Systemic above- and belowground cross talk: hormone-based responses triggered by Heterodera schachtii and shoot herbivores in Arabidopsis thaliana

Identification of genes involved in the tomato root response to Globodera rostochiensis parasitism under varied light conditions

Understanding the intricate interplay between abiotic and biotic stresses is crucial for deciphering plant responses and developing resilient cultivars. Here, we investigate the combined effects of elevated light intensity and nematode infection on tomato seedlings. Chlorophyll fluorescence analysis revealed significant enhancements in PSII quantum yield and photochemical fluorescence quenching under high light conditions. qRT-PCR analysis of stress-related marker genes exhibited differential expression patterns in leaves and roots, indicating robust defense and antioxidant responses. Despite root protection from light, roots showed significant molecular changes, including downregulation of genes associated with oxidative stress and upregulation of genes involved in signaling pathways. Transcriptome analysis uncovered extensive gene expression alterations, with light exerting a dominant influence. Notably, light and nematode response synergistically induced more differentially expressed genes than individual stimuli. Functional categorization of differentially expressed genes upon double stimuli highlighted enrichment in metabolic pathways, biosynthesis of secondary metabolites, and amino acid metabolism, whereas the importance of specific pathogenesis-related pathways decreased. Overall, our study elucidates complex plant responses to combined stresses, emphasizing the importance of integrated approaches for developing stress-resilient crops in the face of changing environmental conditions.

Infestation by the Piercing-Sucking Herbivore Nilaparvata lugens Systemically Triggers JA- and SA-Dependent Defense Responses in Rice

It has been well documented that an infestation of the piercing-sucking herbivore, brown planthopper (BPH), Nilaparvata lugens, activates strong local defenses in rice. However, whether a BPH infestation elicits systemic responses in rice remains largely unknown. In this study, we investigated BPH-induced systemic defenses by detecting the change in expression levels of 12 JA- and/or SA-signaling-responsive marker genes in different rice tissues upon a BPH attack. We found that an infestation of gravid BPH females on rice leaf sheaths significantly increased the local transcript level of all 12 marker genes tested except OsVSP, whose expression was induced only weakly at a later stage of the BPH infestation. Moreover, an infestation of gravid BPH females also systemically up-regulated the transcription levels of three JA-signaling-responsive genes (OsJAZ8, OsJAMyb, and OsPR3), one SA-signaling-responsive gene (OsWRKY62), and two JA- and SA- signaling-responsive genes (OsPR1a and OsPR10a). Our results demonstrate that an infestation of gravid BPH females systemically activates JA- and SA-dependent defenses in rice, which may in turn influence the composition and structure of the community in the rice ecosystem.

Herbivory modifies plant symbiont number and impact on host plant performance in the field

Species interactions are a unifying theme in ecology and evolution. Both fields are currently moving beyond their historical focus on isolated pairwise relationships to understand how ecological communities affect focal interactions. Additional species can modify both the number of interactions and the fitness consequences of each interaction (i.e., selection). Although only selection affects the evolution of the focal interaction, the two are often conflated, limiting our understanding of the evolution of multispecies interactions. We manipulated aboveground herbivory on the legume Medicago lupulina in the field and quantified its effect on number of symbionts and the per-symbiont impact on plant performance in two belowground symbioses: mutualistic rhizobia bacteria (Ensifer meliloti) and parasitic root-knot nematodes (Meloidogyne hapla). We found that herbivores modified the number of rhizobia nodules, as well as the benefit per nodule. However, each effect was specific to a distinct herbivory regime: natural herbivory affected nodule number, whereas leafhoppers (Cicadellidae) weakened the per nodule benefit. We did not detect any effect of herbivory on nematode gall number or the cost of infection. Our data demonstrate that distinguishing between symbiont number from the fitness consequences of symbiosis is crucial to accurately infer how pairwise interactions will evolve in a community.

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.

Aboveground phytochemical responses to belowground herbivory in poplar trees and the consequence for leaf herbivore preference

Belowground (BG) herbivory can influence aboveground (AG) herbivore performance and food preference via changes in plant chemistry. Most evidence for this phenomenon derives from studies in herbaceous plants but studies in woody plants are scarce. Here we investigated whether and how BG herbivory on black poplar (Populus nigra) trees by Melolontha melolontha larvae influences the feeding preference of Lymantria dispar (gypsy moth) caterpillars. In a food choice assay, caterpillars preferred to feed on leaves from trees that had experienced attack by BG herbivores. Therefore, we investigated the effect of BG herbivory on the phytochemical composition of P. nigra trees alone and in combination with AG feeding by L. dispar caterpillars. BG herbivory did not increase systemic AG tree defences like volatile organic compounds, protease inhibitors and salicinoids. Jasmonates and salicylic acid were also not induced by BG herbivory in leaves but abscisic acid concentrations drastically increased together with proline and few other amino acids. Leaf coating experiments with amino acids suggest that proline might be responsible for the caterpillar feeding preference via presumptive phagostimulatory properties. This study shows that BG herbivory in poplar can modify the feeding preference of AG herbivores via phytochemical changes as a consequence of root-to-shoot signaling.

