Plant defense against herbivores: chemical aspects

Deleterious effects of Schinus terebinthifolius Raddi seed flour on cowpea weevil, Callosobruchus maculatus (F.), larval development

Schinus terebinthifolius, Raddi, has been extensively studied due to its anti-inflammatory and antibiotic properties. S. terebinthifolius was also toxic to some insects, however little has been explored about the nature of its insecticide compounds or the toxicity of this plant to insect species. In this work, we investigate the toxicity of S. terebinthifolius seed flour against the insect C. maculatus. S. terebinthifolius seed flour interfered with the post hatch development of the C. maculatus larvae, decreasing larval survival, mass and length. Using DEAE-cellulose chromatography, five protein fractions were isolated, a non-retained fraction (NRF) and four retained fractions, eluted with 0.25, 0.5, 0.7 and 1.0 M NaCl. Proteins with varying molecular masses were observed in all fractions. The majority protein bands were identified by mass spectrometry analysis and among the main identified proteins are 11S globulins (such glycinin), lipoxygenase, chitinases, 7S globulins (vicilins, canavalin and β conglycinin), annexin, catalase and sucrose binding protein. All DEAE-protein fractions were toxic to the insect, interfering with the post hatch larval development and survival. Decreases greater than 90% were observed in the larval mass and length at 20 days after oviposition (DAO) for larvae raised on diet containing 0.5% of some fractions. Alterations in the level of proteins, glucose and in the activity of the enzymes lipases and cysteine proteases were also detected in these larvae. Our results show that seeds of S. terebinthifolius have an arsenal of toxic proteins with potential for the control of the insect C. maculatus.

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

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

A Comparative Analysis of Bombyx mori (Lepidoptera: Bombycidae) β-fructofuranosidase Homologs Reveals Different Post-Translational Regulations in Glyphodes pyloalis Walker (Lepidoptera: Pyralidae)

Simple Summary

The β-fructofuranosidase (β-FFase) encoding gene BmSuc1 regulates the glycometabolism of silkworm larvae, and it participates in the resistance of mulberry alkaloids. However, there is no molecular or biochemical information available about the mulberry pest Glyphodespyloalis Walker β-FFase homologs. In this paper, we have obtained five β-FFase homologous genes in G. pyloalis and characterized the expression and the localization of GpSUC1a in the midgut. The β-FFase activity in the midgut of G. pyloalis larvae and GpSUC1a were both confirmed, while recombinant GpSUC1a displayed little activity as compared with the higher activity of BmSUC1. Some putative N-glycosylation sites were found in GpSUC1a but none in BmSUC1, while there was more methylation in BmSUC1 than in GpSUC1a. The results indicate that such post-translational modifications (PTMs) are differentially supporting that β-FFase are active in these two mulberry feeding caterpillars, and the activation of GpSUC1a may be controlled by a more complex post-translational regulatory system in G. pyloalis larvae. This is the first report on the characterization of β-FFase genes from G. pyloalis and the first comparison of expression regulation between two mulberry feeding insects B. mori and G. pyloalis. Moreover, this research may provide new ideas for the management of mulberry borers.

Abstract

The silk-spinning and Lepidopteran model insect Bombyx mori (Bombycidae) is a mulberry specialist. The BmSuc1 gene is the first β-fructofuranosidase (β-FFase) encoding gene identified in animals, and β-FFase acts as an essential sucrase for glycometabolism modulation in the silkworm larvae, involved in resistance to mulberry alkaloids. Glyphodes pyloalis Walker (Lepidoptera: Pyralidae) is an important mulberry pest leading to heavy economic loss of sericulture. However, no molecular or biochemical information is available about G. pyloalis β-FFase homologs. In this study, five β-FFase homologous genes in G. pyloalis were obtained. The genes GpSuc1a and GpSuc2c were expressed in the midgut; GpSuc2c encodes a truncated polypeptide. The expression and the localization of GpSUC1a in the midgut was characterized. Whereas recombinant GpSUC1a expressed in both Escherichia coli and BmN cells displayed little activity as compared with higher activity of BmSUC1, β-FFase activity in the larval midgut of G. pyloalis and GpSUC1a purified from the midgut were both confirmed. The data suggested that the activation of GpSUC1a is probably controlled by a more complicated post-translational regulation system in G. pyloalis larvae than that of BmSUC1 in B. mori. To study post-translational modifications (PTMs), GpSUC1a and BmSUC1 were purified from larval midguts using immunoprecipitation and subjected to LC-MS to perform PTMs analysis. Some putative N-glycosylated sites were found in GpSUC1a but none in BmSUC1, while there was more methylation in BmSUC1 than in GpSUC1a, indicating that such PTMs were supporting the differential β-FFases activities in these two mulberry feeding caterpillars.

Evolutionary Ecology of Plant-Arthropod Interactions in Light of the "Omics" Sciences: A Broad Guide

Aboveground plant-arthropod interactions are typically complex, involving herbivores, predators, pollinators, and various other guilds that can strongly affect plant fitness, directly or indirectly, and individually, synergistically, or antagonistically. However, little is known about how ongoing natural selection by these interacting guilds shapes the evolution of plants, i.e., how they affect the differential survival and reproduction of genotypes due to differences in phenotypes in an environment. Recent technological advances, including next-generation sequencing, metabolomics, and gene-editing technologies along with traditional experimental approaches (e.g., quantitative genetics experiments), have enabled far more comprehensive exploration of the genes and traits involved in complex ecological interactions. Connecting different levels of biological organization (genes to communities) will enhance the understanding of evolutionary interactions in complex communities, but this requires a multidisciplinary approach. Here, we review traditional and modern methods and concepts, then highlight future avenues for studying the evolution of plant-arthropod interactions (e.g., plant-herbivore-pollinator interactions). Besides promoting a fundamental understanding of plant-associated arthropod communities' genetic background and evolution, such knowledge can also help address many current global environmental challenges.

Find My Way to You: A Comparative Study of Antennal Sensilla and Olfactory Genes in Slug Moth With Different Diet Ranges (Lepidoptera: Limacodidae)

Insects and plants that provide them with foods have coexisted for several hundred million years, which leads to various defense approaches and insect-feeding strategies. The host plant provides insects with food sources, shelter materials, and oviposition sites for phytophagous insects. However, they need to find the most suitable host plants in complicated plant communities. The antenna is the main sensory organ of insects, housing different types of sensilla dedicated to detecting chemical cues, motion, humidity, and temperature. Phytophagous insects with different diets may possess various adaptations in their olfactory system. We selected three species of slug moth (Narosoideus flavidorsalis, Chalcoscelides castaneipars, and Setora postornata) with different diet breadths to detect the structural diversity of antennal sensilla using the scanning electron microscope. A total of nine types of sensilla were identified in these three species, in which two types of sensilla (sensilla uniporous peg and sensilla furcatea) were the first found and reported in Limacodidae. By comparing the number of sensilla types, there was a trend of gradually decreasing the number of sensory types with the gradual expansion of feeding habitats. To better understand the vital roles of olfactory proteins in localizing host plants, we investigated the chemosensory proteins in the antennal transcriptomes of N. flavidorsalis and S. postornata. However, there was no significant correlation between the number of olfactory genes and the increase of antennal sensilla types. Combining antennal morphology, transcriptome analysis, and the prediction of suitable areas, we better understood the olfactory systems with different feeding preferences, which will provide new prospects for plant–insect interactions and population control methods.

