Microbial Mediation of Plant Perception of Herbivores

Plants are subject to attack by an onslaught of microbes and herbivores, yet are able to specifically perceive the threat and mount appropriate defenses. Plants have evolved two primary defense pathways: one regulated by jasmonic acid (JA), which defends against herbivorous insects, the other by salicylic acid (SA), which responds to microbial pathogens and is frequently antagonistic with JA. Chewing herbivores cause massive damage when crushing plant tissues with their mandibles, thus releasing an array of specific cues that may be perceived by the plant, which mobilizes plant defenses.

While specific cues in the oral secretions of herbivores such as caterpillars and beetles trigger plant defenses, we have found that gut bacteria associated with these secretions can trigger the SA pathway, which benefits the herbivore by suppressing JA regulated defenses. These results reveal a new strategy for how herbivores evade plant defenses by using symbiotic bacteria that deceive the plant into perceiving a herbivore threat as microbial, thus resulting in suppression of plant defenses against herbivores.

In another recent study, we have found that insect parasitoids that parasitize caterpillars may indirectly have a strong impact on plant defenses. Along with injecting an egg inside the caterpillar, the parasitoid injects symbiotic polydnaviruses, which disable the caterpillar’s immune system. As part of this immunosuppression, one component in the caterpillar’s saliva known to trigger plant defenses is nearly completely suppressed. These striking findings indicate that a symbiotic virus produced in parasitoids not only causes a massive suppression of the caterpillar’s immune system, but also suppresses the plant’s immunity or defenses against herbivores.

These findings indicate that microbes are the “hidden” players in mediating plant-herbivore interactions. Our evidence from several plant-herbivore systems indicate that insect-associated microbes can have a profound effect on the ability of a plant to perceive herbivores and thus trigger plant defenses.