The evolutionary context for herbivore-induced plant volatiles: beyond the 'cry for help'

Priming grapevines with oregano essential oil vapour results in a metabolomic shift eliciting resistance against downy mildew

BACKGROUND

Priming plants with natural products is extensively studied in the agricultural field to reduce the use of synthetic and copper-based pesticides. Previous studies have shown that Oregano essential oil vapour (OEOV) is an effective priming agent against downy mildew (DM) in grapevine (Vitis vinifera L. cv. Chasselas), activating different transcriptomic regulated defence mechanisms.

RESULTS

In the present study, we complement transcriptomic data with metabolomic insights, confirming some previous regulating patterns and highlighting new mechanisms underlying OEOV-induced resistance. A significant modulation of the phenylpropanoid pathway was noted. The data also confirmed the induction of an oxidative stress response indicated by an up-regulation of reactive oxygen species (ROS)-related genes and a congruent depletion of putative L-glutathione. Interestingly, OEOV promoted the accumulation of organic metabolites such as terpenes and other potential phytoalexins, which could potentially contribute to grapevine innate immune response to Plasmopara viticola.

CONCLUSION

Overall, this study uncovered a diverse influence of OEOV on V. vinifera defence mechanisms against DM, enhancing our comprehension of the mode of action of essential oils. This insight offers various prospects for crafting innovative biocontrol products, fostering a more dynamic and sustainable approach to agriculture.

Rhizobacterial volatile organic compounds: Implications for agricultural ecosystems' nutrient cycling and soil health

Plant growth-promoting rhizobacteria (PGPR) have emerged as key players in sustainable agriculture due to their ability to enhance plant growth, nutrient uptake, and disease resistance. A significant aspect of PGPR is the emission of volatile organic compounds (VOCs), which serve as signaling molecules that influence various physiological processes in plants. This review article explores the complex interactions between rhizobacterial VOCs and soil health, focusing particularly on their role in nutrient cycling within agricultural ecosystems. By investigating the mechanism of production and release of VOCs by rhizobacteria, along with impacts on soil properties and microbial communities. We aim to highlight the potential of rhizobacterial volatile organic compounds (VOCs) for sustainable agricultural management. Additionally, we discuss the role of rhizobacterial VOCs in promoting root growth, nutrient uptake, and enhancing nutrient cycling processes. By providing insights into these mechanisms, this review offers tailored strategies for exploring the potential of rhizobacterial VOCs to optimize nutrient availability, enhance soil fertility, and address environmental challenges in agriculture. Exploring the potential of rhizobacterial VOCs presents an opportunity to establish sustainable and resilient agricultural systems that significantly enhance global food security and promote environmental stewardship.

Chemical Diversity of Mediterranean Seagrasses Volatilome

BACKGROUND/OBJECTIVES

Biogenic volatile organic compounds (BVOCs), extensively studied in terrestrial plants with global emissions around 1 PgC yr-1, are also produced by marine organisms. However, benthic species, especially seagrasses, are understudied despite their global distribution (177,000-600,000 km2). This study aims to examine BVOC emissions from key Mediterranean seagrass species (Cymodocea nodosa, Posidonia oceanica, Zostera noltei, and Zostera marina) in marine and coastal lagoon environments.

METHODS

BVOCs were collected using headspace solid-phase microextraction (HS-SPME) using divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fibers and analyzed by gas chromatography-mass spectrometry (GC-MS).

RESULTS

An important chemical diversity was found with a total of 92 volatile compounds (61 for Z. noltei, 59 for C. nodosa, 55 for P. oceanica, and 51 for Z. marina), from different biosynthetic pathways (e.g., terpenoids, benzenoids, and fatty acid derivatives) and with several types of chemical functions (e.g., alkanes, esters, aldehydes, and ketones) or heteroatoms (e.g., sulfur). No differences in chemical richness or diversity of compounds were observed between species. The four species shared 29 compounds enabling us to establish a specific chemical footprint for Mediterranean marine plants, including compounds like benzaldehyde, benzeneacetaldehyde, 8-heptadecene, heneicosane, heptadecane, nonadecane, octadecane, pentadecane, tetradecane, and tridecanal. PLS-DA and Heatmap show that the four species presented significantly different chemical profiles. The major compounds per species in relative abundance were isopropyl myristate for C. nodosa (25.6%), DMS for P. oceanica (39.3%), pentadecane for Z. marina (42.9%), and heptadecane for Z. noltei (46%).

CONCLUSIONS

These results highlight the potential of BVOCs' emission from seagrass ecosystems and reveal species-specific chemical markers.

Influence of Microbial Treatments on Vine Growth and Must Quality: Preliminary Results

Microorganisms play a crucial role in addressing the challenges related to the increasing detrimental effects of intensive agriculture in vineyards by contributing to various aspects, from maintaining soil health and vine vitality to influencing fermentation and the overall wine features. Among microorganisms, mycorrhizal fungi are widely distributed in both natural and agricultural ecosystems, and their mutually beneficial relationship with most terrestrial plants provides valuable ecological benefits. Nowadays, the wine industry is increasingly moving toward the production of organic wines, highlighting the need for novel and healthier strategies that prioritize both the consumer well-being and the quality of the final wine product. Following our previous study in collaboration with the Bioma SA Company (Quartino, Switzerland), the investigation was continued by extending the organic practice to the cultivation. The present work, indeed, aimed to evaluate the influence of the treatment with mycorrhizal fungi on the metabolism of "Sangiovese" grapevines. In particular, the chemical parameters, including alcohol content, pH, acidity, phenolic composition, and sulfur dioxide, were assessed on the must, while the analysis of the volatile emission was conducted both on whole and pressed grapes, on must, as well as on the grape skins. To the best of our knowledge, this is the first study investigating the mycorrhizal fungi association effect on the quality of "Sangiovese" grapes and, further, its effect on the VOCs emission.

Tritrophic Interactions Mediated by Zoophytophagous Predator-Induced Host Plant Volatiles

The zoophytophagous mirid predator Nesidiocoris tenuis and the ectoparasitoid Stenomesius japonicus are important biological control agents for several agricultural pests including the invasive leafminer, Phthorimaea absoluta, a destructive pest of Solanaceous crops especially tomato in sub-Saharan Africa. However, little is known about how feeding by N. tenuis can influence the tritrophic interactions in the tomato plant. Here, we tested the hypothesis that N. tenuis phytophagy would influence the tritrophic olfactory interactions between the host plant tomato and pest, predator, and parasitoid. In olfactometer assays, P. absoluta females and N. tenuis adults were both attracted to constitutive volatiles released by the tomato plant. Whereas females of P. absoluta avoided volatiles released by N. tenuis-infested plants, S. japonicus females and N. tenuis adults were attracted to the induced volatiles. In coupled gas chromatography-electroantennographic detection (GC-EAD) recordings of intact and N. tenuis-infested plant volatiles, antennae of P. absoluta and S. japonicus females both detected eight components, whereas N. tenuis adults detected seven components which were identified by GC-mass spectrometry (GC-MS) as terpenes and green leaf volatiles (GLVs). Dose-response olfactometer bioassays revealed that the responses of P. absoluta, N. tenuis, and S. japonicus varied with the composition and concentration of blends and individual compounds tested from N tenuis-induced volatiles. Females of P. absoluta showed no preference for an eight-component blend formulated from the individual repellents including hexanal, (Z)-3-hexenyl butanoate, and δ-elemene identified in the volatiles. On the other hand, S. japonicus females were attracted to an eight-component blend including the attractants (E)-2-hexenal, (Z)-3-hexenol, methyl salicylate, β-phellandrene, and (E)-caryophyllene. Likewise, N. tenuis adults were attracted to a seven-component blend including the attractants β-phellandrene, δ-elemene, and (E)-caryophyllene identified in the volatiles. Our findings suggest that there is potential for the use of terpenes and GLVs to manage the insects in the tritrophic interaction.

Arbuscular mycorrhizal fungi influence belowground interactions between a specialist root-feeder and its natural enemy

As primary producers, plants play a central role in mediating interactions across trophic levels. Although plants are the primary food source for herbivorous insects, they can protect themselves from herbivore damage. Many plants produce toxic compounds that directly reduce herbivore feeding, but plants also protect themselves indirectly by attracting natural enemies of the attacking herbivore through volatile signaling. These so-called tri-trophic interactions have historically been documented aboveground in aerial plant parts but are also known to occur belowground in root systems. In addition to herbivores, plants directly interact with other organisms, which can influence the outcomes of tri-trophic interactions. Arbuscular mycorrhizal fungi (AMF) are symbiotic soil microbes that colonize the roots of plants and facilitate nutrient uptake. These microbes can alter plant chemistry and subsequent resistance to herbivores. Few studies, however, have shown how AMF affect tri-trophic interactions above- or belowground. This study examines how AMF colonization affects the emission of root volatiles when plants are under attack by western corn rootworm, a problematic pest of corn, and subsequent attraction of entomopathogenic nematodes, a natural enemy of western corn rootworm. Mycorrhizal fungi increased rootworm survival but decreased larval weight. Differences were detected across root volatile profiles, but there was not a clear link between volatile signaling and nematode behavior. Nematodes were more attracted to non-mycorrhizal plants without rootworms and AMF alone in soil, suggesting that AMF may interfere with cues that are used in combination with volatiles which nematodes use to locate prey.