Patterns of Plumericin Concentration in Leaves of Himatanthus tarapotensis (Apocynaceae) and Its Interactions with Herbivory in the Peruvian Amazon

We explored the concentration patterns of the bioactive metabolite plumericin produced by Himatanthus tarapotensis (Apocynaceae) under different edaphic conditions and variations in rainfall intensity, as well as its potential role in the chemical defense against insect herbivores. Values of plumericin concentration from leaves were obtained by High-Performance Liquid Chromatography, and evaluated as a function of differences in soil types, variation of precipitation, and variation of the abundance of insect herbivores, using first a Repeated Measures Correlation (rmcorr) and then a Generalized Linear Mixed Model (GLMM) analysis. Plumericin concentration is highly variable among plants, but with a significantly higher concentration in plants growing on clay soil compared to that of the white-sand soil habitat (p < 0.001). Plumericin concentration is not affected by precipitation. The caterpillar of Isognathus leachii (Lepidoptera: Sphingidae) is the most conspicuous herbivore of H. tarapotensis, and its presence is continuous but not related to plumericin concentration, probably because of its capacity to elude the chemical defense of this plant. Nevertheless, our multivariate model revealed that plumericin concentration is related to the abundance of Hymenoptera (Formicidae), and this relationship is significantly influenced by the soil parameters of carbon percentage, clay percentage, and phosphorous percentage (p < 0.001). Plumericin is a mediating agent in the interaction between H. tarapotensis and its natural environment. Variation in plumericin concentration would be induced by the abundance of Hymenoptera (Formicidae), probably as a chemical response against these insects, and by differences in soil nutrient availability.

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

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

Foliar Herbivory Reduces Rhizosphere Fungal Diversity and Destabilizes the Co-occurrence Network

Insect herbivores can adversely impact terrestrial plants throughout ontogeny and across various ecosystems. Simultaneously, the effects of foliar herbivory may extend belowground, to the soil microbial community. However, the responses in terms of the diversity, assembly, and stability of rhizosphere fungi to aboveground herbivory remain understudied. Here, using high-throughput sequencing, the effects of foliar insect herbivory on rhizosphere fungal microbes were investigated in a common garden experiment that manipulated herbivory intensity and time from herbivore removal. The number of observed fungal species was reduced by a greater herbivory intensity, with some species evidently sensitive to herbivory intensity and time since herbivore removal. Rhizofungal assembly processes were altered by both herbivory intensity and time since herbivore removal. Further, we found evidence that both factors strongly influenced fungal community stability: a high intensity of herbivory coupled with a shorter time since herbivore removal resulted in low stability. These results suggest that foliar herbivory can adversely alter fungal diversity and stability, which would in turn be harmful for plant health. Fortunately, the effect seems to gradually diminish with time elapsed after herbivore removal. Our findings provide a fresh, in-depth view into the roles of rhizofungi in enhancing the adaption ability of plants under environmental stress.

Identification of Genes Involved in Resistance to High Exogenous 20-Hydroxyecdysone in Spodoptera litura

Simple Summary

20-hydroxyecdysone (20E), the most active insect ecdysteroids, is also a major form of phytoecdysteroids in some plants. The phytoecdysteroid from plant is generally considered as defensive weapon to prevent ingestion by phytophagous insects. Conversely, insects also evolved resistance mechanisms to combat the plant defensive system. In this study, we dissected the molecular mechanism to explain how noctuid pest (Spodoptera litura) resist high dosage of 20E. Besides, comparative transcriptomic analysis using two noctuid insects (S. litura and Helicoverpa armigera) also revealed that different species always ultilized various starategies to tolerate ingested hormone.

Abstract

To prevent their ingestion by phytophagous insects, plants produce secondary metabolites as defensive weapons. Conversely, insects need to counter these metabolites to survive. Different species, though they are closely related, can evolve distinct strategies to resist plant-derived factors. However, the mechanism under this high divergence resistance is still unclear at a molecular level. In this study, we focus on how Spodoptera litura (Lepidoptera; Noctuidae) detoxifies phytoecdysteroids, a class of metabolites capable of disrupting the normal development of insects. Firstly, we find that the S. litura show resistance to artificial foods containing a high level of 20-hydroxyecdysone (20E), the major form of phytoecdysteroids, without any adverse effects on growth and development. Furthermore, a comparative transcriptomic analysis between S. litura and another noctuid insect (Helicoverpa armigera) was performed. Almost all known ecdysteroid degradation pathways including 3-epimerization, 22-phosphorylation, 22-esterification, and 26-hydroxylation were upregulated in the midgut of 20E treated S. litura larvae, whereas only 22-esterification and 26-hydroxylation were enhanced in H. armigera larvae. In summary, though both species belong to the Noctuidae family, they evolved two different strategies to tolerate a high dosage of ingested 20E.

Selection for seed size has uneven effects on specialized metabolite abundance in oat (Avena sativa L.)

Plant breeding strategies to optimize metabolite profiles are necessary to develop health-promoting food crops. In oats (Avena sativa L.), seed metabolites are of interest for their antioxidant properties, yet have not been a direct target of selection in breeding. In a diverse oat germplasm panel spanning a century of breeding, we investigated the degree of variation of these specialized metabolites and how it has been molded by selection for other traits, like yield components. We also ask if these patterns of variation persist in modern breeding pools. Integrating genomic, transcriptomic, metabolomic, and phenotypic analyses for three types of seed specialized metabolites—avenanthramides, avenacins, and avenacosides—we found reduced heritable genetic variation in modern germplasm compared with diverse germplasm, in part due to increased seed size associated with more intensive breeding. Specifically, we found that abundance of avenanthramides increases with seed size, but additional variation is attributable to expression of biosynthetic enzymes. In contrast, avenacoside abundance decreases with seed size and plant breeding intensity. In addition, these different specialized metabolites do not share large-effect loci. Overall, we show that increased seed size associated with intensive plant breeding has uneven effects on the oat seed metabolome, but variation also exists independently of seed size to use in plant breeding. This work broadly contributes to our understanding of how plant breeding has influenced plant traits and tradeoffs between traits (like growth and defense) and the genetic bases of these shifts.

Competing beetles attract egg laying in a hawkmoth

In nature, plant-insect interactions occur in complex settings involving multiple trophic levels, often with multiple species at each level.¹ Herbivore attack of a host plant typically dramatically alters the plant's odor emission in terms of concentration and composition.^2,3 Therefore, a well-adapted herbivore should be able to predict whether a plant is still suitable as a host by judging these changes in the emitted bouquet. Although studies have demonstrated that oviposition preferences of successive insects were affected by previous infestations,^4,5 the underlying molecular and olfactory mechanisms remain unknown. Here, we report that tobacco hawkmoths (Manduca sexta) preferentially oviposit on Jimson weed (Datura wrightii) that is already infested by a specialist, the three-lined potato beetle (Lema daturaphila). Interestingly, the moths' offspring do not benefit directly, as larvae develop more slowly when feeding together with Lema beetles. However, one of M. sexta's main enemies, the parasitoid wasp Cotesia congregata, prefers the headspace of M. sexta-infested plants to that of plants infested by both herbivores. Hence, we conclude that female M. sexta ignore the interspecific competition with beetles and oviposit deliberately on beetle-infested plants to provide their offspring with an enemy-reduced space, thus providing a trade-off that generates a net benefit to the survival and fitness of the subsequent generation. We identify that α-copaene, emitted by beetle-infested Datura, plays a role in this preference. By performing heterologous expression and single-sensillum recordings, we show that odorant receptor (Or35) is involved in α-copaene detection.

