. 1993; Kareiva et al. 1994; Klinger and Elstrand 1994) . Some studies have considered non-target effects of the Bt endotoxin but have used microbial Bt strains rather than plants that produce Bt toxins (Molloy and Jamnback 1981; Flexner et al. 1986; Miller 1990). Only a few studies have used transgenic plants to assess the potential direct or indirect effects of Bt endotoxins on plant and soil ecosystems (Donegan et al. 1995; Donegan et al. 1996a; Donegan et al. 1996b). In addition to the potential effects of Bt endotoxins produced by transgenic plants, there is the possibility that other plant characteristics may be unintentionally altered during the insertion of the transgene (Lange 1990; MacKenzie 1990; The Economist 1990; Jenkins et al. 1991; Gene Exchange 1992; Yamada 1992). These types of alterations in plant characteristics caused by genetic manipulation (e.g., changes in plant enzyme production and biomass), that are independent of expression of the inserted gene, may also produce ecological effects. In this chapter we describe four experiments that investigated the biological and molecular changes in microbial populations following the incorporation of purified B.t.k. endotoxin or B.t.k.-producing cotton into natural soils. Microbial populations were monitored for changes in the total numbers and species composition of culturable bacteria and fungi, in the substrate utilization of the bacterial community and in the total DNA content and DNA fingerprints of the eubacteria.