Premise of the study: Insect-resistant Bacillus thuringiensis (Bt) maize is widely cultivated, yet few studies have examined the interaction of symbiotic arbuscular mycorrhizal fungi (AMF) with different lines of Bt maize. As obligate symbionts, AMF may be sensitive to genetic changes within a plant host. Previous evaluations of the impact of Bt crops on AMF have been inconsistent, and because most studies were conducted under disparate experimental conditions, the results are difficult to compare.

Methods: We evaluate AMF colonization in nine Bt maize lines, differing in number and type of engineered trait, and five corresponding near-isogenic parental (P) base hybrids in greenhouse microcosms. Plants were grown in 50% local agricultural soil with low levels of fertilization, and AMF colonization was evaluated at 60 and 100 d. Nontarget effects of Bt cultivation on AMF colonization were tested in a subsequently planted crop, Glycine max, which was seeded into soil that had been preconditioned for 60 d with Bt or P maize.

Key results: We found that Bt maize had lower levels of AMF colonization in their roots than did the non-Bt parental lines. However, reductions in AMF colonization were not related to the expression of a particular Bt protein. There was no difference in AMF colonization in G. max grown in the Bt- or P-preconditioned soil.

Conclusions: These findings are the first demonstration of a reduction in AMF colonization in multiple Bt maize lines grown under the same experimental conditions and contribute to the growing body of knowledge examining the unanticipated effects of Bt crop cultivation on nontarget soil organisms.