| When fed on by insect herbivores, plants respond dynamically, resulting in far-reaching ecological and economic consequences. For example, when an insect begins to damage leaf tissue, the plant initiates production of within plant signaling molecules (phytohormones) that regulate defenses, including the production of airborne volatile organic compounds (VOCs). These VOCs are attractive to beneficial insects such as predators and parasitoids that feed on the insect pest, resulting in reduced damage to the plant. This specialized attraction of beneficial insects by VOCs released from damaged plants is a well-studied symbiosis and is the foundation for biological control of many economically important crop pests. By utilizing several research tools, we have explored how plants respond to herbivore damage and how aphids affect plant defense responses. First, we found that the pea aphid, Acyrthsiphon pisum, has the ability to attenuate plant defense responses. Pea aphids not only evaded plant defenses, and also inhibited VOCs induced by concurrent damage from caterpillar feeding, but increased emission of aphid alarm pheromone as a result of crowding conditions. This increased emission of alarm pheromone was correlated with alate formation of nearby pea aphids. Additional studies indicated that aphid feeding interfered with induction and accumulation of the defense-related phytohormone jasmonic acid (JA). Lastly, we discovered that aphid honeydew directly interferes with damage-induced phytohormone accumulation. Analysis of pea aphid honeydew indicated high levels of the important defense-related phytohormone salicylic acid (SA) and confirmed the ability of SA to attenuate damage-induced JA. While SA has long been known to interfere with defense-related JA signaling, and has been associated with aphid feeding, this is the first time it has been described as a potential direct mechanism that may be partially responsible for the observed attenuation of plant defense responses. |
