No abstract

Transgenic plants that produce pesticidal proteins will release these proteins into the soil when these plants are incorporated into the soil by tillage or as leaf litter. Little is known about the fate and persistence of transgenic plant pesticidal products in the soil. We used a model system of transgenic cotton that produces Bacillus thuringlensis var. kurstaki δ-endotoxin (Btk toxin) to evaluate the persistence of transgenic pesticides in soil. Purified Btk toxin or transgenic cotton leaves containing Btk toxin were added to soil in five different microcosm experiments in concentrations ranging from 1 to 1600 ng Btk toxin/g soil. The concentration of the extractable Btk toxin was measured for up to 140 days. An initial rapid decline in extractable toxin concentration in the first 14 days, followed by a slower decline, was observed in four of the five experiments. At the end of the experiments, Btk toxin from transgenic plant tissue was undetectable (less than 0.1% of starting concentration) in two of the microcosm experiments and at 3, 16, and 35% of the original amounts in the other experiments. In addition, experiments using γ-irradiated sterilized soil indicated that the observed decline in extractable toxin concentration was due largely to biotic degradation rather than to physical adsorption by the soil.Key words: transgenic plants, Bacillus thuringlensis toxin, risk assessment.

No abstract

The environmental release of genetically engineered (transgenic) plants may be accompanied by ecological effects including changes in the plant-associated microflora. A field release of transgenic potato plants that produce the insecticidal endotoxin of Bacillus thuringiensis var. tenebrionis (Btt) was monitored for changes in total bacterial and fungal populations, fungal species diversity and abundance, and plant pathogen levels. The microflora on three phenological stages of leaves (green, yellow and brown) were compared over the growing season (sample days 0, 21, 42, 63 and 98) for transgenic potato plants, commercial Russet Burbank potato plants treated with systemic insecticide (Di-Syston)and commercial Russet Burbank potato plants treated with microbial Btt (M-Trak). In addition, plant and soil assays were performed to assess disease incidence of Fusarium spp., Pythium spp., Verticillium dahliae, potato leaf roll virus (PLRV) and potato virus Y (PVY). Few significant differences in phylloplane microflora among the plant types were observed and none of the differences were persistent. Total bacterial populations on brown leaves on sample day 21 and on green leaves on sample day 42 were significantly higher on the transgenic potato plants. Total fungal populations on green leaves on sample day 63 were significantly different among the three plant types; lowest levels were on the commercial potato plants treated with systemic insecticide and highest levels were on the commercial potato plants treated with microbial Btt.Differences in fungal species assemblages and diversity were correlated with sampling dates, but relatively consistent among treatments. Alternaria alternata, a common saprophyte on leaves and in soil and leaf litter, was the most commonly isolated fungus species for all the plant treatments. Rhizosphere populations of the soilborne pathogens Pythium spp., Fusarium spp. and V. dahliae did not differ between the transgenic potato plants and the commercial potato plants treated with systemic insecticide. The incidence of tuber infection at the end of the growing season by the plant pathogen g dahliae was highest for the transgenic potato plants but this difference was related to longer viability of the transgenic potato plants. This difference in longevity between the transgenic potato plants and the commercial + systemic insecticide potato plants also made comparison of the incidence of PVY and PLRV problematic. Our results indicate that under field conditions the microflora of transgenic Btt-producing potato plants differed minimally from that of chemically and microbially treated commercial potato plants.