Abstract
Plants are attacked by diverse herbivores and respond with manifold defence responses. To study transcriptional and other early regulation events of these plant responses, herbivory is often simulated to standardize the temporal and spatial dynamics that vary tremendously for natural herbivory. Yet, to what extent such simulations of herbivory are able to elicit the same plant response as real herbivory remains largely undetermined. We examined the transcriptional response of a wild model plant to herbivory by lepidopteran larvae and to a commonly used herbivory simulation by applying the larvae's oral secretions to standardized wounds. We designed a microarray for Solanum dulcamara and showed that the transcriptional responses to real and to simulated herbivory by Spodoptera exigua overlapped moderately by about 40%. Interestingly, certain responses were mimicked better than others; 60% of the genes upregulated but not even a quarter of the genes downregulated by herbivory were similarly affected by application of oral secretions to wounds. While the regulation of genes involved in signalling, defence and water stress was mimicked well by the simulated herbivory, most of the genes related to photosynthesis, carbohydrate- and lipid metabolism were exclusively regulated by real herbivory. Thus, wounding and application of oral secretions decently mimics herbivory-induced defence responses but likely not the reallocation of primary metabolites induced by real herbivory.
Highlights
Plants defend themselves against a great variety of herbivorous insects with a range of traits that are constitutively expressed and/ or inducible upon herbivore attack (Schaller, 2008)
Whereas the W + OS treatment mirrored the upregulation of early responsive genes involved in phytohormone signalling, defence, phenylpropanoid metabolism and water stress reasonably well, it clearly failed to elicit the downregulation of genes related to photosynthesis and lipid metabolism as well as the changes in expression of genes related to carbohydrate metabolism
In addition to phytohormone pathways, we found Gene Ontology (GO) terms related to the respiratory burst that results in the release of reactive oxygen species (ROS) as well as responses to oxidative stress to be enriched in S. dulcamara plants fed on by S. exigua larvae (#20, #24, #34-37)
Summary
Plants defend themselves against a great variety of herbivorous insects with a range of traits that are constitutively expressed and/ or inducible upon herbivore attack (Schaller, 2008). Lima bean for example increases its production of extrafloral nectar to attract ants which defend it from herbivores (Kost & Heil, 2008) Besides such indirect defences, plants produce repellents, antidigestives or toxins that directly deter, slowdown or kill herbivores. Tomato and many other plants respond to herbivory with the production of protease inhibitors that block proteolytic enzymes in the gut of herbivorous insects (Green & Ryan, 1972; Jongsma & Bolter, 1997). In addition to such physiological responses, plants can alter morphological parameters like thickness of the cuticle, the density of defensive.
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