Slug Feeding Triggers Dynamic Metabolomic and Transcriptomic Responses Leading to Induced Resistance in Solanum dulcamara

Steroidal glycoside profile differences among primary roots system and adventitious roots in Solanum dulcamara

Solanum dulcamara primary and adventitious roots showed qualitative and quantitative differences in their steroidal glycosides profile. This opened new venues to evaluate the bioactivity of these molecules in belowground ecosystems. The Solanum genus is characterized by the presence of steroidal glycosides (SGs) that confer herbivore resistance and serve as drug precursors in the pharmaceutical industry. Solanum dulcamara is a self-compatible, sexually reproducing species that produces seeds after buzz-pollination. In addition, primordia on the stem facilitate clonal propagation via adventitious root (AR) formation. ARs contain aerenchyma being developmentally and morphologically different from primary roots (PRs). Therefore, we hypothesized that ARs and PRs have different SG profiles. Aiming to assess differences in SGs profiles in S. dulcamara roots in relation to their origins and morphologies, we used liquid chromatography coupled to electron spray ionization quadruple time of flight mass spectrometry (LC-ESI-qToF-MS) to profile SGs from PRs and ARs of seven S. dulcamara individuals. Mass fragmentation pattern analysis indicated the presence of 31 SG-type structures, including those with spirostans and furostans moieties. We assigned the 31 structures to 9 classes of steroidal aglycons (SAgls) that differ in hydroxylation and degree of unsaturation. We found that SAgls were conjugated with di-, tri- and tetra saccharides whereby one compound contained a malonylated sugar. Principle component analysis showed that SG profiles of PRs and ARs separated on the first principal component, supporting our hypothesis. Specifically, PRs contain higher number of SGs than ARs with some compounds exclusively present in PRs. Our results reveal a high level of novel chemodiversity in PRs and ARs of Solanum dulcamara. The knowledge gained will deepen our understanding of SGs biosynthesis and their functional role in plant-environment interactions.

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.

Induced Local and Systemic Defense Responses in Tomato Underlying Interactions Between the Root-Knot Nematode Meloidogyne incognita and the Potato Aphid Macrosiphum euphorbiae

Plants mediate interactions between different herbivores that attack simultaneously or sequentially aboveground (AG) and belowground (BG) organs. The local and systemic activation of hormonal signaling pathways and the concomitant accumulation of defense metabolites underlie such AG-BG interactions. The main plant-mediated mechanisms regulating these reciprocal interactions via local and systemic induced responses remain poorly understood. We investigated the impact of root infection by the root-knot nematode (RKN) Meloidogyne incognita at different stages of its infection cycle, on tomato leaf defense responses triggered by the potato aphid Macrosiphum euphorbiae. In addition, we analyzed the reverse impact of aphid leaf feeding on the root responses triggered by the RKN. We focused specifically on the signaling pathways regulated by the phytohormones jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), and indole-3-acetic acid (IAA) as well as steroidal glycoalkaloids as induced defense compounds. We found that aphid feeding did not induce AG hormonal signaling, but it repressed steroidal glycoalkaloids related responses in leaves, specifically when feeding on plants in the vegetative stage. Root infection by the RKN impeded the aphid-triggered repression of the steroidal glycoalkaloids-related response AG. In roots, the RKN triggered the SA pathway during the entire infection cycle and the ABA pathway specifically during its reproduction stage. RKN infection also elicited the steroidal glycoalkaloids related gene expression, specifically when it was in the galling stage. Aphid feeding did not systemically alter the RKN-induced defense responses in roots. Our results point to an asymmetrical interaction between M. incognita and Ma. euphorbiae when co-occurring in tomato plants. Moreover, the RKN seems to determine the root defense response regardless of a later occurring attack by the potato aphid AG.