Enemies or allies? Entomopathogenic nematodes increase terpene aldehyde production in cotton roots

Entomopathogenic nematodes (EPNs) are used for the biological control of soil-dwelling insect pests. Recent studies have reported that soil applications of EPNs can induce systemic resistance in plants, thereby increasing plant defenses and resistance to foliar-feeding herbivores. A few studies have also reported that EPN applications can suppress plant-parasitic nematode infections of plant roots. However, these studies have focused only on a few plant species and the underlying mechanisms of systemic defense induction have not been determined. The objective of this study was to determine how exposing roots of cotton plants (Gossypium hirsutum) to EPNs affects systemic defenses in leaves and local defenses in roots. We exposed one set of cotton plants to EPNs (Heterorhabditis bacteriophora) and quantified levels of terpene aldehydes (TAs) in leaf and root tissue. We exposed an additional set of plants to the same EPNs followed by fall armyworm (Spodoptera frugiperda) herbivory. Surprisingly, we found lower levels of TAs in cotton leaves and elevated levels of TAs in roots of EPN-exposed plants that had been fed on by the herbivore, but no difference in the plants that were not fed on. These findings suggest that EPN exposure reduces aboveground defenses but could enhance resistance to belowground-feeding herbivores, pathogens, or plant-parasitic nematodes. Together, these findings can help inform decisions about applying EPNs for biological control.