The dual function of elicitors and effectors from insects: reviewing the ‘arms race’ against plant defenses

This review provides an overview, analysis, and reflection on insect elicitors and effectors (particularly from oral secretions) in the context of the 'arms race' with host plants. Following injury by an insect herbivore, plants rapidly activate induced defenses that may directly or indirectly affect the insect. Such defense pathways are influenced by a multitude of factors; however, cues from the insect's oral secretions are perhaps the most well studied mediators of such plant responses. The relationship between plants and their insect herbivores is often termed an 'evolutionary arms race' of strategies for each organism to either overcome defenses or to avoid attack. However, these compounds that can elicit a plant defense response that is detrimental to the insect may also benefit the physiology or metabolism of an insect species. Indeed, several insect elicitors of plant defenses (such as the fatty acid-amino acid conjugate, volicitin) are known to enhance an insect's ability to obtain nutritionally important compounds from plant tissue. Here we re-examine the well-known elicitors and effectors from chewing insects to demonstrate not only our incomplete understanding of the specific biochemical and molecular cascades involved in these interactions but also to consider the role of these compounds for the insect species itself. Finally, this overview discusses opportunities for research in the field of plant-insect interactions by utilizing tools such as genomics and proteomics to integrate the future study of these interactions through ecological, physiological, and evolutionary disciplines.