Negative regulation of plastidial isoprenoid pathway by herbivore-induced ß-cyclocitral in Arabidopsis thaliana.

Insect damage to plants is known to up-regulate defense and down-regulate growth processes. While there are frequent reports about up-regulation of defense signaling and production of defense metabolites in response to herbivory, much less is understood about the mechanisms by which growth and carbon assimilation are down-regulated. Here we demonstrate that insect herbivory down-regulates the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway in Arabidopsis (Arabidopsis thaliana), a pathway making primarily metabolites for use in photosynthesis. Simulated feeding by the generalist herbivore Spodoptera littoralis suppressed flux through the MEP pathway and decreased steady-state levels of the intermediate 1-deoxy-D-xylulose 5-phosphate (DXP). Simulated herbivory also increased reactive oxygen species content which caused the conversion of ß-carotene to ß-cyclocitral (ßCC). This volatile oxidation product affected the MEP pathway by directly inhibiting DXP synthase (DXS), the rate-controlling enzyme of the MEP pathway in Arabidopsis and inducing plant resistance against S. littoralis ßCC inhibited both DXS transcript accumulation and DXS activity. Molecular models suggested that ßCC binds to DXS at the binding site for the thymine pyrophosphate cofactor and blocks catalysis, which was confirmed by direct assays of ßCC with the purified DXS protein in vitro. Another intermediate of the MEP pathway, 2-C-methyl-D-erythritol-2, 4-cyclodiphosphate, which is known to stimulate salicylate defense signaling, showed greater accumulation and enhanced export out of the plastid in response to simulated herbivory. Together, our work implicates ßCC as a signal of herbivore damage in Arabidopsis that increases defense and decreases flux through the MEP pathway, a pathway involved in growth and carbon assimilation.

RuBPCase activase (RCA) mediates growth-defense trade-offs: silencing RCA redirects jasmonic acid (JA) flux from JA-isoleucine to methyl jasmonate (MeJA) to attenuate induced defense responses in Nicotiana attenuata.

• RuBPCase activase (RCA), an abundant photosynthetic protein, is strongly down-regulated in response to Manduca sexta's oral secretion (OS) in Nicotiana attenuata. RCA-silenced plants are impaired not only in photosynthetic capacity and growth, but also in jasmonic acid-isoleucine (JA-Ile) signaling, and herbivore resistance mediated by JA-Ile-dependent defense traits. These responses are consistent with a resource-based growth-defense trade-off. • As JA + Ile supplementation of OS restored wild-type (WT) levels of JA-Ile, defenses and resistance to M. sexta, but OS supplemented individually with JA or Ile did not, the JA-Ile deficiency of RCA-silenced plants could not be attributed to lower JA or Ile pools or JAR4/6 conjugating activity. Similar levels of JA-Ile derivatives after OS elicitation indicated unaltered JA-Ile turnover, and lower levels of other JA conjugates ruled out competition from other conjugation reactions. • RCA-silenced plants accumulated more methyl jasmonate (MeJA) after OS elicitation, which corresponded to increased jasmonate methyltransferase (JMT) activity. RCA silencing phenocopies JMT overexpression, wherein elevated JMT activity redirects OS-elicited JA flux towards inactive MeJA, creating a JA sink which depletes JA-Ile and its associated defense responses. • Hence, RCA plays an additional non-photosynthetic role in attenuating JA-mediated defenses and their associated costs, potentially allowing plants to anticipate resource-based constraints on growth before they actually occur.

ß-Cyclocitral-Mediated Metabolic Changes Optimize Growth and Defense Responses in Solanum lycopersicum L.

ß-cyclocitral (ßCC) is one of the significant oxidative products of ß-carotene. It primes plants for multiple stress acclimation without compromising plant growth. Metabolic reorganization is necessary to maintain a balance between growth and defense. However, the ßCC-mediated changes in a plant's metabolic network are unknown. Here, we demonstrate how ßCC-induced metabolic changes enable Solanum lycopersicum L. (tomato) plants to promote defense and maintain growth under stress. An analysis of early (0-240 min) and late (72 h) changes in the tomato metabolome after ßCC-treatment using liquid chromatography and tandem mass spectrometry identified 57 compounds. A principal coordinate analysis suggested that ßCC treatment significantly changes the metabolite profile. A variable importance in projection (VIP) analysis revealed 16 and 19 discriminant metabolites from early and late samples, respectively (VIP ≥ 1.0). Upregulated metabolites were mainly amino acids and phytophenols. Pathway enrichment analysis showed that ßCC treatment influenced amino acid metabolism at early and later times; however, phenylpropanoid and isoquinoline biosynthesis were influenced only at the later time. A 66.6% similarity in the upregulated metabolites of ßCC- and simulated-herbivory-treated plants confirmed ßCC's role against herbivores. We conclude that ßCC steers a temporal separation in amino acids and defense metabolite accumulation that optimizes resource allocation to growth and defense.