Herbivore-induced volatiles reduce the susceptibility of neighboring tomato plants to transmission of a whitefly-borne begomovirus

Plant viruses exist in a broader ecological community that includes non-vector herbivores that can impact vector abundance, behavior, and virus transmission within shared host plants. However, little is known about the effects of non-vector herbivore infestation on virus transmission by vector insects on neighboring plants through inter-plant airborne chemicals. In this study, we investigated how volatiles emitted from tomato plants infested with the two-spotted spider mite (Tetranychus urticae) affect the infection of neighboring plants by tomato yellow leaf curl virus (TYLCV) transmitted by whitefly (Bemisia tabaci). Exposure of neighboring tomato plants to volatiles released from T. urticae-infested tomato plants reduced subsequent herbivory as well as TYLCV transmission and infection, and the jasmonic acid signaling pathway was essential for generation of the inter-plant defense signals. We also demonstrated that (E)-β-ocimene and methyl salicylic acid were two volatiles induced by T. urticae that synergistically attenuated TYLCV transmission and infection in tomato. Thus, our findings suggest that plant-plant communication via volatiles likely represents a widespread defensive mechanism that substantially contributes to plant fitness. Understanding such phenomena may help us to predict the occurrence and epidemics of multiple herbivores and viruses in agroecosystems, and ultimately to manage pest and virus outbreaks.

Mathematical modeling predicts that endemics by generalist insects are eradicated if nearly all plants produce constitutive defense

Plants with constitutive defense chemicals exist widely in nature. The phenomenon is backed by abundant data from plant chemical ecology. Sufficient data are also available to conclude that plant defenses act as deterrent and repellent to attacking herbivores, particularly deleterious generalist insects. In the wild, generalist species are usually not endemic, meaning they are not restricted to certain plant species in a region. Therefore, our objective is to inspect theoretically whether evolution of chemical defenses in all plant species eradicate an endemic by any generalist species. The objective is addressed by developing deterministic ordinary differential equations under the following conditions: Plants without constitutive defenses are susceptible to oviposition by generalist insects, while they become defended against generalists by storing chemical defenses. From the models, we explicitly obtain that a generalist-free stable state is only possible if the vast majority of all plant individuals have chemical defenses. The model also allows one to predict the highest possible percentage of undefended plant individuals, which may be considered as free-riders.

Functional Response of Four Phytoseiid Mites to Eggs and First-Instar Larvae of Western Flower Thrips, Frankliniella occidentalis

The predation capacity and functional responses of adult females of the phytoseiid mites Amblyseius largoensis (Muma), Proprioseiopsis lenis (Corpuz and Rimando), Paraphytoseius cracentis (Corpuz and Rimando), and Amblyseius swirskii (Athias-Henriot) were studied on eggs and first instars of the western flower thrips, Frankliniella occidentalis (Pergande), in the laboratory at 25 °C and 30 °C. At both temperatures, the functional response of all four phytoseiid mites was type II to first instars of the thrips. In contrast, when offered thrips eggs, the functional response was type III. At both temperatures tested, A. swirskii had the highest mean daily consumption of first-instar F. occidentalis, followed by A. largoensis, P. cracentis, and P. lenis. Amblyseius largoensis had the shortest handling time and the highest maximum attack rate when first-instar thrips were the prey. When fed on thrips eggs, A. largoensis had the highest mean daily consumption, followed by A. swirskii, P. cracentis, and P. lenis. On thrips eggs, A. swirskii showed the shortest handling time and highest maximum attack rate. Our findings indicate that all four phytoseiids had a better ability to prey on first-instar larvae of F. occidentalis compared to thrips eggs. At 25 and 30 °C, A. largoensis was the better predator on thrips larvae, whereas A. swirskii was superior in consuming eggs of F. occidentalis. Proprioseiopsis lenis was the inferior predator on both thrips larvae and eggs compared to the other phytoseiids tested.

Entomopathogenic Nematodes-Killed Insect Cadavers in the Rhizosphere Activate Plant Direct and Indirect Defences Aboveground

Plants can perceive and respond to external stimuli by activating both direct and indirect defences against herbivores. Soil-dwelling entomopathogenic nematodes (EPNs), natural enemies of root-feeding herbivores, carry symbiotic bacteria that grow and reproduce once inside arthropod hosts. We hypothesized that the metabolites produced by EPN-infected insect cadavers could be perceived by plants, thereby activating plant defences systemically. We tested this hypothesis by adding three EPN-infected Galleria mellonella cadavers to maize plants and testing plant responses against a major maize pest (Spodoptera frugiperda) and one of its parasitoids (Trichogramma dendrolimi). We found that S. frugiperda females deposited fewer, and caterpillars fed less on maize plants growing near EPN-infected cadavers than on control plants. Accordingly, EPN-infected cadavers triggered the systemic accumulation of defence hormones (SA), genes (PR1), and enzymes (SOD, POD, and CAT) in maize leaves. Furthermore, four volatile organic compounds produced by plants exposed to EPN-infected cadavers deterred S. frugiperda caterpillars and female adults. However, these compounds were more attractive to T. dendrolimi parasitoids. Our study enhances the understanding of the intricate relationships within the above- and belowground ecosystems and provides crucial insights for advancing sustainable pest management strategies.

Butterfly eggs prime anti-herbivore defense in an annual but not perennial Arabidopsis species

MAIN CONCLUSION

Unlike Arabidopsis thaliana, defenses of Arabidopsis lyrata against Pieris brassicae larval feeding are not primable by P. brassicae eggs. Thus, egg primability of plant anti-herbivore defenses is not phylogenetically conserved in the genus Arabidopsis. While plant anti-herbivore defenses of the annual species Arabidopsis thaliana were shown to be primable by Pieris brassicae eggs, the primability of the phylogenetically closely related perennial Arabidopsis lyrata has not yet been investigated. Previous studies revealed that closely related wild Brassicaceae plant species, the annual Brassica nigra and the perennial B. oleracea, exhibit an egg-primable defense trait, even though they have different life spans. Here, we tested whether P. brassicae eggs prime anti-herbivore defenses of the perennial A. lyrata. We exposed A. lyrata to P. brassicae eggs and larval feeding and assessed their primability by (i) determining the biomass of P. brassicae larvae after feeding on plants with and without prior P. brassicae egg deposition and (ii) investigating the plant transcriptomic response after egg deposition and/or larval feeding. For comparison, these studies were also conducted with A. thaliana. Consistent with previous findings, A. thaliana's response to prior P. brassicae egg deposition negatively affected conspecific larvae feeding upon A. thaliana. However, this was not observed in A. lyrata. Arabidopsis thaliana responded to P. brassicae eggs with strong transcriptional reprogramming, whereas A. lyrata responses to eggs were negligible. In response to larval feeding, A. lyrata exhibited a greater transcriptome change compared to A. thaliana. Among the strongly feeding-induced A. lyrata genes were those that are egg-primed in feeding-induced A. thaliana, i.e., CAX3, PR1, PR5, and PDF1.4. These results suggest that A. lyrata has evolved a robust feeding response that is independent from prior egg exposure.

