Jasmonate-dependent depletion of soluble sugars compromises plant resistance to Manduca sexta

Heat stress reprograms herbivory-induced defense responses in potato plants

Climate change is predicted to increase the occurrence of extreme weather events such as heatwaves, which may thereby impact the outcome of plant-herbivore interactions. While elevated temperature is known to directly affect herbivore growth, it remains largely unclear if it indirectly influences herbivore performance by affecting the host plant they feed on. In this study, we investigated how transient exposure to high temperature influences plant herbivory-induced defenses at the transcript and metabolic level. To this end, we studied the interaction between potato (Solanum tuberosum) plants and the larvae of the potato tuber moth (Phthorimaea operculella) under different temperature regimes. We found that P. operculella larvae grew heavier on leaves co-stressed by high temperature and insect herbivory than on leaves pre-stressed by herbivory alone. We also observed that high temperature treatments altered phylotranscriptomic patterns upon herbivory, which changed from an evolutionary hourglass pattern, in which transcriptomic responses at early and late time points after elicitation are more variable than the ones in the middle, to a vase pattern. Specifically, transcripts of many herbivory-induced genes in the early and late defense stage were suppressed by HT treatment, whereas those in the intermediate stage peaked earlier. Additionally, we observed that high temperature impaired the induction of jasmonates and defense compounds upon herbivory. Moreover, using jasmonate-reduced (JA-reduced, irAOC) and -elevated (JA-Ile-elevated, irCYP94B3s) potato plants, we showed that high temperature suppresses JA signaling mediated plant-induced defense to herbivore attack. Thus, our study provides evidences on how temperature reprograms plant-induced defense to herbivores.

Volatile-mediated oviposition preference for healthy over root-infested plants by the European corn borer

The selection of oviposition sites by female moths is crucial in shaping their progeny performance and survival, and consequently in determining insect fitness. Selecting suitable plants that promote the performance of the progeny is referred to as the Preference-Performance hypothesis (or 'mother-knows-best'). While root infestation generally reduces the performance of leaf herbivores, little is known about its impact on female oviposition. We investigated whether maize root infestation by the Western corn rootworm (WCR) affects the oviposition preference and larval performance of the European corn borer (ECB). ECB females used leaf volatiles to select healthy plants over WCR-infested plants. Undecane, a compound absent from the volatile bouquet of healthy plants, was the sole compound to be upregulated upon root infestation and acted as a repellent for first oviposition. ECB larvae yet performed better on plants infested below-ground than on healthy plants, suggesting an example of 'bad motherhood'. The increased ECB performance on WCR-infested plants was mirrored by an increased leaf consumption, and no changes in the plant primary or secondary metabolism were detected. Understanding plant-mediated interactions between above- and below-ground herbivores may help to predict oviposition decisions, and ultimately, to manage pest outbreaks in the field.

Girdling behavior of the longhorn beetle modulates the host plant to enhance larval performance

BACKGROUND

Preingestive behavioral modulations of herbivorous insects on the host plant are abundant over insect taxa. Those behaviors are suspected to have functions such as deactivation of host plant defenses, nutrient accumulation, or modulating plant-mediated herbivore interactions. To understand the functional consequence of behavioral modulation of insect herbivore, we studied the girdling behavior of Phytoecia rufiventris Gautier (Lamiinae; Cerambycidae) on its host plant Erigeron annuus L. (Asteraceae) that is performed before endophytic oviposition in the stem.

RESULTS

The girdling behavior significantly increased the larval performance in both field monitoring and lab experiment. The upper part of the girdled stem exhibited lack of jasmonic acid induction upon larval attack, lowered protease inhibitor activity, and accumulated sugars and amino acids in compared to non-girdled stem. The girdling behavior had no effect on the larval performance of a non-girdling longhorn beetle Agapanthia amurensis, which also feeds on the stem of E. annuus during larval phase. However, the girdling behavior decreased the preference of A. amurensis females for oviposition, which enabled P. rufiventris larvae to avoid competition with A. amurensis larvae.

CONCLUSIONS

In conclusion, the girdling behavior modulates plant physiology and morphology to provide a modulated food source for larva and hide it from the competitor. Our study implies that the insect behavior modulations can have multiple functions, providing insights into adaptation of insect behavior in context of plant-herbivore interaction.

Changes in primary metabolite content may affect thrips feeding preference in soybean crops

Past research has characterized the induction of plant defenses in response to chewing insect damage. However, little is known about plant responses to piercing-sucking insects that feed on plant cell-contents like thrips (Caliothrips phaseoli). In this study, we used NMR spectroscopy to measure metabolite changes in response to six days of thrips damage from two field-grown soybean cultivars (cv.), known for their different susceptibility to Caliothrips phaseoli. We observed that thrips damage reduces sucrose concentration in both cultivars, while pinitol, the most abundant leaf soluble carbohydrate, is induced in cv. Charata but not in cv. Williams. Thrips did not show preference for leaves where sucrose or pinitol were externally added, at tested concentration. In addition, we also noted that cv. Charata was less naturally colonized and contained higher levels of trigonelline, tyrosine as well as several compounds that we have not yet identified. We have established that preference-feeding clues are not dependent on the plants major soluble carbohydrates but may depend on other types of compounds or leaf physical characteristics.

OXIDOSQUALENE CYCLASE 1 and 2 influence triterpene biosynthesis and defense in Nicotiana attenuata

Triterpenes are a class of bioactive compounds with diverse biological functions, playing pivotal roles in plant defense against biotic stressors. Oxidosqualene cyclases (OSCs) serve as gatekeepers in the biosynthesis of triterpenes. In this study, we utilized a Nicotiana benthamiana heterologous expression system to characterize NaOSC1 from Nicotiana attenuata as a multifunctional enzyme capable of synthesizing lupeol, dammarenediol II, 3-alpha,20-lupanediol, and 7 other triterpene scaffolds. We also demonstrated that NaOSC2 is, in contrast, a selective enzyme, producing only the β-amyrin scaffold. Through virus-induced gene silencing and in vitro toxicity assays, we elucidated the roles of NaOSC1 and NaOSC2 in the defense of N. attenuata against Manduca sexta larvae. Metabolomic and feature-based molecular network analyses of leaves with silenced NaOSC1 and NaOSC2 unveiled 3 potential triterpene glycoside metabolite clusters. Interestingly, features identified as triterpenes within these clusters displayed a significant negative correlation with larval mass. Our study highlights the pivotal roles of NaOSC1 and NaOSC2 from N. attenuata in the initial steps of triterpene biosynthesis, subsequently influencing defense against M. sexta through the modulation of downstream triterpene glycoside compounds.

Molecular mechanism of salinity and waterlogging tolerance in mangrove Kandelia obovata

Mangrove forests are colloquially referred to as "Earth's kidneys" and serve many important ecological and commercial functions. Salinity and waterlogging stress are the most important abiotic stressors restricting the growth and development of mangroves. Kandelia obovata (K. obovata) is the greatest latitudinally-distributed salt mangrove species in China.Here, morphology and transcriptomics were used to study the response of K. obovata to salt and waterlogging stress. In addition, weighted gene co-expression network analysis of the combined gene expression and phenotypic datasets was used to identify core salinity- and waterlogging-responsive modules. In this study, we observed that both high salinity and waterlogging significantly inhibited growth and development in K. obovata. Notably, growth was negatively correlated with salt concentration and positively correlated with waterlogging duration, and high salinity was significantly more inhibitive than waterlogging. A total of 7, 591 salt-responsive and 228 waterlogging-responsive differentially expressed genes were identified by RNA sequencing. Long-term salt stress was highly correlated with the measured physiological parameters while long-term waterlogging was poorly correlated with these traits. At the same time, 45 salinity-responsive and 16 waterlogging-responsive core genes were identified. All 61 core genes were mainly involved in metabolic and biosynthesis of secondary metabolites pathways. This study provides valuable insight into the molecular mechanisms of salinity and waterlogging tolerance in K. obovata, as well as a useful genetic resource for the improvement of mangrove stress tolerance using molecular breeding techniques.

