CHANGES IN LEVELS, SPECIES AND DNA FINGERPRINTS OF SOIL-MICROORGANISMS ASSOCIATED WITH COTTON EXPRESSING THE BACILLUS-THURINGIENSIS VAR KURSTAKI ENDOTOXIN

An important aspect of the risk assessment of pesticidal transgenic pl ants is the potential for detrimental effects on the soil ecosystem fr om residual plant material following harvesting and tillage. We evalua ted this concern by placing leaves of three different lines of cotton genetically engineered to produce the Bacillus thuringiensis var. kurs taki (B.t.k.) endotoxin in soil and monitoring numbers and species of indigenous soil bacteria and fungi. Four experiments, lasting 28 or 56 days, were performed using combinations of the following treatments: (1) soil only; (2) soil + purified B,t,k. toxin; (3) soil + parental c otton; (4) soil + purified B.t,k, toxin + parental cotton; (5) soil B,t.k, toxin-producing cotton. Two of the three transgenic cotton line s caused a transient increase in total bacterial and fungal population levels that was significantly higher on several sample days in the ex periments than the levels in the other treatments. In contrast, neithe r the third transgenic cotton line nor the purified B.t,k. toxins had any significant effects on the total numbers of bacteria and fungi. Tr ansient changes in bacterial species composition, measured by biochemi cal tests of individual cultures, community substrate utilization and DNA fingerprinting, were also observed in treatments with the two tran sgenic plant lines. The plant line specificity of the response, and th e lack of effects from the purified B,t,k, toxins, suggest that the ob served effects of the two transgenic plant lines on soil microorganism s may not have resulted from the plants' production of B.t,k, toxin, W e suggest that genetic manipulation or tissue culturing of the plants may have produced a change in plant characteristics, aside from B.t.k. toxin production, that can influence growth and species composition o f soil microorganisms.