Molasses Grass Induces Direct and Indirect Defense Responses in Neighbouring Maize Plants

Associational Effects of Desmodium Intercropping on Maize Resistance and Secondary Metabolism

Intercropping is drawing increasing attention as a strategy to increase crop yields and manage pest pressure, however the mechanisms of associational resistance in diversified cropping systems remain controversial. We conducted a controlled experiment to assess the impact of co-planting with silverleaf Desmodium (Desmodium uncinatum) on maize secondary metabolism and resistance to herbivory by the spotted stemborer (Chilo partellus). Maize plants were grown either in the same pot with a Desmodium plant or adjacent to it in a separate pot. Our findings indicate that co-planting with Desmodium influences maize secondary metabolism and herbivore resistance through both above and below-ground mechanisms. Maize growing in the same pot with a Desmodium neighbor was less attractive for oviposition by spotted stemborer adults. However, maize exposed only to above-ground Desmodium cues generally showed increased susceptibility to spotted stemborer herbivory (through both increased oviposition and larval consumption). VOC emissions and tissue secondary metabolite titers were also altered in maize plants exposed to Desmodium cues, with stronger effects being observed when maize and Desmodium shared the same pot. Specifically, benzoxazinoids were strongly suppressed in maize roots by direct contact with a Desmodium neighbor while headspace emissions of short-chain aldehydes and alkylbenzenes were increased. These results imply that direct root contact or soil-borne cues play an important role in mediating associational effects on plant resistance in this system.

Volatile-mediated plant-plant communication and higher-level ecological dynamics

Volatile organic compounds (VOCs) in general and herbivory-induced plant volatiles (HIPVs) in particular are increasingly understood as major mediators of information transfer between plant tissues. Recent findings have moved the field of plant communication closer to a detailed understanding of how plants emit and perceive VOCs and seem to converge on a model that juxtaposes perception and emission mechanisms. These new mechanistic insights help to explain how plants can integrate different types of information and how environmental noise can affect the transmission of information. At the same time, ever-new functions of VOC-mediated plant-plant interactions are being revealed. Chemical information transfer between plants is now known to fundamentally affect plant organismal interactions and, additionally, population, community, and ecosystem dynamics. One of the most exciting new developments places plant-plant interactions along a behavioral continuum with an eavesdropping strategy at one end and mutually beneficial information-sharing among plants within a population at the other. Most importantly and based on recent findings as well as theoretical models, plant populations can be predicted to evolve different communication strategies depending on their interaction environment. We use recent studies from ecological model systems to illustrate this context dependency of plant communication. Moreover, we review recent key findings about the mechanisms and functions of HIPV-mediated information transfer and suggest conceptual links, such as to information theory and behavioral game theory, as valuable tools for a deeper understanding of how plant-plant communication affects ecological and evolutionary dynamics.

Volatile Dimethyl Disulfide from Guava Plants Regulate Developmental Performance of Asian Citrus Psyllid through Activation of Defense Responses in Neighboring Orange Plants

Intercropping with guava (Psidium guajava L.) can assist with the management of Asian citrus psyllid (ACP, Diaphorina citri Kuwayama), the insect vector of the huanglongbing pathogen, in citrus orchards. Sulfur volatiles have a repellent activity and physiological effects, as well as being important components of guava volatiles. In this study, we tested whether the sulfur volatiles emitted by guava plants play a role in plant–plant communications and trigger anti-herbivore activities against ACP in sweet orange plants (Citrus sinensis L. Osbeck). Real-time determination using a proton-transfer-reaction mass spectrometer (PTR-MS) showed that guava plants continuously release methanethiol, dimethyl sulfide (DMS), and dimethyl disulfide (DMDS), and the contents increased rapidly after mechanical damage. The exposure of orange plants to DMDS resulted in the suppression of the developmental performance of ACP. The differential elevation of salicylic acid (SA) levels; the expression of phenylalanine ammonia lyase (PAL), salicylate-O-methyl transferase (SMT), and pathogenesis-related (PR1) genes; the activities of defense-related enzymes PAL, polyphenol oxidase (PPO), and peroxidase (POD); and the total polyphenol content were observed in DMDS-exposed orange plants. The emission of volatiles including myrcene, nonanal, decanal, and methyl salicylate (MeSA) was increased. In addition, phenylpropanoid and flavonoid biosynthesis, and aromatic amino acid (such as phenylalanine, tyrosine, and tryptophan) metabolic pathways were induced. Altogether, our results indicated that DMDS from guava plants can activate defense responses in eavesdropping orange plants and boost their herbivore resistance to ACP, which suggests the possibility of using DMDS as a novel approach for the management of ACP in citrus orchards.