Field-work reveals a novel function for MAX2 in a native tobacco's high-light adaptions

Laboratory studies have revealed that strigolatone (SL) and karrikin (KAR) signalling mediate responses to abiotic and biotic stresses, and reshape branching architecture that could increase reproductive performance and crop yields. To understand the ecological function of SL and KAR signalling, transgenic lines of wild tobacco Nicotiana attenuata, silenced in SL/KAR biosynthesis/signalling were grown in the glasshouse and in two field plots in the Great Basin Desert in Utah over four field seasons. Of the lines silenced in SL and KAR signalling components (irMAX2, irD14, irKAI2 and irD14 × irKAI2 plants), which exhibited the expected increases in shoot branching, only irMAX2 plants showed a strong leaf-bleaching phenotype when grown in the field. In the field, irMAX2 plants had lower sugar and higher leaf amino acid contents, lower lifetime fitness and were more susceptible to herbivore attack compared to wild-type plants. These irMAX2 phenotypes were not observed in glasshouse-grown plants. Transcriptomic analysis revealed dramatic responses to high-light intensity in irMAX2 leaves in the field: lutein contents decreased, and transcriptional responses to high-intensity light, singlet oxygen and hydrogen peroxide increased. PAR and UV-B manipulations in the field revealed that the irMAX2 bleaching phenotype is reversed by decreasing PAR, but not UV-B fluence. We propose that NaMAX2 functions in high-light adaptation and fitness optimisation by regulating high-light responses independently of its roles in the SL and KAR signalling pathways. The work provides another example of the value of studying the function of genes in the complex environments in which plants evolved, namely nature.

Genotypic variation in field-grown maize eliminates trade-offs between resistance, tolerance and growth in response to high pressure from the Asian corn borer

Insect herbivory challenges plant survival, and coordination of the interactions between growth, herbivore resistance/tolerance is a key problem faced by plants. Based on field experiments into resistance to the Asian corn borer (ACB, Ostrinia furnacalis), we selected 10 inbred maize lines, of which five were resistant and five were susceptible to ACB. We conducted ACB larval bioassays, analysed defensive chemicals, phytohormones, and relative gene expression using RNA-seq and qPCR as well as agronomic traits, and found resistant lines had weaker inducibility, but were more resistant after ACB attack than susceptible lines. Resistance was related to high levels of major benzoxazinoids, but was not related to induced levels of JA or JA-Ile. Following combination analyses of transcriptome, metabolome and larval performance data, we discovered three benzoxazinoids biosynthesis-related transcription factors, NAC60, WRKY1 and WRKY46. Protoplast transformation analysis suggested that these may regulate maize defence-growth trade-offs by increasing levels of benzoxazinoids, JA and SA but decreasing IAA. Moreover, the resistance/tolerance-growth trade-offs were not observed in the 10 lines, and genotype-specific metabolic and genetic features probably eliminated the trade-offs. This study highlights the possibility of breeding maize varieties simultaneously with improved defences and higher yield under complex field conditions.

Domestication provides durum wheat with protection from locust herbivory

Lower plant resistance to herbivores following domestication has been suggested as the main cause for higher feeding damage in crops than in wild progenitors. While herbivore compensatory feeding has also been proposed as a possible mechanism for raised damage in crops with low nutritional quality, predictions regarding the effects of plant domestication on nutritional quality for herbivores remain unclear. In particular, data on primary metabolites, even major macronutrients, measured in the organs consumed by herbivores, are scarce. In this study, we used a collection of 10 accessions of wild ancestors and 10 accessions of modern progenies of Triticum turgidum to examine whether feeding damage and selectivity by nymphs of Locusta migratoria primarily depended on five leaf traits related to structural resistance or nutrient profiles. Our results unexpectedly showed that locusts favored wild ancestors over domesticated accessions and that leaf toughness and nitrogen and soluble protein contents increased with the domestication process. Furthermore, the quantitative relationship between soluble protein and digestible carbohydrates was found to poorly meet the specific requirements of the herbivore, in all wheat accessions, both wild and modern. The increase in leaf structural resistance to herbivores in domesticated tetraploid wheat accessions suggested that resource allocation trade-offs between growth and herbivory resistance may have been disrupted by domestication in the vegetative organs of this species. Since domestication did not result in a loss of nutritional quality in the leaves of the tetraploid wheat, our results rather provides evidence for a role of the content of plants in nonnutritive nitrogenous secondary compounds, possibly deterrent or toxic, at least for grasshopper herbivores.

Early land plants: Plentiful but neglected nutritional resources for herbivores?

Plants and herbivores have been engaged in a co-evolutionary arms race for millions of years, during which plants evolved various defenses and other traits to cope with herbivores, whereas herbivores evolved traits to overcome the plants' resistance strategies. Herbivores may also avoid certain plants merely because these lack suitable nutrients for their development. Interestingly, the number of herbivores that attack individual early land plants like mosses and ferns is quite low. Among others, poor nutrient quality has been hypothesized to explain the apparent low herbivory pressure on such plants but still waits for scientific evidences. Here, the nutritive suitability of representative mosses and liverworts (bryophytes) and ferns (pteridophytes) for herbivores was investigated using feeding assays combined with quantifications of nutrients (proteins, amino acids, and sugars). Growth and survival of two polyphagous herbivores, a caterpillar and a snail, were monitored when fed on 15 species of bryophytes and pteridophytes, as well as on maize (Zea mays, angiosperm) used as an external indicative nutritional resource. Overall, our results show that the poor performance of the herbivores on the studied early land plants is not correlated with nutritional quality. The growth and performance of snails and caterpillars fed with these plants were highly variable and independent of nutrient content. These findings arguably dismiss the poor nutrient quality hypothesis as the cause of herbivory deficit in bryophytes and pteridophytes. They suggest the possible presence of early resistance traits that have persisted all through the long evolutionary history of plant-herbivore interactions.

Dichotomous Role of Jasmonic Acid in Modulating Sorghum Defense Against Aphids

The precursors and derivatives of jasmonic acid (JA) contribute to plant protective immunity to insect attack. However, the role of JA in sorghum (Sorghum bicolor) defense against sugarcane aphid (SCA) (Melanaphis sacchari), which is considered a major threat to sorghum production, remains elusive. Sorghum SC265, previously identified as a SCA-resistant genotype among the sorghum nested association mapping founder lines, transiently increased JA at early stages of aphid feeding and deterred aphid settling. Monitoring of aphid feeding behavior using electropenetrography, a technique to unveil feeding process of piercing-sucking insects, revealed that SC265 plants restricted SCA feeding from the phloem sap. However, exogenous application of JA attenuated the resistant phenotype and promoted improved aphid feeding and colonization on SC265 plants. This was further confirmed with sorghum JA-deficient plants, in which JA deficiency promoted aphid settling, however, it also reduced aphid feeding from the phloem sap and curtailed SCA population. Exogenous application of JA caused enhanced feeding and aphid proliferation on JA-deficient plants, suggesting that JA promotes aphid growth and development. SCA feeding on JA-deficient plants altered the sugar metabolism and enhanced the levels of fructose and trehalose compared with wild-type plants. Furthermore, aphid artificial diet containing fructose and trehalose curtailed aphid growth and reproduction. Our findings underscore a previously unknown dichotomous role of JA, which may have opposing effects by deterring aphid settling during the early stage and enhancing aphid proliferative capacity during later stages of aphid colonization on sorghum plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Feeding preference of Rhynchophorus ferrugineus (Oliver) (Coleoptera: Curculionidae) on different date palm cultivars and host biochemical responses to its infestation

To counter the insect infestation, plants respond with wide-ranging and highly dynamic biochemical reactions. Of these, the anti-oxidative activity is poorly understood. The red palm weevil (RPW) Rhynchophorus ferrugineus (Oliver), one of the most widespread pests in Pakistan, prefers to infest date palm Phoenix dactylifera. Our present study investigated the feeding preference of RPW to 11 different date palm cultivars and the results suggested that the Hillawi cultivar was most preferred. Greater infestation rate, fecundity and hatching rate were also recorded from Hillawi and Mozawati than other cultivars. No significant decreases were observed in chlorophyll a, chlorophyll b, total chlorophylls and carotenoids of RPW-infested Hillawi cultivar over un-infested control. In contrast, the contents of enzymatic antioxidants including phenols, proline, hydrogen peroxide, anthocyanin, malondialdehyde, ascorbic acid and glycine betaine showed a drastic increase after RPW infestation, and there was enhanced superoxide dismutase, peroxidase and catalase activities. Furthermore, we recorded the increase of total protein and sugar contents in RPW-infested date palms. These findings offer valuable insight into the antioxidative molecular mechanism of date palms under RPW attack and may contribute to the breeding of insect-resistant crops.

