The role of abscisic acid and water stress in root herbivore-induced leaf resistance

The jasmonate pathway and water loss in rice leaves induced by a stem-borer inhibit leaf-feeder growth

Plant-mediated interactions between herbivores play an important role in regulating the composition of herbivore community. The fall armyworm (FAW), Spodoptera frugiperda, which has become one of the most serious pests on corn in China since it invaded in 2018, has been found feeding rice in the field. However, how FAW interacts with native rice insect pests remains largely unknown. Here, we investigated the interaction between FAW and a resident herbivore, striped stem borer (SSB, Chilo suppressalis) on rice. The infestation of rice leaf sheaths (LSs) by SSB larvae systemically enhanced the level of jasmonic acid (JA), abscisic acid (ABA), and trypsin proteinase inhibitors (TPIs), reduced relative water content (RWC) in leaf blades (LBs), and suppressed the growth of FAW larvae. In contrast, because FAW larvae infested LBs and did not affect defence responses in LSs, they did not influence the performance of SSB larvae. Using different mutants, together with bioassays and chemical analysis, we revealed that SSB-induced suppression of FAW larvae growth depended on both the SSB-activated JA pathway and RWC in LBs, whereas the ABA pathway activated by SSB larvae promoted the growth of FAW larvae by impeding water loss. These results provide new insights into mechanisms underlying plant-mediated interactions between herbivores.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

Root volatile profiles and herbivore preference are mediated by maize domestication, geographic spread, and modern breeding

Domestication affected the abundances and diversity of maize root volatiles more than northward spread and modern breeding, and herbivore preference for roots was correlated with volatile diversity and herbivore resistance. Studies show that herbivore defenses in crops are mediated by domestication, spread, and breeding, among other human-driven processes. They also show that those processes affected chemical communication between crop plants and herbivores. We hypothesized that (i) preference of the herbivore (Diabrotica virgifera virgifera) larvae for embryonic roots of maize (Zea mays mays) would increase and (ii) root volatile diversity would decrease with the crop's domestication, northward spread to present-day USA, and modern breeding. We used Balsas teosinte (Zea mays parviglumis), Mexican and USA landrace maizes, and US inbred maize lines to test these hypotheses. We found that herbivore preference and volatile diversity increased with maize domestication and northward spread but decreased with modern breeding. Additionally, we found that the abundances of single volatiles did not consistently increase or decrease with maize domestication, spread, and breeding; rather, volatiles grouped per their abundances were differentially affected by those processes, and domestication had the greatest effects. Altogether, our results suggested that: the herbivore's preference for maize roots is correlated with volatile diversity and herbivore resistance; changes in abundances of individual volatiles are evident at the level of volatile groups; and maize domestication, but not spread and breeding, affected the abundances of some green leaf volatiles and sesquiterpenes/sesquiterpenoids. In part, we discussed our results in the context of herbivore defense evolution when resources for plant growth and defense vary across environments. We suggested that variability in relative abundance of volatiles may be associated with their local, functional relevance across wild and agricultural environments.

Differential responses of leaf photosynthesis to insect and pathogen outbreaks: A global synthesis

Outbreak of insects or pathogens (referred to as biotic disturbance), which is projected to continually increase in a warmer climate, may profoundly affect plant photosynthesis and production. However, the response of plant photosynthesis to biotic disturbance remains unclear, especially differences in response between insects and pathogens, which hinders the prediction of plant productivity in future climate. In this study, a meta-analysis approach was used to examine effects of insects and pathogens on photosynthetic rate per unit leaf area (Pn) and the associated characteristics from 115 studies. Our results showed that biotic disturbance significantly decreased Pn by 34.8% but increased Rd by 26.2%. Most of parameters associated with Pn were significantly reduced by biotic disturbance, including g_s, T_r, photosynthetic pigments (e.g., a+b, a, and b), and chlorophyll fluorescence properties (Fv/Fm, q_p). The disturbance type (insects vs pathogens) was the most important factor affecting the response of Pn, with a greater decrease in Pn by pathogens (-37.5%) than insects (-28.0%). The response ratio of Pn was positively correlated with that of g_s and Tr for both insects and pathogens, while negatively with C_i and positively with Chl a+b, Φ_PSII, and q_p for only pathogens. In addition, the higher sensitivity of Pn to biotic disturbance in crop than non-crop plants poses a great challenge to agricultural system in the future. The weighted response ratio of Pn and relationships of Pn with other associated paramerters under insect and pathogen disturbance will facilitate vegetation models to integrate the effects of biotic disturbance on primary production, improving predicition of the ecosystem carbon cyling in combining with leaf area measurement.

Belowground and aboveground herbivory differentially affect the transcriptome in roots and shoots of maize

Plants recognize and respond to feeding by herbivorous insects by upregulating their local and systemic defenses. While defense induction by aboveground herbivores has been well studied, far less is known about local and systemic defense responses against attacks by belowground herbivores. Here, we investigated and compared the responses of the maize transcriptome to belowground and aboveground mechanical damage and infestation by two well-adapted herbivores: the soil-dwelling western corn rootworm Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) and the leaf-chewing fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae). In responses to both herbivores, maize plants were found to alter local transcription of genes involved in phytohormone signaling, primary and secondary metabolism. Induction by real herbivore damage was considerably stronger and modified the expression of more genes than mechanical damage. Feeding by the corn rootworm had a strong impact on the shoot transcriptome, including the activation of genes involved in defense and development. By contrast, feeding by the fall armyworm induced only few transcriptional changes in the roots. In conclusion, feeding by a leaf chewer and a root feeder differentially affects the local and systemic defense of maize plants. Besides revealing clear differences in how maize plants respond to feeding by these specialized herbivores, this study reveals several novel genes that may play key roles in plant-insect interactions and thus sets the stage for in depth research into the mechanism that can be exploited for improved crop protection.

Significance statement

Extensive transcriptomic analyses revealed a clear distinction between the gene expression profiles in maize plants upon shoot and root attack, locally as well as distantly from the attacked tissue. This provides detailed insights into the specificity of orchestrated plant defense responses, and the dataset offers a molecular resource for further genetic studies on maize resistance to herbivores and paves the way for novel strategies to enhance maize resistance to pests.