Volatile Signals From Guava Plants Prime Defense Signaling and Increase Jasmonate-Dependent Herbivore Resistance in Neighboring Citrus Plants

Intercropping can reduce agricultural pest incidence and represents an important sustainable alternative to conventional pest control methods. Citrus intercropped with guava (Psidium guajava L.) has a lower incidence of Asian citrus psyllid (ACP, Diaphorina citri Kuwayama) and huanglongbing disease (HLB), but the mechanisms are still unknown. In this study, we tested whether volatile organic compounds (VOCs) emitted by guava plants play a role in plant-plant communications and trigger defense responses in sweet orange (Citrus sinensis L. Osbeck) in the laboratory. The results showed that the behavioral preference and developmental performance of ACP on citrus plants that were exposed to guava VOCs were suppressed. The expression of defense-related pathways involved in early signaling, jasmonate (JA) biosynthesis, protease inhibitor (PI), terpenoid, phenylpropanoid, and flavonoid biosynthesis was induced in guava VOC-exposed citrus plants. Headspace analysis revealed that guava plants constitutively emit high levels of (E)-β-caryophyllene and (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), which can induce the accumulation of JA and promote stronger defense responses of citrus to ACP feeding. In addition, exposure to guava VOCs also increased the indirect defense of citrus by attracting the parasitic wasp Tamarixia radiata. Together, our findings indicate that citrus plants can eavesdrop on the VOC cues emitted by neighboring intact guava plants to boost their JA-dependent anti-herbivore activities. The knowledge gained from this study will provide mechanisms underlying citrus-guava intercropping for the ecological management of insect pests.

Molecular ecology of plant volatiles in interactions with insect herbivores

Plant-derived volatile organic compounds (VOCs) play pivotal roles in interactions with insect herbivores. Individual VOCs can be directly toxic or deterrent, serve as signal molecules to attract natural enemies, and/or be perceived by distal plant tissues as a priming signal to prepare for expected herbivory. Environmental conditions, as well as the specific plant-insect interaction being investigated, strongly influence the observed functions of VOC blends. The complexity of plant-insect chemical communication via VOCs is further enriched by the sophisticated molecular perception mechanisms of insects, which can respond to one or more VOCs and thereby influence insect behavior in a manner that has yet to be fully elucidated. Despite numerous gaps in the current understanding of VOC-mediated plant-insect interactions, successful pest management strategies such as push-pull systems, synthetic odorant traps, and crop cultivars with modified VOC profiles have been developed to supplement chemical pesticide applications and enable more sustainable agricultural practices. Future studies in this field would benefit from examining the responses of both plants and insects in the same experiment to gain a more complete view of these interactive systems. Furthermore, a molecular evolutionary study of key genetic elements of the ecological interaction phenotypes could provide new insights into VOC-mediated plant communication with insect herbivores.

Immunity-associated volatile emissions of β-ionone and nonanal propagate defence responses in neighbouring barley plants

Plants activate biochemical responses to combat stress. (Hemi-)biotrophic pathogens are fended off by systemic acquired resistance (SAR), a primed state allowing plants to respond faster and more strongly upon subsequent infection. Here, we show that SAR-like defences in barley (Hordeum vulgare) are propagated between neighbouring plants, which respond with enhanced resistance to the volatile cues from infected senders. The emissions of the sender plants contained 15 volatile organic compounds (VOCs) associated with infection. Two of these, β-ionone and nonanal, elicited resistance upon plant exposure. Whole-genome transcriptomics analysis confirmed that interplant propagation of defence in barley is established as a form of priming. Although gene expression changes were more pronounced after challenge infection of the receiver plants with Blumeria graminis f. sp. hordei, differential gene expression in response to the volatile cues of the sender plants included an induction of HISTONE DEACETYLASE 2 (HvHDA2) and priming of TETRATRICOPEPTIDE REPEAT-LIKE superfamily protein (HvTPL). Because HvHDA2 and HvTPL transcript accumulation was also enhanced by exposure of barley to β-ionone and nonanal, our data identify both genes as possible defence/priming markers in barley. Our results suggest that VOCs and plant-plant interactions are relevant for possible crop protection strategies priming defence responses in barley.

