Fate of Cry toxins from Bacillus thuringiensis in the soilDevenir des toxines Cry de Bacillus thuringiensis dans le sol
Thesis defended on 14 December 2015 - University of Montpellier and University of Hanoï
The insecticidal properties of Bacillus thuringiensis, discovered by Shigentane Ishiwatari, have been used for decades as biopesticides and this use has been increasing rapidly because of concerns about the negative environmental effects of chemical pesticides. Currently, Bt toxin in the form of both biopesticides and Bt transgenic plants may supplement or replace chemical pesticide. There is little evidence to demonstrate that Bt toxin has any harmful effect to the environment or to human health. Nevertheless, there are concerns that commercial transgenic crops may have harmful impacts on the environment.
After release into soil via root exudation and breakdown of plant residues, Bt toxin interacts with soil particles. The interactions of Bt toxin with soil particles influence its mobility, its bioavailability, its persistence and its toxicity. In this study, we aim to establish the relative importance of biological and physicochemical factors in the determination of the dynamics of detectable Cry proteins in soils, to clarify if adsorbed protein maintains its insecticidal properties and to identify the soil properties that determine the fate of Cry proteins in soil. The results show that Cry proteins have strong affinity on soil surface. However, there was little relationship between affinity for soil or the extraction yield and soil properties including clay content, organic carbon content and soil pH. There was little relationship between the affinity and the extraction yield. The proteins differ in both their affinity for soil and their extraction yields.
An assessment of role of soil and environmental factors in the fate of Cry protein from commercial biopesticide formulation showed a rapid decline of detectable Cry protein subjected to direct sunlight under the laboratory condition, whereas, little effect was observed under field conditions. The half-life of proteins in soil under natural conditions was about one week. Strong temperature effects were observed, but they differed for biopesticide and purified protein, indicating different limiting steps. For biopesticide, the observed decline was due to biological factors, possibly including sporulation.
In contrast for purified proteins, increased temperature enhanced conformational changes of the soil-adsorbed protein, leading to fixation and hence extraction efficiency decreased that decreased with time. Moreover, the study of persistence of various Cry proteins in contrasting soils was carried out by immuno-detection and bioassay showed that extractable toxin decreased with incubation of up to four weeks. Insecticidal activity was still retained in the adsorbed state, but lost after two weeks of incubation at 25°C. The decline in extractable protein and toxicity was much lower at 4°C than 25°C. There was no significant effect of soil sterilization to persistence of Cry toxin indicating that decrease in detectable Cry toxin in soil may be time-dependent fixation of adsorbed protein as well as decreasing solubilization in larva midgut, but not microbial breakdown.
Exposition to Cry in the adsorbed form could have a significant impact on target and even non target insects and should be investigation to determine the potential impact.