Host Plant Species Influences the Composition of Milkweed and Monarch Microbiomes

Plants produce defensive chemicals for protection against insect herbivores that may also alter plant and insect associated microbial communities. However, it is unclear how expression of plant defenses impacts the assembly of insect and plant microbiomes, for example by enhancing communities for microbes that can metabolize defensive chemicals. Monarch butterflies (Danaus plexippus) feed on milkweed species (Asclepias spp.) that vary in production of toxic cardiac glycosides, which could alter associated microbiomes. We therefore sought to understand how different milkweed species, with varying defensive chemical profiles, influence the diversity and composition of monarch and milkweed (root and leaf) bacterial communities. Using a metabarcoding approach, we compared rhizosphere, phyllosphere and monarch microbiomes across two milkweed species (Asclepias curassavica, Asclepias syriaca) and investigated top-down effects of monarch feeding on milkweed microbiomes. Overall, monarch feeding had little effect on host plant microbial communities, but each milkweed species harbored distinct rhizosphere and phyllosphere microbiomes, as did the monarchs feeding on them. There was no difference in diversity between plants species for any of the microbial communities. Taxonomic composition significantly varied between plant species for rhizospheres, phyllospheres, and monarch microbiomes and no dispersion were detected between samples. Interestingly, phyllosphere and monarch microbiomes shared a high proportion of bacterial taxa with the rhizosphere (88.78 and 95.63%, respectively), while phyllosphere and monarch microbiomes had fewer taxa in common. Overall, our results suggest milkweed species select for unique sets of microbial taxa, but to what extent differences in expression of defensive chemicals directly influences microbiome assembly remains to be tested. Host plant species also appears to drive differences in monarch caterpillar microbiomes. Further work is needed to understand how monarchs acquire microbes, for example through horizontal transfer during feeding on leaves or encountering soil when moving on or between host plants.

Balancing nutrients in a toxic environment: the challenge of eating

Insect herbivores can regulate their food intake by mixing food sources with different nutrient content, but face the resulting challenge of ingesting various plant secondary metabolites. How insects deal with toxins in a complex nutrient environment is unclear. Here we investigated the influence of a classic plant secondary metabolite, allyl glucosinolate (sinigrin), and its hydrolyzed product allyl isothiocyanate (AITC), on the development of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) when fed on diets with different protein-to-carbohydrate (p : c) ratios. We also examined the effects of these toxins on larval biochemistry, by chemically analyzing the frass produced by insects feeding on the different diets. As expected, AITC had a greater negative effect than sinigrin on H. armigera life-history traits. However, AITC at low concentration appeared to have a positive effect on some traits. Both sinigrin and AITC-induced detoxification activity in the gut, and the reaction was related to diet protein concentration. High-protein diets can provide the required free amino acid, especially cysteine, needed for the detoxification process. The nutrient content of the diet influences how plant secondary metabolites are handled, and the use of artificial diets in experiments investigating the metabolic fate of plant secondary compounds needs to be carefully evaluated.

Species specific plant‐mediated effects between herbivores converge at high damage intensity

Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant‐mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaf‐feeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root‐feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant‐mediated effects on herbivores. In particular, effects are species‐specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant‐mediated interactions between herbivores, which advances our understanding of the structure and composition of herbivore communities and terrestrial food webs.

Sequestration of Plant Defense Compounds by Insects: From Mechanisms to Insect-Plant Coevolution

Plant defense compounds play a key role in the evolution of insect-plant associations by selecting for behavioral, morphological, and physiological insect adaptations. Sequestration, the ability of herbivorous insects to accumulate plant defense compounds to gain a fitness advantage, represents a complex syndrome of adaptations that has evolved in all major lineages of herbivorous insects and involves various classes of plant defense compounds. In this article, we review progress in understanding how insects selectively accumulate plant defense metabolites and how the evolution of specific resistance mechanisms to these defense compounds enables sequestration. These mechanistic considerations are further integrated into the concept of insect-plant coevolution. Comparative genome and transcriptome analyses, combined with approaches based on analytical chemistry that are centered in phylogenetic frameworks, will help to reveal adaptations underlying the sequestration syndrome, which is essential to understanding the influence of sequestration on insect-plant coevolution.

Beetle-Bacterial Symbioses: Endless Forms Most Functional

Beetles are hosts to a remarkable diversity of bacterial symbionts. In this article, we review the role of these partnerships in promoting beetle fitness following a surge of recent studies characterizing symbiont localization and function across the Coleoptera. Symbiont contributions range from the supplementation of essential nutrients and digestive or detoxifying enzymes to the production of bioactive compounds providing defense against natural enemies. Insights on this functional diversity highlight how symbiosis can expand the host's ecological niche, but also constrain its evolutionary potential by promoting specialization. As bacterial localization can differ within and between beetle clades, we discuss how it corresponds to the microbe's beneficial role and outline the molecular and behavioral mechanisms underlying symbiont translocation and transmission by its holometabolous host. In reviewing this literature, we emphasize how the study of symbiosis can inform our understanding of the phenotypic innovations behind the evolutionary success of beetles.

Females restrict the position of domatia and suffer more herbivory than hermaphrodites in Myriocarpa longipes, a neotropical facultative myrmecophyte

Domatia are hollow structures in plants occupied by ant colonies, in turn ants provide protection against herbivores. In plants, competition for resources has driven sex-related changes in the patterns of resource allocation to life-history traits and defence traits. The resource-competition hypothesis (RCH) proposes that female plants due to their higher investment in reproduction will allocate fewer resources to defence production, showing greater herbivore damage than other sexual forms. We hypothesise the existence of sex-related differences in defensive traits of domatia-bearing plants, being female plants less defended due to differences in domatia traits, such as size, number of domatia and their position, exhibiting more herbivore damage than hermaphrodite plants of Myriocarpa longipes, a facultative neotropical myrmecophyte. We found eight species of ants inhabiting domatia; some species co-inhabited the same plant, even the same branch. Our results are consistent with the predictions of RCH, as female plants had ant-inhabited domatia restricted to the middle position of their branches and exhibited greater herbivore damage in leaves than hermaphrodites. However, we did not find differences in domatia size and leaf area between sexual forms. Our study provides evidence for intersexual differences in domatia position and herbivory in a facultative ant–plant mutualism in M. longipes. We highlight the importance of considering the plant sex in ant–plant interactions. Differences in resource allocation related to sexual reproduction could influence the outcome of ant–plant interactions.

Pseudomonas syringae senses polyphenols via phosphorelay crosstalk to inhibit virulence

Bacteria use a variety of mechanisms, such as two-component regulatory systems (TCSs), to rapidly sense and respond to distinct conditions and signals in their host organisms. For example, a type III secretion system (T3SS) is a key determinant of the virulence of the model plant pathogen Pseudomonas syringae and contains the TCS RhpRS as a key regulator. However, the plant-derived compound targeting RhpRS remains unknown. Here, we report that RhpRS directly interacts with polyphenols and responds by switching off P. syringae T3SS via crosstalk with alternative histidine kinases. We identify three natural polyphenols that induce the expression of the rhpRS operon in an RhpS-dependent manner. The presence of these three specific polyphenols inhibits the phosphatase activity of RhpS, thus suppressing T3SS activation in T3SS-inducing conditions. The Pro40 residue of RhpS is essential to respond to these polyphenols. In addition, three non-cognate histidine kinases cooperatively phosphorylate RhpR and antagonize the rhpS mutant phenotype. This work illustrates that plant polyphenols can directly target P. syringae RhpRS, which results in bacterial virulence being switched off via a phosphorylation-related crosstalk.

