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

## TL;DR

Plants exposed to insect threat cues boost their defenses against unrelated herbivores, showing a lasting primed response.

## Contribution

This study reveals that plant defenses primed by a specialist herbivore's volatile cue enhance resistance to unrelated generalist herbivores.

## Key findings

- Primed plants showed increased expression of defense-related genes during herbivory by an unrelated insect.
- Hundreds of defense-related genes exhibited a rise and fall in expression in primed plants over 48 hours.
- The priming effect suggests that plants can mount stronger, short-term defenses against diverse herbivores.

## Abstract

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.

Plants can detect airborne compounds emitted by insects, which signals that a threat is nearby—these cues ‘prime’ the plants’ defence. We found that if plants detect a threat cue from one insect, they show heightened defences even against an unrelated generalist insect.