Defense Molecules of the Invasive Plant Species Ageratum conyzoides

Ageratum conyzoides L. is native to Tropical America, and it has naturalized in many other tropical, subtropical, and temperate countries in South America, Central and Southern Africa, South and East Asia, Eastern Austria, and Europe. The population of the species has increased dramatically as an invasive alien species, and it causes significant problems in agriculture and natural ecosystems. The life history traits of Ageratum conyzoides, such as its short life cycle, early reproductive maturity, prolific seed production, and high adaptive ability to various environmental conditions, may contribute to its naturalization and increasing population. Possible evidence of the molecules involved in the defense of Ageratum conyzoides against its natural enemies, such as herbivore insects and fungal pathogens, and the allelochemicals involved in its competitive ability against neighboring plant species has been accumulated in the literature. The volatiles, essential oils, extracts, residues, and/or rhizosphere soil of Ageratum conyzoides show insecticidal, fungicidal, nematocidal, and allelopathic activity. The pyrrolizidine alkaloids lycopsamine and echinatine, found in the species, are highly toxic and show insecticidal activity. Benzopyran derivatives precocenes I and II show inhibitory activity against insect juvenile hormone biosynthesis and trichothecene mycotoxin biosynthesis. A mixture of volatiles emitted from Ageratum conyzoides, such as β-caryophyllene, β-bisabolene, and β-farnesene, may work as herbivore-induced plant volatiles, which are involved in the indirect defense function against herbivore insects. Flavonoids, such as nobiletin, eupalestin, 5'-methoxynobiletin, 5,6,7,3',4',5'-hexamethoxyflavone, and 5,6,8,3,4',5'-hexamethoxyflavone, show inhibitory activity against the spore germination of pathogenic fungi. The benzoic acid and cinnamic acid derivatives found in the species, such as protocatechuic acid, gallic acid, p-coumaric acid, p-hydroxybenzoic acid, and ferulic acid, may act as allelopathic agents, causing the germination and growth inhibition of competitive plant species. These molecules produced by Ageratum conyzoides may act as defense molecules against its natural enemies and as allelochemicals against neighboring plant species, and they may contribute to the naturalization of the increasing population of Ageratum conyzoides in new habitats as an invasive plant species. This article presents the first review focusing on the defense function and allelopathy of Ageratum conyzoides.

Field evidence for the role of plant volatiles induced by caterpillar oral secretion in prey localization by predatory social wasps

One assumed function of herbivore-induced plant volatiles (HIPVs) is to attract natural enemies of the inducing herbivores. Field evidence for this is scarce. In addition, the assumption that elicitors in oral secretions that trigger the volatile emissions are essential for the attraction of natural enemies has not yet been demonstrated under field conditions. After observing predatory social wasps removing caterpillars from maize plants, we hypothesized that these wasps use HIPVs to locate their prey. To test this, we conducted an experiment that simultaneously explored the importance of caterpillar oral secretions in the interaction. Spodoptera caterpillars pinned onto mechanically damaged plants treated with oral secretion were more likely to be attacked by wasps compared with caterpillars on plants that were only mechanically wounded. Both of the latter treatments were considerably more attractive than plants only treated with oral secretion or left untreated. Subsequent analyses of headspace volatiles confirmed differences in emitted volatiles that likely account for the differential predation across treatments. These findings highlight the importance of HIPVs in prey localization by social wasps, hitherto underappreciated potential biocontrol agents and provide evidence for the role that elicitors play in inducing attractive odour blends.

The endosymbiont Serratia symbiotica improves aphid fitness by disrupting the predation strategy of ladybeetle larvae

Aphids, the important global agricultural pests, harbor abundant resources of symbionts that can improve the host adaptability to environmental conditions, also control the interactions between host aphid and natural enemy, resulting in a significant decrease in efficiency of biological control. The facultative symbiont Serratia symbiotica has a strong symbiotic association with its aphid hosts, a relationship that is known to interfere with host-parasitoid interactions. We hypothesized that Serratia may also influence other trophic interactions by interfering with the physiology and behavior of major predators to provide host aphid defense. To test this hypothesis, we investigated the effects of Serratia on the host aphid Acyrthosiphon pisum and its predator, the ladybeetle Propylaea japonica. First, the prevalence of Serratia in different A. pisum colonies was confirmed by amplicon sequencing. We then showed that harboring Serratia improved host aphid growth and fecundity but reduced longevity. Finally, our research demonstrated that Serratia defends aphids against P. japonica by impeding the predator's development and predation capacity, and modulating its foraging behavior. Our findings reveal that facultative symbiont Serratia improves aphid fitness by disrupting the predation strategy of ladybeetle larvae, offering new insight into the interactions between aphids and their predators, and providing the basis of a new biological control strategy for aphid pests involving the targeting of endosymbionts.

Surviving adversity: Exploring the presence of Lunularia cruciata (L.) Dum. on metal-polluted mining waste

The tailings dump of Barraxiutta (Sardinia, Italy) contains considerable concentrations of heavy metals and, consequently, is scarcely colonized by plants. However, wild populations of the liverwort Lunularia cruciata (L.) Dum. form dense and healthy-looking carpets on this tailing dump. L. cruciata colonizing the tailing dump was compared with a control population growing in a pristine environment in terms of: (i) pollutant content, (ii) photochemical efficiency, and (iii) volatile secondary metabolites in thalli extracts. L. cruciata maintained optimal photosynthesis despite containing considerable amounts of soil pollutants in its thalli and had higher sesquiterpene content compared to control plants. Sesquiterpenes have a role in plant stress resistance and adaptation to adverse environments. In the present study, we propose enhanced sesquiterpenes featuring Contaminated L. cruciata as a defence strategy implemented in the post-mining environment.

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.

The frequency and chemical phenotype of neighboring plants determine the effects of intraspecific plant diversity

Associational effects, whereby plants influence the biotic interactions of their neighbors, are an important component of plant-insect interactions. Plant chemistry has been hypothesized to mediate these interactions. The role of chemistry in associational effects, however, has been unclear in part because the diversity of plant chemistry makes it difficult to tease apart the importance and roles of particular classes of compounds. We examined the chemical ecology of associational effects using backcross-bred plants of the Solanum pennellii introgression lines. We used eight genotypes from the introgression line system to establish 14 unique neighborhood treatments that maximized differences in acyl sugars, proteinase inhibitor, and terpene chemical diversity. We found that the chemical traits of the neighboring plant, rather than simply the number of introgression lines within a neighborhood, influenced insect abundance on focal plants. Furthermore, within-chemical class diversity had contrasting effects on herbivore and predator abundances, and depended on the frequency of neighboring plant chemotypes. Notably, we found insect mobility-flying versus crawling-played a key role in insect response to phytochemistry. We highlight that the frequency and chemical phenotype of plant neighbors underlie associational effects and suggest this may be an important mechanism in maintaining intraspecific phytochemical variation within plant populations.

The sky is not the limit: Successful foliar application of Steinernema spp. entomopathogenic nematodes to control Lepidopteran caterpillars

Entomopathogenic nematodes (EPNs) are ubiquitous soil-thriving organisms that use chemical cues to seek and infect soil-dwelling arthropods, yielding various levels of biological control. Going beyond soil application, scientists and practitioners started exploring the option of applying EPNs onto the foliage of crops in attempts to manage leaf-dwelling insect pests as well. Despite some success, particularly with protective formulations, it remains uncertain whether EPNs could indeed survive the phyllospheric environment, and successfully control foliar insect pests. In this context, we tested the potential of commercially produced Steinernema feltiae and S. carpocapsae, two of the most commonly used EPNs in the field of biological control, in controlling Lepidopteran foliar pests of economic importance, i.e. Tuta absoluta and Spodoptera spp. caterpillars as models. We first tested the survival and efficacy of both EPN species against the Lepidopteran caterpillars when applied onto tomato, sweet pepper and lettuce leaves, under controlled conditions and in commercial greenhouse conditions, respectively. Subsequently, we explored the behavioural responses of the EPNs to environmental cues typically encountered in the phyllosphere, and analysed plant volatile organic compounds (VOCs). Our results show that both S. feltiae and S. carpocapsae successfully survived and infected the foliar caterpillars, reaching similar level of control to a standard chemical pesticide in commercial practices. Remarkably, both EPN species survived and remained effective up to four days in the phyllosphere, and needed only a few hours to successfully penetrate the caterpillars. Interestingly, S. feltiae was attracted to VOCs from tomato plants, and tended to prefer those from caterpillar-induced plants, suggesting that the nematodes may actively forage toward its host, although it has never been exposed to leaf-borne volatiles during its evolution. The present study shows the high potential of steinernematids in managing major foliar pests in greenhouses and in becoming a key player in foliar biological control. In particular, the discovery that EPNs use foliar VOCs to locate caterpillar hosts opens up new opportunities in terms of application techniques and affordable effective doses.

Plant Immunity Modulation in Arbuscular Mycorrhizal Symbiosis and Its Impact on Pathogens and Pests

Arbuscular mycorrhizal (AM) symbiosis is the oldest and most widespread mutualistic association on Earth and involves plants and soil fungi belonging to Glomeromycotina. A complex molecular, cellular, and genetic developmental program enables partner recognition, fungal accommodation in plant tissues, and activation of symbiotic functions such as transfer of phosphorus in exchange for carbohydrates and lipids. AM fungi, as ancient obligate biotrophs, have evolved strategies to circumvent plant defense responses to guarantee an intimate and long-lasting mutualism. They are among those root-associated microorganisms able to boost plants' ability to cope with biotic stresses leading to mycorrhiza-induced resistance (MIR), which can be effective across diverse hosts and against different attackers. Here, we examine the molecular mechanisms underlying the modulation of plant immunity during colonization by AM fungi and at the onset and display of MIR against belowground and aboveground pests and pathogens. Understanding the MIR efficiency spectrum and its regulation is of great importance to optimizing the biotechnological application of these beneficial microbes for sustainable crop protection.