Genome-Wide Analysis of WRKY Gene Family and the Dynamic Responses of Key WRKY Genes Involved in Ostrinia furnacalis Attack in Zea mays

WRKY transcription factors comprise one of the largest gene families and serve as key regulators of plant defenses against herbivore attack. However, studies related to the roles of WRKY genes in response to herbivory are limited in maize. In this study, a total of 128 putative maize WRKY genes (ZmWRKYs) were identified from the new maize genome (v4). These genes were divided into seven subgroups (groups I, IIa–e, and III) based on phylogenomic analysis, with distinct motif compositions in each subgroup. Syntenic analysis revealed that 72 (56.3%) of the genes were derived from either segmental or tandem duplication events (69 and 3, respectively), suggesting a pivotal role of segmental duplication in the expansion of the ZmWRKY family. Importantly, transcriptional regulation prediction showed that six key WRKY genes contribute to four major defense-related pathways: L-phenylalanine biosynthesis II and flavonoid, benzoxazinoid, and jasmonic acid (JA) biosynthesis. These key WRKY genes were strongly induced in commercial maize (Jingke968) infested with the Asian corn borer, Ostrinia furnacalis, for 0, 2, 4, 12 and 24 h in the field, and their expression levels were highly correlated with predicted target genes, suggesting that these genes have important functions in the response to O. furnacalis. Our results provide a comprehensive understanding of the WRKY gene family based on the new assembly of the maize genome and lay the foundation for further studies into functional characteristics of ZmWRKY genes in commercial maize defenses against O. furnacalis in the field.

Metabolic response to larval herbivory in three Physalis species

The Physalis genus includes species of commercial importance due to their ornamental, edible and medicinal properties. These qualities stem from their variety of biologically active compounds. We performed a metabolomic analysis of three Physalis species, i.e., P. angulata, P. grisea, and P. philadelphica, differing in domestication stage and cultivation practices, to determine the degree of inter-species metabolite variation and to test the hypothesis that these related species mount a common metabolomic response to foliar damage caused by Trichoplusia ni larvae. The results indicated that the metabolomic differences detected in the leaves of these species were species-specific and remained even after T. ni herbivory. They also show that each Physalis species displayed a unique response to insect herbivory. This study highlighted the metabolite variation present in Physalis spp. and the persistence of this variability when faced with biotic stressors. Furthermore, it sets an experimental precedent from which highly species-specific metabolites could be identified and subsequently used for plant breeding programs designed to increase insect resistance in Physalis and related plant species.

Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species

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

Comparative analyses of responses to exogenous and endogenous antiherbivore elicitors enable a forward genetics approach to identify maize gene candidates mediating sensitivity to herbivore-associated molecular patterns

Crop damage by herbivorous insects remains a significant contributor to annual yield reductions. Following attack, maize (Zea mays) responds to herbivore-associated molecular patterns (HAMPs) and damage-associated molecular patterns (DAMPs), activating dynamic direct and indirect antiherbivore defense responses. To define underlying signaling processes, comparative analyses between plant elicitor peptide (Pep) DAMPs and fatty acid-amino acid conjugate (FAC) HAMPs were conducted. RNA sequencing analysis of early transcriptional changes following Pep and FAC treatments revealed quantitative differences in the strength of response yet a high degree of qualitative similarity, providing evidence for shared signaling pathways. In further comparisons of FAC and Pep responses across diverse maize inbred lines, we identified Mo17 as part of a small subset of lines displaying selective FAC insensitivity. Genetic mapping for FAC sensitivity using the intermated B73 × Mo17 population identified a single locus on chromosome 4 associated with FAC sensitivity. Pursuit of multiple fine-mapping approaches further narrowed the locus to 19 candidate genes. The top candidate gene identified, termed FAC SENSITIVITY ASSOCIATED (ZmFACS), encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) that belongs to the same family as a rice (Oryza sativa) receptor gene previously associated with the activation of induced responses to diverse Lepidoptera. Consistent with reduced sensitivity, ZmFACS expression was significantly lower in Mo17 as compared to B73. Transient heterologous expression of ZmFACS in Nicotiana benthamiana resulted in a significantly increased FAC-elicited response. Together, our results provide useful resources for studying early elicitor-induced antiherbivore responses in maize and approaches to discover gene candidates underlying HAMP sensitivity in grain crops.

Total Biosynthesis of the Tubulin-Binding Alkaloid Colchicine

Colchicine (1) is a bioactive plant alkaloid from Colchicum and Gloriosa species that is used as a pharmaceutical treatment for inflammatory diseases, including gouty arthritis and familial Mediterranean fever. The activity of this alkaloid is attributed to its ability to bind tubulin dimers and inhibit microtubule assembly, which not only promotes anti-inflammatory effects, but also makes colchicine a potent mitotic poison. The biochemical origins of colchicine biosynthesis have been investigated for over 50 years, but only recently has the underlying enzymatic machinery become clear. Here, we report the discovery of multiple pathway enzymes from Gloriosa superba that allows for the reconstitution of a complete metabolic route to 1. This includes three enzymes that process a previously established tropolone-containing intermediate into 1 via tailoring of the nitrogen atom. We further demonstrate the total biosynthesis of enantiopure (-)-1 from primary metabolites via heterologous production in a model plant, thus enabling future efforts for the metabolic engineering of this medicinal alkaloid. Additionally, our results provide insight into the timing and tissue specificity for the late stage modifications required in colchicine biosynthesis, which are likely connected to the biological functions for this class of medicinal alkaloids in native producing plants.

Volatiles Emission by Crotalaria nitens after Insect Attack

Plants are known to increase the emission of volatile organic compounds upon the damage of phytophagous insects. However, very little is known about the composition and temporal dynamics of volatiles released by wild plants of the genus Crotalaria (Fabaceae) attacked with the specialist lepidopteran caterpillar Utetheisa ornatrix (Linnaeus) (Erebidae). In this work, the herbivore-induced plant volatiles (HIPV) emitted by Crotalaria nitens Kunth plants were isolated with solid phase micro-extraction and the conventional purge and trap technique, and their identification was carried out by GC/MS. The poly-dimethylsiloxane/divinylbenzene fiber showed higher affinity for the extraction of apolar compounds (e.g., trans-β-caryophyllene) compared to the Porapak™-Q adsorbent from the purge & trap method that extracted more polar compounds (e.g., trans-nerolidol and indole). The compounds emitted by C. nitens were mainly green leaf volatile substances, terpenoids, aromatics, and aldoximes (isobutyraldoxime and 2-methylbutyraldoxime), whose maximum emission was six hours after the attack. The attack by caterpillars significantly increased the volatile compounds emission in the C. nitens leaves compared to those subjected to mechanical damage. This result indicated that the U. ornatrix caterpillar is responsible for generating a specific response in C. nitens plants. It was demonstrated that HIPVs repelled conspecific moths from attacked plants and favored oviposition in those without damage. The results showed the importance of volatiles in plant-insect interactions, as well as the choice of appropriate extraction and analytical methods for their study.

