Direct effects of insecticides on plants: Neonicotinoids alter transcriptomic profiles of cotton and corn

Insecticides can have a range of unintended effects on non-target organisms. In particular, neonicotinoids, a widely used class of insecticides, modify plant defense gene regulation. This likely increases host plant vulnerability to outbreaks of the non-susceptible two spotted spider mite, Tetranychus schoenei (Acari: Tetranychidae). Neonicotinoid treatments also potentially benefit plant development, which may further provision spider mite outbreaks. To determine the extent of these neonicotinoid mediated endogenous changes, we used RNA-Seq to develop a broader picture of the differential gene expression cascades that may increase plant quality and susceptibility for spider mite infestations. We exposed commercial varieties of corn, Zea mays L., and cotton, Gossypium hirsutum L., to a neonicotinoid seed treatment (Thiamethoxam or Clothianidin), a foliar treatment (Imidacloprid) or omitted the neonicotinoid application. We extracted total RNA from leaf tissue, five weeks after germination, for three replicates of each of the six treatments. These were sequenced on an Illumina HiSeq 2500 system. We mapped the reads, assembled the transcripts, and compared gene expression amongst our treatments within each plant species. We then selected a subset of differentially expressed genes and verified the patterns through qRT-PCR. We found that for both plant species, foliar and seed neonicotinoid treatments modified the expression of several genes involved in defensive and metabolic pathways.