Leafminer attack accelerates the development of soil-dwelling conspecific pupae via plant-mediated changes in belowground volatiles

Herbivore population dynamics are strongly influenced by the interactions established through their shared host. Such plant-mediated interactions can occur between different herbivore species and different life developmental stages of the same herbivore. However, whether these interactions occur between leaf-feeding herbivores and their soil-dwelling pupae is unknown. We studied whether tomato (Solanum lycopersicum) leaf herbivory by the American serpentine leafminer Liriomyza trifolii affects the performance of conspecific pupae exposed to the soil headspace of the plant. To gain mechanistic insights, we performed insect bioassays with the jasmonate-deficient tomato mutant def-1 and its wild-type, along with phytohormones, gene expression and root volatiles analyses. Belowground volatiles accelerated leafminer metamorphosis when wild-type plants were attacked aboveground by conspecifics. The opposite pattern was observed for def-1 plants, in which aboveground herbivory slowed metamorphosis. Leafminer attack induced jasmonate and abscisic acid accumulation and modulated volatile production in tomato roots in a def-1-dependent manner. Our results demonstrate that aboveground herbivory triggers changes in root defence signalling and expression, which can directly or indirectly via changes in soil or microbial volatiles, alter pupal development time. This finding expands the repertoire of plant-herbivore interactions to herbivory-induced modulation of metamorphosis, with potential consequences for plant and herbivore community dynamics.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Maize biochemistry in response to root herbivory was mediated by domestication, spread, and breeding

With domestication, northward spread, and breeding, maize defence against root-herbivores relied on induced defences, decreasing levels of phytohormones involved in resistance, and increasing levels of a phytohormone involved in tolerance. We addressed whether a suite of maize (Zea mays mays) phytohormones and metabolites involved in herbivore defence were mediated by three successive processes: domestication, spread to North America, and modern breeding. With those processes, and following theoretical predictions, we expected to find: a change in defence strategy from reliance on induced defences to reliance on constitutive defences; decreasing levels of phytohormones involved in herbivore resistance, and; increasing levels of a phytohormone involved in herbivore tolerance. We tested those predictions by comparing phytohormone levels in seedlings exposed to root herbivory by Diabrotica virgifera virgifera among four plant types encompassing those processes: the maize ancestor Balsas teosinte (Zea mays parviglumis), Mexican maize landraces, USA maize landraces, and USA inbred maize cultivars. With domestication, maize transitioned from reliance on induced defences in teosinte to reliance on constitutive defences in maize, as predicted. One subset of metabolites putatively involved in herbivory defence (13-oxylipins) was suppressed with domestication, as predicted, though another was enhanced (9-oxylipins), and both were variably affected by spread and breeding. A phytohormone (indole-3-acetic acid) involved in tolerance was enhanced with domestication, and with spread and breeding, as predicted. These changes are consistent with documented changes in herbivory resistance and tolerance, and occurred coincidentally with cultivation in increasingly resource-rich environments, i.e., from wild to highly enriched agricultural environments. We concluded that herbivore defence evolution in crops may be mediated by processes spanning thousands of generations, e.g., domestication and spread, as well as by processes spanning tens of generations, e.g., breeding and agricultural intensification.

Water Dipping of Auxin Coated Chrysanthemum Cuttings Confers Protection against Insect Herbivores

Simple Summary

Cultivated chrysanthemums are one of the most economically important ornamental greenhouse crops worldwide. Classical breeding programs have mainly focused on improving aesthetic characteristics to meet the continuous increasing customer demands for new flower varieties. Consequently, commercial cultivars often lack insect resistance traits. Among the most important production constraints are biotic foes, in particular thrips and leaf miner infestations form a prominent hazard during its vegetative state. To maintain the desired aesthetic characteristics, clonal commercial propagation is aided by the use of auxin hormones for root promotion. This study aims to evaluate the potential of root promoting auxins in antiherbivore defenses. We demonstrate that water dipping of unrooted basal cut ends, coated with the commercial rooting hormone indole-3-butyric acid (IBA), conferred protection in chrysanthemum against thrips and leaf miner. Our findings add an interesting twist to the traditional role of auxins. We advocate a new twist of auxins beyond its traditional role in rooting in order to maximize plant yield by reducing herbivory through feasible, cost-effective water dipping treatments.

Abstract

Auxins are commonly used for commercial propagation of chrysanthemums by stem cuttings. Recent studies imply that these root-promoting hormones also affect plant defense responses. The underlying motive of this study stems from the serendipitous observation that water dipping of auxin-coated cuttings beneficially affected thrips herbivory. Therefore, the primary objective of this investigation was to explore the role of indole-3-butyric acid (IBA) in relation to herbivore susceptibility in chrysanthemum. We observed contrasting findings concerning the physical presence of IBA and it’s role in promoting susceptibility of cuttings to thrips, which may in part be explained by the phenotypical variations of cuttings generated from mother plants. Nonetheless, we repeatedly demonstrated considerable protection, in some experiments up to 37%, against thrips and leaf miner upon water dipping of IBA-coated cuttings. Assessment of polyphenol oxidase activity (PPO), 14 days after dipping treatment, suggests that neither direct induction nor priming of plant defenses are involved. Future experiments aimed at understanding the early signaling events may help to explain the underlying mechanisms involved in conferring herbivore protection. We propose a dual role for auxins in early integrated pest management strategies to maximize plant development and minimize herbivory through feasible, cost-effective water dipping treatments.

Spodoptera frugiperda Caterpillars Suppress Herbivore-Induced Volatile Emissions in Maize

The vast spectrum of inducible plant defenses can have direct negative effects on herbivores, or indirect effects, for instance in the form of herbivore-induced plant volatiles (HIPVs) that attract natural enemies. Various arthropods have evolved ways to suppress plant defenses. To test whether this is the case for caterpillar-induced HIPVs, we compared the volatile induction by Spodoptera frugiperda (Lepidoptera: Noctuidae), which is particularly well adapted to feed on maize (Zea mays), with the induction by three more generalist noctuid larvae. We tested the hypothesis that S. frugiperda suppresses HIPV emissions in maize, and thereby reduces attractiveness to natural enemies. HIPV emissions triggered by S. frugiperda when feeding on maize were indeed found to be significantly weaker than by Spodoptera littoralis, Spodoptera exigua, and Helicoverpa armigera. The suppression seems specific for maize, as we found no evidence for this when S. frugiperda caterpillars fed on cotton (Gossypium herbaceum). Artificially damaged maize plants treated with larval regurgitant revealed that HIPV suppression may be related to factors in the caterpillars' oral secretions. We also found evidence that differential physical damage that the caterpillars inflict on maize leaves may play a role. The suppressed induction of HIPVs had no apparent consequences for the attraction of a common parasitoid of S. frugiperda, Cotesia marginiventris (Hymenoptera: Braconidae). Nevertheless, the ability to manipulate the defenses of its main host plant may have contributed to the success of S. frugiperda as a major pest of maize, especially in Africa and Asia, which it has recently invaded.

