THE ROLE OF GIBBERELLIC ACID IN APHID-PLANT-ARBUSCULAR MYCORRHIZAL FUNGUS INTERACTIONS

Arbuscular mycorrhizal (AM) fungi form beneficial associations with the roots of most terrestrial plants, which is mostly characterized by increased plant nutrient acquisition (phosphate and nitrogen) by the fungi in exchange for carbon and lipids from the plants. These associations can also enhance plant coping mechanisms to abiotic and biotic stresses. However, insect herbivores, such as aphids, can benefit from AM fungi-plant symbioses by altering plant defenses that are driven mainly by the phytohormones jasmonic acid (JA) and salicylic acid (SA). Nevertheless, other phytohormones can modulate plant defenses by altering the balance of the JA and SA signaling pathways, such as the growth promoting phytohormone gibberellic acid (GA). Although GA signaling plays an important role in modulating plant defenses during plant-pathogen interactions, its role in insect-plant and insect-plant-beneficial microbe interactions remains largely unknown. Therefore, the current study set out to investigate the following two objectives: 1) examine the role of exogenous GA application in regulating plant defenses during aphid-plant-AM fungus interactions, and 2) evaluate the role of GA signaling in modulating plant defenses during aphid-plant interactions. In objective 1, we found that exogenous GA negatively affected aphid fitness on GA-treated mycorrhizal plants 7-days post pea aphid feeding. Exogenous GA had a synergistic effect on JA-related plant gene expression in shoots, and increased JA and SA levels in leaves and petioles after 36 hours of aphid feeding. Pea aphid feeding for 36 hours induced SA-related defense gene expression and increased SA levels in leaves and petioles. In addition, root colonization by the AM fungus resulted in induced JA- and SA-related plant defense gene expression in shoots and roots, but decreased SA levels in leaves and petioles after 36 hours of aphid feeding. In objective 2, we found that GA signaling had an antagonistic effect on JA-related defense signaling after 36 hours of aphid feeding. In summary, this study serves as a foundation for future studies examining the role of GA in regulating plant defenses during insect-plant and insect-plant-AM fungus interactions