Induced airborne volatiles coordinate systemic defense responses to insect herbivores

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 plants. While a captivating hypothesis, HIPV-mediated plant-plant signaling is controversial because the distance between emitter and recipient plants may be large and ecologically dubious.  However, the concept that such HIPVs provide signaling cues over shorter distances within a plant has been overlooked.  Plant internal wound signal pathways follow assimilate transport and are thus restricted by vascular architecture, limiting systemic responses to herbivores.  HIPVs may overcome vascular constraints on systemic signaling and provide wound signals to otherwise isolated leaves; that is, plants may “self-prime.”  Here I show that poplar (Populus deltoides x nigra) leaves, connected by air contact to non-orthosticious herbivore-damaged leaves on the same stem (within 10 cm), respond vigorously when attacked by herbivores.  Self-priming offers an alternative hypothesis for HIPV-mediated plant signaling over short, ecologically-relevant transport distances, alleviating a central criticism of volatiles in plant signaling.