Effect of nitrogen on Bt gene expression in corn roots, resulting trait performance against corn rootworms (Diabrotica spp.), and transgenic hybrid performance

Abstract

This study examined the impact of nitrogen rates on the expression of corn rootworm (Diabrotica spp.) resistant traits in transgenic corn engineered to express one or more of Bt (Bacillus thuringiensis Berliner) Cry protein(s). The resulting protection against corn rootworm, and the comparative performance of hybrids containing zero, one or multiple (pyramided) corn rootworm traits were evaluated. Recommended nitrogen application rates have been suggested based on economics, but not for optimal expression of Bt Cry proteins. Specifically, this experiment explored the need to shift N rates to optimize Bt trait expression, corn rootworm protection provided by different traits, and a possible revision of nitrogen requirements for optimal yield with pyramided traits. The experiment featured a factorial treatment arrangement in a split-plot randomized complete block design with six nitrogen rates as the main plots and three hybrids differing in corn rootworm traits as the sub-plots. Corn roots were sampled at the beginning of and just after peak larval feeding and run through an Enzyme-linked Immunosorbant Assay (ELISA) to determine gene expression levels. Resulting root injury and adult emergence were measured to assess impacts on larval survival and damage. Root injury generally decreased with nitrogen rate in all hybrids while Bt gene expression, for Cry3Bb1, increased. Nitrogen rate did not affect expression of Cry34 Ab1/Cry35Ab1. Below-ground biomass was found to be highest in the Round-UpTM Ready hybrid at growth stage V6, along with nitrogen uptake. Nitrogen uptake was similar in VT TripleTM to the Round-Up Ready hybrid, which were both significantly higher than the Smart StaxTM hybrid. Yield for the Smart Stax hybrid did not plateau with the nitrogen rates applied at one site in this study, while the unprotected Round-Up Ready hybrid could not take advantage of the higher N rates. Implications of these results for growers in terms of adjusting nitrogen application rates in fields to obtain better gene expression, optimizing hybrid protection from corn rootworm traits, and reducing the risk of resistance are discussed.