Functional Studies of Plant Latex as a Rich Source of Bioactive Compounds: Focus on Proteins and Alkaloids

Latex, a sticky emulsion produced by specialized cells called laticifers, is a crucial part of a plant's defense system against herbivory and pathogens. It consists of a broad spectrum of active compounds, which are beneficial not only for plants, but for human health as well, enough to mention the use of morphine or codeine from poppy latex. Here, we reviewed latex's general role in plant physiology and the significance of particular compounds (alkaloids and proteins) to its defense system with the example of Chelidonium majus L. from the poppy family. We further attempt to present latex chemicals used so far in medicine and then focus on functional studies of proteins and other compounds with potential pharmacological activities using modern techniques such as CRISPR/Cas9 gene editing. Despite the centuries-old tradition of using latex-bearing plants in therapies, there are still a lot of promising molecules waiting to be explored.

Impacts of Constitutive and Induced Benzoxazinoids Levels on Wheat Resistance to the Grain Aphid (Sitobion avenae)

Benzoxazinoids are important secondary metabolites in gramineae plants and have inhibitory and toxic effects against a wide range of herbivore pests. However, the relationship between benzoxazinoid level and plant resistance to aphids remains controversial. In this study, we investigated the relationship between benzoxazinoids composition and concentration in wheat leaves and the resistance to the grain aphid Sitobion avenae. Overall, six benzoxazinoids were detected and identified by mass spectrometry based metabolites profiling, including three lactams, two hydroxamic acids, and one methyl derivative. The constitutive levels of these benzoxazinoids were significantly different among the wheat varieties/lines. However, none of these benzoxazinoids exhibited considerable correlation with aphid resistance. S. avenae feeding elevated the level of 2-O-β-D-glucopyranosyloxy-4,7-dimethoxy-(2H)-1,4-benzoxazin-3(4H)-one (HDMBOA-Glc) and reduced the level of 2-O-β-D-glucopyranosyloxy-4-hydroxy-7-(2H)-methoxy-1,4-benzoxazin-3(4H)-one (DIMBOA-Glc) in some of the wheat varieties/lines. Moreover, aphid-induced level of DIMBOA-Glc was positively related with callose deposition, which was closely associated with aphid resistance. Wheat leaves infiltrated with DIMBOA-Glc caused a noticeable increase of callose deposition and the effect was in a dose dependent manner. This study suggests that the constitutive level of benzoxazinoids has limited impact on S. avenae. Aphid feeding can affect the balance of benzoxazinoids metabolism and the dynamic level of benzoxazinoids can act as a signal of callose deposition for S. avenae resistance. This study will extend our understanding of aphid–wheat interaction and provides new insights in aphid-resistance wheat breeding.

Concerted impacts of antiherbivore defenses and opportunistic Serratia pathogens on the fall armyworm (Spodoptera frugiperda)

Insects frequently confront different microbial assemblages. Bacteria inhabiting an insect gut are often commensal, but some can become pathogenic when the insect is compromised from different stressors. Herbivores are often confronted by various forms of plant resistance, but how defenses generate opportunistic microbial infections from residents in the gut are not well understood. In this study, we evaluated the pathogenic tendencies of Serratia isolated from the digestive system of healthy fall armyworm larvae (Spodoptera frugiperda) and how it interfaces with plant defenses. We initially selected Serratia strains that varied in their direct expression of virulence factors. Inoculation of the different isolates into the fall armyworm body cavity indicated differing levels of pathogenicity, with some strains exhibiting no effects while others causing mortality 24 h after injection. Oral inoculations of pathogens on larvae provided artificial diets caused marginal (< 7%) mortality. However, when insects were provided different maize genotypes, mortality from Serratia increased and was higher on plants exhibiting elevated levels of herbivore resistance (< 50% mortality). Maize defenses facilitated an initial invasion of pathogenic Serratia into the larval hemocoel¸ which was capable of overcoming insect antimicrobial defenses. Tomato and soybean further indicated elevated mortality due to Serratia compared to artificial diets and differences between plant genotypes. Our results indicate plants can facilitate the incipient emergence of pathobionts within gut of fall armyworm. The ability of resident gut bacteria to switch from a commensal to pathogenic lifestyle has significant ramifications for the host and is likely a broader phenomenon in multitrophic interactions facilitated by plant defenses.

Dual-species origin of an adaptive chemical defense polymorphism

Allopolyploid speciation and chemical defense diversification are two of the most characteristic features of plant evolution; although the former has likely shaped the latter, this has rarely been documented. Here we document allopolyploidy-mediated chemical defense evolution in the origin of cyanogenesis (HCN release upon tissue damage) in white clover (Trifolium repens). We combined linkage mapping of the loci that control cyanogenesis (Ac, controlling production of cyanogenic glucosides; and Li, controlling production of their hydrolyzing enzyme linamarase) with genome sequence comparisons between white clover, a recently evolved allotetraploid, and its diploid progenitors (Trifolium pallescens, Trifolium occidentale). The Ac locus (a three-gene cluster comprising the cyanogenic glucoside pathway) is derived from T. occidentale; it maps to linkage group 2O (occidentale subgenome) and is orthologous to a highly similar cluster in the T. occidentale reference genome. By contrast, Li maps to linkage group 4P (pallescens subgenome), indicating an origin in the other progenitor species. These results indicate that cyanogenesis evolved in white clover as a product of the interspecific hybridization that created the species. This allopolyploidization-derived chemical defense, together with subsequent selection on intraspecific cyanogenesis variation, appears to have contributed to white clover's ecological success as a globally distributed weed species.

Plant secondary metabolite, daphnetin reduces extracellular polysaccharides production and virulence factors of Ralstonia solanacearum

Plants deploy a variety of secondary metabolites to fend off pathogen attack. Certain plants could accumulate coumarins in response to infection of bacteria, fungi, virus and oomycetes. Although coumarins are generally considered toxic to microbes, the exact mechanisms are often unknown. Here, we showed that a plant secondary metabolite daphnetin functions primarily by inhibiting Ralstonia solanacearum extracellular polysaccharides (EPS) production and biofilm formation in vitro, through suppressing genes expression of xpsR, epsE, epsB and lexM. Indeed, daphnetin significantly impaired virulence of R. solanacearum on tobacco plants. Transcriptional analysis suggested that daphnetin suppresses EPS synthesis cluster genes expression through transcriptional regulator XpsR. And daphnetin alter mainly virulence factors genes involved in type III secretion system, and type IV secretion system. R. solanacearum lacking EPS synthesis genes (epsB and epsC) that do not produce EPS, showed less virulence on tobacco plants. Molecular docking results indicated that the critical residues of domain in the binding pocket of the EpsB protein interact with daphnetin via conventional hydrogen bonding and hydrophobic interactions. Collectively, we found that daphnetin has potential as a novel virulence inhibitor of R. solanacearum, directly regulates EPS synthesis genes expression.

Volatiles Induced from Hypolepis punctata (Dennstaedtiaceae) by Herbivores Attract Sclomina erinacea (Hemiptera: Reduviidae): Clear Evidence of Indirect Defense in Fern

Simple Summary

Plants have developed many kinds of indirect defense mechanisms against herbivorous insects, through which the natural enemies are attracted by herbivore-induced plant volatiles to prey on these phytophagous species. There have been many reports regarding this mechanism in spermatophytes, but little is known in ferns. In this study, the relationship between the fern species Hypolepis punctata and the predatory insect Sclomina erinacea was studied. Employing field observations, plant volatile analysis, electrophysiological experiments, and behavioral experiments, we found that not only the plant can attract the assassin bug in the field, but compared with the healthy individuals, the fern being damaged by herbivorous insects also emitted several novel volatile organic compounds, which were strongly attractive to the assassin bug. The results indicate that ferns may also have indirect defense mechanisms using volatile organic compounds, and it is the first report of an indirect defense mechanism in fern.