Characterisation of aphid antixenosis in aphid-resistant ancestor wheat, Triticum monococcum

BACKGROUND

Due to the increasing presence of insecticide resistance across cereal aphid populations, new aphid management strategies, including the engineering of host resistance to aphids into commercial wheat varieties, are required. Previous studies have identified ancestor wheat, Triticum monococcum accessions MDR045 and MDR049, with resistance against the grain aphid, Sitobion avenae. To test the hypothesis that resistance can be accounted for by antixenosis (reduced attractiveness of host plants) via the release of repellent volatile organic compounds (VOCs), we explored the response of S. avenae to MDR045 and MDR049 following S. avenae herbivory, using behaviour and electrophysiology experiments.

RESULTS

In four-arm olfactometry assays, alate S. avenae showed aphid density-dependent reduced preference to VOC extracts from T. monococcum MDR045 and MDR049. By contrast, alate S. avenae showed aphid density-dependent increased preference to extracts from aphid-susceptible hexaploid wheat, Triticum aestivum var. Solstice and T. monococcum MDR037. Coupled gas chromatography-electroantennography (GC-EAG), using the antennae of alate S. avenae, located 24 electrophysiologically active compounds across all tested accessions. Synthetic blends created from 21 identified EAG-active compounds confirmed bioactivity of corresponding VOC extracts in four-arm olfactometry assays against alate S. avenae.

CONCLUSION

Our data suggest that resistance of T. monococcum MDR045 and MDR049 to S. avenae can be at least partially accounted for by antixenosis through antennal perception of specific repellent VOC blends induced by S. avenae feeding behaviour. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Exogenous stimulation of Tanacetum vulgare roots with pipecolic acid leads to tissue-specific responses in terpenoid composition

Tanacetum vulgare L., tansy, is a perennial plant with highly variable terpenoid composition, with mono- and sesquiterpenoids being the most abundant. The high diversity of terpenoids plays an important role in mediating ecological interactions. However, the distribution of terpenoids in different tissues and inducibility of terpenoids in these tissues via biotic stress are poorly understood. We investigated changes in terpenoid profiles and concentrations in different organs following treatment of roots with pipecolic acid (Pip), a non-proteinogenic amino acid that triggers defence responses leading to induce systemic resistance (SAR) in plants. Tansy leaves and midribs contained mainly monoterpenoids, while coarse and fine roots contained mainly sesquiterpenoids. Rhizomes contained terpenoid profiles of both midribs and roots but also unique compounds. Treatment with Pip led to an increase in concentrations of mono- and sesquiterpenoids in all tissues except rhizomes. However, significantly more sesquiterpenoids was formed in root tissues in response to Pip treatment, compared to shoots. The metabolic atlas for terpenoids presented here shows that there is exceptionally strong differentiation of terpenoid patterns and terpenoid content in different tissues of tansy. This, together with differential inducibility by Pip, suggests that the chemical diversity of terpenoids may play an important role in tansy ecological interactions and defence against biotic stressors that feed on below- and aboveground organs.

TkJAZs-TkMYC2-TkSRPP/REF Regulates the Biosynthesis of Natural Rubber in Taraxacum kok-saghyz

Taraxacum kok-saghyz (TKS) is a natural rubber (NR)-producing plant and a model plant for studying the biosynthesis of NR. Analyzing and studying the biosynthetic mechanism of NR is an important way to cultivate high-yield rubber TKS varieties. JAZ proteins, which belong to the Jasmonate ZIM domain family, function as negative regulators in the jasmonic acid (JA) signal transduction pathway. MYC2 is typically regarded as a regulatory factor for the target genes of JAZ proteins; JAZ proteins indirectly influence the gene expression regulated by MYC2 by modulating its activity. Theoretically, JAZ is expected to participate in growth, development, and responses to environmental cues related to rubber and biomass accumulation in TKS, all of which rely on the interaction between JAZ and MYC2. In this study, we identified 11 TkJAZs through homology searching of the TKS genomes and bioinformatics analyses. Subcellular localization, Y2H, and BiFC analysis demonstrate that TkJAZs and TkMYC2 are localized in the nucleus, with all TkJAZs and TkMYC2 showing nuclear colocalization interactions. Overexpression of TkMYC2 in TKS inhibited leaf development, promoted root growth, and simultaneously increased NR production. RNA-seq and qRT-PCR analysis revealed that the TkSRPP/REF genes exhibit varying degrees of upregulation compared to the wild type, upregulating the TkREF1 gene by 3.7-fold, suggesting that TkMYC2 regulates the synthesis of NR by modulating the TkSRPP/REF genes.

Plant volatile-triggered defense in citrus against biotic stressors

Plants employ sophisticated defense mechanisms, including releasing volatile organic compounds, to defend against biotic and abiotic stresses. These compounds play a crucial role in plant defense by attracting natural enemies and facilitating communication between plants to activate defense mechanisms. However, there has been no research on how exposure to these compounds activates defense mechanisms in citrus plants. To elucidate the underlying mechanisms governing citrus defensive activation, we conducted a molecular analysis of the rootstock Citrange carrizo [a hybrid of Citrus sinensis × Poncirus trifoliata] in response to defense activation by the volatile (Z)-3-hexenyl propanoate [(Z)-3-HP], utilizing a groundbreaking transcriptomic analysis involving the genomes of both parental species. Our results revealed significant gene expression changes, notably the overexpression of genes related to plant immunity, antioxidant activity, defense against herbivores, and tolerance to abiotic stress. Significantly, P. trifoliata contributed most notably to the hybrid's gene expression profile in response to (Z)-3-HP. Additionally, plants exposed to (Z)-3-HP repelled several citrus pests, attracted natural predators, and led to diminished performance of two key citrus pests. Our study emphasizes the complex molecular basis of volatile-triggered defenses in citrus and highlights the potential of plant volatiles in pest control strategies.

Pathogenicity and induced resistance in Larix kaempferi and Larix olgensis inoculated with Endoconidiophora fujiensis

With climate warming and economic globalization, insect-microbe assemblages are becoming increasingly responsible for various devastating forest diseases worldwide. Japanese larch (Larix kaempferi) is extensively cultivated in China because of its high survival rate, rapid maturation and robust mechanical properties. Endoconidiophora fujiensis, an ophiostomatoid fungus associated with Ips subelongatus, has been identified as a lethal pathogen of L. kaempferi in Japan. However, there is a dearth of research on the pathogenicity of E. fujiensis in larches in China. Therefore, we investigated the pathogenicity of E. fujiensis in introduced L. kaempferi and indigenous larch (Larix olgensis) trees and compared the induced resistance responses to the pathogen in both tree species in terms of physiology and gene expression. Five-year-old saplings and 25-year-old adult trees of L. olgensis and L. kaempferi were inoculated in parallel during the same growing season. Endoconidiophora fujiensis exhibited high pathogenicity in both larch species, but particularly in L. kaempferi compared with L. olgensis adult trees; adult L. olgensis was more resistant to E. fujiensis than adult L. kaempferi, which was reflected in higher accumulation of defensive monoterpenes, such as myrcene, 3-carene and limonene and the earlier induction of defense genes catalase (CAT) and pathogenesis-related protein 1 (PR1). This study contributes to our understanding of the interactions between bark beetle-associated ophiostomatoid fungi and host larches, from phenotypic responses to alterations in secondary metabolites via defense- and metabolism-related gene activation, providing a valuable foundation for the management of larch diseases and pests in the future.

Bergamotenes: A comprehensive compile of their natural occurrence, biosynthesis, toxicity, therapeutic merits and agricultural applications

Sesquiterpenoids constitute the largest subgroup of terpenoids that have numerous applications in pharmaceutical, flavor, and fragrance industries as well as biofuels. Bergamotenes, a type of bicyclic sesquiterpenes, are found in plants, insects, and fungi with α-trans-bergamotene as the most abundant compound. Bergamotenes and their related structures (Bergamotane sesquiterpenoids) have been shown to possess diverse biological activities such as antioxidant, anti-inflammatory, immunosuppressive, cytotoxic, antimicrobial, antidiabetic, and insecticidal effects. However, studies on their biotechnological potential are still limited. This review compiles the characteristics of bergamotenes and their related structures in terms of occurrence, biosynthesis pathways, and biological activities. It further discusses their functionalities and potential applications in pharmaceutical, nutraceuticals, cosmeceuticals, and pest management sectors. This review also opens novel perspectives in identifying and harnessing bergamotenes for pharmaceutical and agricultural purposes.