Bottom-Up Forces in Agroecosystems and Their Potential Impact on Arthropod Pest Management

Bottom-up effects are major ecological forces in crop-arthropod pest-natural enemy multitrophic interactions. Over the past two decades, bottom-up effects have been considered key levers for optimizing integrated pest management (IPM). Irrigation, fertilization, crop resistance, habitat manipulation, organic management practices, and landscape characteristics have all been shown to trigger marked bottom-up effects and thus impact pest management. In this review, we summarize current knowledge on the role of bottom-up effects in pest management and the associated mechanisms, and discuss several key study cases showing how bottom-up effects practically promote natural pest control. Bottom-up effects on IPM also contribute to sustainable intensification of agriculture in the context of agricultural transition and climate change. Finally, we highlight new research priorities in this important area. Together with top-down forces (biological control), future advances in understanding ecological mechanisms underlying key bottom-up forces could pave the way for developing novel pest management strategies and new optimized IPM programs.

Cover Crop Soil Legacies Alter Phytochemistry and Resistance to Fall Armyworm (Lepidoptera: Noctuidae) in Maize

Plant-soil feedbacks can mediate aboveground plant-herbivore interactions by impacting plant chemistry. Given that soil legacies and agricultural practices are closely tied, a better understanding of soil legacy cascades and their application in pest management are needed. We tested how cover crop legacies alter resistance to fall armyworm (Spodoptera frugiperda Smith, Lepidoptera: Noctuidae) in maize (Zea mays L., Poales: Poaceae). We compared herbivore performance and behavior of fall armyworm larvae on maize grown after four cover crop treatments: a leguminous mycorrhizal cover crop (pea: Pisum sativum L., Fabales: Fabaceae), a nonleguminous mycorrhizal cover crop (triticale: x Triticosecale Wittm. Ex A. Camus, Poales: Poaceae), a nonleguminous nonmycorrhizal cover crop (radish: Raphanus sativus L., Brassicales: Brassicaceae), and no cover crops (fallow). Soil inorganic N was highest in pea treatments and lowest in triticale treatments, while maize AMF colonization was greatest when grown after mycorrhizal cover crops compared to nonmycorrhizal or no cover crops. Cover crop legacies altered the emission of maize volatiles and fall armyworm larvae oriented toward odors emitted by maize grown after radish more frequently than triticale in olfactometer assays. Additionally, larvae performed better and consumed more leaf tissue when feeding on maize grown after radish and poorest on plants grown after triticale. When damaged by fall armyworm, maize grown after triticale expressed higher levels of lipoxygenase-3 (lox3), while plants grown after radish upregulated maize proteinase inhibitor (mpi) gene expression. Our results highlight the importance of appropriate cover crop selection and suggest that triticale could strengthen maize resistance to fall armyworm.

Antagonist effects of the leek Allium porrum as a companion plant on aphid host plant colonization

Combining a non-host plant (companion plant or CP) with a target cultivated plant is considered as a promising strategy to reduce pest pressure. Among the companion plants (CP) commonly used in integrated systems, those belonging to the Amaryllidaceae family (chives, garlic, onion, leek) exhibit characteristics related to certain volatile organic compounds (VOCs) with promising repellent potentialities. The aim of this work was to investigate the potential disruption of sweet pepper (host plant) colonization by the green peach aphid (Myzus persicae) when exposed to leek (Allium porrum) as a CP. Retention/dispersion, EPG and clip-cage/Petri dish laboratory experiments were thus performed to study the effect of leek VOCs on aphid settlement/migration, feeding behavior and life history traits parameters, respectively. This work revealed that leek as a CP had a negative effect on aphid feeding behavior, by disturbing the balance between phloem and xylem sap ingestion, but had no influence concerning aphid settlement. Surprisingly, leek as a CP triggered some unexpected probiotic effects on certain life history traits such as aphid survival, biomass, and fecundity, suggesting a possible hormetic effect of leek VOCs on aphid physiology. The possibility of experience-induced preference of aphids for leek VOCs was also discussed.

Metabolomics as an Emerging Tool for the Study of Plant-Pathogen Interactions

Plants defend themselves from most microbial attacks via mechanisms including cell wall fortification, production of antimicrobial compounds, and generation of reactive oxygen species. Successful pathogens overcome these host defenses, as well as obtain nutrients from the host. Perturbations of plant metabolism play a central role in determining the outcome of attempted infections. Metabolomic analyses, for example between healthy, newly infected and diseased or resistant plants, have the potential to reveal perturbations to signaling or output pathways with key roles in determining the outcome of a plant-microbe interaction. However, application of this -omic and its tools in plant pathology studies is lagging relative to genomic and transcriptomic methods. Thus, it is imperative to bring the power of metabolomics to bear on the study of plant resistance/susceptibility. This review discusses metabolomics studies that link changes in primary or specialized metabolism to the defense responses of plants against bacterial, fungal, nematode, and viral pathogens. Also examined are cases where metabolomics unveils virulence mechanisms used by pathogens. Finally, how integrating metabolomics with other -omics can advance plant pathology research is discussed.