Evolution of the herbivore associated elicitor induced early defence signalling networks in Nicotiana. Nicotiana

Background: Herbivore-induced defences are widespread in plants and can increase plant fitness when attacked. During herbivore attack, plant-perceived herbivore-associated elicitors (HAE) elicit a sophisticated suite of defenses that are mediated by the rapid activation of HAE-induced early defence signalling (EDS) that includes the transient accumulation of jasmonic acid (JA) and its derivatives. Although several genes are known to be involved in HAE-induced defence signalling, the complete HAE-induced EDS network and its evolution remains unstudied.Results: We used a comparative transcriptomic approach to analyse 60 HAE-induced leaf transcriptomic early defence responses among six closely related Nicotiana species, each of which have evolved highly specific and divergent HAE-induced defence responses. Using co-expression gene network analysis, we characterised a key co-expression gene network (M4 module), which is largely independent of HAE-induced JA accumulations but co-activated with the HAE-induced defence responses within and among the Nicotiana species. The functional significance of the M4 module in HAE-induced EDS is supported by: 1) functional evidence based on previously characterised genes and, 2) silencing three newly identified candidate genes from the M4 module by virus-induced gene silencing and demonstrating their influence on HAE-induced JA accumulation and metabolism. Using a phylogenomics approach, we provide evidence that gene retention after a genome-wide triplication event in Solanaceae contributed to the evolution of this HAE-induced EDS network.Conclusions: We present the first HAE-induced EDS co-expression network and provide new insights into the molecular mechanisms and evolution of herbivore induced defences.