A field evaluation of arbuscular mycorrhizal fungal colonization in split plots of conventional and genetically modified Bt maize

Background/Question/Methods

Genetically modified maize expressing Bacillus thuringiensis (Bt) insecticidal toxins is cultivated worldwide, but recent studies have reported that Bt maize may have non-target effects on symbiotic soil organisms. Arbuscular mycorrhizal fungi (AMF) are obligate symbionts that require a suitable host for nutrition and reproduction, and therefore may be sensitive to alterations in the physiology or rhizosphere of some genetically modified plants. In this field study, we used a split-plot design to evaluate AMF colonization and growth responses of seven different Bt maize lines, differing in number and type of engineered trait, and their corresponding non-Bt parental isolines. Plants were grown in plots that had been cultivated in the previous year with either Bt or non-Btmaize to determine whether there was a plot history effect on AMF colonization. Plants were harvested 60 days after planting, and data were collected on above and belowground biomass, chlorophyll content of leaves, and % AMF colonization in roots. Soil chemistry and texture data were also recorded.

Results/Conclusions

Few significant differences in AMF colonization or growth response were detected between the Bt and non-Bt parental maize lines (most p > 0.05) at the 60 day harvest. Plot appeared to have the greatest effect on AMF colonization, indicating spatial heterogeneity in AMF propagules throughout the field site. There also appeared to be a positive feedback effect where Bt plants had a higher biomass (p < 0.05) when they were grown in plots that were cultivated with Bt maize the previous year. Similarly, non-Bt parental isolines grew better in plots that had been previously cultivated with non-Bt maize (p < 0.05). This pattern may be mediated by the soil microbial community and/or soil chemistry, but this remains to be tested. Results of this study indicate that the symbiotic relationship between maize and AMF is dynamic and that differences in AMF colonization between cultivars may be influenced by propagule distribution in the field, plot history, soil conditions, and other biotic and abiotic factors. Additional field studies will be conducted to verify these results over a multi-year time period.