Independent action between DvSnf7 RNA and Cry3Bb1 protein in southern corn rootworm, Diabrotica undecimpunctata howardi and Colorado potato beetle, Leptinotarsa decemlineata.

Steven L Levine,Joshua P Uffman,Peter D Jensen,Jianguo Tan,Pamela M Bachman,Geoffrey M Mueller,Kun Yan Zhu

doi:10.1371/journal.pone.0118622

Abstract

In recent years, corn rootworm (CRW)-resistant maize events producing two or more CRW-active Bt proteins have been commercialized to enhance efficacy against the target pest(s) by providing multiple modes of action (MoA). The maize hybrid MON 87411 has been developed that produces the CRW-active Cry3Bb1 Bt protein (hereafter Cry3Bb1) and expresses a RNAi-mediated MoA that also targets CRW. As part of an environmental risk assessment for MON 87411, the potential for an interaction between the CRW-active DvSnf7 RNA (hereafter DvSnf7) and Cry3Bb1 was assessed in 12-day diet incorporation bioassays with the southern corn rootworm (SCR, Diabrotica undecimpunctata howardi). The potential for an interaction between DvSnf7 and Cry3Bb1 was evaluated with two established experimental approaches. The first approach evaluated each substance alone and in combination over three different response levels. For all three response levels, observed responses were shown to be additive and not significantly different from predicted responses under the assumption of independent action. The second approach evaluated the potential for a fixed sub-lethal concentration of Cry3Bb1 to decrease the median lethal concentration (LC50) of DvSnf7 and vice-versa. With this approach, the LC50 value of DvSnf7 was not altered by a sub-lethal concentration of Cry3Bb1 and vice-versa. In addition, the potential for an interaction between the Cry3Bb1 and DvSnf7 was tested with Colorado potato beetle (CPB, Leptinotarsa decemlineata), which is sensitive to Cry3Bb1 but not DvSnf7. CPB assays also demonstrated that DvSnf7 does not alter the activity of Cry3Bb1. The results from this study provide multiple lines of evidence that DvSnf7 and Cry3Bb1 produced in MON 87411 have independent action.

Highlights

Engineered (GE) insect-resistant crops that produce two or more insect-resistant traits either through co-expression of both traits in a single event or through conventional breeding have become progressively more common in commercial cultivation
In the United States and many other countries, regulatory approval for import or cultivation of Genetically engineered (GE) crops with combined insect-resistant traits may require developing data to demonstrate the lack of a synergistic interaction between the plant incorporated protectants (PIPs) that would impact the safety assessment [2, 3]
Biological activity was confirmed by diet incorporation bioassay against CPB with a 7-day LC50 of 0.60 μg/ml diet, which is in the LC50 range of reported activity for CPB in a comparable 7-day assay [35]

Summary

Introduction

Engineered (GE) insect-resistant crops that produce two or more insect-resistant traits either through co-expression of both traits in a single event or through conventional breeding have become progressively more common in commercial cultivation. In 2009, the FIFRA SAP (Federal Insecticide, Fungicide and Rodenticide Act Scientific Advisory Panel) recommended for new combinations of PIPs that have been previously registered as individual events with a proven safety record, that a synergy less than ten-fold will not require additional NTO testing [3]. The rationale for this recommendation may reflect the need to achieve a safety factor of !10 to 100 times the expected environmental concentration, which is the margin of safety generally required for Tier I NTO assessments. It is unclear why this recommendation differed for PIPs with different or novel modes of action and may reflect a lack of a proven safety record

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Conclusion