Silencing the alarm: an insect salivary enzyme closes plant stomata and inhibits volatile release

Secret Weapon of Insects: The Oral Secretion Cocktail and Its Modulation of Host Immunity

Plants and insects have co-existed for almost 400 million years and their interactions can be beneficial or harmful, thus reflecting their intricate co-evolutionary dynamics. Many herbivorous arthropods cause tremendous crop loss, impacting the agro-economy worldwide. Plants possess an arsenal of chemical defenses that comprise diverse secondary metabolites that help protect against harmful herbivorous arthropods. In response, the strategies that herbivores use to cope with plant defenses can be behavioral, or molecular and/or biochemical of which salivary secretions are a key determinant. Insect salivary secretions/oral secretions (OSs) play a crucial role in plant immunity as they contain several biologically active elicitors and effector proteins that modulate plants' defense responses. Using this oral secretion cocktail, insects overcome plant natural defenses to allow successful feeding. However, a lack of knowledge of the nature of the signals present in oral secretion cocktails has resulted in reduced mechanistic knowledge of their cellular perception. In this review, we discuss the latest knowledge on herbivore oral secretion derived elicitors and effectors and various mechanisms involved in plant defense modulation. Identification of novel herbivore-released molecules and their plant targets should pave the way for understanding the intricate strategies employed by both herbivorous arthropods and plants in their interactions.

Spider mite herbivory induces an ABA-driven stomatal defense

Arthropod herbivory poses a serious threat to crop yield, prompting plants to employ intricate defense mechanisms against pest feeding. The generalist pest 2-spotted spider mite (Tetranychus urticae) inflicts rapid damage and remains challenging due to its broad target range. In this study, we explored the Arabidopsis (Arabidopsis thaliana) response to T. urticae infestation, revealing the induction of abscisic acid (ABA), a hormone typically associated with abiotic stress adaptation, and stomatal closure during water stress. Leveraging a Forster resonance energy transfer (FRET)-based ABA biosensor (nlsABACUS2-400n), we observed elevated ABA levels in various leaf cell types postmite feeding. While ABA's role in pest resistance or susceptibility has been debated, an ABA-deficient mutant exhibited increased mite infestation alongside intact canonical biotic stress signaling, indicating an independent function of ABA in mite defense. We established that ABA-triggered stomatal closure effectively hinders mite feeding and minimizes leaf cell damage through genetic and pharmacological interventions targeting ABA levels, ABA signaling, stomatal aperture, and density. This study underscores the critical interplay between biotic and abiotic stresses in plants, highlighting how the vulnerability to mite infestation arising from open stomata, crucial for transpiration and photosynthesis, reinforces the intricate relationship between these stress types.

An effector of Phthorimaea absoluta oral secretions inhibits host plant defense

Insects have evolved effectors to regulate host defenses for efficient feeding, yet their impact on chewing insects, like the tomato leaf miner (Phthorimaea absoluta), a significant pest, is poorly understood. We used RNAi to target the REPAT38 gene in larvae, monitoring changes at 0.5, 1, 2, and 4 h in leaf stomata, plant hormone concentrations (jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-Ile), salicylic acid (SA), ethylene (ET), and abscisic acid (ABA)), and 12 hormone-responsive genes to explore the molecular mechanism of REPAT38-mediated plant-insect interactions. The results showed that the effector induced stomatal closure at 0.5 h and inhibited the synthesis of JA, ET, and ABA at 1 h. Additionally, seven plant hormone-responsive genes-AOC, MYC2, ACS1A, PAL, PR1, EIL2, and SRK2E-were inhibited at various time points. Our data suggest that REPAT38, as an effector with conserved functions, can weaken tomato host defenses and conducive to insect adaptation to host plants.

Spodoptera frugiperda Salivary Glucose Oxidase Reduces the Release of Green Leaf Volatiles and Increases Terpene Emission from Maize

The intricate relationships between plants and insects are essential for understanding ecological dynamics. Among these interactions, HIPVs serve as a pivotal defense mechanism. Our findings reveal the highly conserved nature of the GOX gene within the Lepidoptera order, highly expressed in the salivary glands of S. frugiperda, and its role in mediating maize's defense responses. Notably, salivary GOX activity expression significantly decreases subsequent gene knockout. The presence of GOX in the saliva of S. frugiperda significantly modulates the emission of HIPVs during maize consumption. This research delineates that GOX selectively inhibits the emission of certain green leaf volatiles (GLVs) while concurrently enhancing the release of terpene volatiles. This study unveils a novel mechanism whereby S. frugiperda utilizes GOX proteins in OS to modulate volatile emissions from maize, offering fresh perspectives on the adaptive evolution of phytophagous insects and their interactions with their preferred host plants.