Comparative Tolerance Levels of Maize Landraces and a Hybrid to Natural Infestation of Fall Armyworm

Simple Summary

Exploiting the tolerance of plants against herbivorous insects is a viable pest management alternative, especially where conventional controls are ineffective. For example, due to the inefficacy of currently adopted practices, new strategies and methods are needed for Spodoptera frugiperda management in maize. This study evaluated the tolerance levels of maize landraces and a conventional hybrid under natural infestation of S. frugiperda. We found promising sources of tolerance among the landraces, evident as tolerance indices that varied across the landraces and hybrid we evaluated.

Abstract

Insect pests such as Spodoptera frugiperda cause significant losses to maize (Zea mays mays). Control of S. frugiperda is difficult, but the use of insect resistant cultivars, including tolerant cultivars, is a promising alternative, and landraces are a potential source of insect resistance. This study investigated tolerance to S. frugiperda in five Brazilian landraces, Amarelão, Aztequinha, Branco Antigo, Palha Roxa, and São Pedro, in relation to one conventional (non-Bt) hybrid, BM207, under field conditions. We assessed tolerance as the ratio of insecticide-free to insecticide-protected plants for plant height, stem diameter, and leaf chlorophyll content at two plant stages. Tolerance ratios varied across the maize genotypes, but inconsistently across plant variables, and cluster analysis revealed three groups based on tolerance ratios. A first group contained genotypes similarly tolerant to S. frugiperda, BM207, Palha Roxa, São Pedro, and Aztequinha, while the second and third groups each contained single genotypes, Amarelão, and Branco Antigo, which were considered not tolerant. Overall, the landraces Palha Roxa, São Pedro, and Aztequinha compared favorably to BM207 in terms of tolerance, and therefore may be valuable for management of this pest, and as germplasm sources to improve tolerance in other cultivars.

Interplay of phytohormones facilitate sorghum tolerance to aphids

Interactions among phytohormones are essential for providing tolerance of sorghum plants to aphids. Plant's encounter with insect herbivores trigger defense signaling networks that fine-tune plant resistance to insect pests. Although it is well established that phytohormones contribute to antixenotic- and antibiotic-mediated resistance to insect pests, their role in conditioning plant tolerance, the most durable and promising category of host plant resistance, is largely unknown. Here, we screened a panel of sorghum (Sorghum bicolor) inbred lines to identify and characterize sorghum tolerance to sugarcane aphids (SCA; Melanaphis sacchari Zehntner), a relatively new and devastating pest of sorghum in the United States. Our results suggest that the sorghum genotype SC35, the aphid-tolerant line identified among the sorghum genotypes, displayed minimal plant biomass loss and a robust photosynthetic machinery, despite supporting higher aphid population. Phytohormone analysis revealed significantly higher basal levels of 12-oxo-phytodienoic acid, a precursor in the jasmonic acid biosynthesis pathway, in the sorghum SCA-tolerant SC35 plants. Salicylic acid accumulation appeared as a generalized plant response to aphids in sorghum plants, however, SCA feeding-induced salicylic acid levels were unaltered in the sorghum tolerant genotype. Conversely, basal levels of abscisic acid and aphid feeding-induced cytokinins were accumulated in the SCA-tolerant sorghum genotype. Our findings imply that the aphid-tolerant sorghum genotype tightly controls the relationship among phytohormones, as well as provide significant insights into the underlying mechanisms that contribute to plant tolerance to sap-sucking aphids.

The downside of metabolic diversity: Postingestive rearrangements by specialized insects

Deploying toxins in complex mixtures is thought to be advantageous and is observed during antagonistic interactions in nature. Toxin mixtures are widely utilized in medicine and pest control, as they are thought to slow the evolution of detoxification counterresponses in the targeted organisms. Here we show that caterpillars rearrange key constituents of two distinct plant defense pathways to postingestively disable the defensive properties of both pathways. Specifically, phenolic esters of quinic acid, chlorogenic acids (CAs), potent herbivore and ultraviolet (UV) defenses, are reesterified to decorate particular sugars of 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) and prevent their respective anti–herbivore defense functions. This was discovered through the employment of comparative metabolomics of the leaves of Nicotiana attenuata and the frass of this native tobacco’s specialist herbivore, Manduca sexta larvae. Feeding caterpillars on leaves of transgenic plants abrogated in each of the two pathways, separately and together, revealed that one of the fully characterized frass conjugates, caffeoylated HGL-DTG, originated from ingested CA and HGL-DTGs and that both had negative effects on the defensive function of the other compound class, as revealed by rates of larval mass gain. This negative defensive synergy was further explored in 183 N. attenuata natural accessions, which revealed a strong negative covariance between the two defense pathways. Further mapping analyses in a biparental recombinant inbred line (RIL) population imputed quantitative trait loci (QTLs) for the two pathways at distinct genomic locations. The postingestive repurposing of defense metabolism constituents reveals a downside of deploying toxins in mixtures, a downside which plants in nature have evolved to counter.

Jasmonate: A hormone of primary importance for plant metabolism

Over the years, jasmonates (JAs) have become recognized as one of the main plant hormones that regulate stress responses by activating defense programs and the production of specialized metabolites. High JA levels have been associated with reduced plant growth, supposedly as a result of the reallocation of carbon sources from primary growth to the biosynthesis of defense compounds. Recent advances suggest however that tight regulatory networks integrate several sensing pathways to steer plant metabolism, and thereby drive the trade-off between growth and defense. In this review, we discuss how JA influences primary metabolism and how it is connected to light-regulated processes, nutrient sensing and energy metabolism. Finally, we speculate that JA, in a conceptual parallelism with adrenaline for humans, overall boosts cellular processes to keep up with an increased metabolic demand during harsh times.

Changes in Polar Metabolites Content during Natural and Methyl-Jasmonate-Promoted Senescence of Ginkgo biloba Leaves

The present study clarified changes in the contents of polar metabolites (amino acids, organic acids, saccharides, cyclitols, and phosphoric acid) in leaf senescence in Ginkgo biloba with or without the application of methyl jasmonate (JA-Me) in comparison with those in naturally senescent leaf blades and petioles. The contents of most amino acids and citric and malic acids were significantly higher in abaxially, and that of myo-inositol was lower in abaxially JA-Me-treated leaves than in adaxially JA-Me-treated and naturally senescent leaves. The levels of succinic and fumaric acids in leaves treated adaxially substantially high, but not in naturally senescent leaves. In contrast, sucrose, glucose, and fructose contents were much lower in leaf blades and petioles treated abaxially with JA-Me than those treated adaxially. The levels of these saccharides were also lower compared with those in naturally senescent leaves. Shikimic acid and quinic acid were present at high levels in leaf blades and petioles of G. biloba. In leaves naturally senescent, their levels were higher compared to green leaves. The shikimic acid content was also higher in the organs of naturally yellow leaves than in those treated with JA-Me. These results strongly suggest that JA-Me applied abaxially significantly enhanced processes of primary metabolism during senescence of G. biloba compared with those applied adaxially. The changes in polar metabolites in relation to natural senescence were also discussed.

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.

