Defense priming against insect herbivores by plants using volatile cues may be in the genes

Airborne herbivore-induced plant volatiles (HIPVs) play critical roles as chemical mediators of ecological interactions in terrestrial systems. One longstanding hypothesis that still resonates in modern plant-herbivore interactions 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. Defense priming is a physiological process by which a plant responds to an environmental cue by preparing to more quickly or aggressively respond to future biotic or abiotic stress. I conducted a series of experiments to explore defense priming. 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 green leaf volatile (GLV) cis-3-hexenyl acetate (z3HAC) 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 oxylipin signaling, which suggests that HIPVs prime the major signaling pathway used against herbivores. Further, HIPVs primed direct defenses and also the release of three of the five most abundant herbivore-induced volatile terpene compounds, which can be indirect defenses. The results were further supported by microarray analysis; thus, a transcriptome-level modification evidently underlies the phenomenon of defense priming. I will discuss the results in the context of how such signals may be adaptive and ecologically important.