Developing Oak Buds Produce Volatile Emissions in Response to Herbivory by Freshly Hatched Caterpillars

Plant responses to damage by insectivorous herbivores are well-documented in mature leaves. The resulting herbivore-induced plant volatiles (HIPVs) protect the plant by attracting carnivorous arthropods and even some insectivorous vertebrates, to parasitize or consume the plant invaders. However, very little is known about plant production of HIPVs in developing buds, particularly when herbivorous insects are too small to be considered a prey item. It is additionally unclear whether plants respond differently to generalist and specialist chewing insects that overlap in distribution. Therefore, we compared HIPV production of Downy oak (Quercus pubescens Willd.) buds infested with freshly hatched caterpillars of Tortrix viridana (specialist) and Operophtera brumata (generalist), against uninfested buds. Of the compounds identified in both years of the experiment, we found that (Z)-hex-3-enyl acetate, (E)-β-ocimene, acetophenone, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), methyl salicylate, α-copaene, α-humulene, (E)-caryophyllene, and (E,E)-α-farnesene appeared to be higher in infested buds compared to controls. We found no difference in HIPV production between the specialist and the generalist herbivores. Production of HIPVs was also associated with leaf damage, with higher HIPV production in more severely attacked buds. Thus, our study shows that oak trees already start responding to insect herbivory before leaves are developed, by producing compounds similar to those found in damaged mature leaves. Future work should focus on how Downy oak may benefit from initiating alarm cues at a time when carnivorous arthropods and insectivorous vertebrates are unable to use herbivorous insects as host or food.

Stomatal closure prevents xylem transport of green leaf volatiles and impairs their systemic function in plants

Plants perceive environmental stresses as whole organisms via distant signals conveying danger messages through their vasculature. In parallel to vascular transport, airborne plant volatile compounds, including green leaf volatiles (GLVs), can bypass the lack of vascular connection. However, some small volatile compounds move through the vasculature; such vascular transport is little known about GLVs. Here we illustrate GLV alcohols as solutes move within xylem vessels in Zea mays. We describe GLV alcohols, including Z-3-hexen-ol and its isomer E-3-hexen-ol, which is not synthesized in maize, moving through the transpiration stream via xylem vessels. Since transpiration is mediated by the stomatal aperture, closing stomata by two independent methods diminishes the transport of GLV alcohol and its isomer. In addition, the lower transport of GLV alcohols impairs their function in inducing terpenoid biosynthesis, suggesting that xylem transport of GLV alcohols plays a significant role in their systemic function. Our study suggests that GLV alcohols, in addition to airborne signals, are transported through xylem vessels. Our findings can be critical in future studies about the perception and function of these compounds in plants.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Glucose Oxidase of a Crucifer-Specialized Insect: A Potential Role in Suppressing Plant Defense via Modulating Antagonistic Plant Hormones

Glucose oxidase (GOX) is a representative compound found in most insect saliva that can suppress plant-defensive responses. However, little is known about the origin and role of GOX in the crucifer-specialized pest Plutella xylostella. In this study, we showed obvious regurgitation from the larval gut of P. xylostella and identified abundant peptides highly similar to known GOX. Three PxGOX genes were verified with PxGOX2 preferentially expressed in the gut. The heterologously expressed PxGOX2 confirmed its function to be a GOX, and it was detected in plant wounds together with the gut regurgitant. Further experiments revealed that PxGOX2 functioned as an effector and may suppress defensive responses in plant through the production of H2O2, which modulates levels of antagonistic salicylic acid and jasmonic acid. However, excessive H2O2 in the host plant may be neutralized by peroxidase, thus forming defensive feedback. Our findings provided new insights into understanding the GOX-mediated insect-plant interactions.