Transcriptomic Profiling Reveals Shared Signalling Networks Between Flower Development and Herbivory-Induced Responses in Tomato

Most flowering plants must defend themselves against herbivores for survival and attract pollinators for reproduction. Although traits involved in plant defence and pollinator attraction are often localised in leaves and flowers, respectively, they will show a diffuse evolution if they share the same molecular machinery and regulatory networks. We performed RNA-sequencing to characterise and compare transcriptomic changes involved in herbivory-induced defences and flower development, in tomato leaves and flowers, respectively. We found that both the herbivory-induced responses and flower development involved alterations in jasmonic acid signalling, suppression of primary metabolism and reprogramming of secondary metabolism. We identified 411 genes that were involved in both processes, a number significantly higher than expected by chance. Genetic manipulation of key regulators of induced defences also led to the expression changes in the same genes in both leaves and flowers. Targeted metabolomic analysis showed that among closely related tomato species, jasmonic acid and α-tomatine are correlated in flower buds and herbivory-induced leaves. These findings suggest that herbivory-induced responses and flower development share a common molecular machinery and likely have coevolved in nature.

Reduced host plant growth and increased tyrosine-derived secondary metabolites under climate change and negative consequences on its specialist herbivore

Compositive changes in climatic factors, e.g., carbon dioxide (CO₂) and precipitation frequency and intensity, affect the strength of species interactions via responses in plants. Therefore, understanding the effects of climate change on plant-herbivore interactions is important to maintain biodiversity as about 70% of insects are herbivorous. However, the interactive effects of CO₂ and precipitation on plants and consequences for herbivores are poorly understood. Here, we examined how elevated CO₂ and increased watering frequency affect the growth and resistance responses of Aristolochia contorta and the growth performance of its specialist herbivore, Sericinus montela. Elevated CO₂ suppressed growth with decreased photosynthesis ability, and increased resistance in plants. In contrast, increased watering frequency partly ameliorated the negative effects of high CO₂. Growth performance of specialist herbivores decreased under elevated CO₂ condition as a consequence of increased resistance in plants. Due to the significant effects of CO₂, we suggest that both the quantity and the quality of host plants as a food would decline, and the growth performance of its specialist herbivore might be threatened as climate change progresses.

The plant metabolome guides fitness-relevant foraging decisions of a specialist herbivore

Plants produce complex mixtures of primary and secondary metabolites. Herbivores use these metabolites as behavioral cues to increase their fitness. However, how herbivores combine and integrate different metabolite classes into fitness-relevant foraging decisions in planta is poorly understood. We developed a molecular manipulative approach to modulate the availability of sugars and benzoxazinoid secondary metabolites as foraging cues for a specialist maize herbivore, the western corn rootworm. By disrupting sugar perception in the western corn rootworm and benzoxazinoid production in maize, we show that sugars and benzoxazinoids act as distinct and dynamically combined mediators of short-distance host finding and acceptance. While sugars improve the capacity of rootworm larvae to find a host plant and to distinguish postembryonic from less nutritious embryonic roots, benzoxazinoids are specifically required for the latter. Host acceptance in the form of root damage is increased by benzoxazinoids and sugars in an additive manner. This pattern is driven by increasing damage to postembryonic roots in the presence of benzoxazinoids and sugars. Benzoxazinoid- and sugar-mediated foraging directly improves western corn rootworm growth and survival. Interestingly, western corn rootworm larvae retain a substantial fraction of their capacity to feed and survive on maize plants even when both classes of chemical cues are almost completely absent. This study unravels fine-grained differentiation and combination of primary and secondary metabolites into herbivore foraging and documents how the capacity to compensate for the lack of important chemical cues enables a specialist herbivore to survive within unpredictable metabolic landscapes.

Controlled hydroxylations of diterpenoids allow for plant chemical defense without autotoxicity

Many plant specialized metabolites function in herbivore defense, and abrogating particular steps in their biosynthetic pathways frequently causes autotoxicity. However, the molecular mechanisms underlying their defense and autotoxicity remain unclear. Here, we show that silencing two cytochrome P450s involved in diterpene biosynthesis in the wild tobacco Nicotiana attenuata causes severe autotoxicity symptoms that result from the inhibition of sphingolipid biosynthesis by noncontrolled hydroxylated diterpene derivatives. Moreover, the diterpenes' defensive function is achieved by inhibiting herbivore sphingolipid biosynthesis through postingestive backbone hydroxylation products. Thus, by regulating metabolic modifications, tobacco plants avoid autotoxicity and gain herbivore defense. The postdigestive duet that occurs between plants and their insect herbivores can reflect the plant's solutions to the "toxic waste dump" problem of using potent chemical defenses.

Protein-carbohydrate regulation and nutritionally mediated responses to Bt are affected by caterpillar population history

BACKGROUND

The widespread adoption of genetically modified crops, including Bacillius thuringensis (Bt) crops that target chewing insects, has transformed agricultural pest management. This increased use of Bt has raised concerns about the onset of resistance amongst target pests. Recent studies have shown that for some caterpillars, nutritional foraging (e.g. the ratio of proteins and carbohydrates consumed) can affect the insect susceptibility to the Bt toxin Cry1Ac. However, studies on both nutritional foraging and Bt susceptibility tend to rely on laboratory colonies without specifically addressing physiological differences that may occur between populations of the same species. Here, we used choice assays, no choice assays and dose response assays to address two overarching questions: Do populations of Spodoptera frugiperda (J.E. Smith) vary in their protein-carbohydrate foraging behavior? and Does protein-carbohydrate intake impact S. frugiperda's susceptibility to the Bt toxin Cry1F?

RESULTS

All three of our S. frugiperda populations actively regulated their protein-carbohydrate intake, but we observed significant differences between populations with respect to their self-selected protein-carbohydrate intake. We also found that feeding at the protein-carbohydrate intake target slightly increased Cry1F susceptibility for one S. frugiperda population, but had no effect on the other two populations.

CONCLUSIONS

Our findings indicate that inherent differences exist in the nutritional physiology of three S. frugiperda populations, possibly related to the time spent in culture. This suggests that population-level differences are an important consideration when drawing parallels between field-collected and laboratory-reared insects.

Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Jasmonoyl-l-isoleucine and allene oxide cyclase-derived jasmonates differently regulate gibberellin metabolism in herbivory-induced inhibition of plant growth

The trade-off between plant growth and resistance to herbivory is thought to be at least partly mediated by the interactions between jasmonates and gibberellins (GAs). Insect herbivory activates jasmonate biosynthesis and signaling, and plant growth is concomitantly inhibited. Whether or not the herbivory-induced jasmonates suppress the accumulation of GAs and thus reduce plant growth, and which jasmonates are functional in this process, remain unclear. In this study, we show that herbivory-induced stunted growth of Nicotiana attenuata was completely dependent on allene oxide cyclase (AOC) and coronatine insensitive1 (COI1), which encode a JA biosynthetic enzyme and the receptor, respectively, but only partially dependent on jasmonic acid-isoleucine conjugate (JA-Ile), the bioactive jasmonate. Quantification of GAs and exogenous treatments indicated that herbivory-induced growth inhibition was caused by GA4 deficiency, and that the reduction in GA4 accumulation was strongly associated with both decreased concentrations of GA biosynthetic gene transcripts and transcriptional activation of GA catabolic genes. We further show that JA-Ile only positively regulated the levels of GA catabolic genes, while the accumulation of GA biosynthetic gene transcripts was controlled by certain AOC-derived jasmonate(s) rather than by JA-Ile. This work sheds light on the mechanisms by which plants adapt to herbivory by using intricate phytohormone signaling and transcriptional regulatory networks.

Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Early Molecular Responses of Tomato to Combined Moderate Water Stress and Tomato Red Spider Mite Tetranychus evansi Attack

Interaction between plants and their environment is changing as a consequence of the climate change and global warming, increasing the performance and dispersal of some pest species which become invasive species. Tetranychus evansi also known as the tomato red spider mite, is an invasive species which has been reported to increase its performance when feeding in the tomato cultivar Moneymaker (MM) under water deficit conditions. In order to clarify the underlying molecular events involved, we examined early plant molecular changes occurring on MM during T. evansi infestation alone or in combination with moderate drought stress. Hormonal profiling of MM plants showed an increase in abscisic acid (ABA) levels in drought-stressed plants while salicylic acid (SA) levels were higher in drought-stressed plants infested with T. evansi, indicating that SA is involved in the regulation of plant responses to this stress combination. Changes in the expression of ABA-dependent DREB2, NCED1, and RAB18 genes confirmed the presence of drought-dependent molecular responses in tomato plants and indicated that these responses could be modulated by the tomato red spider mite. Tomato metabolic profiling identified 42 differentially altered compounds produced by T. evansi attack, moderate drought stress, and/or their combination, reinforcing the idea of putative manipulation of tomato plant responses by tomato red spider mite. Altogether, these results indicate that the tomato red spider mite acts modulating plant responses to moderate drought stress by interfering with the ABA and SA hormonal responses, providing new insights into the early events occurring on plant biotic and abiotic stress interaction.

Regrowth strategies of Leymus chinensis in response to different grazing intensities

In temperate grassland ecosystems, grazing can affect plant growth by foraging, trampling, and excretion. The ability of dominant plant species to regrow after grazing is critical, since it allows the regeneration of photosynthetic tissues to support growth. We conducted a field experiment to evaluate the effects of different grazing intensities (control, light, medium, and heavy) on the physiological and biochemical responses of Leymus chinensis and the carbon (C) sources utilized during regrowth. Light grazing promoted regrowth and photoassimilate storage of L. chinensis, by increasing the net photosynthetic rate (P_n ), photosynthetic quenching, light interception, sugar accumulation, sucrose synthase activities, and fructose supply from stems. At medium grazing intensity, L. chinensis had low P_n , light interception, and sugar accumulation, but higher expression of a sucrose transporter gene (LcSUT1) and water-use efficiency, which reflected a tendency to store C in belowground to promote survival. This strategy was associated with regulation by abscisic acid (ABA), jasmonate, and salicylic acid (SA) signaling. However, L. chinensis tolerated heavy grazing by increased ABA and jasmonate-induced promotion of C assimilation and osmotic adjustment, combined with photoprotection against photo-oxidation, suggesting a strategy based on regrowth. In addition, stems were the main C source organs and energy supply rather than roots. Simultaneously, SA represented a weaker defense than ABA and jasmonate. Therefore, L. chinensis adopted different strategies for regrowth under different grazing intensities, and light grazing promoted regrowth the most. Our results demonstrate the regulation of C reserves utilization by phytohormones, and this regulation provides an explanation for recent results about grazing responses.

Silencing JA hydroxylases in Nicotiana attenuata enhances jasmonic acid-isoleucine-mediated defenses against Spodoptera litura

Jasmonic acid (JA) plays important roles in plant resistance to insect herbivores. One important derivative of JA is 12-OH-JA, which is produced by two independent pathways: direct hydroxylation of JA by jasmonate-induced oxygenases (JOXs) or hydrolyzation of 12-OH-JA-Ile.Yet the function of 12-OH-JA in plant-herbivore interactions remains largely unknown. In this study, we silenced four JOX homologs independently in the wild tobacco Nicotiana attenuata by virus-induced gene silencing (VIGS), and found that all four JOX homologs are involved in JA hydroxylation. Simultaneously silencing the four JA hydroxylases in VIGS-NaJOXs plants decreased herbivory-induced 12-OH-JA by 33%, but JA and JA-Ile levels increased by 45% and 30%, respectively, compared to those in control plants. Compared to direct hydroxylation from JA, hydrolyzation from 12-OH-JA-Ile is equally important for herbivory-induced 12-OH-JA accumulation: in the 12-OH-JA-Ile deficient irJAR4/6 plants, 12-OH-JA decreased 34%. Moreover, VIGS-NaJOXs plants exhibited enhanced resistance to the generalist herbivore Spodoptera litura. The poor larval performance was strongly correlated with high levels of several JA-Ile-dependent direct defense metabolites in the VIGS-NaJOXs plants. When we simultaneously silenced all four JA hydroxylases in the JA-Ile-deficient irJAR4/6 background, the enhanced herbivore resistance diminished, demonstrating that enhanced herbivore resistance resulted from elevated JA-Ile levels. Given that silencing these NaJOX-like genes did not detectably alter plant growth but highly increased plant defense levels, we propose that JOX genes are potential targets for genetic improvement of herbivore-resistant crops.

The Role of Plant-Associated Microbes in Mediating Host-Plant Selection by Insect Herbivores

There is increasing evidence that plant-associated microorganisms play important roles in shaping interactions between plants and insect herbivores. Studies of both pathogenic and beneficial plant microbes have documented wide-ranging effects on herbivore behavior and performance. Some studies, for example, have reported enhanced insect-repellent traits or reduced performance of herbivores on microbe-associated plants, while others have documented increased herbivore attraction or performance. Insect herbivores frequently rely on plant cues during foraging and oviposition, suggesting that plant-associated microbes affecting these cues can indirectly influence herbivore preference. We review and synthesize recent literature to provide new insights into the ways pathogenic and beneficial plant-associated microbes alter visual, olfactory, and gustatory cues of plants that affect host-plant selection by insect herbivores. We discuss the underlying mechanisms, ecological implications, and future directions for studies of plant-microbial symbionts that indirectly influence herbivore behavior by altering plant traits.

The nutritional landscape of host plants for a specialist insect herbivore

Nutrition has far-reaching effects on both the ecology and evolution of species. A substantial body of work has examined the role of host plant quality on insect herbivores, with a particular focus on specialist-generalist dynamics, the interaction of growth and other physiological attributes on fitness and tritrophic effects. Measures of plant quality usually involve one or two axes of nutritional space: typically secondary metabolites or elemental proxies (N and C) of protein and carbohydrates, respectively.Here, we describe the nutrient space of seven host plants of the specialist insect herbivore, Manduca sexta, using an approach that measures physiologically relevant sources of nutrition, soluble protein and digestible carbohydrates. We show that plant species differ markedly in their nutrient content, offering developing insect herbivores a range of available nutrient spaces that also depend on the age of the leaves being consumed.The majority of host-plant species produce diets that are suboptimal to the herbivore, likely resulting in varying levels of compensatory feeding for M. sexta to reach target levels of protein to ensure successful growth and development. Low-quality diets can also impact immune function leading to complex patterns of optimization of plant resources that maximizes both growth and the ability to defend from parasitoids and pathogens. This study is the first to quantify the nutrient space of a suite of host plants used by an insect herbivore using physiologically relevant measures of nutrition.

Jasmonates-Mediated Rewiring of Central Metabolism Regulates Adaptive Responses

The lipid-derived hormones jasmonates (JAs) play key functions in a wide range of physiological and developmental processes that regulate growth, secondary metabolism and defense against biotic and abiotic stresses. In this connection, biosynthesis, tissue-specific distribution, metabolism, perception, signaling of JAs have been the target of extensive studies. In recent years, the involvement of JAs signaling pathway in the regulation of growth and adaptive responses to environmental challenges has been further examined. However, JAs-mediated mechanisms underlying the transition from 'growth mode' to 'adaptive mode' remain ambiguous. Combined analysis of transgenic lines deficient in JAs signaling in conjunction with the data from JAs-treated plants revealed the function of these hormones in rewiring of central metabolism. The collective data illustrate JAs-mediated decrease in the levels of metabolites associated with active growth such as sucrose, raffinose, orotate, citrate, malate, and an increase in phosphorylated hexoses, responsible for the suppression of growth and photosynthesis, concurrent with the induction of protective metabolites, such as aromatic and branched-chain amino acids, and aspartate family of metabolites. This finding provides an insight into the function of JAs in shifting the central metabolism from the production of growth-promoting metabolites to protective compounds and expands our understanding of the role of JAs in resource allocation in response to environmental challenges.

