Up-regulation of apoptotic- and cell survival-related gene pathways following exposures of western corn rootworm to B. thuringiensis crystalline pesticidal proteins in transgenic maize roots

Abstract

Background Resistance of pest insect species to insecticides, including B. thuringiensis (Bt) pesticidal proteins expressed by transgenic plants, is a threat to global food security. Despite the western corn rootworm, Diabrotica virgifera virgifera, being a major pest of maize and having populations showing increasing levels of resistance to hybrids expressing Bt pesticidal proteins, the cell mechanisms leading to mortality are not fully understood. Results Twenty unique RNA-seq libraries from the Bt susceptible D. v. virgifera inbred line Ped12, representing all growth stages and a range of different adult and larval exposures, were assembled into a reference transcriptome. Ten-day exposures of Ped12 larvae to transgenic Bt Cry3Bb1 and Gpp34/Tpp35Ab1 maize roots showed significant differential expression of 1055 and 1374 transcripts, respectively, compared to cohorts on non-Bt maize. Among these, 696 were differentially expressed in both Cry3Bb1 and Gpp34/Tpp35Ab1 maize exposures. Differentially-expressed transcripts encoded protein domains putatively involved in detoxification, metabolism, binding, and transport, were, in part, shared among transcripts that changed significantly following exposures to the entomopathogens Heterorhabditis bacteriophora and Metarhizium anisopliae. Differentially expressed transcripts in common between Bt and entomopathogen treatments encode proteins in general stress response pathways, including putative Bt binding receptors from the ATP binding cassette transporter superfamily. Putative caspases, pro- and anti-apoptotic factors, as well as endoplasmic reticulum (ER) stress-response factors were identified among transcripts uniquely up-regulated following exposure to either Bt protein. Conclusions Our study suggests that the up-regulation of genes involved in ER stress management and apoptotic progression may be important in determining cell fate following exposure of susceptible D. v. virgifera larvae to Bt maize roots. This study provides novel insights into insect response to Bt intoxication, and a possible framework for future investigations of resistance mechanisms.

Toxicometabolomic profiling of resistant and susceptible western corn rootworm larvae feeding on Bt maize seedlings

“…Multiple metabolic pathways associated with significantly altered metabolites in the susceptible and resistant insects were identified. Our results are consistent with recent similar toxicogenomics studies conducted on WCR larvae fed Bt proteins or Bt-expressing maize which reported elevated transcriptional levels of cellular pathways associated with stress, detoxification, free radical damage repair, apoptosis, autophagy, repair pathways, and ABC transporters (Cry3Bb1 and Gpp34/Tpp35Ab1 50 , eCry3.1Ab 33 , Cry3Bb1 32 , Gpp34Ab1, Tpp35Ab1 and Gpp34/TppAb1 51 ). Validation of hypotheses generated with toxicometabolomic methods will require additional experimentation to determine which, if any, of the metabolites participate in WCR intoxication by Bt toxins.…”

A draft Diabrotica virgifera virgifera genome: insights into control and host plant adaption by a major maize pest insect

“…Regardless, 50 P450s were predicted with a majority being up-regulated (78%) or putatively belonging to clan CYP6. Interestingly, 22 were shared in comparisons of maize with other plant exposure and starvation treatments, where such shared expression was previously attributed to general D. v. virgifera stress responses [64]. The arthropod cytochrome P450 family is comprised of 6 clans involved in oxidation of a variety of endogenous and exogenous substrates [128,129].…”

“…Interestingly, although only 17 ABC transporters were among the DEGs across all treatment comparisons to maize at 12-h exposure, 15 (88.2%) were ABCC members. Functional characterization of these genes will be required to determine any importance in response to consumption of different plant materials, or general stress responses of D. v. virgifera as hypothesized earlier [64].…”

“…In a second round, the BLAT query and L-RNA-Scaffolder steps were performed as described above, except queries were made with 116,070 transcripts from a D. v. virgifera reference transcriptome (TSA accession ERZ1775117.1) [64], and the merged set of FASTA data from round 1 were used as the BLAT database. The resulting round 2 scaffold and non-scaffolded FASTA output were again joined.…”

“…Differences in larval performance and host range are associated with changes in expression of a suite of transcripts encoding detoxification enzymes and proteases [58][59][60], including cytochrome P450 monooxygenases and ATP binding cassette (ABC) transporters. Prior studies document the association of differentially-expressed D. v. virgifera transcripts with larval [61][62][63] or exposures [64] to Bt, resistance to chemical insecticides [65,66], and between adults feeding on maize or G. max [67]. However, investigations into larval D. v. virgifera adaptations to maize and non-maize grass hosts have yet to be conducted.…”