Endocytosis-mediated entry of a caterpillar effector into plants is countered by Jasmonate

Insects and pathogens release effectors into plant cells to weaken the host defense or immune response. While the imports of some bacterial and fungal effectors into plants have been previously characterized, the mechanisms of how caterpillar effectors enter plant cells remain a mystery. Using live cell imaging and real-time protein tracking, we show that HARP1, an effector from the oral secretions of cotton bollworm (Helicoverpa armigera), enters plant cells via protein-mediated endocytosis. The entry of HARP1 into a plant cell depends on its interaction with vesicle trafficking components including CTL1, PATL2, and TET8. The plant defense hormone jasmonate (JA) restricts HARP1 import by inhibiting endocytosis and HARP1 loading into endosomes. Combined with the previous report that HARP1 inhibits JA signaling output in host plants, it unveils that the effector and JA establish a defense and counter-defense loop reflecting the robust arms race between plants and insects.

Caterpillar Salivary Glucose Oxidase Decreases Green Leaf Volatile Emission and Increases Terpene Emission from Maize

Caterpillar salivary glucose oxidase (GOX) can function as both an elicitor or as an effector of plant defense responses depending upon the system. Treatment with GOX reduces the stomatal aperture of tomato and soybean leaves, thereby reducing the emission of volatile organic compounds (VOCs), that are important indirect defense responses of plants by attracting natural enemies of the caterpillars. Here we examined the effect of fungal GOX (fungal glucose oxidases have been used to determine specificity in defense response elicitation) on stomatal closure of maize leaves and on the volatile emission pattern whole maize plants. We also used salivary gland homogenate from wild-type and CRISPR-Cas9 Helicoverpa zea mutants deficient in GOX activity to determine the effect caterpillar saliva with and without GOX had on maize volatile emission. Collecting volatiles at 2-hour intervals allowed us to examine the changes in emission over time. Fungal GOX reduced the stomatal aperture in maize leaves, which may have influenced the observed significant reduction in total green leaf volatile (GLV) emission. Furthermore, fungal GOX significantly increased the emission of several key terpenes: linalool, DMNT, and Z-β-farnesene from maize, while salivary gland homogenate from wild type (WT; GOX+) H. zea increased the emission of α-pinene, β-pinene, and ocimene compared to H. zea unable to synthesize GOX. This study addressed a significant knowledge gap about the effect of GOX on maize volatiles and provides a baseline for further research on the effect of GOX on the regulation of terpene synthase genes and their relation to terpene volatile emission.

Plant Stomata: An Unrealized Possibility in Plant Defense against Invading Pathogens and Stress Tolerance

Stomata are crucial structures in plants that play a primary role in the infection process during a pathogen's attack, as they act as points of access for invading pathogens to enter host tissues. Recent evidence has revealed that stomata are integral to the plant defense system and can actively impede invading pathogens by triggering plant defense responses. Stomata interact with diverse pathogen virulence factors, granting them the capacity to influence plant susceptibility and resistance. Moreover, recent studies focusing on the environmental and microbial regulation of stomatal closure and opening have shed light on the epidemiology of bacterial diseases in plants. Bacteria and fungi can induce stomatal closure using pathogen-associated molecular patterns (PAMPs), effectively preventing entry through these openings and positioning stomata as a critical component of the plant's innate immune system; however, despite this defense mechanism, some microorganisms have evolved strategies to overcome stomatal protection. Interestingly, recent research supports the hypothesis that stomatal closure caused by PAMPs may function as a more robust barrier against pathogen infection than previously believed. On the other hand, plant stomatal closure is also regulated by factors such as abscisic acid and Ca2+-permeable channels, which will also be discussed in this review. Therefore, this review aims to discuss various roles of stomata during biotic and abiotic stress, such as insects and water stress, and with specific context to pathogens and their strategies for evading stomatal defense, subverting plant resistance, and overcoming challenges faced by infectious propagules. These pathogens must navigate specific plant tissues and counteract various constitutive and inducible resistance mechanisms, making the role of stomata in plant defense an essential area of study.

A salivary GMC oxidoreductase of Manduca sexta re-arranges the green leaf volatile profile of its host plant

Green leaf volatiles (GLVs) are short-chain oxylipins that are emitted from plants in response to stress. Previous studies have shown that oral secretions (OS) of the tobacco hornworm Manduca sexta, introduced into plant wounds during feeding, catalyze the re-arrangement of GLVs from Z-3- to E-2-isomers. This change in the volatile signal however is bittersweet for the insect as it can be used by their natural enemies, as a prey location cue. Here we show that (3Z):(2E)-hexenal isomerase (Hi-1) in M. sexta's OS catalyzes the conversion of the GLV Z-3-hexenal to E-2-hexenal. Hi-1 mutants that were raised on a GLV-free diet showed developmental disorders, indicating that Hi-1 also metabolizes other substrates important for the insect's development. Phylogenetic analysis placed Hi-1 within the GMCβ-subfamily and showed that Hi-1 homologs from other lepidopterans could catalyze similar reactions. Our results indicate that Hi-1 not only modulates the plant's GLV-bouquet but also functions in insect development.

