Beneficial worm allies warn plants of parasite attack belowground and reduce aboveground herbivore preference and performance

1. We investigated responses of tomato (Solanum lycopersicum) to belowground presence of two functional guilds of nematodes - plant parasite (Meloidogyne javanica) and entomopathogens (Heterorhabditis bacteriophora, Steinernema feltiae, and S. carpocapsae) - as well as a leaf mining insect (Tuta absoluta) aboveground. Our results indicate that entomopathogenic nematodes (EPNs): 1) induced plant defense responses, 2) reduced root knot nematode (RKN) infestation belowground and 3) reduced herbivore (T. absoluta) host preference and performance aboveground. 2. Concurrently, we investigated the plant signaling mechanisms underlying these interactions using biochemical and transcriptome analyses. We found that both entomopathogen and parasite triggered immune responses in plant roots with shared gene expression. Tomato plants responded similarly to presence of RKN or EPN in the rootzone, by rapidly activating polyphenol oxidase (PPO) and guaiacol peroxidase (GP) activity in roots, but simultaneously suppressed this activity in aboveground tissues.  3. We quantified changes in expression of candidate resistance genes in tomato that may play essential roles in defense response to RKN, which were also coincidentally triggered with EPN. For example, PR-14 expression was greater in plants inoculated with EPN than in plants co-inoculated with both nematode functional guilds. Overall, EPN inoculation directly mediated enhanced plant defense and reduced subsequent RKN infection. Likewise, EPNs may modulate plant defense against RKN invasion, in part, by suppressing active expression of antioxidant enzymes. 4. Inoculation of tomato roots with EPNs belowground reduced both host preference and performance of the aboveground herbivore, T. absoluta.  Inoculations of roots with EPN also triggered an immune response in tomato via up-regulated phenylpropanoid metabolism and synthesis of protease inhibitors (PIs) in plant tissues, which could explain an observed decrease in egg laying and developmental performance exhibited by herbivores on EPN-inoculated plants. 5. Synthesis. Our results add to a growing body of evidence indicating that subterranean EPNs activate systemic acquired resistance (SAR) and/or induced systemic resistance (ISR) in plants with concomitant antagonistic effects on temporally co-occurring subterranean plant pathogenic nematodes and terrestrial herbivores.