RNA-seq of eight different poplar clones reveals conserved up-regulation of gene expression in response to insect herbivory.

Background: Herbivorous insects can have a profound impact on plant growth performance. In some years, canopy damage in poplar plantations exceeds 50% of the total leaf surface, thereby possibly compromising carbon fixation and biomass yield. To assess the transcriptional response of elite poplar clones to insect feeding and to test whether this response varies between different genotypes, we performed an RNA-sequencing experiment. We deeply sequenced the transcriptomes of eight elite clones belonging to three poplar species (Populus trichocarpa, P. nigra and P. maximowiczii), under Phratora vitellinae feeding and control conditions. This allowed us to precisely quantify transcript levels of about 24,000 expressed genes.

Results: Our data reveal a striking overall up-regulation of gene expression under insect attack in all eight poplar clones studied. The up-regulated genes were markedly enriched for the biological process 'regulation of transcription' indicating a highly concerted restructuring of the transcriptome. A search for potential cis-regulatory elements (CREs) that may be involved in this process identified the G-box (CACGTG) as the most significant motif in the promoters of the induced genes. In line with the role of the G-box in jasmonate (JA)-mediated activation of gene expression by MYC2, several genes involved in JA biosynthesis and signaling were up-regulated in our dataset. A co-expression network analysis additionally highlighted WRKY transcription factors. Within the most prominent expression module, WRKYs were strongly overrepresented and occupied several network hubs. Finally, the insect-induced genes comprised several protein families known to be involved in plant defenses, e.g. cytochrome P450s, chitinases and protease inhibitors.

Conclusions: Our data represent a comprehensive characterization of the transcriptional response of selected elite poplar clones to insect herbivory. Our results suggest that the concerted up-regulation of gene expression is controlled by JA signaling and WRKY transcription factors, and activates several defense mechanisms. Our data highlight potential targets of selection and may thus contribute to breeding insect-resistant poplar clones.