The Circadian Clock Gene PHYTOCLOCK1 Mediates the Diurnal Emission of the Anti-Insect Volatile Benzyl Nitrile from Damaged Tea (Camellia sinensis) Plants

Benzyl nitrile from tea plants attacked by various pests displays a diurnal pattern, which may be closely regulated by the endogenous circadian clock. However, the molecular mechanism by the circadian clock of tea plants that regulates the biosynthesis and release of volatiles remains unclear. In this study, the circadian clock gene CsPCL1 can activate both the expression of the benzyl nitrile biosynthesis-related gene CsCYP79 and the jasmonic acid signaling-related transcription factor CsMYC2 involved in upregulating CsCYP79 gene, thereby resulting in the accumulation and release of benzyl nitrile. Therefore, the anti-insect function of benzyl nitrile was explored in the laboratory. The application of slow-release beads of benzyl nitrile in tea plantations significantly reduced the number of tea geometrids and had positive effects on the yield of fresh tea leaves. These findings reveal the potential utility of herbivore-induced plant volatiles for the green control of pests in tea plantations.

Differential effects of plant-beneficial fungi on the attraction of the egg parasitoid Trissolcus basalis in response to Nezara viridula egg deposition

There is increasing evidence that plant-associated microorganisms play important roles in defending plants against insect herbivores through both direct and indirect mechanisms. While previous research has shown that these microbes can modify the behaviour and performance of insect herbivores and their natural enemies, little is known about their effect on egg parasitoids which utilize oviposition-induced plant volatiles to locate their hosts. In this study, we investigated how root inoculation of sweet pepper (Capsicum annuum) with the plant-beneficial fungi Beauveria bassiana ARSEF 3097 or Trichoderma harzianum T22 influences the olfactory behaviour of the egg parasitoid Trissolcus basalis following egg deposition by its host Nezara viridula. Olfactometer assays showed that inoculation by T. harzianum significantly enhanced the attraction of the egg parasitoid, while B. bassiana had the opposite effect. However, no variation was observed in the chemical composition of plant volatiles. Additionally, fitness-related traits of the parasitoids (wasp body size) were not altered by any of the two fungi, suggesting that fungal inoculation did not indirectly affect host quality. Altogether, our results indicate that plant inoculation with T. harzianum T22 can be used to enhance attraction of egg parasitoids, which could be a promising strategy in manipulating early plant responses against pest species and improving sustainable crop protection. From a more fundamental point of view, our findings highlight the importance of taking into account the role of microorganisms when studying the intricate interactions between plants, herbivores and their associated egg parasitoids.

Plant-Entomopathogenic Fungi Interaction: Recent Progress and Future Prospects on Endophytism-Mediated Growth Promotion and Biocontrol

Entomopathogenic fungi, often acknowledged primarily for their insecticidal properties, fulfill diverse roles within ecosystems. These roles encompass endophytism, antagonism against plant diseases, promotion of the growth of plants, and inhabitation of the rhizosphere, occurring both naturally and upon artificial inoculation, as substantiated by a growing body of contemporary research. Numerous studies have highlighted the beneficial aspects of endophytic colonization. This review aims to systematically organize information concerning the direct (nutrient acquisition and production of phytohormones) and indirect (resistance induction, antibiotic and secondary metabolite production, siderophore production, and mitigation of abiotic and biotic stresses) implications of endophytic colonization. Furthermore, a thorough discussion of these mechanisms is provided. Several challenges, including isolation complexities, classification of novel strains, and the impact of terrestrial location, vegetation type, and anthropogenic reluctance to use fungal entomopathogens, have been recognized as hurdles. However, recent advancements in biotechnology within microbial research hold promising solutions to many of these challenges. Ultimately, the current constraints delineate potential future avenues for leveraging endophytic fungal entomopathogens as dual microbial control agents.

Recruitment of beneficial cucumber rhizosphere microbes mediated by amino acid secretion induced by biocontrol Bacillus subtilis isolate 1JN2

Introduction

At present, the use of beneficial microorganisms to control cucumber Fusarium wilt is a widely used method, and the rhizosphere microecological reset is one of the mechanisms involved. However, how biocontrol strains reshape cucumber rhizosphere microecology remains to be further studied.

Methods

The composition changes of cucumber root exudates induced by biocontrol strain 1JN2, the microbial ecology of cucumber rhizosphere and the colonization ability of biocontrol strain 1JN2 in cucumber rhizosphere were analyzed through UHPLC-MS/MS analysis, Illumina high-throughput sequencing and SEM, respectively.

Results

First, cucumber plants treated with biocontrol Bacillus 1JN2 reduced the disease severity of Fusarium wilt by 60%. Significant changes in cucumber root exudates were found after 1JN2 inoculation and the contents of four amino acids including glutamine, tryptophan, glycine and glutamic acid were significantly increased. Second, It was found that the bacterial diversity in the rhizosphere of cucumber was significantly increased in both the strain treatment group and the amino acid mixture treatment group, The number of Bacillus was the largest in all dominant populations, exceeded 20% in all treatment groups. The bacteria of Hydrogenispora and Vicinamibacteria were significantly increased after treatment.

Discussion

Overall, the results demonstrated that amino acid substances in cucumber root exudates induced by biocontrol strain 1JN2 can shift the cucumber root microenvironment and prevent the occurrence of Fusarium wilt disease.

Ozone alters the chemical signal required for plant - insect pollination: The case of the Mediterranean fig tree and its specific pollinator

Tropospheric ozone (O3) is likely to affect the chemical signal emitted by flowers to attract their pollinators through its effects on the emission of volatile organic compounds (VOCs) and its high reactivity with these compounds in the atmosphere. We investigated these possible effects using a plant-pollinator interaction where the VOCs responsible for pollinator attraction are known and which is commonly exposed to high O3 concentration episodes: the Mediterranean fig tree (Ficus carica) and its unique pollinator, the fig wasp (Blastophaga psenes). In controlled conditions, we exposed fig trees bearing receptive figs to a high-O3 episode (5 h) of 200 ppb and analyzed VOC emission. In addition, we investigated the chemical reactions occurring in the atmosphere between O3 and pollinator-attractive VOCs using real-time monitoring. Finally, we tested the response of fig wasps to the chemical signal when exposed to increasing O3 mixing ratios (0, 40, 80, 120 and 200 ppb). The exposure of the fig tree to high O3 levels induced a significant decrease in leaf stomatal conductance, a limited change in the emission by receptive figs of VOCs not involved in pollinator attraction, but a major change in the relative abundances of the compounds among pollinator-attractive VOCs in O3-enriched atmosphere. Fig VOCs reacted with O3 in the atmosphere even at the lowest level tested (40 ppb) and the resulting changes in VOC composition significantly disrupted the attraction of the specific pollinator. These results strongly suggest that current O3 episodes are probably already affecting the interaction between the fig tree and its specific pollinator.

Odours of caterpillar-infested trees increase testosterone concentrations in male great tits

Trees release Herbivore-Induced Plant Volatiles (HIPVs) into the air in response to damage inflicted by insects. It is known that songbirds use those compounds to locate their prey, but more recently the idea emerged that songbirds could also use those odours as cues in their reproductive decisions, as early spring HIPVs may contain information about the seasonal timing and abundance of insects. We exposed pre-breeding great tits (Parus major) to the odours of caterpillar-infested trees under controlled conditions, and monitored reproduction (timing of egg laying, number of eggs, egg size) and two of its main hormonal drivers (testosterone and 17β-estradiol in males and females, respectively). We found that females exposed to HIPVs did not advance their laying dates, nor laid larger clutches, or larger eggs compared to control females. 17β-estradiol concentrations in females were also similar between experimental and control birds. However, males exposed to HIPVs had higher testosterone concentrations during the egg-laying period. Our study supports the hypothesis that insectivorous songbirds are able to detect minute amounts of plant odours. The sole manipulation of plant scents was not sufficient to lure females into a higher reproductive investment, but males increased their reproductive effort in response to a novel source of information for seasonal breeding birds.