Abstract

Plants have evolved various self-defense mechanisms against insect feeding. There are many reports regarding both direct and indirect defense mechanisms in seed-plant. However, only direct defenses on ferns were considered and the indirect defense mechanism has never been reported. In this study, it was observed that the fern Hypolepis punctata can attract the assassin bug Sclomina erinacea in the field. We collected and analyzed volatiles from H. punctata healthy individuals and the ones wounded by Bertula hadenalis, using dynamic headspace and GC-MS. We recorded the electroantennogram responses of antennae of S. erinacea to different standards of volatile compounds identified from the GC-MS analysis. We also analyzed the behavior of male and female S. erinacea adults in response to volatiles collected from H. punctata using a Y-tube olfactometer. The results showed that a number of volatile compounds were produced when the fern was damaged by B. hadenalis. Electroantennography and Y-tube olfactometer results showed that some herbivore-induced volatiles and volatiles from undamaged leaves could attract S. erinacea. Our research suggests that H. punctata can attract insect predators by releasing herbivory-induced volatile organic compounds, and for the first time we found ferns may also have indirect defense mechanisms using volatile organic compounds.

Soil chemistry determines whether defensive plant secondary metabolites promote or suppress herbivore growth

Plant secondary (or specialized) metabolites mediate important interactions in both the rhizosphere and the phyllosphere. If and how such compartmentalized functions interact to determine plant-environment interactions is not well understood. Here, we investigated how the dual role of maize benzoxazinoids as leaf defenses and root siderophores shapes the interaction between maize and a major global insect pest, the fall armyworm. We find that benzoxazinoids suppress fall armyworm growth when plants are grown in soils with very low available iron but enhance growth in soils with higher available iron. Manipulation experiments confirm that benzoxazinoids suppress herbivore growth under iron-deficient conditions and in the presence of chelated iron but enhance herbivore growth in the presence of free iron in the growth medium. This reversal of the protective effect of benzoxazinoids is not associated with major changes in plant primary metabolism. Plant defense activation is modulated by the interplay between soil iron and benzoxazinoids but does not explain fall armyworm performance. Instead, increased iron supply to the fall armyworm by benzoxazinoids in the presence of free iron enhances larval performance. This work identifies soil chemistry as a decisive factor for the impact of plant secondary metabolites on herbivore growth. It also demonstrates how the multifunctionality of plant secondary metabolites drives interactions between abiotic and biotic factors, with potential consequences for plant resistance in variable environments.

Carnivorous Nepenthes x ventrata plants use a naphthoquinone as phytoanticipin against herbivory

Carnivorous plants feed on animal prey, mainly insects, to get additional nutrients. This carnivorous syndrome is widely investigated and reported. In contrast, reports on herbivores feeding on carnivorous plants and related defenses of the plants under attack are rare. Here, we studied the interaction of a pitcher plant, Nepenthes x ventrata, with a generalist lepidopteran herbivore, Spodoptera littoralis, using a combination of LC/MS-based chemical analytics, choice and feeding assays. Chemical defenses in N. x ventrata leaves were analyzed upon S. littoralis feeding. A naphthoquinone, plumbagin, was identified in Nepenthes defense against herbivores and as the compound mainly responsible for the finding that S. littoralis larvae gained almost no weight when feeding on Nepenthes leaves. Plumbagin is constitutively present but further 3-fold increased upon long-term (> 1 day) feeding. Moreover, in parallel de novo induced trypsin protease inhibitor (TI) activity was identified. In contrast to TI activity, enhanced plumbagin levels were not phytohormone inducible, not even by defense-related jasmonates although upon herbivory their level increased more than 50-fold in the case of the bioactive jasmonic acid-isoleucine. We conclude that Nepenthes is efficiently protected against insect herbivores by naphthoquinones acting as phytoanticipins, which is supported by additional inducible defenses. The regulation of these defenses remains to be investigated.

Boosting of Antioxidants and Alkaloids in Catharanthus roseus Suspension Cultures Using Silver Nanoparticles with Expression of CrMPK3 and STR Genes

Global agricultural systems are under unprecedented pressures due to climate change. Advanced nano-engineering can help increase crop yields while ensuring sustainability. Nanotechnology improves agricultural productivity by boosting input efficiency and reducing waste. Alkaloids as one of the numerous secondary metabolites that serve variety of cellular functions essential for physiological processes. This study tests the competence of silver nanoparticles (AgNPs) in boosting alkaloids accumulation in Catharanthus roseus suspension cultures in relation to the expression of C. roseus Mitogen Activated Protein Kinase 3 (CrMPK3) and Strictosidine Synthase (STR) genes. Five concentrations (5, 10, 15, 20 and 25 mg·L-1) of AgNPs were utilized in addition to deionized water as control. Results reflected binary positive correlations among AgNPs concentration, oxidative stress indicated with increase in hydrogen peroxide and malondialdehyde contents, activities of ascorbate peroxidase and superoxide dismutase, expression of the regulatory gene CrMPK3 and the alkaloid biosynthetic gene STR as well as alkaloids accumulation. These correlations add to the growing evidence that AgNPs can trigger the accumulation of alkaloids in plant cells through a signaling pathway that involves hydrogen peroxide and MAPKs, leading to up-regulation of the biosynthetic genes, including STR gene.

A beta-glucosidase of an insect herbivore determines both toxicity and deterrence of a dandelion defense metabolite

Gut enzymes can metabolize plant defense compounds and thereby affect the growth and fitness of insect herbivores. Whether these enzymes also influence feeding preference is largely unknown. We studied the metabolization of taraxinic acid β-D-glucopyranosyl ester (TA-G), a sesquiterpene lactone of the common dandelion (Taraxacum officinale) that deters its major root herbivore, the common cockchafer larva (Melolontha melolontha). We have demonstrated that TA-G is rapidly deglucosylated and conjugated to glutathione in the insect gut. A broad-spectrum M. melolontha β-glucosidase, Mm_bGlc17, is sufficient and necessary for TA-G deglucosylation. Using cross-species RNA interference, we have shown that Mm_bGlc17 reduces TA-G toxicity. Furthermore, Mm_bGlc17 is required for the preference of M. melolontha larvae for TA-G-deficient plants. Thus, herbivore metabolism modulates both the toxicity and deterrence of a plant defense compound. Our work illustrates the multifaceted roles of insect digestive enzymes as mediators of plant-herbivore interactions.

Plants produce certain substances to fend off attackers like plant-feeding insects. To stop these compounds from damaging their own cells, plants often attach sugar molecules to them. When an insect tries to eat the plant, the plant removes the stabilizing sugar, ‘activating’ the compounds and making them toxic or foul-tasting. Curiously, some insects remove the sugar themselves, but it is unclear what consequences this has, especially for insect behavior.