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.

Transcriptomic and Phytochemical Analyses Reveal Root-Mediated Resource-Based Defense Response to Leaf Herbivory by Ectropis oblique in Tea Plant ( Camellia sinensis)

Leaf herbivory on tea plants ( Camellia sinensis) by tea geometrids ( Ectropis oblique) severely threaten the yield and quality of tea. In previous work, we found that local defense response was induced in damaged leaves by geometrids at the transcriptome level. Here, we investigated the systemic response triggered in undamaged roots and the potential role of roots in response to leaf herbivory. Comparative transcriptome analysis and carbohydrate dynamics indicated that leaf herbivory activated systemic carbon reallocation to enhance resource investment for local secondary metabolism. The crucial role of jasmonic acid and the involvement of other potential hormone signals for local and systemic signaling networks were supported by phytohormone quantification and dynamic expression analysis of phytohormone-related genes. This work represents a deep understanding of the interaction of tea plants and geometrids from the perspective of systems biology and reveals that tea plants have evolved an intricate root-mediated resource-based resistance strategy to cope with geometrid attack.

Defence-related priming and responses to recurring drought: Two manifestations of plant transcriptional memory mediated by the ABA and JA signalling pathways

Collective evidence from agricultural practices and from scientific research has demonstrated that plants can alter their phenotypic responses to repeated biotic and abiotic stresses or their elicitors. A coordinated reaction at the organismal, cellular, and genome levels has suggested that plants can "remember" an earlier stress and modify their future responses, accordingly. Stress memory may increase a plant's survival chances by improving its tolerance/avoidance abilities and may provide a mechanism for acclimation and adaptation. Understanding the mechanisms that regulate plant stress memory is not only an intellectually challenging topic but has important implications for agricultural practices as well. Here, I focus exclusively on specific aspects of the transcription memory in response to recurring dehydration stresses and the memory-type responses to insect damage in a process known as "priming." The questions discussed are (a) whether/how the two memory phenomena are connected at the level of transcriptional regulation; (b) how differential transcription is achieved mechanistically under a repeated stress; and (c) whether similar molecular and/or epigenetic mechanisms are involved. Possible biological relevance of transcriptional stress memory and its preservation in plant evolution are also discussed.

The Asian corn borer Ostrinia furnacalis feeding increases the direct and indirect defence of mid-whorl stage commercial maize in the field

The Asian corn borer (Ostrinia furnacalis Guenée) is a destructive pest of maize (Zea mays L.). Despite large-scale commercial maize production, little is known about the defensive responses of field-grown commercial maize to O. furnacalis herbivory, and how these responses result in direct and indirect defence against this pest. To elucidate the maize transcriptome response to O. furnacalis feeding, leaves of maize hybrid Jingke968 were infested with O. furnacalis for 0, 2, 4, 12 and 24 h. Ostrinia furnacalis feeding elicited stronger and more rapid changes in the defence-related gene expression (i.e. after 2 h), and more differentially expressed genes (DEGs) were up-regulated than down-regulated at all times post-induction (i.e. 2, 4, 12 and 24 h) in the O. furnacalis pre-infested maize plants. KEGG pathway analysis indicated that the DEGs in the O. furnacalis pre-infested maize are involved in benzoxazinoids, phytohormones, volatiles, and other metabolic pathways related to maize resistance to herbivores. In addition, the maize leaves previously infested by O. furnacalis for 24 h showed an obvious inhibition of the subsequent O. furnacalis performance, and maize volatiles induced by O. furnacalis feeding for 24 and 48 h attracted the parasitic wasp, Macrocentrus cingulum Brischke. The increased direct and indirect defences induced by O. furnacalis feeding were correlated with O. furnacalis-induced phytohormones, benzoxazinoids, and volatiles. Together, our findings provide new insights into how commercial maize orchestrates its transcriptome and metabolome to directly and indirectly defend against O. furnacalis at the mid-whorl stage in the field.

Root JA Induction Modifies Glucosinolate Profiles and Increases Subsequent Aboveground Resistance to Herbivore Attack in Cardamine hirsuta

Alteration and induction of plant secondary metabolites after herbivore attack have been shown in almost all the studied plant species. Induction can be at the local site of damage, or systemic, such as from roots to shoots. In addition to immediate induction, previous herbivore bouts have been shown to "prime" the plants for a stronger and faster response only after a subsequent attack happens. Whereas several studies revealed a link between root herbivory and increased resistance against aboveground (AG) herbivory, the evidence of root defense priming against subsequent AG herbivory is currently lacking. To address this gap, we induced Cardamine hirsuta roots by applying jasmonic acid (JA), and, after a time lag, we subjected both control and JA-treated plants to AG herbivory by the generalist herbivore Spodoptera littoralis. We addressed the effect of root JA addition on AG herbivore resistance by measuring larval weight gain and tested the effect of root induction on abundance and composition of glucosinolates (GSLs) in shoots, prior, and after subsequent herbivory. We observed a strong positive effect of root induction on the resistance against AG herbivory. The overall abundance and identity of GSLs was globally affected by JA induction and by herbivore feeding, independently, and we found a significant correlation between larval growth and the shoot GSL profiles only after AG herbivory, 11 days after induction in roots. Contrary to expectations of priming, we observed that JA induction in roots altered the GSLs profile in the leaves that was maintained through time. This initial modification was sufficient to maintain a lower caterpillar weight gain, even 11 days post-root induction. Altogether, we show that prior root defense induction increases AG insect resistance by modifying and maintaining variation in GSL profiles during insect feeding.

Plant iron acquisition strategy exploited by an insect herbivore

Insect herbivores depend on their host plants to acquire macro- and micronutrients. Here we asked how a specialist herbivore and damaging maize pest, the western corn rootworm, finds and accesses plant-derived micronutrients. We show that the root-feeding larvae use complexes between iron and benzoxazinoid secondary metabolites to identify maize as a host, to forage within the maize root system, and to increase their growth. Maize plants use these same benzoxazinoids for protection against generalist herbivores and, as shown here, for iron uptake. We identify an iron transporter that allows the corn rootworm to benefit from complexes between iron and benzoxazinoids. Thus, foraging for an essential plant-derived complex between a micronutrient and a secondary metabolite shapes the interaction between maize and a specialist herbivore.