Flowering-mediated root-fungus symbiosis loss is related to jasmonate-dependent root soluble sugar deprivation

The role of flowering in root-fungal symbiosis is not well understood. Because flowering and fungal symbionts are supported by carbohydrates, we hypothesized that flowering modulates root-beneficial fungal associations through alterations in carbohydrate metabolism and transport. We monitored fungal colonization and soluble sugars in the roots of Arabidopsis thaliana following inoculation with a mutualistic fungus Phomopsis liquidambari across different plant developmental stages. Jasmonate signalling of wild-type plants, sugar transport, and root invertase of wild-type and jasmonate-insensitive plants were exploited to assess whether and how jasmonate-dependent sugar dynamics are involved in flowering-mediated fungal colonization alterations. We found that flowering restricts root-fungal colonization and activates root jasmonate signalling upon fungal inoculation. Jasmonates reduce the constitutive and fungus-induced accumulation of root glucose and fructose at the flowering stage. Further experiments with sugar transport and metabolism mutant lines revealed that root glucose and fructose positively influence fungal colonization. Diurnal, jasmonate-dependent inhibitions of sugar transport and soluble invertase activity were identified as likely mechanisms for flowering-mediated root sugar depletion upon fungal inoculation. Collectively, our results reveal that flowering drives root-fungus cooperation loss, which is related to jasmonate-dependent root soluble sugar depletion. Limiting the spread of root-fungal colonization may direct more resources to flower development.

First evidence of protein-carbohydrate regulation in a plant bug (Lygus hesperus)

Lygus bugs are highly polyphagous piercing/sucking insects found throughout North America. Collectively, they have been reported to feed on over 330 plant species (one of the broadest host range ever documented for a group of insects); they also feed on many economically important crops. Despite its prevalence across North America and status as a common pest in many agroecosystems, very little is known about how Lygus bugs regulate their intake of nutrients. In reality, little is known about nutrient regulation for most hemipterans, specifically non-phloem feeding species in the suborder Heteroptera. This likely reflects difficulties in developing adequate artificial diets for insects with piercing/sucking mouthparts. There is, however, an artificial diet for L. hersperus, and in this study we modified it and performed choice and no-choice experiments to determine how L. hesperus regulates its intake of two macronutrients - protein (p) and carbohydrates (c) - that are tightly linked to survival and performance in other insect herbivores. In choice experiments L. hesperus was allowed to select between two foods with different protein:carbohydrate ratios. We documented strong regulation for protein and carbohydrates, with late instar nymphs selecting a slightly protein-biased intake target (protein-carbohydrate ratio = 1.5:1). We also performed no-choice experiments, where nymphs were restricted to a single food. Here, the protein-carbohydrate ratio of their food had a strong impact on survival, which was highest for nymphs reared on the treatment with a protein-carbohydrate ratio closest to the self-selected intake target (determined by the choice experiments), but no significant impact on developmental time or mass gain. Our data are the first of their kind for a non-phloem feeding hemipteran and provide a starting point for more broadly understanding and further investigating the nutritional ecology/physiology of Lygus bugs. Our study also provides a framework for exploring nutrient regulation in other hemipterans and for optimizing artificial diets for piercing/sucking insects, especially heteropterans.

Fall armyworm (Spodoptera frugiperda Smith) feeding elicits differential defense responses in upland and lowland switchgrass

Switchgrass (Panicum virgatum L.) is a low input, high biomass perennial grass being developed for the bioenergy sector. Upland and lowland cultivars can differ in their responses to insect herbivory. Fall armyworm [FAW; Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae)] is a generalist pest of many plant species and can feed on switchgrass as well. Here, in two different trials, FAW larval mass were significantly reduced when fed on lowland cultivar Kanlow relative to larvae fed on upland cultivar Summer plants after 10 days. Hormone content of plants indicated elevated levels of the plant defense hormone jasmonic acid (JA) and its bioactive conjugate JA-Ile although significant differences were not observed. Conversely, the precursor to JA, 12-oxo-phytodienoic acid (OPDA) levels were significantly different between FAW fed Summer and Kanlow plants raising the possibility of differential signaling by OPDA in the two cultivars. Global transcriptome analysis revealed a stronger response in Kanlow plant relative to Summer plants. Among these changes were a preferential upregulation of several branches of terpenoid and phenylpropanoid biosynthesis in Kanlow plants suggesting that enhanced biosynthesis or accumulation of antifeedants could have negatively impacted FAW larval mass gain on Kanlow plants relative to Summer plants. A comparison of the switchgrass-FAW RNA-Seq dataset to those from maize-FAW and switchgrass-aphid interactions revealed that key components of plant responses to herbivory, including induction of JA biosynthesis, key transcription factors and JA-inducible genes were apparently conserved in switchgrass and maize. In addition, these data affirm earlier studies with FAW and aphids that the cultivar Kanlow can provide useful genetics for the breeding of switchgrass germplasm with improved insect resistance.

Growth-defense trade-off regulated by hormones in grass plants growing under different grazing intensities

Herbivory creates conflicts between a plant's need to allocate resources for growth and defense. It is not yet clear how plants rebalance resource utilization between growth and defense in response to increasing grazing intensity. We measured characteristics of the primary and secondary metabolism of Leymus chinensis at five levels of grazing intensity (control, light, moderate, heavy and extremely heavy). Furthermore, we evaluated hormone signaling by quantifying the impact of key hormones on plant growth and defense. Under light grazing intensity, indole-3-acetic acid and jasmonates appeared to promote the growth of L. chinensis through a high photosynthetic rate, high water-use efficiency and high soluble protein contents, whereas abscisic acid decreased these properties. Under moderate grazing intensity, L. chinensis had a low photosynthetic capacity but greater production of secondary metabolites (tannins, total flavonoids and total phenols), possibly induced by salicylic acid. When the grazing pressure further intensified, L. chinensis translocated more carbohydrates to its roots in order to survive and regrow. Leymus chinensis therefore exhibited a trade-off between growth and defense in order to survive and reproduce under herbivory. Plants developed different mechanisms to enhance their grazing tolerance by means of hormonal regulation.

Waterlogging tolerance rendered by oxylipin-mediated metabolic reprogramming in Arabidopsis

Environmental stresses induce production of oxylipins synthesized by the two main biosynthetic branches, allene oxide synthase (AOS) and hydroperoxide lyase (HPL). Here, we investigate how waterlogging-mediated alteration of AOS- and HPL-derived metabolic profile results in modulation of central metabolism and ultimately enhanced tolerance to this environmental stress in Arabidopsis thaliana. Waterlogging leads to increased levels of AOS- and HPL-derived metabolites, and studies of genotypes lacking either one or both branches further support the key function of these oxylipins in waterlogging tolerance. Targeted quantitative metabolic profiling revealed oxylipin-dependent alterations in selected primary metabolites, and glycolytic and citric acid cycle intermediates, as well as a prominent shift in sucrose cleavage, hexose activation, the methionine salvage pathway, shikimate pathway, antioxidant system, and energy metabolism in genotypes differing in the presence of one or both functional branches of the oxylipin biosynthesis pathway. Interestingly, despite some distinct metabolic alterations caused specifically by individual branches, overexpression of HPL partially or fully alleviates the majority of altered metabolic profiles observed in AOS-depleted lines. Collectively, these data identify the key role of AOS- and HPL-derived oxylipins in altering central metabolism, and further provide a metabolic platform targeted at identification of gene candidates for enhancing plant tolerance to waterlogging.

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

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

Endophyte Infection and Methyl Jasmonate Treatment Increased the Resistance of Achnatherum sibiricum to Insect Herbivores Independently

Alkaloids are usually thought to be responsible for protecting endophyte-infected (EI) grasses from their herbivores. For EI grasses that produce few alkaloids, can endophyte infection enhance their resistance to herbivores? Related studies are limited. In the Inner Mongolian steppe, Achnatherum sibiricum is highly infected by Epichloë endophytes, but produces few alkaloids. Locusts are the common insect herbivores of grasses. In this study, A. sibiricum was used as plant material. Methyl jasmonate (MJ, when applied exogenously, can induce responses similar to herbivore damage) treatment was performed. The effects of endophyte infection and MJ treatment on the resistance of A. sibiricum to Locusta migratoria were studied. We found that locusts preferred EF (endophyte-free) plants to EI plants in both choice and no-choice feeding experiments. Endophyte infection enhanced the resistance of A. sibiricum to locusts. Endophyte infection decreased soluble sugar concentrations, while it increased the total phenolic content and phenylalanine ammonia lyase (PAL) activity, which may contribute to the resistance of A. sibiricum to locusts. There was an interaction effect between MJ treatment and endophyte infection on the growth of the host. MJ treatment was a negative regulator of the plant growth-promoting effects of endophyte infection. There was no interaction effect between MJ treatment and endophyte infection on the defense characteristics of the host. In groups not exposed to locusts, MJ treatment and endophyte infection had a similar effect in decreasing the soluble sugar content, while increasing the total phenolic content and the PAL activity. In groups exposed to locusts, the effect of MJ treatment on the above characteristics disappeared, while the effect of endophyte infection became more obvious. All of these results suggest that even for endophytes producing few alkaloids, they could still increase the resistance of native grasses to insect herbivores. Furthermore, endophyte infection might mediate the defense responses of the host, independent of jasmonic acid (JA) pathways.