Common mycorrhizal network: the predominant socialist and capitalist responses of possible plant-plant and plant-microbe interactions for sustainable agriculture

Plants engage in a variety of interactions, including sharing nutrients through common mycorrhizal networks (CMNs), which are facilitated by arbuscular mycorrhizal fungi (AMF). These networks can promote the establishment, growth, and distribution of limited nutrients that are important for plant growth, which in turn benefits the entire network of plants. Interactions between plants and microbes in the rhizosphere are complex and can either be socialist or capitalist in nature, and the knowledge of these interactions is equally important for the progress of sustainable agricultural practice. In the socialist network, resources are distributed more evenly, providing benefits for all connected plants, such as symbiosis. For example, direct or indirect transfer of nutrients to plants, direct stimulation of growth through phytohormones, antagonism toward pathogenic microorganisms, and mitigation of stresses. For the capitalist network, AMF would be privately controlled for the profit of certain groups of plants, hence increasing competition between connected plants. Such plant interactions invading by microbes act as saprophytic and cause necrotrophy in the colonizing plants. In the first case, an excess of the nutritional resources may be donated to the receiver plants by direct transfer. In the second case, an unequal distribution of resources occurs, which certainly favor individual groups and increases competition between interactions. This largely depends on which of these responses is predominant ("socialist" or "capitalist") at the moment plants are connected. Therefore, some plant species might benefit from CMNs more than others, depending on the fungal species and plant species involved in the association. Nevertheless, benefits and disadvantages from the interactions between the connected plants are hard to distinguish in nature once most of the plants are colonized simultaneously by multiple fungal species, each with its own cost-benefits. Classifying plant-microbe interactions based on their habitat specificity, such as their presence on leaf surfaces (phyllospheric), within plant tissues (endophytic), on root surfaces (rhizospheric), or as surface-dwelling organisms (epiphytic), helps to highlight the dense and intricate connections between plants and microbes that occur both above and below ground. In these complex relationships, microbes often engage in mutualistic interactions where both parties derive mutual benefits, exemplifying the socialistic or capitalistic nature of these interactions. This review discusses the ubiquity, functioning, and management interventions of different types of plant-plant and plant-microbe interactions in CMNs, and how they promote plant growth and address environmental challenges for sustainable agriculture.

Specialist reassociation and residence time modulate the evolution of defense in invasive plants: A meta-analysis

Invasive plants typically escape specialist herbivores but are often attacked by generalist herbivores in their introduced ranges. The shifting defense hypothesis suggests that this will cause invasive plants to evolve lower resistance against specialists, higher resistance against generalists, and greater tolerance to herbivore damage. However, the duration and direction of selective pressures can shape the evolutionary responses of resistance and tolerance for invasive plants. Two critical factors are (1) residence time (length of time that an invasive species has been in its introduced range) and (2) specialist herbivore reassociation (attack by purposely or accidentally introduced specialists). Yet, these two factors have not been considered simultaneously in previous quantitative syntheses. Here, we performed a meta-analysis with 367 effect sizes from 70 studies of 35 invasive plant species from native and invasive populations. We tested how the residence time of invasive plant species and specialist reassociation in their introduced ranges affected evolutionary responses of defenses against specialists and generalists, including herbivore resistance traits (physical barriers, digestibility reducers and toxins), resistance effects (performance of and damage caused by specialists or generalists) and tolerance to damage (from specialists or generalists). We found that residence time and specialist reassociation each significantly altered digestibility reducers, specialist performance, generalist damage, and tolerance to specialist damage. Furthermore, residence time and specialist reassociation strongly altered toxins and generalist performance, respectively. When we restricted consideration to invasive plant species with both longer residence times and no reassociation with specialists, invasive populations had lower resistance to specialists, similar resistance to generalists, and higher tolerance to damage from both herbivore types, compared with native populations. We conclude that the duration and direction of selective pressure shape the evolutionary responses of invasive plants. Under long-term (long residence time) and stable (no specialist reassociation) selective pressure, invasive plants generally decrease resistance to specialists and increase tolerance to generalist damage that provides mixed support for the shifting defense hypothesis.

How Plant Toxins Cause Early Larval Mortality in Herbivorous Insects: An Explanation by Modeling the Net Energy Curve

Plants store chemical defenses that act as toxins against herbivores, such as toxic isothiocyanates (ITCs) in Brassica plants, hydrolyzed from glucosinolate (GLS) precursors. The fitness of herbivorous larvae can be strongly affected by these toxins, causing immature death. We modeled this phenomenon using a set of ordinary differential equations and established a direct relationship between feeding, toxin exposure, and the net energy of a larva, where the fitness of an organism is proportional to its net energy according to optimal foraging theory. Optimal foraging theory is widely used in ecology to model the feeding and searching behavior of organisms. Although feeding provides energy gain, plant toxins and foraging cause energy loss for the larvae. Our equations explain that toxin exposure and foraging can sharply reduce larval net energy to zero at an instar. Since herbivory needs energy, the only choice left for a larva is to stop feeding at that time point. If that is significantly earlier than the end of the last instar stage, the larva dies without food. Thus, we show that plant toxins can cause immature death in larvae from the perspective of optimal foraging theory.

UGT89AC1-mediated quercetin glucosylation is induced upon herbivore damage and enhances Camellia sinensis resistance to insect feeding

Quercetin is a key flavonol in tea plants (Camellia sinensis (L.) O. Kuntze) with various health benefits, and it often occurs in the form of glucosides. The roles of quercetin and its glucosylated forms in plant defense are generally not well-studied, and remain unknown in the defense of tea. Here, we found higher contents of quercetin glucosides and a decline of the aglucone upon Ectropis grisescens (E. grisescens) infestation of tea. Nine UGTs were strongly induced, among which UGT89AC1 exhibited the highest activity toward quercetin in vitro and in vivo. The mass of E. grisescens larvae that fed on plants with repressed UGT89AC1 or varieties with lower levels of UGT89AC1 was significantly lower than that of larvae fed on controls. Artificial diet supplemented with quercetin glucoside also reduced the larval growth rate, whereas artificial diet supplemented with free quercetin had no significant effect on larval growth. UGT89AC1 was located in both the cytoplasm and nucleus, and its expression was modulated by JA, JA-ILE, and MeJA. These findings demonstrate that quercetin glucosylation serves a defensive role in tea against herbivory. Our results also provide novel insights into the ecological relevance of flavonoid glycosides under biotic stress in plants.

Exploring the co-operativity of secretory structures for defense and pollination in flowering plants

MAIN CONCLUSION

In flowers multiple secretory systems cooperate to deliver specialized metabolites to support specific roles in defence and pollination. The collective roles of cell types, enzymes, and transporters are discussed. The interplay between reproductive strategies and defense mechanisms in flowering plants has long been recognized, with trade-offs between investment in defense and reproduction predicted. Glandular trichomes and secretory cavities or ducts, which are epidermal and internal structures, play a pivotal role in the secretion, accumulation, and transport of specialized secondary metabolites, and contribute significantly to defense and pollination. Recent investigations have revealed an intricate connection between these two structures, whereby specialized volatile and non-volatile metabolites are exchanged, collectively shaping their respective ecological functions. However, a comprehensive understanding of this profound integration remains largely elusive. In this review, we explore the secretory systems and associated secondary metabolism primarily in Asteraceous species to propose potential shared mechanisms facilitating the directional translocation of these metabolites to diverse destinations. We summarize recent advances in our understanding of the cooperativity between epidermal and internal secretory structures in the biosynthesis, secretion, accumulation, and emission of terpenes, providing specific well-documented examples from pyrethrum (Tanacetum cinerariifolium). Pyrethrum is renowned for its natural pyrethrin insecticides, which accumulate in the flower head, and more recently, for emitting an aphid alarm pheromone. These examples highlight the diverse specializations of secondary metabolism in pyrethrum and raise intriguing questions regarding the regulation of production and translocation of these compounds within and between its various epidermal and internal secretory systems, spanning multiple tissues, to serve distinct ecological purposes. By discussing the cooperative nature of secretory structures in flowering plants, this review sheds light on the intricate mechanisms underlying the ecological roles of terpenes in defense and pollination.

Terpene Synthase Gene Family in Chinese Chestnut (Castanea mollissima BL.) Harbors Two Sesquiterpene Synthase Genes Implicated in Defense against Gall Wasp Dryocosmus kuriphilus

Chinese chestnut (Castanea mollissima BL.) is a well-known fruit tree that has been cultivated in East Asia for millennia. Leaves and buds of the plant can become seriously infested by the gall wasp Dryocosmus kuriphilus (GWDK), which results in gall formation and associated significant losses in fruit production. Herbivore-induced terpenes have been reported to play an important role in plant-herbivory interactions, and in this study, we show that upon herbivory by GWDK, four terpene-related compounds were significantly induced, while the concentrations of these four compounds in intact buds were relatively low. Among these compounds, (E)-nerolidol and (E, E)-α-farnesene have frequently been reported to be involved in plant herbivory defenses, which suggests direct and/or indirect functions in chestnut GWDK defenses. Candidate terpene synthase (TPS) genes that may account for (E)-nerolidol and (E, E)-α-farnesene terpene biosynthesis were characterized by transcriptomics and phylogenetic approaches, which revealed altered transcript levels for two TPSs: CmAFS, a TPS-g subfamily member, and CmNES/AFS, a TPS-b clade member. Both genes were dramatically upregulated in gene expression upon GWDK infestation. Furthermore, Agrobacterium tumefaciens-mediated transient overexpression in Nicotiana benthamiana showed that CmAFS catalyzed the formation of (E, E)-α-farnesene, while CmNES/AFS showed dual (E)-nerolidol and (E, E)-α-farnesene synthase activity. Biochemical assays of the recombinant CmAFS and CmNES/AFS proteins confirmed their catalytic activity in vitro, and the enzymatic products were consistent with two of the major volatile compounds released upon GWDK-infested chestnut buds. Subcellular localization demonstrated that CmAFS and CmNES/AFS were both localized in the cytoplasm, the primary compartment for sesquiterpene synthesis. In summary, we show that two novel sesquiterpene synthase genes CmAFS and CmNES/AFS are inducible by herbivory and can account for the elevated accumulation of (E, E)-α-farnesene and (E)-nerolidol upon GWDK infestation and may be implicated in chestnut defense against GWDK herbivores.