Dynamic environmental interactions shaped by vegetative plant volatiles

Covering: up to November 2022Plants shape terrestrial ecosystems through physical and chemical interactions. Plant-derived volatile organic compounds in particular influence the behavior and performance of other organisms. In this review, we discuss how vegetative plant volatiles derived from leaves, stems and roots are produced and released into the environment, how their production and release is modified by abiotic and biotic factors, and how they influence other organisms. Vegetative plant volatiles are derived from different biosynthesis and degradation pathways and are released via distinct routes. Both biosynthesis and release are regulated by other organisms as well as abiotic factors. In turn, vegetative plant volatiles modify the physiology and the behavior of a wide range of organisms, from microbes to mammals. Several concepts and frameworks can help to explain and predict the evolution and ecology of vegetative plant volatile emission patterns of specific pathways: multifunctionality of specialized metabolites, chemical communication displays and the information arms race, and volatile physiochemistry. We discuss how these frameworks can be leveraged to understand the evolution and expression patterns of vegetative plant volatiles. The multifaceted roles of vegetative plant volatiles provide fertile grounds to understand ecosystem dynamics and harness their power for sustainable agriculture.

A highly accumulated secretory protein from cotton bollworm interacts with basic helix-loop-helix transcription factors to dampen plant defense

Caterpillar oral secretion (OS) contains active molecules that modulate plant defense signaling. We isolated an effector-like protein (Highly Accumulated Secretory Protein 1, HAS1) from cotton bollworm (Helicoverpa armigera) that is the most highly accumulated secretory protein of the nondigestive components in OS and belongs to venom R-like protein. Elimination of HAS1 by plant-mediated RNA interference reduced the suppression of OS on the defense response in plants. Plants expressing HAS1 are more susceptible to insect herbivory accompanied by the reduced expressions of multiple defense genes. HAS1 binds to the basic helix-loop-helix (bHLH) transcription factors, including GoPGF involved in pigmented gland formation and defense compounds biosynthesis in cotton and MYC3/MYC4 the main regulators in jasmonate (JA) signaling in Arabidopsis. The binding activity is required for HAS1 to inhibit the activation of bHLHs on plant defense gene expressions. Together with our previous study that another venom R-like protein HARP1 in cotton bollworm OS blocks JA signaling by interacting with JASMONATE-ZIM-domain repressors, we conclude that the venom R-like proteins in OS interfere with plant defense in a dual suppression manner. Considering the venom proteins in parasitic wasp assault the immune system of its host animal, our investigation reveals their conserved function in carnivorous and herbivorous insects.

Undercover operation: Belowground insect herbivory modifies systemic plant defense and repels aboveground foraging insect herbivores

Plants attacked by insects may induce defenses locally in attacked plant tissues and/or systemically in non-attacked tissues, such as aboveground herbivory affecting belowground roots or belowground herbivory modifying aboveground tissues (i.e., cross-compartment systemic defense). Through induced systemic plant defenses, above-and belowground insect herbivores indirectly interact when feeding on a shared host plant. However, determining the systemic effects of herbivory on cross-compartment plant tissues and cascading consequences for herbivore communities remains underexplored. The goal of this study was to determine how belowground striped cucumber beetle (Acalymma vittatum) larval herbivory alters aboveground zucchini squash (Cucurbita pepo subsp. pepo) defenses and interactions with herbivores, including adult cucumber beetles and squash bugs (Anasa tristis). To explore this question, field and laboratory experiments were conducted to compare responses of aboveground herbivores to belowground larvae-damaged plants and non-damaged control plants. We also characterized changes in defensive chemicals and nutritional content of aboveground plant structures following belowground herbivory. We discovered belowground herbivory enhanced aboveground plant resistance and deterred aboveground foraging herbivores. We also found that larvae-damaged plants emitted higher amounts of a key volatile compound, (E)-β-ocimene, compared to non-damaged controls. Further investigation suggests that other mechanisms, such as plant nutrient content, may additionally contribute to aboveground herbivore foraging decisions. Collectively, our findings underscore connections between above-and belowground herbivore communities as mediated through induced systemic defenses of a shared host plant. Specifically, these findings indicate that belowground larval herbivory systemically enhances plant defenses and deters a suite of aboveground herbivores, suggesting larvae may manipulate aboveground plant defenses for their own benefit, while plants may benefit from enhanced systemic defenses against multi-herbivore attack.

