Influence of Endosymbionts in Mediating Plant Defense Responses to Herbivory by the Pea Aphid, Acyrthosiphon pisum

Abstract

Growing evidence suggests that symbiotic microbes that colonize different tissues and organs, known as endosymbionts, may influence the behavioral and physiological traits exhibited by insects as they interact with plants. Endosymbionts and insect hosts share a mutual interest in combating plant defenses, and research is demonstrating that endosymbionts can manipulate phytohormone-dependent signaling pathways that regulate a variety of defenses against herbivorous insects. Using the pea aphid, Acyrthosiphon pisum, and the aphid endosymbiont, Regiella insecticola, this study tested the hypothesis that endosymbionts can modify pea plant defenses to the benefit of aphid hosts. Defense-related activities associated with major molecular-signaling pathways were examined to determine the effects of a R. insecticola-aphid unification in plant enzymatic activity and gene expression after a period of aphid infestation. Later, aphid nymph and adult stages were assessed to compare parameters of survival and fecundity between aphids with and without R. insecticola. Interestingly, plant enzymatic activity and gene expression levels were always highest in response to infestation by aphids lacking R. insecticola and lowest after infestation by aphids harboring R. insecticola. However, benefits to aphid survivability and reproduction on plants with defenses suppressed were not as clear, and although in some cases aphids harboring R. insecticola outperformed other aphid treatments, the results were never statistically significant. Nevertheless, these results suggest that R. insecticola, through unidentified mechanisms, suppress pea plant defenses intended to deter A. pisum. While it can only be speculated, other underlying mechanisms may be responsible for aphid recovery from plant defenses or more elusive traits of aphids may be promoted by R. insecticola which may explain the unremarkable effect of suppressed plant defenses in aphid performance.