Fate of the insecticidal Cry1Ab protein of GM crops in two agricultural soils as revealed by ¹⁴C-tracer studies

Insecticidal delta-endotoxins of Bacillus thuringiensis are among the most abundant recombinant proteins released by genetically modified (GM) crops into agricultural soils worldwide. However, there is still controversy about their degradation and accumulation in soils. In this study, (14)C-labelled Cry1Ab protein was applied to soil microcosms at two concentrations (14 and 50 μg g(-1) soil) to quantify the mineralization of Cry1Ab, its incorporation into the soil microbial biomass, and its persistence in two soils which strongly differed in their texture but not in silt or pH. Furthermore, ELISA was used to quantify Cry1Ab and its potential immunoreactive breakdown products in aqueous soil extracts. In both soils, (14)CO2-production was initially very high and then declined during a total monitoring period of up to 135 days. A total of 16 to 23 % of the (14)C activity was incorporated after 29 to 37 days into the soil microbial biomass, indicating that Cry1Ab protein was utilized by microorganisms as a growth substrate. Adsorption in the clay-rich soil was the most important factor limiting microbial degradation; as indicated by higher degradation rates in the more sandy soil, extremely low concentrations of immunoreactive Cry1Ab molecules in the soils' aqueous extracts and a higher amount of (14)C activity bound to the soil with more clay. Ecological risk assessments of Bt-crops should therefore consider that the very low concentrations of extractable Cry1Ab do not reflect the actual elimination of the protein from soils but that, on the other hand, desorbed proteins mineralize quickly due to efficient microbial degradation.

Microbes as targets and mediators of allelopathy in plants

Studies of allelopathy in terrestrial systems have experienced tremendous growth as interest has risen in describing biochemical mechanisms responsible for structuring plant communities, determining agricultural and forest productivity, and explaining invasive behaviors in introduced organisms. While early criticisms of allelopathy involved issues with allelochemical production, stability, and degradation in soils, an understanding of the chemical ecology of soils and its microbial inhabitants has been increasingly incorporated in studies of allelopathy, and recognized as an essential predictor of the outcome of allelopathic interactions between plants. Microbes can mediate interactions in a number of ways with both positive and negative outcomes for surrounding plants and plant communities. In this review, we examine cases where soil microbes are the target of allelopathic plants leading to indirect effects on competing plants, provide examples where microbes play either a protective effect on plants against allelopathic competitors or enhance allelopathic effects, and we provide examples where soil microbial communities have changed through time in response to allelopathic plants with known or potential effects on plant communities. We focus primarily on interactions involving wild plants in natural systems, using case studies of some of the world's most notorious invasive plants, but we also provide selected examples from agriculturally managed systems. Allelopathic interactions between plants cannot be fully understood without considering microbial participants, and we conclude with suggestions for future research.

Mineralization of the allelochemical sorgoleone in soil

The allelochemical sorgoleone is produced in and released from the root hairs of sorghum (Sorghum bicolor). Studies have confirmed that it is the release of sorgoleone that causes the phytotoxic properties of sorghum, and sorgoleone has a potential to become a new natural herbicide, or the weed suppressive activity of sorghum can be utilized in integrated weed management. Since sorgoleone is released into soil, knowledge of the fate of sorgoleone in soil is essential if it is to be utilized as an herbicide. Fate studies will characterize the persistence and mobility of the compound. Three types of radioactively labelled sorgoleone were produced and used to study mineralization (complete degradation to CO(2)) of this lipid benzoquinone in four soils, two from the United States of America (Mississippi) and two from Denmark. The studies showed that sorgoleone was mineralized in all soils tested. The methoxy group of sorgoleone was readily mineralized, whereas mineralization of the remaining molecule was slower. Mineralization kinetics indicated that microorganisms in American soils were able to use sorgoleone as a source of energy.

Variation in the chemical constituents of soybean due to the industrial pollution

The two varieties of soybean (Soybean Bragg and Soybean JS-71-05) were collected from an industrial site (IS) and from a non-industrial site (NIS) for the study of their chemical composition and fatty acids profiles by gas liquid chromatography (GLC). These studies revealed large changes in the major and minor fatty acids of the soybean seeds due to the effect of chemical pollutants. There was a significant decrease in the amounts of major and minor fatty acids, such as myristic acid (14:0), palmitic acid (16:0) stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2), and linolenic acid (18:3), in the seeds from industrial site. The changes in the chemical composition due to chemical pollutants showed mixed results.