Influence of endosymbionts in mediating plant defense responses to herbivory by the pea aphid, Acyrthosiphon pisum

Microbial endosymbioses of insects are extremely common and exhibit a diverse nature ranging from mutualistic to pathogenic as well as obligate to facultative.  The known benefits of mutualistic endosymbionts of insects include broadening the range of plants suitable for their host’s diet, enhancing host thermal tolerance, and inhibition of the development and killing of parasitoid larvae within the host’s body.  Also, recent evidence suggests that endosymbionts may enable their insect hosts to tolerate or hinder plant resistance to herbivory.  However, studies examining endosymbiont-mediated suppression of plant defenses are limited. Here we hypothesized that endosymbionts may manipulate plant signaling pathways involved in the induction of a variety of defenses against herbivores. Experiments are under execution using the pea aphid (Acyrthosiphon pisum) and pea plants to evaluate whether endosymbionts have a role in altering plant defenses to the benefit of their aphid hosts. Comparisons of the activity of various defensive enzymes, transcript levels of different defense related genes, and volatile organic compound emissions will be made between plants infested with aphids harboring certain facultative endosymbionts to those infested with aphids lacking these facultative endosymbionts. Using a novel model system, this study investigates whether the defenses conferred by facultative endosymbionts of pea aphids extend beyond the plant-insect interface, such that the bacteria are involved in the manipulation of plant defenses against herbivory to ameliorate the harmful effects exerted on their hosts.