Unravelling the Complexity of Plant Defense Induced by a Simultaneous and Sequential Mite and Aphid Infestation

In natural and agricultural conditions, plants are attacked by a community of herbivores, including aphids and mites. The green peach aphid and the two-spotted spider mite, both economically important pests, may share the same plant. Therefore, an important question arises as to how plants integrate signals induced by dual herbivore attack into the optimal defensive response. We showed that regardless of which attacker was first, 24 h of infestation allowed for efficient priming of the Arabidopsis defense, which decreased the reproductive performance of one of the subsequent herbivores. The expression analysis of several defense-related genes demonstrated that the individual impact of mite and aphid feeding spread systematically, engaging the salicylic acid (SA) and jasmonic acid (JA) signaling pathways. Interestingly, aphids feeding on the systemic leaf of the plant simultaneously attacked by mites, efficiently reduced the magnitude of the SA and JA activation, whereas mites feeding remotely increased the aphid-induced SA marker gene expression, while the JA-dependent response was completely abolished. We also indicated that the weaker performance of mites and aphids in double infestation essays might be attributed to aliphatic glucosinolates. Our report is the first to provide molecular data on signaling cross-talk when representatives of two distinct taxonomical classes within the phylum Arthropoda co-infest the same plant.

Neonicotinoid Insecticides Alter the Transcriptome of Soybean and Decrease Plant Resistance

Neonicotinoids are widely used systemic insecticides that have been associated with spider mite outbreaks on diverse plants. These insecticides have complex effects on plant physiology, which have been speculated to drive enhanced performance of spider mites. We used RNA-Seq to explore how neonicotinoids modify gene expression in soybean thereby lowering plant resistance. We exposed soybean (Glycine max L.) to two neonicotinoid insecticides, thiamethoxam applied to seeds and imidacloprid applied as a soil drench, and we exposed a subset of these plants to spider mites (Tetranychus cinnabarinus). Applications of both insecticides downregulated genes involved in plant-pathogen interactions, phytohormone pathways, phenylpropanoid pathway, and cell wall biosynthesis. These effects were especially pronounced in plants exposed to thiamethoxam. Introduction of spider mites restored induction of genes in these pathways in plants treated with imidacloprid, while expression of genes involved in phenylpropanoid synthesis, in particular, remained downregulated in thiamethoxam-treated plants. Our outcomes indicate that both insecticides suppress genes in pathways relevant to plant⁻arthropod interactions, and suppression of genes involved in cell wall synthesis may explain lower plant resistance to spider mites, cell-content feeders. These effects appear to be particularly significant when plants are exposed to neonicotinoids applied to soybean seeds.

Induction of Jasmonic Acid-Associated Defenses by Thrips Alters Host Suitability for Conspecifics and Correlates with Increased Trichome Densities in Tomato

Plant defenses inducible by herbivorous arthropods can determine performance of subsequent feeding herbivores. We investigated how infestation of tomato (Solanum lycopersicum) plants with the Western flower thrips (Frankliniella occidentalis) alters host plant suitability and foraging decisions of their conspecifics. We explored the role of delayed-induced jasmonic acid (JA)-mediated plant defense responses in thrips preference by using the tomato mutant def-1, impaired in JA biosynthesis. In particular, we investigated the effect of thrips infestation on trichome-associated tomato defenses. The results showed that when offered a choice, thrips preferred non-infested plants over infested wild-type plants, while no differences were observed in def-1. Exogenous application of methyl jasmonate restored the repellency effect in def-1. Gene expression analysis showed induction of the JA defense signaling pathway in wild-type plants, while activating the ethylene signaling pathway in both genotypes. Activation of JA defenses led to increases in type-VI leaf glandular trichome densities in the wild type, augmenting the production of trichome-associated volatiles, i.e. terpenes. Our study revealed that plant-mediated intraspecific interactions between thrips are determined by JA-mediated defenses in tomato. We report that insects can alter not only trichome densities but also the allelochemicals produced therein, and that this response might depend on the magnitude and/or type of the induction.

Plant-Mediated Systemic Interactions Between Pathogens, Parasitic Nematodes, and Herbivores Above- and Belowground

Plants are important mediators of interactions between aboveground (AG) and belowground (BG) pathogens, arthropod herbivores, and nematodes (phytophages). We highlight recent progress in our understanding of within- and cross-compartment plant responses to these groups of phytophages in terms of altered resource dynamics and defense signaling and activation. We review studies documenting the outcome of cross-compartment interactions between these phytophage groups and show patterns of cross-compartment facilitation as well as cross-compartment induced resistance. Studies involving soilborne pathogens and foliar nematodes are scant. We further highlight the important role of defense signaling loops between shoots and roots to activate a full resistance complement. Moreover, manipulation of such loops by phytophages affects systemic interactions with other plant feeders. Finally, cross-compartment-induced changes in root defenses and root exudates extend systemic defense loops into the rhizosphere, enhancing or reducing recruitment of microbes that induce systemic resistance but also affecting interactions with root-feeding phytophages.