Arbuscular mycorrhizal fungi (AMF) can facilitate nutrient uptake and enhance systemic resistance of their plant hosts. Entomopathogenic nematodes (EPNs) are commonly applied as biological control agents against root-feeding insects and were recently shown to enhance plant systemic defenses. Although these plant mutualists are both commonly deployed in agriculture, there is little research examining the systemic effects of combined AMF and EPN inoculations within the same plant. This is of interest because defense signaling pathways associated with systemic resistance can be antagonistic or act synergistically. For example, EPN inoculations have the potential to inhibit later AMF colonization, and pre-existing AMF association could alter the systemic effects of EPNs. The aim of this study was to fill knowledge gaps regarding the interacting effects of AMF and EPNs on plant systemic defenses through a series of experiments examining plant defense responses and herbivore performance. Squash plants (Cucurbita pepo) were inoculated with either AMF (Rhizophagus clarus), EPNs (Heterorhabditis bacteriophora), a combination of AMF followed by EPN inoculations, or remained non-inoculated controls. Plants were then challenged with the insect herbivore Acalymma vittatum and the resulting leaf herbivory levels and defense-related phytohormone levels (jasmonic acid, salicylic acid, and abscisic acid) were measured. This research provides new insights into the roles of mutualists in shaping plant defenses and expands our understanding of how AMF and EPNs can be applied for agricultural pest management.
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