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.

While many phenylpropanoid pathway-derived molecules act as physical and chemical barriers to pests and pathogens, comparatively little is known about their role in regulating plant immunity. To explore this research field, we transiently perturbed the phenylpropanoid pathway through application of the CINNAMIC ACID-4-HYDROXYLASE (C4H) inhibitor piperonylic acid (PA). Using bioassays involving diverse pests and pathogens, we show that transient C4H inhibition triggers systemic, broad-spectrum resistance in higher plant without affecting growth. PA treatment enhances tomato (Solanum lycopersicum) resistance in field and laboratory conditions, thereby illustrating the potential of phenylpropanoid pathway perturbation in crop protection. At the molecular level, transcriptome and metabolome analyses reveal that transient C4H inhibition in tomato reprograms phenylpropanoid and flavonoid metabolism, systemically induces immune signaling and pathogenesis-related genes, and locally affects reactive oxygen species metabolism. Furthermore, C4H inhibition primes cell wall modification and phenolic compound accumulation in response to root-knot nematode infection. Although PA treatment induces local accumulation of the phytohormone salicylic acid, the PA resistance phenotype is preserved in tomato plants expressing the salicylic acid-degrading NahG construct. Together, our results demonstrate that transient phenylpropanoid pathway perturbation is a conserved inducer of plant resistance and thus highlight the crucial regulatory role of this pathway in plant immunity.