A New Model of the Mechanisms behind Glucose Oxidase Action in Plant Insect Interactions

Plants and insect herbivores are in a constant co-evolutionary arms race. Plants are always under the threat of insect herbivory and need to employ defenses against insect herbivores, which in turn employ counter defense strategies. The salivary enzyme glucose oxidase (GOX), found in many caterpillar species, has been documented to attenuate defenses in plants such as Nicotiana tabacum (cultivated tobacco). However, in Solanum lycopersicum (cultivated tomato), glucose oxidase elicits defensive responses. Multiple mechanisms have been proposed for how GOX affects plant signaling, but there is still considerable disagreement about which is correct. In this review, we review existing models on the mode of GOX action and propose a new model to fill in research gaps and better explain the mechanism behind GOX action. Our model, coined the "ROS Threshold-Dependent Defense Toggle Model", proposes that whether a plant activates jasmonic acid or salicylic acid-mediated defenses depends on the amount and persistence of hydrogen peroxide whose levels are dependent upon ROS-scavenging capabilities of the plant. We also emphasize the use of cultivated tomato as a model system to test our proposed model.