A field evaluation of arbuscular mycorrhizal fungal colonization in multiple transgenic Bt maize lines

Background/Question/Methods

Genetically modified maize is cultivated globally, yet the effects of transgenic maize lines expressing Bacillus thuringiensis (Bt) insecticidal toxins on symbiotic soil organisms are poorly understood. Previous greenhouse studies revealed that some lines of Bt maize exhibit reduced arbuscular mycorrhizal fungi (AMF) colonization in their roots compared to their non-Bt parental isolines. If certain lines of transgenic maize consistently exhibit reduced AMF colonization in field studies, then abundance and diversity of AMF propagules in the soil ecosystem may be affected over time. In this field study, we evaluated AMF colonization and growth response of 12 different Bt and non-Bt maize lines at three different time points (60, 90, and 130 days after planting) and investigated whether Bt maize cultivation has a negative impact on AMF diversity and abundance in the soil. Soil samples were collected from each plot at the beginning and end of the field season to investigate whether spore abundance or diversity was reduced in the Bt plots after one growing season. Maize plants were harvested at three different physiological time points to examine temporal differences in AMF colonization rates in each Bt and non-Bt maize line and potential differences in yield at the end of the season.

Results/Conclusions

There was no overall effect of Bt maize cultivation on AMF spore abundance or diversity detected in soil after one growing season (all Bt cultivars vs all non-Bt cultivars, p >0.05). Variations in total spore counts per plot at the end of the field season were not associated with Bt treatment per se, but appeared to be cultivar specific (total spore count as affected by cultivar, p = 0.02) and influenced by plot x cultivar interactions (p = 0.03). Few significant differences in root colonization or growth response were detected between each Bt and non-Bt parental isoline (most p >0.05) at the 60, 90, and 130 day harvests. When an AMF or growth response was detected, it was usually plant line specific and not a general effect of the Bt cultivars. Results of this study indicate that the symbiotic relationship between maize and AMF is dynamic and that differences in colonization at each time point are likely due to a plant-line specific response and not a generalized Bt effect. Additional field studies will be conducted to verify the ecological significance of these results over a multi-year time period.