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 how exposure to one mutualist affects plant interactions with another. This is of interest because both AMF and EPNs can activate systemic defense pathways in host plants, which could alter plant receptivity to other mutualists. The aim of this study was to fill knowledge gaps regarding interacting effects of AMF and EPNs by examining how EPN exposure effects host plant receptivity to AMF colonization. Squash plants (Cucurbita pepo) were treated with EPNs (Heterorhabditis bacteriophora) prior to inoculation with AMF sourced from wild texas squash (Cucurbita melopepo ssp. texana). We predicted that prior EPN exposure would induce systemic defenses that limit AMF colonization. Plants were exposed to either free-living EPN infective juveniles (EPN IJs), EPN-infected insect cadavers, or were non-exposed control plants. Roots were harvested 48 h post exposure and levels of defense hormones jasmonic acid and salicylic acid were quantified. In a separate trial, EPN-exposed plants were inoculated with AMF and harvested after ~5 weeks. AMF colonization was quantified using ink-and-vinegar staining and a gridline intersect method. This research expands our understanding of complex plant-mutualist interactions and may yield insights that allow for the most effective use of AMF and EPNs in agricultural pest management.
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