Stomata at the crossroad of molecular interaction between biotic and abiotic stress responses in plants

Increasing global food production is threatened by harsh environmental conditions along with biotic stresses, requiring massive new research into integrated stress resistance in plants. Stomata play a pivotal role in response to many biotic and abiotic stresses, but their orchestrated interactions at the molecular, physiological, and biochemical levels were less investigated. Here, we reviewed the influence of drought, pathogen, and insect herbivory on stomata to provide a comprehensive overview in the context of stomatal regulation. We also summarized the molecular mechanisms of stomatal response triggered by these stresses. To further investigate the effect of stomata-herbivore interaction at a transcriptional level, integrated transcriptome studies from different plant species attacked by different pests revealed evidence of the crosstalk between abiotic and biotic stress. Comprehensive understanding of the involvement of stomata in some plant-herbivore interactions may be an essential step towards herbivores' manipulation of plants, which provides insights for the development of integrated pest management strategies. Moreover, we proposed that stomata can function as important modulators of plant response to stress combination, representing an exciting frontier of plant science with a broad and precise view of plant biotic interactions.

Volatile uptake, transport, perception, and signaling shape a plant's nose

Herbivore-induced plant volatiles regulate defenses in undamaged neighboring plants. Understanding the mechanisms by which plant volatiles are taken up, perceived, and translated into canonical defense signaling pathways is an important frontier of knowledge. Volatiles can enter plants through stomata and the cuticle. They are likely perceived by membrane-associated receptors as well as intracellular receptors. The latter likely involves metabolization and transport across cell membranes by volatile transporters. Translation of volatiles into defense priming and induction typically involves mitogen-activated protein kinases (MAPKs), WRKY transcription factors, and jasmonates. We propose that the broad range of molecular processes involved in volatile signaling will likely result in substantial spatiotemporal and ontogenetic variation in plant responsiveness to volatiles, with important consequences for plant–environment interactions.

Ethyl Vinyl Ketone Activates K+ Efflux to Regulate Stomatal Closure by MRP4-Dependent eATP Accumulation Working Upstream of H2O2 Burst in Arabidopsis

Plants regulate stomatal mobility to limit water loss and improve pathogen resistance. Ethyl vinyl ketone (evk) is referred to as a reactive electrophilic substance (RES). In this paper, we found that evk can mediate stomatal closure and that evk-induced stomatal closure by increasing guard cell K⁺ efflux. To investigate the role of eATP, and H₂O₂ in evk-regulated K⁺ efflux, we used Arabidopsis wild-type (WT), mutant lines of mrp4, mrp5, dorn1.3 and rbohd/f. Non-invasive micro-test technology (NMT) data showed that evk-induced K⁺ efflux was diminished in mrp4, rbohd/f, and dorn1.3 mutant, which means eATP and H₂O₂ work upstream of evk-induced K⁺ efflux. According to the eATP content assay, evk stimulated eATP production mainly by MRP4. In mrp4 and mrp5 mutant groups and the ABC transporter inhibitor glibenclamide (Gli)-pretreated group, evk-regulated stomatal closure and eATP buildup were diminished, especially in the mrp4 group. According to qRT-PCR and eATP concentration results, evk regulates both relative gene expressions of MRP4/5 and eATP concentration in rbohd/f and WT group. According to the confocal data, evk-induced H₂O₂ production was lower in mrp4, mrp5 mutants, which implied that eATP works upstream of H₂O₂. Moreover, NADPH-dependent H₂O₂ burst is regulated by DORN1. A yeast two-hybrid assay, firefly luciferase complementation imaging assay, bimolecular fluorescence complementation assay, and pulldown assay showed that the interaction between DORN1 and RBOHF can be realized, which means DORN1 may control H₂O₂ burst by regulating RBOHF through interaction. This study reveals that evk-induced stomatal closure requires MRP4-dependent eATP accumulation and subsequent H₂O₂ accumulation to regulate K⁺ efflux.

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.

