Transcriptome restructuring underlies plant defense priming against insect herbivores

Airborne herbivore-induced plant volatiles (HIPVs) play critical roles as chemical mediators of ecological interactions in terrestrial systems. One longstanding hypothesis is that HIPVs act as wound signals between and within plants. Recently, a phenomenon termed “priming” has been applied to plant-herbivore interactions and renewed interest in the chemical ecology that mediates such interactions. Priming is a physiological process by which a plant, in response to an environmental cue, prepares to respond to future biotic or abiotic stress. However, particularly in woody plants, the ability for HIPVs to serve as signaling cues are largely unknown. I conducted a series of experiments to explore defense priming in a model woody plant. Leaves of hybrid poplar (Populus deltoides x nigra) saplings were exposed in vivo to naturally wound-emitted concentrations of the HIPVs and, in separate experiments, to specific volatile compounds and then subsequently fed upon by gypsy moth larvae (Lymantria dispar L.). Relative to controls, leaves exposed to HIPVs had higher endogenous levels of jasmonic acid and linolenic acid following gypsy moth feeding and transcripts of genes involved in phytohormone signaling genes, which suggests that HIPVs prime the major signaling pathway used against herbivores. Further, HIPVs primed direct defense genes and herbivore-induced volatile terpene compounds, which can be indirect defenses. Microarray analysis of volatile-exposed leaves indicates that a transcriptome-level modification underlies the phenomenon of defense priming, and I will discuss the results in the context of how such signals may be adaptive and therefore ecologically important.