Whole-cell biosensors to monitor and assess the effects of transgenic crops on soil health
04 / 2009 - 04 / 2012
Commercial cultivation of genetically modified (GM) crops is expected to increase in the coming years and also the type and number of genetic traits introduced into crop plants will expand substantially. Consequently, there is an urgent need to monitor and evaluate the potential adverse effects of GM crops on the environment. GM crops may exert unwanted non-target effects on soil microorganisms and influence key soil processes, including nutrient cycling and regulation of plant pathogens. Given the intimate association between plant roots and soil, effects of GM crops on soil microorganisms are to be expected although the nature of these effects and their ecological consequences are largely unknown. In several studies with GM crops, shifts in the soil microflora have been documented but the meaning of these changes for soil ecosystem functioning is lacking. The overall goal of this project is to identify and develop whole-cell bacterial biosensors for monitoring and predicting the effects of GM crops on soil health.
Whole-cell biosensors are bacterial strains designed for the detection of physical, chemical and/or biological signals. In these biosensors, a natural regulatory circuit (promoter and/or transcriptional regulator) is fused with a promoterless gene coding for an output signal that can be detected, calibrated and interpreted. Whole-cell biosensors respond to bioavailable chemicals and nutrients, and are easily exposed to microscale heterogeneities and gradients such as those prevailing in soil and rhizosphere environments. Therefore, whole-cell biosensors represent a powerful first-line screening for soil perturbations induced by GM crops. Since it is not feasible to monitor all microbial components of soil ecosystems for their responsiveness to a GM crop, this project focuses on the identification of key bacterial species involved in the disease suppression function of soils. The development of a generic set of biosensors will be accomplished by i) exploiting current knowledge of specific antagonistic bacterial species and traits, and ii) identifying novel bacterial species involved in the disease suppression function of soils. The first generation of biosensors will comprise members of the bacterial genera Pseudomonas and Bacillus. Antagonistic strains of these beneficial bacteria represent excellent microbial indicators for monitoring the effects of GM crops on soil health because they occur in physically and chemically diverse soils, are distributed worldwide and can be found on roots of multiple agricultural crops as well as on native plant species. In this project, we will also identify novel bacterial species involved in the disease suppression function of soils by employing a cultivation-independent approach, involving a combination of suppression subtractive hybridization and microarray-based community profiling. This approach has never been adopted for identifying bacterial species that are unique or highly enriched in suppressive soils and will significantly expand the fundamental knowledge on microorganisms and mechanisms involved in the disease suppression function of soils.
To investigate the responsiveness of the developed whole-cell biosensors to soil and rhizosphere perturbations, proof-of-principle experiments will be conducted with oats and oat mutants in which the production of antimicrobial compounds in the rhizosphere is disrupted. To link the responses of the whole-cell biosensors with changes in soil health, quantitative dose-response relationships will be established that couple population densities and gene expression of the bacterial biosensors with the level of pathogen control. This simultaneous assessment of both population density and gene expression of the biosensors is easy and fast, and provides quantitative tools for public and private sectors to predict soil health. The generic biosensors developed in this project will be applicable to any crop and soil type, and thus are very suitable as an early warning and support tool in risk assessment studies with any transgenic crop, including those yet to be developed.