Airborne herbivore-induced plant volatiles (HIPVs) play critical roles as chemical mediators of ecological interactions in terrestrial systems. Of the possible ecological roles of HIPVs, a longstanding hypothesis that still resonates in modern ecological research 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 some environmental cue, prepares to more quickly or aggressively respond to future biotic or abiotic stress. However, particularly in woody plants, whether HIPVs serve as signaling cues to prime plants is 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 the specific green leaf volatile (GLV) cis-3-hexenyl acetate (z3HAC) and then subsequently fed upon by gypsy moth larvae (Lymantria dispar L.). Volatiles were collected throughout the experiments, and leaf tissue was collected to measure phytohormone levels, direct and indirect defenses, and expression of defense-related genes.
Results/Conclusions
Relative to controls, leaves exposed to HIPVs had higher endogenous levels of jasmonic acid, linolenic acid, and transcripts of genes involved in phytohormone signaling (lipoxygenase 1) following gypsy moth feeding, which suggests that HIPVs prime the major signaling pathway used against herbivores. Further, HIPVs primed some direct defenses (a Kunitz proteinase inhibitor). In addition, HIPVs, and z3HAC specifically, primed the release of three of the five most abundant herbivore-induced volatile terpene compounds. Such a comprehensive priming response discussed here suggests an adaptive benefit to detecting HIPVs as a wound signal. Thus, woody plants can detect and use HIPVs as signaling cues to prime defenses before actually experiencing damage, and HIPVs may therefore have important ecological functions in arboreal ecosystems. However, z3HAC is a commonly emitted GLV, and I will discuss my results in the context of how reliable HIPV signals may be in nature and in what contexts such signals may be adaptive and therefore ecologically important.