Dandelions, Taraxacum officinale, make high concentrations of a sugar-containing defense compound in their roots called taraxinic acid β-D-glucopyranosyl ester, or TA-G for short. TA-G deters the larvae of the Maybug – a pest also known as the common cockchafer or the doodlebug – from eating dandelion roots. When Maybug larvae do eat TA-G, it is found in their systems without its sugar. However, it is unclear whether it is the plant or the larva that removes the sugar. A second open question is how the sugar removal process affects the behavior of the Maybug larvae.

Using chemical analysis and genetic manipulation, Huber et al. investigated what happens when Maybug larvae eat TA-G. This revealed that the acidity levels in the larvae’s digestive system deactivate the proteins from the dandelion that would normally remove the sugar from TA-G. However, rather than leaving the compound intact, larvae remove the sugar from TA-G themselves. They do this using a digestive enzyme, known as a beta-glucosidase, that cuts through sugar. Removing the sugar from TA-G made the compound less toxic, allowing the larvae to grow bigger, but it also increased TA-G’s deterrent effects, making the larvae less likely to eat the roots.

Any organism that eats plants, including humans, must deal with chemicals like TA-G in their food. Once inside the body, enzymes can change these chemicals, altering their effects. This happens with many medicines, too. In the future, it might be possible to design compounds that activate only in certain species, or under certain conditions. Further studies in different systems may aid the development of new methods of pest control, or new drug treatments.

Performance of two species of whiteflies is unaffected by glucosinolate profile in Brassica plants

BACKGROUND

While plant glucosinolates are known to impart resistance to many insects, their role in the interactions between plants and many phloem-feeding insects such as whiteflies are poorly understood. The whitefly Bemisia tabaci complex comprises many cryptic species that differ in the ability to utilize Brassica plants. However, whether Brassica plants-specific traits such as glucosinolates determine differences of whiteflies in colonizing Brassica plants remains in question.

RESULTS

We first observed performance of two whitefly species MEAM1 and Asia II 3, which differ obviously in their ability to colonize Brassica plants, on four cultivars of three Brassica species that vary in glucosinolate profile. We found that the life history characteristics of each of the two whitefly species seems to be only marginally affected by cultivar. We next used wild-type Arabidopsis plants and mutants defective in glucosinolate biosynthesis or hydrolysis to explore the effects of glucosinolates on the whitefly. We found that fecundity and development of immature stages of neither of the two whitefly species differ significantly between wild-type and mutants.

CONCLUSION

The data suggest that glucosinolates may have little effect on the oviposition by adults and the survival and development of immature stages of MEAM1 and Asia II 3 whiteflies. The marked differences in colonizing Brassica crops between the two whitefly species are likely due to plant traits other than glucosinolates. © 2021 Society of Chemical Industry.

Phytopathogenic infection alters rice-pest-parasitoid tri-trophic interactions

BACKGROUND

Plant pathogens and pests often occur together, causing damage while interfering with plant growth. The effects of phytopathogenic infections on plant-herbivore-natural enemy tri-trophic interactions (TTIs) have been extensively investigated, but little is known about how the interval of infection influences such relationships. Here, the effect of rice plants infected by the phytopathogen Rhizoctonia solani on the herbivorous rice brown planthopper (BPH) and associated egg parasitoid Anagrus nilaparvatae over a temporal scale was examined.

RESULTS

Our results showed that rice plants infected by R. solani showed increased volatile profiles and significantly attracted BPH and A. nilaparvatae at 5-15 days post infection (DPI) and 5-10 DPI, respectively, when compared with healthy plants. Jasmonic acid and salicylic acid content decreased significantly in BPH-damaged plants after 15 DPI, whereas oxalic acid accumulated soon after 5 DPI when compared with healthy plants. To adapt to adverse environment, BPH laid more eggs and developed into macropterous adults. Under field conditions, R. solani infection had no substantial effect on the arthropod community when compared with healthy plants.

CONCLUSION

Taken together, R. solani infection altered rice-pest-parasitoid TTIs over a temporal scale. This result will shed more light on our understanding of plant pathogen-insect cross-talk essential for developing novel pest management strategies. © 2021 Society of Chemical Industry.

Neurotoxic Zanthoxylum chalybeum root constituents invoke mosquito larval growth retardation through ecdysteroidogenic CYP450s transcriptional perturbations

Intracellular effects exerted by phytochemicals eliciting insect growth-retarding responses during vector control intervention remain largely underexplored. We studied the effects of Zanthoxylum chalybeum Engl. (Rutaceae) (ZCE) root derivatives against malaria (Anopheles gambiae) and arbovirus vector (Aedes aegypti) larvae to decipher possible molecular targets. We report dose-dependent biphasic effects on larval response, with transient exposure to ZCE and its bioactive fraction (ZCFr.5) inhibiting acetylcholinesterase (AChE) activity, inducing larval lethality and growth retardation at sublethal doses. Half-maximal lethal concentrations (LC₅₀) for ZCE against An. gambiae and Ae. aegypti larvae after 24-h exposure were 9.00 ppm and 12.26 ppm, respectively. The active fraction ZCFr.5 exerted LC₅₀ of 1.58 ppm and 3.21 ppm for An. gambiae and Ae. aegypti larvae, respectively. Inhibition of AChE was potentially linked to larval toxicity afforded by 2-tridecanone, palmitic acid (hexadecanoic acid), linoleic acid ((Z,Z)-9,12-octadecadienoic acid), sesamin, β-caryophyllene among other compounds identified in the bioactive fraction. In addition, the phenotypic larval retardation induced by ZCE root constituents was exerted through transcriptional modulation of ecdysteroidogenic CYP450 genes. Collectively, these findings provide an explorative avenue for developing potential mosquito control agents from Z. chalybeum root constituents.

Comparative Leaf Volatile Profiles of Two Contrasting Mandarin Cultivars against Candidatus Liberibacter asiaticus Infection Illustrate Huanglongbing Tolerance Mechanisms

Huanglongbing (HLB), presumably caused by Candidatus Liberibacter asiaticus (CaLas), is a devastating citrus disease worldwide. While all citrus are affected by HLB, some cultivars display greater tolerance; however, the underlying mechanisms are not fully understood. Here, volatile changes in HLB-tolerant LB8-9 Sugar Belle (SB) and HLB-sensitive Murcott mandarins after CaLas infection were comprehensively compared to determine if specific volatiles are associated with HLB responses and to discern the underlying tolerance mechanisms. These cultivars emitted qualitatively and quantitatively different volatiles in response to HLB induced by artificial graft or natural psyllid inoculation. Increasing amounts of total volatiles and de novo-synthesized new volatiles were two key responses to HLB of both cultivars. Markers potentially associated with HLB and host susceptibility were identified. Terpenoid biosynthetic pathway, green leaf volatile, and thymol metabolic pathways responsive to CaLas infection were dramatically altered. SB mandarin allows simultaneous defense and growth, contributing to its greater HLB tolerance.