The dual function of elicitors and effectors from insects: reviewing the 'arms race' against plant defenses

This review provides an overview, analysis, and reflection on insect elicitors and effectors (particularly from oral secretions) in the context of the 'arms race' with host plants. Following injury by an insect herbivore, plants rapidly activate induced defenses that may directly or indirectly affect the insect. Such defense pathways are influenced by a multitude of factors; however, cues from the insect's oral secretions are perhaps the most well studied mediators of such plant responses. The relationship between plants and their insect herbivores is often termed an 'evolutionary arms race' of strategies for each organism to either overcome defenses or to avoid attack. However, these compounds that can elicit a plant defense response that is detrimental to the insect may also benefit the physiology or metabolism of an insect species. Indeed, several insect elicitors of plant defenses (such as the fatty acid-amino acid conjugate, volicitin) are known to enhance an insect's ability to obtain nutritionally important compounds from plant tissue. Here we re-examine the well-known elicitors and effectors from chewing insects to demonstrate not only our incomplete understanding of the specific biochemical and molecular cascades involved in these interactions but also to consider the role of these compounds for the insect species itself. Finally, this overview discusses opportunities for research in the field of plant-insect interactions by utilizing tools such as genomics and proteomics to integrate the future study of these interactions through ecological, physiological, and evolutionary disciplines.

Disentangling the Potato Tuber Moth-Induced Early-Defense Response by Simulated Herbivory in Potato Plants

Plants rely on the perception of a multitude of herbivory-associated cues (HACs) to activate their defense response to insect herbivores. These stimuli are mainly derived from three functional components, namely, mechanical damage, insect-associated microbe, and insect's chemical cues. While simulated herbivory integrating these stimuli is widely exploited for complementing actual herbivory in clarifying the details of plant-herbivore interaction, breaking down these stimuli and identifying the mechanisms of plant responses associated with them have been less explored. In this study, the components of potato tuber moth (Phthorimaea operculella, PTM) herbivory were reorganized in a cumulative way and their impacts on the early defense responses of potato leaf were characterized. We found that simulated and actual herbivory of PTM triggered similar patterns of phytohormonal and transcriptomic responses in potato leaf. Moreover, the microbe in the PTM herbivory stimuli is associated with the regulation of the phytohormones jasmonic acid (JA) and abscisic acid (ABA) since reducing the microbe in HAC could reduce JA while increasing ABA. In addition, seven robust gene modules were identified to illustrate how potato plants respond to different PTM herbivory stimuli when herbivory components increased. Significantly, we found that mechanical damage mainly activated JA-mediated signaling; PTM-derived HACs contributed much more to potato early-defense response and induced signaling molecules such as multiple protein kinases; orally secreted bacteria stimuli could antagonize PTM-derived HACs and modulate plant defense, including repressing phenylpropanoid biosynthesis. Our study broadened the understanding of how potato plants integrate the responses to a multitude of stimuli upon PTM herbivory and evidenced that insect-associated microbes greatly modulated the plants response to insect herbivory.

Molecular tug-of-war: Plant immune recognition of herbivory

Plant defense responses against insect herbivores are induced through wound-induced signaling and the specific perception of herbivore-associated molecular patterns (HAMPs). In addition, herbivores can deliver effectors that suppress plant immunity. Here we review plant immune recognition of HAMPs and effectors, and argue that these initial molecular interactions upon a plant-herbivore encounter mediate and structure effective resistance. While the number of distinct HAMPs and effectors from both chewing and piercing-sucking herbivores has expanded rapidly with omics-enabled approaches, paired receptors and targets in the host are still not well characterized. Herbivore-derived effectors may also be recognized as HAMPs depending on the host plant species, potentially through the evolution of novel immune receptor functions. We compile examples of HAMPs and effectors where natural variation between species may inform evolutionary patterns and mechanisms of plant-herbivore interactions. Finally, we discuss the combined effects of wounding and HAMP recognition, and review potential signaling hubs, which may integrate both sensing functions. Understanding the precise mechanisms for plant sensing of herbivores will be critical for engineering resistance in agriculture.

