Persistence and impact on microorganisms of Bacillus thuringiensis proteins in some Zimbabwean soils

Abstract

The persistence of the Bacillus thuringiensis subsp. kurstaki (Btk) toxin (Cry1Ab protein) from Bt maize (MON810, Yieldgard ) residues incorporated in a vertisol (739 g clay kg −1 ) was investigated. The maize residues were incubated in the soil for 4 weeks, and activity of the toxin in the residues was bioassayed using larvae of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae). Corrected mortality of P. xylostella in the bioassays decreased from 76% to 30% in less than a week of incubation in the soil. In addition to the above observations, the effects of Btk, Bt subsp. israelensis (Bti), and Bt subsp. tenebrionis (Btt) proteins on the soil microbiota were examined using a vertisol, an alfisol, and an oxisol. The pre-incubated soils (7 days after moisture adjustment) were treated with crystal proteins of Btk, Bti, and Btt and incubated for further a 7-day period. Microbial biomass carbon (MBC) and counts of culturable bacteria and fungi were determined. The proteins did not show effects on MBC or bacterial and fungal counts, possibly as a result of adsorption of the proteins on soil particles, which could have rendered the proteins inaccessible for microbial utilization. Microbial biomass carbon and counts arranged in decreasing order were vertisol>oxisol>alfisol, similar to the amounts of organic C and clay in the soils. However, bacteria and fungi counts were higher in the vertisol than in the alfisol and the oxisol soils. Our observations suggest that larvicidal proteins produced by different subspecies of Bt and Bt maize could persist in tropical soils as a result of adsorption on soil clays but that there were no observable effect on the soil microbiota.

No Evidence of an Impact on the Rhizosphere Diazotroph Community by the Expression of Bacillus thuringiensis Cry1Ab Toxin by Bt White Spruce

“…This study is the first to evaluate the impact of Bt white spruce, or any other Bt plant, on nitrogenfixing communities. No effect associated with the presence of the B. thuringiensis transgene was observed in this study, which is in agreement with other recent studies evaluating the impact on overall bacterial diversity (1,3,4,10,12,23,26,32). On the other hand, significant differences were found between diazotroph communities associated with white spruce in natural stands and those associated with trees in a plantation.…”

“…Particular attention was paid to studying the impact of Bt plants, which constitutively express the Cry1Ab insecticidal toxin of Bacillus thuringiensis, on rhizosphere microbial communities. Although studies demonstrating a negative impact of Bt crops on soil microbiota exist (7,44), more studies showing no effect have been published (1,3,4,10,12,23,26,32).…”

Cry3Bb1 protein from Bacillus thuringiensis in root exudates and biomass of transgenic corn does not persist in soil

“…However, Muchaonyerwa et al (2004) reported that the Cry1Ab protein from Bt corn persisted in tropical soils for only several weeks without losing insecticidal activity, and other studies with Bt corn and Bt cotton (Cry1Ab and Cry1Ac proteins, respectively) indicated that these proteins do not persist and are generally degraded in soil in ca. 20 days (Ream et al 1994;Sims and Holden 1996;Palm et al 1996;Hopkins and Gregorich 2003).…”

Persistence of the pesticidalBacillus thuringiensisprotein expressed in Bt maize plant materials in two soils of the Central Eastern Cape, South Africa

“…Sundaram (1996) reported higher sorption of a protein from Bt subspecies kurstaki on a clay than a sand soil. Muchaonyerwa et al (2004) also reported that the bioactivity of Bt maize residues added and incubated in a vertisol (760 g clay kg-I) in a pot experiment at room temperature, decreased rapidly from 76% to 30% in one week and remained essentially constant for the remaining three weeks of the experi-S. Afr. Tydskr.…”