Influence of drought on plant performance through changes in belowground tritrophic interactions

Climate change is predicted to increase the risk of drought in many temperate agroecosystems. While the impact of drought on aboveground plant-herbivore-natural enemy interactions has been studied, little is known about its effects on belowground tritrophic interactions and root defense chemistry. We investigated the effects of low soil moisture on the interaction between maize, the western corn rootworm (WCR, Diabrotica virgifera), and soil-borne natural enemies of WCR. In a manipulative field experiment, reduced soil moisture and WCR attack reduced plant performance and increased benzoxazinoid levels. The negative effects of WCR on cob dry weight and silk emergence were strongest at low moisture levels. Inoculation with entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora) was ineffective in controlling WCR, and the EPNs died rapidly in the warm and dry soil. However, ants of the species Solenopsis molesta invaded the experiment, were more abundant in WCR-infested pots and predated WCR independently of soil moisture. Ant presence increased root and shoot biomass and was associated with attenuated moisture-dependent effects of WCR on maize cob weight. Our study suggests that apart from directly reducing plant performance, drought can also increase the negative effects of root herbivores such as WCR. It furthermore identifies S. molesta as a natural enemy of WCR that can protect maize plants from the negative impact of herbivory under drought stress. Robust herbivore natural enemies may play an important role in buffering the impact of climate change on plant-herbivore interactions.

Can Plant Defence Mechanisms Provide New Approaches for the Sustainable Control of the Two-Spotted Spider Mite Tetranychus urticae?

Tetranychus urticae (T. urticae) Koch is a cosmopolitan, polyphagous mite which causes economic losses in both agricultural and ornamental plants. Some traits of T. urticae hamper its management, including a short life cycle, arrhenotokous parthenogenesis, its haplodiploid sex determination system, and its extraordinary ability to adapt to different hosts and environmental conditions. Currently, the use of chemical and biological control are the major control methods used against this mite. In recent years, some studies have focused on plant defence mechanisms against herbivores. Various families of plant compounds (such as flavonoids, glucosinolates, or acyl sugars) have been shown to behave as acaricides. Plants can be induced upon appropriate stimuli to increase their resistance against spider mites. This knowledge, together with the understanding of mechanisms by which T. urticae detoxifies and adapts to pesticides, may complement the control of this pest. Herein, we describe plant volatile compounds (VOCs) with repellent activity, and new findings about defence priming against spider mites, which interfere with the T. urticae performance. The use of VOCs and defence priming can be integrated into current management practices and reduce the damage caused by T. urticae in the field by implementing new, more sustainable crop management tools.

Response of Maize Hybrids With and Without Rootworm- and Drought-Tolerance to Rootworm Infestation Under Well-Watered and Drought Conditions

Anecdotal data in the past have suggested that the effect of the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), on maize yield is greater under drought and the effect of drought is greater under rootworm infestations, but no field experiments have controlled both moisture and rootworm levels. Field studies were conducted in 2012, 2013, and 2014 with treatments in a factorial arrangement of western corn rootworm infestation levels, and maize hybrids (with and without tolerance to drought and rootworm feeding). The experiment was repeated under well-watered and drought conditions in adjacent plots. Leaf water potential and stomatal conductance data suggested significant plant stress was achieved in the drought plots toward the end of the season each year and maize hybrids only played a minor role. In particular, in 2012 and 2013 yield was dramatically lower for the drought experiment than for the well-watered experiment. However, the impacts of rootworm infestation level and maize hybrids on water potential, stomatal conductance, and yield were variable across years and between experiments. In fact, the only year that the main effect of rootworm infestation levels significantly impacted yield was in 2014, when an extremely high infestation level was added and this was only for the well-watered portion of the experiment. Overall, rootworm infestation level played a relatively minor role in maize productivity and it did not appear that soil moisture level influenced that to a large degree.

Drought stress leads to systemic induced susceptibility to a nectrotrophic fungus associated with mountain pine beetle in Pinus banksiana seedlings

Conifers have complex defense responses to initial attacks by insects and pathogens that can have cascading effects on success of subsequent colonizers. However, drought can affect a plant's ability to respond to biotic agents by potentially altering the resources needed for the energetically costly production of induced defense chemicals. We investigated the impact of reduced water on induced chemical defenses of jack pine (Pinus banksiana) seedlings from initial attack by biotic agents and resistance to subsequent challenge inoculation with a pathogenic fungal associate of mountain pine beetle (Dendroctonus ponderosae), Grosmannia clavigera. Applications of phytohormones (methyl salicylate and methyl jasmonate) and G. clavigera were used for initial induction of defenses. Monoterpene concentrations varied with initial induction from fungal and phytohormone application while watering treatment had no effect. Seedlings treated with G. clavigera and methyl jasmonate had the greatest monoterpene concentrations compared to the control and methyl salicylate-treated seedlings. However, the monoterpene response to the challenge inoculation varied with watering treatments, not with prior induction treatments, with lower monoterpene concentrations in fungal lesions on seedlings in the low to moderate watering treatments compared to normal watering treatment. Furthermore, prior induction from phytohormones resulted in systemic cross-induction of resistance to G. clavigera under normal watering treatment but susceptibility under low watering treatment. Seedlings stressed by low water conditions, which also had lower stomatal conductance than seedlings in the normal watering treatment, likely allocated resources to initial defense response but were left unable to acquire further resources for subsequent responses. Our results demonstrate that drought can affect interactions among tree-infesting organisms through systemic cross-induction of susceptibility.

What happens in the pith stays in the pith: tissue-localized defense responses facilitate chemical niche differentiation between two spatially separated herbivores

Herbivore attack is known to elicit systemic defense responses that spread throughout the host plant and influence the performance of other herbivores. While these plant-mediated indirect competitive interactions are well described, and the co-existence of herbivores from different feeding guilds is common, the mechanisms of co-existence are poorly understood. In both field and glasshouse experiments with a native tobacco, Nicotiana attenuata, we found no evidence of negative interactions when plants were simultaneously attacked by two spatially separated herbivores: a leaf chewer Manduca sexta and a stem borer Trichobaris mucorea. T. mucorea attack elicited jasmonic acid (JA) and jasmonoyl-l-isoleucine bursts in the pith of attacked stems similar to those that occur in leaves when M. sexta attacks N. attenuata leaves. Pith chlorogenic acid (CGA) levels increased 1000-fold to levels 6-fold higher than leaf levels after T. mucorea attack; these increases in pith CGA levels, which did not occur in M. sexta-attacked leaves, required JA signaling. With plants silenced in CGA biosynthesis (irHQT plants), CGA, as well as other caffeic acid conjugates, was demonstrated in both glasshouse and field experiments to function as a direct defense protecting piths against T. mucorea attack, but not against leaf chewers or sucking insects. T. mucorea attack does not systemically activate JA signaling in leaves, while M. sexta leaf-attack transiently induces detectable but minor pith JA levels that are dwarfed by local responses. We conclude that tissue-localized defense responses allow tissue-specialized herbivores to share the same host and occupy different chemical defense niches in the same hostplant.