Convergent evolution of a metabolic switch between aphid and caterpillar resistance in cereals

Tailoring defense responses to different attackers is important for plant performance. Plants can use secondary metabolites with dual functions in resistance and defense signaling to mount herbivore-specific responses. To date, the specificity and evolution of this mechanism are unclear. Here, we studied the functional architecture, specificity, and genetic basis of defense regulation by benzoxazinoids in cereals. We document that DIMBOA-Glc induces callose as an aphid resistance factor in wheat. O-methylation of DIMBOA-Glc to HDMBOA-Glc increases plant resistance to caterpillars but reduces callose inducibility and resistance to aphids. DIMBOA-Glc induces callose in wheat and maize, but not in Arabidopsis, while the glucosinolate 4MO-I3M does the opposite. We identify a wheat O-methyltransferase (TaBX10) that is induced by caterpillar feeding and converts DIMBOA-Glc to HDMBOA-Glc in vitro. While the core pathway of benzoxazinoid biosynthesis is conserved between wheat and maize, the wheat genome does not contain close homologs of the maize DIMBOA-Glc O-methyltransferase genes, and TaBx10 is only distantly related. Thus, the functional architecture of herbivore-specific defense regulation is similar in maize and wheat, but the regulating biosynthetic genes likely evolved separately. This study shows how two different cereal species independently achieved herbivore-specific defense activation by regulating secondary metabolite production.

NAC transcription factor ONAC066 positively regulates disease resistance by suppressing the ABA signaling pathway in rice

This is the first time to dissect the mechanism of NACs-mediated disease resistance in plants using metabolomic approach and discover the involvement of ABA signaling pathway in NACs-mediated disease resistance. NAC transcription factors have been validated as important regulators in stress responses, but their molecular mechanisms in plant disease resistance are still largely unknown. Here we report that the NAC gene ONAC066 (LOC_Os01g09550) is significantly activated by rice blast infection. ONAC066 is ubiquitously expressed and this protein is localized in the nucleus. Overexpression of ONAC066 quantitatively enhances resistance to blast disease and bacterial blight in rice. The transcript levels of PR genes are also dramatically induced in ONAC066 overexpressing plants. Exogenous abscisic acid (ABA) strongly activates the transcription of ONAC066 in rice. Further analysis shows that overexpression of ONAC066 remarkably suppresses the expression of ABA-related genes, whereas there are no obvious differences for salicylic acid (SA) and jasmonic acid (JA)-related genes between wild-type and ONAC066 overexpressing plants. Consistently, lower endogenous ABA levels are identified in ONAC066 overexpressing plants compared with wild-type plants before and after blast inoculation, while no significant differences are observed for the SA and JA levels. Yeast one-hybrid assays demonstrate that ONAC066 directly binds to the promoters of LIP9 and NCED4 to modulate their expression. Moreover, the metabolomic study reveals that the ONAC066 overexpressing plants accumulated higher contents of soluble sugars and amino acids both before and after pathogen attack, when compared to wild-type plants. Taken together, our results suggest that ONAC066 positively regulates rice resistance to blast and bacterial blight, and ONAC066 exerts its functions on disease resistance by modulating of ABA signaling pathway, sugars and amino acids accumulation in rice.

Spodoptera litura-mediated chemical defense is differentially modulated in older and younger systemic leaves of Solanum lycopersicum

Metabolite profiling, biochemical assays, and transcript analysis revealed differential modulation of specific induced defense responses in local, older, and younger systemic leaves in Solanum lycopersicum upon Spodoptera litura herbivory. Plants reconfigure their metabolome upon herbivory to induce production of defense metabolites involved in both direct and indirect defenses against insect herbivores. Herbivory mediated leaf-to-leaf systemic induction pattern of primary and non-volatile secondary metabolites is not well studied in tomato. Here, we show that, in cultivated tomato Solanum lycopersicum herbivory by generalist insect, Spodoptera litura results in differential alteration of primary metabolites, majorly sugars and amino acids and specific secondary metabolites in local, younger, and older systemic leaves. Cluster analysis of 55 metabolites identified by GC-MS showed correlation between local and younger systemic leaves. Re-allocation of primary metabolites like glucose and amino acids from the local to systemic leaf was observed. Secondary metabolites chlorogenic acid, caffeic acid, and catechin were significantly induced during herbivory in systemic leaves. Among specific secondary metabolites, chlorogenic acid and catechin significantly inhibits S. litura larval growth in all stages. Local leaf exhibited increased lignin accumulation upon herbivory. Differential alteration of induced defense responses like reactive oxygen species, polyphenol oxidase activity, proteinase inhibitor, cell wall metabolites, and lignin accumulation was observed in systemic leaves. The metabolite alteration also resulted in increased defense in systemic leaves. Thus, comparative analysis of metabolites in local and systemic leaves of tomato revealed a constant re-allocation of primary metabolites to systemic leaves and differential induction of secondary metabolites and induced defenses upon herbivory.

Quantifying Plant Soluble Protein and Digestible Carbohydrate Content, Using Corn (Zea mays) As an Exemplar

Elemental data are commonly used to infer plant quality as a resource to herbivores. However, the ubiquity of carbon in biomolecules, the presence of nitrogen-containing plant defensive compounds, and variation in species-specific correlations between nitrogen and plant protein content all limit the accuracy of these inferences. Additionally, research focused on plant and/or herbivore physiology require a level of accuracy that is not achieved using generalized correlations. The methods presented here offer researchers a clear and rapid protocol for directly measuring plant soluble proteins and digestible carbohydrates, the two plant macronutrients most closely tied to animal physiological performance. The protocols combine well characterized colorimetric assays with optimized plant-specific digestion steps to provide precise and reproducible results. Our analyses of different sweet corn tissues show that these assays have the sensitivity to detect variation in plant soluble protein and digestible carbohydrate content across multiple spatial scales. These include between-plant differences across growing regions and plant species or varieties, as well as within-plant differences in tissue type and even positional differences within the same tissue. Combining soluble protein and digestible carbohydrate content with elemental data also has the potential to provide new opportunities in plant biology to connect plant mineral nutrition with plant physiological processes. These analyses also help generate the soluble protein and digestible carbohydrate data needed to study nutritional ecology, plant-herbivore interactions and food-web dynamics, which will in turn enhance physiology and ecological research.

Resolution of growth-defense conflict: mechanistic insights from jasmonate signaling

Induced plant resistance depends on the production of specialized metabolites that repel attack by biotic aggressors and is often associated with reduced growth of vegetative tissues. Despite progress in understanding the signal transduction networks that control growth-defense tradeoffs, much remains to be learned about how growth rate is coordinated with changes in metabolism during growth-to-defense transitions. Here, we highlight recent advances in jasmonate research to suggest how a major branch of plant immunity is dynamically regulated to calibrate growth-defense balance with shifts in carbon availability. We review evidence that diminished growth, as an integral facet of induced resistance, may optimize the temporal and spatial expression of defense compounds without compromising other critical roles of central metabolism. New insights into the evolution of jasmonate signaling further suggest that opposing selective pressures associated with too much or too little defense may have shaped the emergence of a modular jasmonate pathway that integrates primary and specialized metabolism through the control of repressor-transcription factor complexes. A better understanding of the mechanistic basis of growth-defense balance has important implications for boosting plant productivity, including insights into how these tradeoffs may be uncoupled for agricultural improvement.