Genetic trade-offs underlie divergent life history strategies for local adaptation in white clover

Local adaptation is common in plants, yet characterization of its underlying genetic basis is rare in herbaceous perennials. Moreover, while many plant species exhibit intraspecific chemical defence polymorphisms, their importance for local adaptation remains poorly understood. We examined the genetic architecture of local adaptation in a perennial, obligately-outcrossing herbaceous legume, white clover (Trifolium repens). This widespread species displays a well-studied chemical defence polymorphism for cyanogenesis (HCN release following tissue damage) and has evolved climate-associated cyanogenesis clines throughout its range. Two biparental F₂  mapping populations, derived from three parents collected in environments spanning the U.S. latitudinal species range (Duluth, MN, St. Louis, MO and Gainesville, FL), were grown in triplicate for two years in reciprocal common garden experiments in the parental environments (6,012 total plants). Vegetative growth and reproductive fitness traits displayed trade-offs across reciprocal environments, indicating local adaptation. Genetic mapping of fitness traits revealed a genetic architecture characterized by allelic trade-offs between environments, with 100% and 80% of fitness QTL in the two mapping populations showing significant QTL×E interactions, consistent with antagonistic pleiotropy. Across the genome there were three hotspots of QTL colocalization. Unexpectedly, we found little evidence that the cyanogenesis polymorphism contributes to local adaptation. Instead, divergent life history strategies in reciprocal environments were major fitness determinants: selection favoured early investment in flowering at the cost of multiyear survival in the southernmost site versus delayed flowering and multiyear persistence in the northern environments. Our findings demonstrate that multilocus genetic trade-offs contribute to contrasting life history characteristics that allow for local adaptation in this outcrossing herbaceous perennial.

Elicitation of biomolecules as host defense arsenals during insect attacks on tea plants (Camellia sinensis (L.) Kuntze)

The most consumed and economically important beverage plant, tea (Camellia sinensis), and its pests have coevolved so as to maintain the plant-insect interaction. In this review, findings of different research groups on pest responsive tolerance mechanisms that exist in tea manifested through the production of secondary metabolites and their inducers are presented. The phytochemicals of C. sinensis have been categorized into volatiles, nonvolatiles, enzymes, and phytohormones for convenience. Two types of pests, namely the piercing-sucking pests and chewing pests, are associated with tea. Both the insect groups can trigger the production of those metabolites and inducers through several primary and secondary biosynthetic pathways. These induced biomolecules can act as insect repellents and most of them are associated with lowering the nutrient quality of plant tissue and increasing the indigestibility in the pest's gut. Moreover, some of them also act as predator attractants of particular pests. The herbivore-induced plant volatiles secreted from tea plants during pest infestation were (E)-nerolidol, α-farnesene, (Z)-3-hexenol, (E)-4,8-dimethyl-1,3,7-nonatriene, indole, benzyl nitrile (BN), linalool, and ocimenes. The nonvolatiles like theaflavin and L-theanine were increased in response to the herbivore attack. Simultaneously, S-adenosyl-L-methionine synthase, caffeine synthase activities were affected, whereas flavonoid synthesis and wax formation were elevated. Defense responsive enzymes like peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase, ascorbate peroxidase, and catalase are involved in pest prevention mechanisms. Phytohormones like jasmonic acid, salicylic acid, abscisic acid, and ethylene act as the modulator of the defense system. The objective of this review is to discuss the defensive roles of these metabolites and their inducers against pest infestation in tea with an aim to develop environmentally sustainable pesticides in the future.Key points• Herbivore-induced volatile signals and their effects on neighboring tea plant protection• Stereochemical conversion of volatiles, effects of nonvolatiles, expression of defense-responsive enzymes, and phytohormones due to pest attack• Improved understanding of metabolites for bio-sustainable pesticide development.

Fruit secondary metabolites shape seed dispersal effectiveness

Plant secondary metabolites (PSMs) play a central role in seed dispersal and fruit defense, with potential for large impacts on plant fitness and demography. Yet because PSMs can have multiple interactive functions across seed dispersal stages, we must systematically study their effects to determine the net consequences for plant fitness. To tackle this issue, we integrate the role of fruit PSMs into the seed dispersal effectiveness (SDE) framework. We describe PSM effects on the quantity and quality of animal-mediated seed dispersal, both in pairwise interactions and diverse disperser communities, as well as trade-offs that occur across dispersal stages. By doing so, this review provides structure to a rapidly growing field and yields insights into a critical process shaping plant populations.

Pyrrole alkaloids from Solanum rostratum and their chemical defense function against Henosepilachna vigintioctomaculata

Three pairs of novel enantiomeric pyrrole alkaloids (1a/1b, 2a/2b, 3a/3b) were isolated from the leaves of Solanum rostratum and their structures were determined via NMR analyses and ECD calculation. All the enantiomers displayed different levels of antifeedant and growth-inhibitory activities against Henosepilachna vigintioctomaculata (a noxious herbivore for Solanaceae), especially 1a and 2a. Interestingly, the results showed enantioselectivity, in which that the pyrrole alkaloids with R configuration at C-2' showed stronger chemical defense function than their enantiomers.

Sportive lemurs elevate their metabolic rate during challenging seasons and do not enter regular heterothermy

Animals experience seasonal changes of environmental and ecological conditions in most habitats. Fluctuations in ambient temperature have a strong influence on thermoregulation, particularly on small endothermic mammals. However, different mammalian species cope differently with these changes. Understanding the physiological responses of organisms to different seasons and analysing the mechanisms that account for intra- and inter-specific differences and the ecological consequences of these variations is important to predict species responses to climatic changes. Consequences of climatic changes will be most pronounced in climatically already challenging habitats, such as the dry regions of western Madagascar. We aimed to identify the seasonal responses and adaptive possibilities in energy budgeting of Lepilemur edwardsi, a small primate of this habitat, by measuring metabolic rate (MR; open-flow respiratory) and skin temperature in the field during different seasons. Resting metabolism was generally low, but our study did not detect any signs of regular heterothermic episodes, despite the fact that these are known in other sympatrically living lemurs with a similar lifestyle. Surprisingly, L. edwardsi responded by elevating its resting MR in the poor-resourced dry season, compared to the better-resourced wet season, presumably to master detoxification of their increasingly toxic diet. As body mass decreased over this time, this strategy is obviously not energetically balanced on the long term. This is cause for concern, as it suggests that L. edwardsi has a very small leeway to adjust to changing conditions as experienced due to climate change, as dry season are expected to become longer and hotter, straining water budgets and food quality even more. Moreover, our findings highlight the importance of studying physiological parameters directly in the field and under differing climatic conditions.

Contribution of sample processing to gut microbiome analysis in the model Lepidoptera, silkworm Bombyx mori

Microbes that live inside insects play various roles in host biology, ranging from nutrient supplementation to host defense. Although Lepidoptera (butterflies and moths) are one of the most diverse insect taxa and important in natural ecosystems, their microbiotas are little-studied, and to understand their structure and function, it is necessary to identify potential factors that affect microbiome analysis. Using a model organism, the silkworm Bombyx mori, we investigated the effects of different sample types (whole gut, gut content, gut tissue, starvation, or frass) and metagenomic DNA extraction methodologies (small-scale versus large-scale) on the composition and diversity of the caterpillar gut microbial communities. High-throughput 16S rRNA gene sequencing and computational analysis of the resulting data unraveled that DNA extraction has a large effect on the outcome of metagenomic analysis: significant biases were observed in estimates of community diversity and in the ratio between Gram-positive and Gram-negative bacteria. Furthermore, bacterial communities differed significantly among sample types. The gut content and whole gut samples differed least, both had a higher percentage of Enterococcus and Acinetobacter species; whereas the frass and starvation samples differed substantially from the whole gut and were poor representatives of the gut microbiome. Thus, we recommend a small-scale DNA extraction methodology for sampling the whole gut under normal insect rearing conditions whenever possible, as this approach provides the most accurate assessment of the gut microbiome. Our study highlights that evaluation of the optimal sample-processing approach should be the first step taken to confidently assess the contributions of microbiota to Lepidoptera.