Persistence in soil of transgenic plant produced Bacillus thuringiensis var. kurstaki delta-endotoxin
Authors: Palm, C. J., Schaller, D.L., Donegan, K.K., Seidler, R.J.
Journal, Volume, Year: Canadian Journal of Microbiology, volume 42:1258-1262, 1996
Purified Btk toxin or transgenic cotton leaves containing Btk toxin were added to soil in five different microcosm experiments. Experiments ranged from 28 to 140 days and the initial Btk toxin concentrations ranged from 1 to 1,600 ng/g soil. In four of the five experiments, there was a rapid decline in extractable toxin concentration by day 14, followed by a more gradual rate of decline. The authors stated that the transgenic plant Btk toxin in plant tissue degraded faster than the purified toxin. Btk toxin from transgenic cotton leaves was undetectable in two of the experiments and 3, 16, and 35% of the original concentrations in the other experiments. Also, experiments using sterilized soil indicated that the observed decline in extractable toxin concentrations was due to biotic degradation rather than adsorption by soil.
This is one of the few papers that has evaluated Bt toxin using natural soil and Bt toxin from transgenic crops. The problem remains in analyzing concentrations of Bt toxin in soil. For example, in this study, the authors reported and measured only the extractable Btk toxin concentrations. In addition, the recovery efficiency for the Btk toxins varies greatly from different soils as stated by Palm et al. (1994). The statement that the transgenically-produced protein degraded faster is hard to support from the data shown. In the graph on p. 1260, the control plant + purified toxin treatment and one transgenic line (249) start with similar levels of protein and decay at similar rates. The purified toxin alone treatment starts with a much lower amount of protein, and does not appear to decay slower than the second transgenic line (247), which also starts with a similarly low amount of protein.