A Volatile Cue From a Specialist Herbivore Primes Gene Expression Against Biotic Stress in Tall Goldenrod ( <i>Solidago altissima</i> L.)

Insect-derived molecular cues can prime plant defences against herbivore attack. The genes that are sensitive to priming, and how their expression changes on the scale of days, have not been fully resolved. Moreover, priming may affect interactions with insects that are not the source of the priming cue. We primed tall goldenrod (Solidago altissima) plants by exposure to the volatile emission of a specialist herbivore, the goldenrod gall fly (Eurosta solidaginis) then subjected the plants to 48 h of herbivory from an unrelated generalist, corn earworm (Helicoverpa zea). Using RNA sequencing, we identified transcriptome-wide gene expression patterns between exposed and unexposed plants. We identified biotic stress-associated genes that were more abundant during herbivory in primed plants, including defence-related transcription factors, thaumatin-like receptors and chitinases. We observed a surprising rise and fall in expression of hundreds of defence-related genes in a 48-h phase in primed damaged plants only. Our results support the hypothesis that primed defences are stronger than typical induced defences and suggest that primed defences target herbivores in the short term. We show that the threat cue from a specialist can affect plant defences against an unrelated herbivore.