A mechanism for sequence specificity in plant-mediated interactions between herbivores

Herbivore communities are shaped by indirect plant-mediated interactions whose outcomes are strongly dependent on the sequence of herbivore arrival. However, the mechanisms underlying sequence specificity are poorly understood. We examined the mechanisms that govern sequence-specific effects of the interaction between two specialist maize herbivores, the leaf feeder Spodoptera frugiperda and the root feeder Diabrotica virgifera virgifera. In the field, S. frugiperda reduces D. v. virgifera abundance, but only when it arrives on the plant first. In behavioral experiments, D. v. virgifera larvae continued feeding on plants that they had infested before leaf infestation, but refused to initiate feeding on plants that were infested by S. frugiperda before their arrival. Changes in root-emitted volatiles were sufficient to elicit this sequence-specific behavior. Root volatile and headspace mixing experiments showed that early-arriving D. v. virgifera larvae suppressed S. frugiperda-induced volatile repellents, which led to the maintenance of host attractiveness to D. v. virgifera. Our study provides a physiological and behavioral mechanism for sequence specificity in plant-mediated interactions and suggests that physiological canalization of behaviorally active metabolites can drive sequence specificity and result in strongly diverging herbivore distribution patterns.

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.

How plants handle multiple stresses: hormonal interactions underlying responses to abiotic stress and insect herbivory

Adaptive plant responses to specific abiotic stresses or biotic agents are fine-tuned by a network of hormonal signaling cascades, including abscisic acid (ABA), ethylene, jasmonic acid (JA) and salicylic acid. Moreover, hormonal cross-talk modulates plant responses to abiotic stresses and defenses against insect herbivores when they occur simultaneously. How such interactions affect plant responses under multiple stresses, however, is less understood, even though this may frequently occur in natural environments. Here, we review our current knowledge on how hormonal signaling regulates abiotic stress responses and defenses against insects, and discuss the few recent studies that attempted to dissect hormonal interactions occurring under simultaneous abiotic stress and herbivory. Based on this we hypothesize that drought stress enhances insect resistance due to synergistic interactions between JA and ABA signaling. Responses to flooding or waterlogging involve ethylene signaling, which likely reduces plant resistance to chewing herbivores due to its negative cross-talk with JA. However, the outcome of interactions between biotic and abiotic stress signaling is often plant and/or insect species-dependent and cannot simply be predicted based on general knowledge on the involvement of signaling pathways in single stress responses. More experimental data on non-model plant and insect species are needed to reveal general patterns and better understand the molecular mechanisms allowing plants to optimize their responses in complex environments.

Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

Drought and flooding have distinct effects on herbivore-induced responses and resistance in Solanum dulcamara

In the field, biotic and abiotic stresses frequently co-occur. As a consequence, common molecular signalling pathways governing adaptive responses to individual stresses can interact, resulting in compromised phenotypes. How plant signalling pathways interact under combined stresses is poorly understood. To assess this, we studied the consequence of drought and soil flooding on resistance of Solanum dulcamara to Spodoptera exigua and their effects on hormonal and transcriptomic profiles. The results showed that S. exigua larvae performed less well on drought-stressed plants than on well-watered and flooded plants. Both drought and insect feeding increased abscisic acid and jasmonic acid (JA) levels, whereas flooding did not induce JA accumulation. RNA sequencing analyses corroborated this pattern: drought and herbivory induced many biological processes that were repressed by flooding. When applied in combination, drought and herbivory had an additive effect on specific processes involved in secondary metabolism and defence responses, including protease inhibitor activity. In conclusion, drought and flooding have distinct effects on herbivore-induced responses and resistance. Especially, the interaction between abscisic acid and JA signalling may be important to optimize plant responses to combined drought and insect herbivory, making drought-stressed plants more resistant to insects than well-watered and flooded plants.

Similar local, but different systemic, metabolomic responses of closely related pine subspecies to folivory by caterpillars of the processionary moth

Plants respond locally and systemically to herbivore attack. Most of the research conducted on plant-herbivore relationships at element and molecular levels have focused on the elemental composition or/and certain molecular compounds or specific families of defence metabolites showing that herbivores tend to select plant individuals or species with higher nutrient concentrations and avoid those with higher levels of defence compounds. We performed stoichiometric and metabolomics, both local and systemic, analyses in two subspecies of Pinus sylvestris under attack from caterpillars of the pine processionary moth, an important pest in the Mediterranean Basin. Both pine subspecies responded locally to folivory mainly by increasing relative concentrations of terpenes and some phenolics. Systemic responses differed between pine subspecies, and most of the metabolites presented intermediate concentrations between those of the affected parts and unattacked trees. Our results support the hypothesis that foliar nutrient concentrations are not a key factor for plant selection by adult female processionary moths for oviposition, since folivory was not associated with any of the elements analysed. Phenolic compounds generally did not increase in the attacked trees, questioning the suggestion of induction of phenolics following folivory attack and the anti-feeding properties of phenolics. Herbivory attack produced a general systemic shift in pines, in both primary and secondary metabolism, which was less intense and chemically different from the local responses. Local pine responses were similar between pine subspecies, while systemic responses were more distant.

Climate Change and Defense against Pathogens in Plants

Most reviews of climate change are epidemiological, focusing on impact assessment and risk mapping. However, there are many reports of the effects of environmental stress factors on defense mechanisms in plants against pathogens. We review those representative of key climate change-related stresses to determine whether there are any patterns or trends in adaptation responses. We recognize the complexity of climate change itself and the multitrophic nature of the complex biological interactions of plants, microbes, soil, and the environment and, therefore, the difficulty of reductionist dissection approaches to resolving the problems. We review host defense genes, germplasm, and environmental interactions in different types of organisms but find no significant group-specific trends. Similarly, we review by host defense mechanism type and by host-pathogen trophic relationship but identify no dominating mechanism for stress response. However, we do identify core stress response mechanisms playing key roles in multiple response pathways whether to biotic or abiotic stress. We suggest that these should be central to mechanistic climate change plant defense research. We also recognize biodiversity, heterogeneity, and the need for understanding stress in a true systems biology approach as being essential components of progressing our understanding of and response to climate change.