Salivary surprise: Symmerista caterpillars anoint petioles with red saliva after clipping leaves

After feeding on a tree leaf, caterpillars in ten families sever the petiole and allow the remaining leaf fragment to fall to the ground. Previous researchers proposed that the caterpillars thereby reduced bird predation by eliminating visual evidence of feeding. In this study, 26 species of caterpillars in five families were filmed clipping leaves. Caterpillar behavior did not conform to the visual cue hypothesis. Some caterpillars clipped midribs and petioles repeatedly even though a single clip would suffice to reduce visual cues for birds. Every caterpillar that clipped a leaf rubbed its spinneret (which secretes saliva from the labial glands) over the petiole or midrib stub. In the notodontids Symmerista albifrons and S. leucitys, petiole stubs were bathed in red fluid. Cauterizing the spinneret eliminated fluid application. Dissections documented that the anterior portion of their labial glands contained red pigment, thereby confirming that the red secretion is saliva. When applied to petiole stubs, the red pigment in Symmerista saliva travelled several mm in five minutes within the petiole xylem demonstrating the potential for rapid movement of salivary constituents into the plant. In diverse caterpillars, including species that clip leaves, saliva contains substances reported to suppress plant defenses. Thus, leaf clipping likely functions primarily not to remove visual cues, but to introduce salivary constituents into the plant that prevent defenses from being mobilized in nearby leaves where the caterpillar feeds next.

Stomata-mediated interactions between plants, herbivores, and the environment

Stomata play a central role in plant responses to abiotic and biotic stresses. Existing knowledge regarding the roles of stomata in plant stress is centered on abiotic stresses and plant-pathogen interactions, but how stomata influence plant-herbivore interactions remains largely unclear. Here, we summarize the functions of stomata in plant-insect interactions and highlight recent discoveries of how herbivores manipulate plant stomata. Because stomata are linked to interrelated physiological processes in plants, herbivory-induced changes in stomatal dynamics might have cellular, organismic, and/or even community-level impacts. We summarize our current understanding of how stomata mediate plant responses to herbivory and environmental stimuli, propose how herbivores may influence these responses, and identify key knowledge gaps in plant-herbivore interactions.

Characterized constituents of insect herbivore oral secretions and their influence on the regulation of plant defenses

For more than 350 million years, there have been ongoing dynamic interactions between plants and insects. In several cases, insects cause-specific feeding damage with ensuing herbivore-associated molecular patterns that invoke characteristic defense responses. During feeding on plant tissue, insects release oral secretions (OSs) containing a repertoire of molecules affecting plant defense (effectors). Some of these OS components might elicit a defense response to combat insect attacks (elicitors), while some might curb the plant defenses (suppressors). Few reports suggest that the synthesis and function of OS components might depend on the host plant and associated microorganisms. We review these intricate plant-insect interactions, during which there is a continuous exchange of molecules between plants and feeding insects along with the associated microorganisms. We further provide a list of commonly identified inducible plant produced defensive molecules released upon insect attack as well as in response to OS treatments of the plants. Thus, we describe how plants specialized and defense-related metabolism is modulated at innumerable phases by OS during plant-insect interactions. A molecular understanding of these complex interactions will provide a means to design eco-friendly crop protection strategies.

New approaches to improve crop tolerance to biotic and abiotic stresses

During the last years, a great effort has been dedicated at the development and employment of diverse approaches for achieving more stress-tolerant and climate-flexible crops and sustainable yield increases to meet the food and energy demands of the future. The ongoing climate change is in fact leading to more frequent extreme events with a negative impact on food production, such as increased temperatures, drought, and soil salinization as well as invasive arthropod pests and diseases. In this review, diverse "green strategies" (e.g., chemical priming, root-associated microorganisms), and advanced technologies (e.g., genome editing, high-throughput phenotyping) are described on the basis of the most recent research evidence. Particularly, attention has been focused on the potential use in a context of sustainable and climate-smart agriculture (the so called "next agriculture generation") to improve plant tolerance and resilience to abiotic and biotic stresses. In addition, the gap between the results obtained in controlled experiments and those from application of these technologies in real field conditions (lab to field step) is also discussed.

Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species

In response to herbivory, plants emit volatile compounds that play important roles in plant defense. Herbivore-induced plant volatiles (HIPVs) can deter herbivores, recruit natural enemies, and warn other plants of possible herbivore attack. Following HIPV detection, neighboring plants often respond by enhancing their anti-herbivore defenses, but a recent study found that herbivores can manipulate HIPV-interplant communication for their own benefit and suppress defenses in neighboring plants. Herbivores induce species-specific blends of HIPVs and how these different blends affect the specificity of plant defense responses remains unclear. Here we assessed how HIPVs from zucchini plants (Cucurbita pepo) challenged with different herbivore species affect resistance in neighboring plants. Volatile "emitter" plants were damaged by one of three herbivore species: saltmarsh caterpillars (Estigmene acrea), squash bugs (Anasa tristis), or striped cucumber beetles (Acalymma vittatum), or were left as undamaged controls. Neighboring "receiver" plants were exposed to HIPVs or control volatiles and then challenged by the associated herbivore species. As measures of plant resistance, we quantified herbivore feeding damage and defense-related phytohormones in receivers. We found that the three herbivore species induced different HIPV blends from squash plants. HIPVs induced by saltmarsh caterpillars suppressed defenses in receivers, leading to greater herbivory and lower defense induction compared to controls. In contrast, HIPVs induced by cucumber beetles and squash bugs did not affect plant resistance to subsequent herbivory in receivers. Our study shows that herbivore species identity affects volatile-mediated interplant communication in zucchini, revealing a new example of herbivore defense suppression through volatile cues.

Differential Levels of Fatty Acid-Amino Acid Conjugates in the Oral Secretions of Lepidopteran Larvae Account for the Different Profiles of Volatiles

BACKGROUND

Plants have evolved sophisticated defense responses to insect herbivore attack, which often involve elicitors in the insects' oral secretions. The major eliciting compounds in insect oral secretions across different species and their potency in inducing volatile emissions have not yet been fully characterized and compared.

RESULTS

Seven lepidopteran insects with variable duration of association with maize were selected, five species known as pests for a long time (Ostrinia furnacalis, Spodoptera exigua, Spodoptera litura, Mythimna separata, and Helicoverpa armigera) and two newly emerging pests (Athetis lepigone and Athetis dissimilis). Oral secretions of the newly emerging pests have the highest total contents of Fatty Acid-Amino Acid Conjugates (FACs), and their relative composition was well separated from that of the other five species in principal compound analysis. Redundancy analyses suggested that higher quantity of FACs was mainly responsible for the increases in maize volatiles, of which (E)-3,8-dimethyl-1,4,7-nonatriene (DMNT) and (E, E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) were the most strongly inducible compounds. Adding FACs to the oral secretion of S. litura larvae significantly increased the emissions of TMTT and DMNT, confirming the key role of FACs in inducing volatile emissions in maize plants. Additional experiments with artificial diet spiked with linolenic acid suggested that variation in FACs is due to differences in internal FAC degradation and fatty acid excretion.

CONCLUSION

Compared with two newly emerging pests A. lepigone and A. dissimilis, the long-term pests could diminish the volatile emission by maize through reducing the FAC content in their oral secretions, which may lower the risk of attracting natural enemies.

Light dominates the diurnal emissions of herbivore-induced volatiles in wild tobacco

BACKGROUND

Timing is everything when it comes to the fitness outcome of a plant's ecological interactions, and accurate timing is particularly relevant for interactions with herbivores or mutualists that are based on ephemeral emissions of volatile organic compounds. Previous studies of the wild tobacco N. attenuata have found associations between the diurnal timing of volatile emissions, and daytime predation of herbivores by their natural enemies.

RESULTS

Here, we investigated the role of light in regulating two biosynthetic groups of volatiles, terpenoids and green leaf volatiles (GLVs), which dominate the herbivore-induced bouquet of N. attenuata. Light deprivation strongly suppressed terpenoid emissions while enhancing GLV emissions, albeit with a time lag. Silencing the expression of photoreceptor genes did not alter terpenoid emission rhythms, but silencing expression of the phytochrome gene, NaPhyB1, disordered the emission of the GLV (Z)-3-hexenyl acetate. External abscisic acid (ABA) treatments increased stomatal resistance, but did not truncate the emission of terpenoid volatiles (recovered in the headspace). However, ABA treatment enhanced GLV emissions and leaf internal pools (recovered from tissue), and reduced internal linalool pools. In contrast to the pattern of diurnal terpenoid emissions and nocturnal GLV emissions, transcripts of herbivore-induced plant volatile (HIPV) biosynthetic genes peaked during the day. The promotor regions of these genes were populated with various cis-acting regulatory elements involved in light-, stress-, phytohormone- and circadian regulation.

CONCLUSIONS

This research provides insights into the complexity of the mechanisms involved in the regulation of HIPV bouquets, a mechanistic complexity which rivals the functional complexity of HIPVs, which includes repelling herbivores, calling for body guards, and attracting pollinators.