Cytokinin transfer by a free-living mirid to Nicotiana attenuata recapitulates a strategy of endophytic insects

Endophytic insects provide the textbook examples of herbivores that manipulate their host plant's physiology, putatively altering source/sink relationships by transferring cytokinins (CK) to create 'green islands' that increase the nutritional value of infested tissues. However, unambiguous demonstrations of CK transfer are lacking. Here we show that feeding by the free-living herbivore Tupiocoris notatus on Nicotiana attenuata is characterized by stable nutrient levels, increased CK levels and alterations in CK-related transcript levels in attacked leaves, in striking similarity to endophytic insects. Using 15N-isotope labeling, we demonstrate that the CK N6-isopentenyladenine (IP) is transferred from insects to plants via their oral secretions. In the field, T. notatus preferentially attacks leaves with transgenically increased CK levels; plants with abrogated CK-perception are less tolerant of T. notatus feeding damage. We infer that this free-living insect uses CKs to manipulate source/sink relationships to increase food quality and minimize the fitness consequences of its feeding.

Functional variation in a key defense gene structures herbivore communities and alters plant performance

Plant genetic diversity structures animal communities and affects plant population productivity. However, few studies have investigated which traits are involved and the mechanisms mediating these effects. We studied the consequences of varying the expression of a single biosynthetic gene in jasmonate (JA) defense hormones, which are essential for defense against herbivores but constrain plant growth, in experimental mesocosm populations of wild tobacco (Nicotiana attenuata) plants under attack from three native herbivores. Empoasca leafhoppers preferentially attack JA-deficient N. attenuata plants in nature, and the specialist Tupiocoris notatus mirids avoid Empoasca-damaged plants. However, in experimental mesocosm populations having equal numbers of wild-type (WT) and JA-deficient plants that are silenced in the expression of the biosynthetic gene lipoxygenase 3 (LOX3), Empoasca sp. attacked both genotypes. Empoasca sp. damage, rather than JA, determined T. notatus damage, which was reduced in mixed populations. The growth of specialist Manduca sexta larvae was reduced on WT vs. asLOX3 monocultures, but differed in mixtures depending on caterpillar density. However, seed capsule number remained similar for WT and asLOX3 plants in mixtures, not in monocultures, in two experimental scenarios reflecting high and low caterpillar attack. At high caterpillar density, WT plants growing in mixtures produced more seed capsules than those growing in monocultures while seed production of asLOX3 plants did not differ by population type. However, at low caterpillar density, asLOX3 plants growing in mixed populations produced more seed capsules than those growing in monoculture, while seed capsule production did not differ for WT by population type. Thus, mixed populations had a more stable output of seed capsules under the two scenarios. This may result from a balance between JA-mediated herbivore defense and plant competitive ability in mixed populations.

Targeted Metabolomic and Transcriptomic Analyses of "Red Russian" Kale (Brassicae napus var. pabularia) Following Methyl Jasmonate Treatment and Larval Infestation by the Cabbage Looper (Trichoplusia ni Hübner)

Methyl jasmonate (MeJA), synthesized in the jasmonic acid (JA) pathway, has been found to upregulate glucosinolate (GS) biosynthesis in plant species of the Brassicaceae family. Exogenous application of MeJA has shown to increase tissue GS concentrations and the formation of myrosinase-mediated GS hydrolysis products (GSHPs). In vitro and in vivo assays have demonstrated the potential health-promoting effects of certain GSHPs. MeJA is also known to elicit and induce genes associated with defense mechanisms to insect herbivory in Brassica species. To investigate the relationship between MeJA-induced GS biosynthesis and insect defense, three treatments were applied to "Red Russian" kale (Brassicae napus var. pabularia) seedlings: (1) a 250 µM MeJA leaf spray treatment; (2) leaf infestation with larvae of the cabbage looper (Trichoplusia ni (Hübner)); (3) control treatment (neither larval infestation nor MeJA application). Samples of leaf tissue from the three treatments were then assayed for changes in GS and GSHP concentrations, GS gene biosynthesis expression, and myrosinase activity. Major differences were observed between the three treatments in the levels of GS accumulation and GS gene expression. The insect-damaged samples showed significantly lower aliphatic GS accumulation, while both MeJA and T. ni infestation treatments induced greater accumulation of indolyl GS. The gene expression levels of CYP81F4, MYB34, and MYB122 were significantly upregulated in samples treated with MeJA and insects compared to the control group, which explained the increased indolyl GS concentration. The results suggest that the metabolic changes promoted by MeJA application and the insect herbivory response share common mechanisms of induction. This work provides potentially useful information for kale pest control and nutritional quality.

Protein-carbohydrate regulation in Helicoverpa amigera and H. punctigera and how diet protein-carbohydrate content affects insect susceptibility to Bt toxins

Many animals, including insects, demonstrate a remarkable ability to regulate their intake of key macronutrients (e.g., soluble protein and digestible carbohydrates), which allows them to optimize fitness and performance. Additionally, regulating the intake of these two macronutrients enhances an animal's ability to defend itself against pathogens, mitigate the effects of secondary plant metabolites, and decrease susceptibility to toxins. In this study, we first compared how Bt-resistant and -susceptible lines of Helicoverpa armigera and Helicoverpa punctigera regulate their intake of protein (p) and digestible carbohydrates (c). We found that there was no difference in the self-selected protein-carbohydrate intake target between resistant and susceptible genotypes of either species. We then explored the extent to which food protein-carbohydrate content altered the susceptibility of these species to three Bt toxins: Cry1Ac, Cry2Ab, and Vip3Aa. We found that H. armigera on diets that had protein-carbohydrate profiles that matched their self-selected protein-carbohydrate intake target were significantly less susceptible to Cry1Ac. In contrast, diet protein-carbohydrate content did not affect H. punctigera susceptibility to Cry1Ac. For both H. armigera and H. punctigera, susceptibility to Cry2Ab and Vip3Aa toxins did not change as a function of diet protein-carbohydrate profile. These results, when combined with earlier work on H. zea, suggest food protein-carbohydrate content can modify susceptibility to some Bt toxins, but not others. An increased understanding of how the nutritional environment can modify susceptibility to different Bt toxins could help improve pest management and resistance management practices.

Aboveground herbivory induced jasmonates disproportionately reduce plant reproductive potential by facilitating root nematode infestation

Different plant feeders, including insects and parasitic nematodes, can influence each other by triggering systemic changes in their shared host plants. In most cases, however, the underlying mechanisms are unclear, and the consequences for plant fitness are not well understood. We studied the interaction between leaf feeding Manduca sexta caterpillars and root parasitic nematodes in Nicotiana attenuata. Simulated M. sexta attack increased the abundance of root parasitic nematodes in the field and facilitated Meloidogyne incognita reproduction in the glasshouse. Intact jasmonate biosynthesis was found to be required for both effects. Flower counts revealed that the jasmonate-dependent facilitation of nematode infestation following simulated leaf attack reduces the plant's reproductive potential to a greater degree than would be expected from the additive effects of the individual stresses. This work reveals that jasmonates mediate the interaction between a leaf herbivore and root parasitic nematodes and illustrates how plant-mediated interactions can alter plant's reproductive potential. The selection pressure resulting from the demonstrated fitness effects is likely to influence the evolution of plant defense traits in nature.

Plant Defenses against Herbivory: Closing the Fitness Gap

Many morphological and chemical features of plants are classified as plant defenses against herbivores. By definition, plant defenses should increase a plant's fitness (i.e., its contribution to the gene pool of the next generation) as a function of herbivory. Over the past years, substantial progress has been made in understanding and manipulating the mechanistic basis of many putative plant defense traits. However, most plant defenses are still characterized by proximate variables such as herbivore performance or plant damage rather than actual fitness. Determining fitness benefits as a function of herbivory therefore remains a major knowledge gap that must be filled to understand the ecology and evolution of plant defenses.