Simultaneous expression of regulatory genes associated with specific drought-adaptive traits improves drought adaptation in peanut

Adaptation of crops to drought-prone rain-fed conditions can be achieved by improving plant traits such as efficient water mining (by superior root characters) and cellular-level tolerance mechanisms. Pyramiding these drought-adaptive traits by simultaneous expression of genes regulating drought-adaptive mechanisms has phenomenal relevance in improving stress tolerance. In this study, we provide evidence that peanut transgenic plants expressing Alfalfa zinc finger 1 (Alfin1), a root growth-associated transcription factor gene, Pennisetum glaucum heat-shock factor (PgHSF4) and Pea DNA helicase (PDH45) involved in protein turnover and protection showed improved tolerance, higher growth and productivity under drought stress conditions. Stable integration of all the transgenes was noticed in transgenic lines. The transgenic lines showed higher root growth, cooler crop canopy air temperature difference (less CCATD) and higher relative water content (RWC) under drought stress. Low proline levels in transgenic lines substantiate the maintenance of higher water status. The survival and recovery of transgenic lines was significantly higher under gradual moisture stress conditions with higher biomass. Transgenic lines also showed significant tolerance to ethrel-induced senescence and methyl viologen-induced oxidative stress. Several stress-responsive genes such as heat-shock proteins (HSPs), RING box protein-1 (RBX1), Aldose reductase, late embryogenesis abundant-5 (LEA5) and proline-rich protein-2 (PRP2), a gene involved in root growth, showed enhanced expression under stress in transgenic lines. Thus, the simultaneous expression of regulatory genes contributing for drought-adaptive traits can improve crop adaptation and productivity under water-limited conditions.

A conserved pattern in plant-mediated interactions between herbivores

Plant‐mediated interactions between herbivores are important determinants of community structure and plant performance in natural and agricultural systems. Current research suggests that the outcome of the interactions is determined by herbivore and plant identity, which may result in stochastic patterns that impede adaptive evolution and agricultural exploitation. However, few studies have systemically investigated specificity versus general patterns in a given plant system by varying the identity of all involved players. We investigated the influence of herbivore identity and plant genotype on the interaction between leaf‐chewing and root‐feeding herbivores in maize using a partial factorial design. We assessed the influence of leaf induction by oral secretions of six different chewing herbivores on the response of nine different maize genotypes and three different root feeders. Contrary to our expectations, we found a highly conserved pattern across all three dimensions of specificity: The majority of leaf herbivores elicited a negative behavioral response from the different root feeders in the large majority of tested plant genotypes. No facilitation was observed in any of the treatment combinations. However, the oral secretions of one leaf feeder and the responses of two maize genotypes did not elicit a response from a root‐feeding herbivore. Together, these results suggest that plant‐mediated interactions in the investigated system follow a general pattern, but that a degree of specificity is nevertheless present. Our study shows that within a given plant species, plant‐mediated interactions between herbivores of the same feeding guild can be stable. This stability opens up the possibility of adaptations by associated organisms and suggests that plant‐mediated interactions may contribute more strongly to evolutionary dynamics in terrestrial (agro)ecosystems than previously assumed.

Leaf-mining by Phyllonorycter blancardella reprograms the host-leaf transcriptome to modulate phytohormones associated with nutrient mobilization and plant defense

Phytohormones have long been hypothesized to play a key role in the interactions between plant-manipulating organisms and their host-plants such as insect-plant interactions that lead to gall or 'green-islands' induction. However, mechanistic understanding of how phytohormones operate in these plant reconfigurations is lacking due to limited information on the molecular and biochemical phytohormonal modulation following attack by plant-manipulating insects. In an attempt to fill this gap, the present study provides an extensive characterization of how the leaf-miner Phyllonorycter blancardella modulates the major phytohormones and the transcriptional activity of plant cells in leaves of Malus domestica. We show here, that cytokinins strongly accumulate in mined tissues despite a weak expression of plant cytokinin-related genes. Leaf-mining is also associated with enhanced biosynthesis of jasmonic acid precursors but not the active form, a weak alteration of the salicylic acid pathway and a clear inhibition of the abscisic acid pathway. Our study consolidates previous results suggesting that insects may produce and deliver cytokinins to the plant as a strategy to manipulate the physiology of the leaf to create a favorable nutritional environment. We also demonstrate that leaf-mining by P. blancardella leads to a strong reprogramming of the plant phytohormonal balance associated with increased nutrient mobilization, inhibition of leaf senescence and mitigation of plant direct and indirect defense.

Accumulation of terpenoid phytoalexins in maize roots is associated with drought tolerance

Maize (Zea mays) production, which is of global agro-economic importance, is largely limited by herbivore pests, pathogens and environmental conditions, such as drought. Zealexins and kauralexins belong to two recently identified families of acidic terpenoid phytoalexins in maize that mediate defence against both pathogen and insect attacks in aboveground tissues. However, little is known about their function in belowground organs and their potential to counter abiotic stress. In this study, we show that zealexins and kauralexins accumulate in roots in response to both biotic and abiotic stress including, Diabrotica balteata herbivory, Fusarium verticillioides infection, drought and high salinity. We find that the quantity of drought-induced phytoalexins is positively correlated with the root-to-shoot ratio of different maize varieties, and further demonstrate that mutant an2 plants deficient in kauralexin production are more sensitive to drought. The induction of phytoalexins in response to drought is root specific and does not influence phytoalexin levels aboveground; however, the accumulation of phytoalexins in one tissue may influence the induction capacity of other tissues.

Priming maize resistance by its neighbors: activating 1,4-benzoxazine-3-ones synthesis and defense gene expression to alleviate leaf disease

Plant disease can be effectively suppressed in intercropping systems. Our previous study demonstrated that neighboring maize plants can restrict the spread of soil-borne pathogens of pepper plants by secreting defense compounds into the soil. However, whether maize plant can receive benefits from its neighboring pepper plants in an intercropping system is little attention. We examined the effects of maize roots treated with elicitors from the pepper pathogen Phytophthora capsici and pepper root exudates on the synthesis of 1,4-benzoxazine-3-ones (BXs), the expression of defense-related genes in maize, and their ability to alleviate the severity of southern corn leaf blight (SCLB) caused by Bipolaris maydis. We found that SCLB was significantly reduced after the above treatments. The contents of 1,4-benzoxazine-3-ones (BXs: DIBOA, DIMBOA, and MBOA) and the expression levels of BX synthesis and defense genes in maize roots and shoots were up-regulated. DIMBOA and MBOA effectively inhibited the mycelium growth of Bipolaris maydis at physiological concentrations in maize shoots. Further studies suggested that the defense related pathways or genes in maize roots and shoots were activated by elicitors from the P. capsici or pepper root exudates. In conclusion, maize increased the levels of BXs and defense gene expression both in roots and shoots after being triggered by root exudates and pathogen from neighboring pepper plants, eventually enhancing its resistance.