Root-associated bacterial communities and root metabolite composition are linked to nitrogen use efficiency in sorghum

The development of cereal crops with high nitrogen use efficiency (NUE) is a priority for worldwide agriculture. In addition to conventional plant breeding and genetic engineering, the use of the plant microbiome offers another approach to improving crop NUE. To gain insight into the bacterial communities associated with sorghum lines that differ in NUE, a field experiment was designed comparing 24 diverse Sorghum bicolor lines under sufficient and deficient nitrogen (N). Amplicon sequencing and untargeted gas chromatography-mass spectrometry were used to characterize the bacterial communities and the root metabolome associated with sorghum genotypes varying in sensitivity to low N. We demonstrated that N stress and sorghum type (energy, sweet, and grain sorghum) significantly impacted the root-associated bacterial communities and root metabolite composition of sorghum. We found a positive correlation between sorghum NUE and bacterial richness and diversity in the rhizosphere. The greater alpha diversity in high NUE lines was associated with the decreased abundance of a dominant bacterial taxon, Pseudomonas. Multiple strong correlations were detected between root metabolites and rhizosphere bacterial communities in response to low N stress. This indicates that the shift in the sorghum microbiome due to low N is associated with the root metabolites of the host plant. Taken together, our findings suggest that host genetic regulation of root metabolites plays a role in defining the root-associated microbiome of sorghum genotypes differing in NUE and tolerance to low N stress.IMPORTANCEThe development of crops that are more nitrogen use-efficient (NUE) is critical for the future of the enhanced sustainability of agriculture worldwide. This objective has been pursued mainly through plant breeding and plant molecular engineering, but these approaches have had only limited success. Therefore, a different strategy that leverages soil microbes needs to be fully explored because it is known that soil microbes improve plant growth through multiple mechanisms. To design approaches that use the soil microbiome to increase NUE, it will first be essential to understand the relationship among soil microbes, root metabolites, and crop productivity. Using this approach, we demonstrated that certain key metabolites and specific microbes are associated with high and low sorghum NUE in a field study. This important information provides a new path forward for developing crop genotypes that have increased NUE through the positive contribution of soil microbes.

Cowpea volatiles induced by beet armyworm or fall armyworm differentially prime maize plants

Exposure to herbivore-induced plant volatiles (HIPVs) is known to enhance the defense responses in plants. This so-called priming effect has only been marginally studied in intercropping systems. We tested whether HIPVs from cowpea, which often serves as an intercrop alongside maize, can prime herbivore-induced volatile emissions in maize. Conventional volatile collection assays and real-time mass spectrometry revealed that maize plants that were exposed to HIPVs from cowpea infested with Spodoptera exigua caterpillars emitted more than control plants when they themselves were subsequently damaged by the same pest. The enhanced emission was only evident on the first day after infestation. Maize plants that were exposed to HIPVs from cowpea infested by S. frugiperda larvae showed no priming effect and released considerably less upon S. frugiperda infestation than upon S. exigua infestation. The latter may be explained by the fact that S. frugiperda is particularly well adapted to feed on maize and is known to suppress maize HIPV emissions. Our results imply that HIPVs from cowpea, depending on the inducing insect herbivore, may strongly prime maize plants. This deserves further investigation, also in other intercropping systems, as it can have important consequences for tritrophic interactions and crop protection.

Comparative analysis of defensive secondary metabolites in wild teosinte and cultivated maize under flooding and herbivory stress

Climate change is driving an alarming increase in the frequency and intensity of abiotic and biotic stress factors, negatively impacting plant development and agricultural productivity. To survive, plants respond by inducing changes in below and aboveground metabolism with concomitant alterations in defensive secondary metabolites. While plant responses to the isolated stresses of flooding and insect herbivory have been extensively studied, much less is known about their response in combination. Wild relatives of cultivated plants with robust stress tolerance traits provide an excellent system for comparing how diverse plant species respond to combinatorial stress, and provide insight into potential germplasms for stress-tolerant hybrids. In this study, we compared the below and aboveground changes in the secondary metabolites of maize (Zea mays) and a flood-tolerant wild relative Nicaraguan teosinte (Zea nicaraguensis) in response to flooding, insect herbivory, and their combination. Root tissue was analyzed for changes in belowground metabolism. Leaf total phenolic content and headspace volatile organic compound emission were analyzed for changes in aboveground secondary metabolism. Results revealed significant differences in the root metabolome profiles of teosinte and maize. Notably, the accumulation of the flavonoids apigenin, naringenin, and luteolin during flooding and herbivory differentiated teosinte from maize. Aboveground, terpenes, including trans-α-bergamotene and (E)-4,8-dimethylnona-1,3,7-triene, shaped compositional differences in their volatile profiles between flooding, herbivory, and their combination. Taken together, these results suggest teosinte may be more tolerant than maize due to dynamic metabolic changes during flooding and herbivory that help relieve stress and influence plant-insect interactions.

Effects of far-red light on the behaviour and reproduction of the zoophytophagous predator Macrolophus pygmaeus and its interaction with a whitefly herbivore

Plants can detect neighbouring plants through a reduction in the ratio between red and far-red light (R:FR). This provides a signal of plant-plant competition and induces rapid plant growth while inhibiting defence against biotic stress, two interlinked responses designated as the shade avoidance syndrome (SAS). Consequently, the SAS can influence plant-herbivore interactions that could cascade to higher trophic levels. However, little is known about how the expression of the SAS can influence tritrophic interactions. We investigated whether changes in R:FR affect the emission of herbivore-induced plant volatiles (HIPVs), and whether these changes influence the attraction of the zoophytophagous predator Macrolophus pygmaeus. We also studied how the expression of the SAS and subsequent inhibition of plant defences affects the reproduction of M. pygmaeus in both the presence and absence of the greenhouse whitefly (WF) (Trialeurodes vaporariorum) as arthropod prey. The results show that changes in R:FR have little effect on HIPV emissions and predator attraction. However, a reduction in R:FR leads to increased reproduction of both the predator and the WFs. We discuss that shade avoidance responses can increase the population development of M. pygmaeus through a combination of reduced plant defences and increased herbivore densities.

Allelopathy and allelobiosis: efficient and economical alternatives in agroecosystems

Chemical interactions in plants often involve plant allelopathy and allelobiosis. Allelopathy is an ecological phenomenon leading to interference among organisms, while allelobiosis is the transmission of information among organisms. Crop failures and low yields caused by inappropriate management can be related to both allelopathy and allelobiosis. Therefore, research on these two phenomena and the role of chemical substances in both processes will help us to understand and upgrade agroecosystems. In this review, substances involved in allelopathy and allelobiosis in plants are summarized. The influence of environmental factors on the generation and spread of these substances is discussed, and relationships between allelopathy and allelobiosis in interspecific, intraspecific, plant-micro-organism, plant-insect, and mechanisms, are summarized. Furthermore, recent results on allelopathy and allelobiosis in agroecosystem are summarized and will provide a reference for the future application of allelopathy and allelobiosis in agroecosystem.

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.

Volatiles from cotton aphid (Aphis gossypii) infested plants attract the natural enemy Hippodamia variegata

The Aphis gossypii is a major threat of cotton worldwide due to its short life cycle and rapid reproduction. Chemical control is the primary method used to manage the cotton aphid, which has significant environmental impacts. Therefore, prioritizing eco-friendly alternatives is essential for managing the cotton aphid. The ladybird, Hippodamia variegata, is a predominant predator of the cotton aphid. Its performance in cotton plantation is directly linked to chemical communication, where volatile compounds emitted from aphid-infested plants play important roles in successful predation. Here, we comprehensively studied the chemical interaction between the pest, natural enemy and host plants by analyzing the volatile profiles of aphid-infested cotton plants using gas chromatography-mass spectrometry (GC-MS). We then utilized the identified volatile compounds in electrophysiological recording (EAG) and behavioral assays. Through behavioral tests, we initially demonstrated the clear preference of both larvae and adults of H. variegata for aphid-infested plants. Subsequently, 13 compounds, namely α-pinene, cis-3-hexenyl acetate, 4-ethyl-1-octyn-3-ol, β-ocimene, dodecane, E-β-farnesene, decanal, methyl salicylate, β-caryophyllene, α-humulene, farnesol, DMNT, and TMTT were identified from aphid-infested plants. All these compounds were electrophysiologically active and induced detectable EAG responses in larvae and adults. Y-tube olfactometer assays indicated that, with few exceptions for larvae, all identified chemicals were attractive to H. variegata, particularly at the highest tested concentration (100 mg/ml). The outcomes of this study establish a practical foundation for developing attractants for H. variegata and open avenues for potential advancements in aphid management strategies by understanding the details of chemical communication at a tritrophic level.

