Transcriptome profiling reveals differential gene expression of detoxification enzymes in a hemimetabolous tobacco pest after feeding on jasmonate-silenced Nicotiana attenuata plants

Cristina M. Crava, Christoph Brütting, Ian T. Baldwin

The evolutionary arms race between plants and insects has driven the co-evolution of sophisticated defense mechanisms used by plants to deter herbivores and equally sophisticated strategies that enable phytophagous insects to rapidly detoxify the plant's defense metabolites. In this study, we identify the genetic determinants that enable the mirid, Tupiocoris notatus, to feed on its well-defended host plant, Nicotiana attenuata, an outstanding model for plant-insect interaction studies. We used an RNAseq approach to evaluate the global gene expression of T. notatus after feeding on a transgenic N. attenuata line which does not accumulate jasmonic acid (JA) after herbivory, and consequently accumulates very low levels of defense metabolites. Using Illumina sequencing, we generated a de novo assembled transcriptome which resulted in 63,062 contigs (putative transcript isoforms) contained in 42,610 isotigs (putative identified genes). Differential expression analysis based on RSEM-estimated transcript abundances identified 82 differentially expressed (DE) transcripts between T. notatus fed on wild-type and the defenseless plants. The same analysis conducted with Corset-estimated transcript abundances identified 59 DE clusters containing 85 transcripts. In both analyses, a larger number of DE transcripts were found down-regulated in mirids feeding on JA-silenced plants (around 70%). Among these down-regulated transcripts we identified seven transcripts possibly involved in the detoxification of N. attenuata defense metabolite, specifically, one glutathione-S-transferase (GST), one UDP-glucosyltransferase (UGT), five cytochrome P450 (P450s), and six serine proteases. Real-time quantitative PCR confirmed the down-regulation for six transcripts (encoding GST, UGT and four P450s) and revealed that their expression was only slightly decreased in mirids feeding on another N. attenuata transgenic line specifically silenced in the accumulation of diterpene glycosides, one of the many classes of JA-mediated defenses in N. attenuata. The results provide a transcriptional overview of the changes in a specialist hemimetabolous insect associated with feeding on host plants depleted in chemical defenses. Overall, the analysis reveals that T. notatus responses to host plant defenses are narrow and engages P450 detoxification pathways. It further identifies candidate genes which can be tested in future experiments to understand their role in shaping the T. notatus-N. attenuata interaction.