Jasmonates induce both defense responses and communication in monocotyledonous and dicotyledonous plants

Jasmonic acid (JA) and its derivatives (jasmonates, JAs) are phytohormones with essential roles in plant defense against pathogenesis and herbivorous arthropods. Both the up- and down-regulation of defense responses are dependent on signaling pathways mediated by JAs as well as other stress hormones (e.g. salicylic acid), generally those involving the transcriptional and post-transcriptional regulation of transcription factors via protein modification and epigenetic regulation. In addition to the typical model plant Arabidopsis (a dicotyledon), advances in genetics research have made rice a model monocot in which innovative pest control traits can be introduced and whose JA signaling pathway can be studied. In this review, we introduce the dynamic functions of JAs in plant defense strategy using defensive substances (e.g. indole alkaloids and terpenoid phytoalexins) and airborne signals (e.g. green leaf volatiles and volatile terpenes) in response to biotrophic and necrotrophic pathogens as well as above-ground and below-ground herbivores. We then discuss the important issue of how the mutualism of herbivorous arthropods with viruses or bacteria can cause cross-talk between JA and other phytohormones to counter the defense systems.

Induced carbon reallocation and compensatory growth as root herbivore tolerance mechanisms

Upon attack by leaf herbivores, many plants reallocate photoassimilates below ground. However, little is known about how plants respond when the roots themselves come under attack. We investigated induced resource allocation in maize plants that are infested by the larvae Western corn rootworm Diabrotica virgifera virgifera. Using radioactive (11) CO(2), we demonstrate that root-attacked maize plants allocate more new (11) C carbon from source leaves to stems, but not to roots. Reduced meristematic activity and reduced invertase activity in attacked maize root systems are identified as possible drivers of this shoot reallocation response. The increased allocation of photoassimilates to stems is shown to be associated with a marked thickening of these tissues and increased growth of stem-borne crown roots. A strong quantitative correlation between stem thickness and root regrowth across different watering levels suggests that retaining photoassimilates in the shoots may help root-attacked plants to compensate for the loss of belowground tissues. Taken together, our results indicate that induced tolerance may be an important strategy of plants to withstand belowground attack. Furthermore, root herbivore-induced carbon reallocation needs to be taken into account when studying plant-mediated interactions between herbivores.

Species-specific defence responses facilitate conspecifics and inhibit heterospecifics in above-belowground herbivore interactions

Conspecific and heterospecific aboveground and belowground herbivores often occur together in nature and their interactions may determine community structure. Here we show how aboveground adults and belowground larvae of the tallow tree specialist beetle Bikasha collaris and multiple heterospecific aboveground species interact to determine herbivore performance. Conspecific aboveground adults facilitate belowground larvae, but other aboveground damage inhibits larvae or has no effect. Belowground larvae increase conspecific adult feeding, but decrease heterospecific aboveground insect feeding and abundance. Chemical analyses and experiments with plant populations varying in phenolics show that all these positive and negative effects on insects are closely related to root and shoot tannin concentrations. Our results show that specific plant herbivore responses allow herbivore facilitation and inhibition to co-occur, likely shaping diverse aboveground and belowground communities. Considering species-specific responses of plants is critical for teasing apart inter- and intraspecific interactions in aboveground and belowground compartments.

It is unclear how herbivores determine community structure. Here the authors show how interactions between aboveground adults and belowground larvae of a tree flea beetle and multiple heterospecific aboveground species interact via plant defence responses to determine herbivore performance.

MYB34, MYB51, and MYB122 distinctly regulate indolic glucosinolate biosynthesis in Arabidopsis thaliana

The MYB34, MYB51, and MYB122 transcription factors are known to regulate indolic glucosinolate (IG) biosynthesis in Arabidopsis thaliana. To determine the distinct regulatory potential of MYB34, MYB51, and MYB122, the accumulation of IGs in different parts of plants and upon treatment with plant hormones were analyzed in A. thaliana seedlings. It was shown that MYB34, MYB51, and MYB122 act together to control the biosynthesis of I3M in shoots and roots, with MYB34 controlling biosynthesis of IGs mainly in the roots, MYB51 regulating biosynthesis in shoots, and MYB122 having an accessory role in the biosynthesis of IGs. Analysis of glucosinolate levels in seedlings of myb34, myb51, myb122, myb34 myb51 double, and myb34 myb51 myb122 triple knockout mutants grown in the presence of abscisic acid (ABA), salicylic acid (SA), jasmonate (JA), or ethylene (ET) revealed that: (1) MYB51 is the central regulator of IG synthesis upon SA and ET signaling, (2) MYB34 is the key regulator upon ABA and JA signaling, and (3) MYB122 plays only a minor role in JA/ET-induced glucosinolate biosynthesis. The myb34 myb51 myb122 triple mutant is devoid of IGs, indicating that these three MYB factors are indispensable for IG production under standard growth conditions.

Belowground induction by Delia radicum or phytohormones affect aboveground herbivore communities on field-grown broccoli

Induced plant defence in response to phytophagous insects is a well described phenomenon. However, so far little is known about the effect of induced plant responses on subsequently colonizing herbivores in the field. Broccoli plants were induced in the belowground compartment using (i) infestation by the root-herbivore Delia radicum, (ii) root application of jasmonic acid (JA) or (iii) root application of salicylic acid (SA). The abundance of D. radicum and six aboveground herbivores displaying contrasting levels of host specialization were surveyed for 5 weeks. Our study showed that the response of herbivores was found to differ from one another, depending on the herbivore species, its degree of specialization and the root treatment. The abundance of the root herbivore D. radicum and particularly the number of emerging adults was decreased by both phytohormone treatments, while the number of D. radicum eggs was increased on conspecific infested plants. The root infestation exhibited moderate effects on the aboveground community. The abundance of the aphid Brevicoryne brassicae was strongly increased on D. radicum infested plants, but the other species were not impacted. Root hormone applications exhibited a strong effect on the abundance of specialist foliar herbivores. A higher number of B. brassicae and Pieris brassicae and a lower number of Plutella xylostella were found on JA treated plants. On SA treated plants we observed a decrease of the abundance of B. brassicae, Pi. rapae, and P. xylostella. Surprisingly, generalist species, Mamestra brassicae and Myzus persicae were not affected by root induction treatments. Finally, root treatments had no significant effect on either glucosinolate (GLS) profiles of the heads or on plant quality parameters. These results are discussed from the perspective of below- aboveground interactions and adaptations of phytophagous insects to induced plant responses according to their trophic specialization level.