Selection pressure by specialist and generalist insect herbivores leads to optimal constitutive plant defense. A mathematical model

Brassicaceae plants have the glucosinolate-myrosinase defense system, jointly active against herbivory. However, constitutive glucosinolate (GLS) defense is observed to occur at levels that do not deter all insects from feeding. That prompts the question of why Brassicaceae plants have not evolved a higher constitutive defense. The answer may lie in the contrasting relationship between plant defense and host plant preference of specialist and generalist herbivores. GLS content increases a plant's susceptibility to specialist insects. In contrast, generalists are deterred by the plant GLSs. Although GLSs can attract the natural enemies (predators and parasitoids) of these herbivores, enemies can reduce herbivore pressure to some extent only. So, plants can be overrun by specialists if GLS content is too high, whereas generalists can invade the plants if it is too low. Therefore, an optimal constitutive plant defense can minimize the overall herbivore pressure. To explain the optimal defense theoretically, we model the contrasting host selection behavior of insect herbivores and the emergence of their natural enemies by non-autonomous ordinary differential equations, where the independent variable is the plant GLS concentration. From the model, we quantify the optimal amount of GLSs, which minimizes total herbivore (specialists and generalists) pressure. That quite successfully explains the evolution of constitutive defense in plants from the perspective of optimality theory.

Plant Variety, Mycorrhization, and Herbivory Influence Induced Volatile Emissions and Plant Growth Characteristics in Tomato

Plants produce a range of volatile organic compounds (VOCs) that mediate vital ecological interactions between herbivorous insects, their natural enemies, plants, and soil dwelling organisms including arbuscular mycorrhizal fungi (AMF). The composition, quantity, and quality of the emitted VOCs can vary and is influenced by numerous factors such as plant species, variety (cultivar), plant developmental stage, root colonization by soil microbes, as well as the insect developmental stage, and level of specialization of the attacking herbivore. Understanding factors shaping VOC emissions is important and can be leveraged to enhance plant health and pest resistance. In this greenhouse study, we evaluated the influence of plant variety, mycorrhizal colonization, herbivory, and their interactions on the composition of emitted volatiles in tomato plants (Solanum lycopersicum L.). Four tomato varieties from two breeding histories (two heirlooms and two hybrids), were used. Tomato plants were inoculated with a commercial inoculum blend consisting of four species of AMF. Plants were also subjected to herbivory by Manduca sexta (Lepidoptera: Sphingidae L.) five weeks after transplanting. Headspace volatiles were collected from inoculated and non-inoculated plants with and without herbivores using solid phase-microextraction. Volatile profiles consisted of 21 different volatiles in detectable quantities. These included monoterpenes, sesquiterpenes, and alkane hydrocarbons. We documented a strong plant variety effect on VOC emissions. AMF colonization and herbivory suppressed VOC emissions. Plant biomass was improved by colonization of AMF. Our results show that mycorrhization, herbivory and plant variety can alter tomato plant VOC emissions and further shape volatile-mediated insect and plant interactions.

Flavonoid production in tomato mediates both direct and indirect plant defences against whiteflies in tritrophic interactions

BACKGROUND

The role of plant flavonoids in direct defences against chewing and sap-sucking herbivorous insects has been extensively characterized. However, little is known about flavonoid-mediated tritrophic interactions between plants, herbivorous insects and natural enemies. In this study, we investigated how flavonoids modulate plant-insect interactions in a tritrophic system involving near-isogenic lines (NILs) of cultivated tomato (Solanum lycopersicum) with high (line NIL-purple hypocotyl [PH]) and low (line NIL-green hypocotyl [GH]) flavonoid levels, with a generalist herbivore whitefly (Bemisia tabaci) and its predatory bug (Orius sauteri).

RESULTS

By contrasting levels of tomato flavonoids (direct defence) while manipulating the presence of predators (indirect defence), we found that high production of flavonoids in tomato was associated with a higher inducibility of direct defences and a stronger plant resistance to whitefly infestation and stimulated the emissions of induced volatile organic compounds, thereby increasing the attractiveness of B. tabaci-infested plants to the predator O. sauteri. Furthermore, suppression of B. tabaci population growth and enhancement of plant growth were mediated directly by the high production of flavonoids and indirectly by the attraction of O. sauteri, and the combined effects were larger than each effect individually.

CONCLUSION

Our results show that high flavonoid production in tomato enhances herbivore-induced direct and indirect defences to better defend against herbivores in tritrophic interactions. Thus, the development of transgenic plants may present an opportunity to utilize the beneficial role of flavonoids in integrated pest management, while simultaneously maintaining or improving resistance against other pests and pathogens. © 2023 Society of Chemical Industry.

Heterologous expression of PtAAS1 reveals the metabolic potential of the common plant metabolite phenylacetaldehyde for auxin synthesis in planta

Aromatic aldehydes and amines are common plant metabolites involved in several specialized metabolite biosynthesis pathways. Recently, we showed that the aromatic aldehyde synthase PtAAS1 and the aromatic amino acid decarboxylase PtAADC1 contribute to the herbivory-induced formation of volatile 2-phenylethanol and its glucoside 2-phenylethyl-β-D-glucopyranoside in Populus trichocarpa. To unravel alternative metabolic fates of phenylacetaldehyde and 2-phenylethylamine beyond alcohol and alcohol glucoside formation, we heterologously expressed PtAAS1 and PtAADC1 in Nicotiana benthamiana and analyzed plant extracts using untargeted LC-qTOF-MS and targeted LC-MS/MS analysis. While the metabolomes of PtAADC1-expressing plants did not significantly differ from those of control plants, expression of PtAAS1 resulted in the accumulation of phenylacetic acid (PAA) and PAA-amino acid conjugates, identified as PAA-aspartate and PAA-glutamate. Herbivory-damaged poplar leaves revealed significantly induced accumulation of PAA-Asp, while levels of PAA remained unaltered upon herbivory. Transcriptome analysis showed that members of auxin-amido synthetase GH3 genes involved in the conjugation of auxins with amino acids were significantly upregulated upon herbivory in P. trichocarpa leaves. Overall, our data indicates that phenylacetaldehyde generated by poplar PtAAS1 serves as a hub metabolite linking the biosynthesis of volatile, non-volatile herbivory-induced specialized metabolites, and phytohormones, suggesting that plant growth and defense can be balanced on a metabolic level.

Biological control potential of a laboratory selected generalist parasitoid versus a co-evolved specialist parasitoid against the invasive Drosophila suzukii

A few generations of laboratory selection can increase the developmental success of native parasitoids on invasive targets. However, for this approach to be used more widely for biological control, we need to understand if the improved performance of native species, achieved under artificial laboratory conditions, translates to improved control in more natural environments. It is also unknown what the biocontrol potential of laboratory selected generalist native parasitoids may be compared to co-evolved specialists that are typically introduced for biological control of invasive species. To assess how rearing in artificial diet affected host finding ability in natural hosts, we used laboratory selected (adapted) and nonadapted populations of the generalist native parasitoid Trichopria drosophilae to parasitize the invasive fly, Drosophila suzukii in three different fruit types. In a separate experiment, we compared the effectiveness of adapted and nonadapted populations of T. drosophilae in raspberries with a co-evolved specialist larval parasitoid Ganaspis brasiliensis from Asia that was recently approved for release in the USA. More adult parasitoids emerged in each fruit type of the adapted compared to the nonadapted population of T. drosophilae. D. suzukii emergence rates were reduced on average by 85% by the adapted T. drosophilae population indicating that the artificial rearing conditions did not significantly impair the ability of parasitoids to locate and attack hosts in natural hosts. The specialist G. brasiliensis had higher adult emergence than the adapted population of T. drosophilae; however, both parasitoid species were able to reduce D. suzukii populations to the same extent. These results show that despite the lower developmental success of the laboratory selected T. drosophilae, they killed the same proportion of D. suzukii as G. brasiliensis when host choice was restricted. In nature, where host choices are available, specialist and generalist parasitoids will be unlikely to exhibit the same biocontrol potential.