Root infection by the nematode Meloidogyne incognita modulates leaf antiherbivore defenses and plant resistance to Spodoptera exigua

Studies on plant-mediated interactions between root parasitic nematodes and aboveground herbivores are rapidly increasing. However, outcomes for the interacting organisms vary, and the mechanisms involved remain ambiguous. We hypothesized that the impact of root infection by the root-knot nematode Meloidogyne incognita on the performance of the aboveground caterpillar Spodoptera exigua is modulated by the nematode's infection cycle. We challenged root-knot nematode-infected tomato plants with caterpillars when the nematode's infection cycle was at the invasion, galling, and reproduction stages. We found that M. incognita root infection enhanced S. exigua performance during the galling stage, while it did not affect the caterpillar's performance at the invasion and reproduction stages. Molecular and chemical analyses performed at the different stages of the nematode infection cycle revealed that M. incognita root infection systemically affected the jasmonic acid-, salicylic acid-, and abscisic acid-related responses, as well as the changes in the leaf metabolome triggered during S. exigua feeding. The M. incognita-induced leaf responses varied over the nematode's root infection cycle. These findings suggest that specific leaf responses triggered systemically by the nematode at its different life-cycle stages underlie the differential impact of M. incognita on plant resistance against the caterpillar S. exigua.

Leaf herbivory counteracts nematode-triggered repression of jasmonate-related defenses in tomato roots

Shoot herbivores may influence the communities of herbivores associated with the roots via inducible defenses. However, the molecular mechanisms and hormonal signaling underpinning the systemic impact of leaf herbivory on root-induced responses against nematodes remain poorly understood. By using tomato (Solanum lycopersicum) as a model plant, we explored the impact of leaf herbivory by Manduca sexta on the performance of the root knot nematode Meloidogyne incognita. By performing glasshouse bioassays, we found that leaf herbivory reduced M. incognita performance in the roots. By analyzing the root expression profile of a set of oxylipin-related marker genes and jasmonate root content, we show that leaf herbivory systemically activates the 13-Lipoxigenase (LOX) and 9-LOX branches of the oxylipin pathway in roots and counteracts the M. incognita-triggered repression of the 13-LOX branch. By using untargeted metabolomics, we also found that leaf herbivory counteracts the M. incognita-mediated repression of putative root chemical defenses. To explore the signaling involved in this shoot-to-root interaction, we performed glasshouse bioassays with grafted plants compromised in jasmonate synthesis or perception, specifically in their shoots. We demonstrated the importance of an intact shoot jasmonate perception, whereas having an intact jasmonate biosynthesis pathway was not essential for this shoot-to-root interaction. Our results highlight the impact of leaf herbivory on the ability of M. incognita to manipulate root defenses and point to an important role for the jasmonate signaling pathway in shoot-to-root signaling.