Facilitation and inhibition: changes in plant nitrogen and secondary metabolites mediate interactions between above-ground and below-ground herbivores

To date, it remains unclear how herbivore-induced changes in plant primary and secondary metabolites impact above-ground and below-ground herbivore interactions. Here, we report effects of above-ground (adult) and below-ground (larval) feeding by Bikasha collaris on nitrogen and secondary chemicals in shoots and roots of Triadica sebifera to explain reciprocal above-ground and below-ground insect interactions. Plants increased root tannins with below-ground herbivory, but above-ground herbivory prevented this increase and larval survival doubled. Above-ground herbivory elevated root nitrogen, probably contributing to increased larval survival. However, plants increased foliar tannins with above-ground herbivory and below-ground herbivory amplified this increase, and adult survival decreased. As either foliar or root tannins increased, foliar flavonoids decreased, suggesting a trade-off between these chemicals. Together, these results show that plant chemicals mediate contrasting effects of conspecific larval and adult insects, whereas insects may take advantage of plant responses to facilitate their offspring performance, which may influence population dynamics.

Drought and root herbivory interact to alter the response of above-ground parasitoids to aphid infested plants and associated plant volatile signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/below-ground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40-55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and below-ground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought.

Soil abiotic factors influence interactions between belowground herbivores and plant roots

Root herbivores are important ecosystem drivers and agricultural pests, and, possibly as a consequence, plants protect their roots using a variety of defensive strategies. One aspect that distinguishes belowground from aboveground plant-insect interactions is that roots are constantly exposed to a set of soil-specific abiotic factors. These factors can profoundly influence root resistance, and, consequently, the outcome of the interaction with belowground feeders. In this review, we synthesize the current literature on the impact of soil moisture, nutrients, and texture on root-herbivore interactions. We show that soil abiotic factors influence the interaction by modulating herbivore abundance and behaviour, root growth and resistance, beneficial microorganisms, as well as natural enemies of the herbivores. We suggest that abiotic heterogeneity may explain the high variability that is often encountered in root-herbivore systems. We also propose that under abiotic stress, the relative fitness value of the roots and the potential negative impact of herbivory increases, which may lead to a higher defensive investment and an increased recruitment of beneficial microorganisms by the plant. At the same time, both root-feeding herbivores and natural enemies are likely to decrease in abundance under extreme environmental conditions, leading to a context- and species-specific impact on plant fitness. Only by using tightly controlled experiments that include soil abiotic heterogeneity will it be possible to understand the impact of root feeders on an ecosystem scale and to develop predictive models for pest occurrence and impact.

Long distance root-shoot signalling in plant-insect community interactions

Plants mediate interactions between insects, including leaf- and root-feeders; yet the underlying mechanisms and connection with ecological theory remain unresolved. In this review, based on novel insights into long-distance (i.e., leaf-leaf, root-shoot) defence signalling, we explore the role of phytohormones in driving broad-scale patterns of aboveground-belowground interactions that can be extrapolated to general plant-insect relationships. We propose that the outcome of intra-feeding guild interactions is generally negative due to induction of similar phytohormonal pathways, whereas between-guild interactions are often positive due to negative signal crosstalk. However, not all outcomes could be explained by feeding guild; we argue that future studies should target ecologically representative plant-insect systems, distinguish subguilds, and include plant growth hormones to improve our understanding of plant-mediated interactions.

UVB radiation and 17-hydroxygeranyllinalool diterpene glycosides provide durable resistance against mirid (Tupiocoris notatus) attack in field-grown Nicotiana attenuata plants

Depending on geographical location, plants are exposed to variable amounts of UVB radiation and herbivore attack. Because the role(s) of UVB in the priming and/or accumulation of plant defence metabolites against herbivores are not well understood, we used field-grown Nicotiana attenuata plants to explore the effects of UVB on herbivore performance. Consistent with previous reports, UVB-exposed plants accumulated higher levels of ultraviolet (UV)-absorbing compounds (rutin, chlorogenic acid, crypto-chlorogenic acid and dicaffeoylspermidine). Furthermore, UVB increased the accumulation of jasmonic acid, jasmonoyl-L-isoleucine and abscisic acid, all phytohormones which regulate plant defence against biotic and abiotic stress. In herbivore bioassays, N. attenuata plants experimentally protected from UVB were more infested by mirids in three consecutive field seasons. Among defence metabolites measured, 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) showed strongly altered accumulation patterns. While constitutive HGL-DTGs levels were higher under UVB, N. attenuata plants exposed to mirid bugs (Tupiocoris notatus) had still more HGL-DTGs under UVB, and mirids preferred to feed on HGL-DTGs-silenced plants when other UVB protecting factors were eliminated by UVB filters. We conclude that UVB exposure not only stimulates UV protective screens but also affects plant defence mechanisms, such as HGL-DTGs accumulation, and modulates ecological interactions of N. attenuata with its herbivores in nature.

Foliar herbivory triggers local and long distance defense responses in maize

Many studies have documented the induction of belowground defenses in plants in response to aboveground herbivory and vice versa, but the genes and signaling molecules mediating systemic induction are not well understood. We performed comparative microarray analysis on maize whorl and root tissues from the insect resistant inbred Mp708 in response to foliar feeding by fall armyworm (Spodoptera frugiperda) caterpillars. Although Mp708 has elevated jasmonic acid (JA) levels prior to herbivory, genes involved in JA biosynthesis were up-regulated in whorls in response to fall armyworm feeding. Alternatively, genes possibly involved in regulating ethylene (ET) perception and signaling were up-regulated in roots following foliar herbivory. Transcript levels of genes encoding proteins involved in direct defenses against herbivores were enhanced both in roots and leaves, but transcriptional factors and genes involved in various biosynthetic pathways were selectively down-regulated in the whorl. The results indicate that foliar herbivory by fall armyworm changes root gene expression pathways suggesting profound long distance signaling. Tissue specific induction and suppression of JA and ET signaling pathway genes provides a clue to their possible roles in signaling between the two distant tissue types that eventually triggers defense responses in the roots in response to foliar herbivory.