Effects of herbivory on carotenoid biosynthesis and breakdown.

The carotenoid content of plants may be impacted by stress with major consequences for photosynthesis and photoprotection. Most carotenoid stress research, however, has concentrated on abiotic stresses, and we know little about how biological stresses, such as herbivory, alter profiles of plant carotenoids and their degradation products. For example, carotenoid derivatives such as ß-cyclocitral and ß-ionone have been recently shown to act as signals in plant growth and protection against oxidative stress and herbivory. To understand how carotenoid composition is influenced by herbivory, changes in biosynthesis and degradation should be investigated. This chapter describes methods to simulate herbivory in a simple reproducible fashion and to assess carotenoid biosynthesis and degradation. Carotenoid biosynthesis depends on precursors provided by the methylerythritol 4-phosphate (MEP) pathway, which converts pyruvate and glyceraldehyde-3-phosphate to the five-carbon units used for construction of larger isoprenoids. We present protocols to quantify the activity of the first enzyme of the MEP pathway, deoxy-xylulose 5-phosphate synthase (DXS), usually assumed to be rate-controlling, and to estimate the concentration of the first intermediate of the pathway, deoxy-xylulose 5-phosphate (DXP). We also discuss procedures to measure the formation of volatile carotenoid breakdown products after herbivory. To monitor the activity of carotenoid-specific biosynthetic enzymes, such as phytoene synthase, protocols are available elsewhere in this volume (Wurtzel, 2022b).

Assessing the Impact of Geographical Distribution and Genetic Diversity on Metabolic Profiles of a Medicinal Plant, Embelia ribes Burm. f.

The extensive use of Embelia ribes Burm. f. (Embelia) in tribal medicine proclaimed global attention as a promising candidate in complementary and alternative medicine. The knowledge of chemical blends is a prerequisite for the selection of raw materials for herbal medicine formulations; however, the influence of geographical distance and genetic diversity on the metabolome of Embelia fruits is unknown. Therefore, we collected Embelia fruits from four locations across the Western Ghats of India and analyzed the metabolic profile and genotypic diversity of Embelia fruits by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and inter simple sequence repeats (ISSR), respectively. LC-MS/MS analysis yielded 583 compounds; however, the trimmed data resulted in 149 compounds. Further, MS/MS analysis identified 36 compounds, among which we reported 30 compounds for the first time from Embelia. These compounds belong to 11 compound classes that suggest location-specific chemical blends of Embelia fruits. Multivariate analysis showed 94% compound diversity across the accessions. ISSR analysis suggests 95% polymorphism across the accessions. A significant positive correlation (80%) between metabolomics and genotypic data matrices validates the genotype's influence in tuning Embelia's metabolic profiles. We conclude that the chemical profiles of Embelia are location-specific, which can be explored for the selection of herbal trade sustainably.

ß-Cyclocitral, a Master Regulator of Multiple Stress-Responsive Genes in Solanum lycopersicum L. Plants.

ß-cyclocitral (ßCC), a major apocarotenoid of ß-carotene, enhances plants' defense against environmental stresses. However, the knowledge of ßCC's involvement in the complex stress-signaling network is limited. Here we demonstrate how ßCC reprograms the transcriptional responses that enable Solanum lycopersicum L. (tomato) plants to endure a plethora of environmental stresses. Comparative transcriptome analysis of control and ßCC-treated tomato plants was done by generating RNA sequences in the BGISEQ-500 platform. The trimmed sequences were mapped on the tomato reference genome that identifies 211 protein-coding differentially expressed genes. Gene ontology and Kyoto Encyclopedia of Genes and Genomes analysis and their enrichment uncovered that only upregulated genes are attributed to the stress response. Moreover, 80% of the upregulated genes are functionally related to abiotic and biotic stresses. Co-functional analysis of stress-responsive genes revealed a network of 18 genes that code for heat shock proteins, transcription factors (TFs), and calcium-binding proteins. The upregulation of jasmonic acid (JA)-dependent TFs (MYC2, MYB44, ERFs) but not the JA biosynthetic genes is surprising. However, the upregulation of DREB3, an abscisic acid (ABA)-independent TF, validates the unaltered expression of ABA biosynthetic genes. We conclude that ßCC treatment upregulates multiple stress-responsive genes without eliciting JA and ABA biosynthesis.

Detoxification of hostplant's chemical defence rather than its anti-predator co-option drives ß-glucosidase-mediated lepidopteran counteradaptation.

The evolutionary plant-herbivore arms race sometimes gives rise to remarkably unique adaptation strategies. Here we report one such strategy in the lepidopteran herbivore Manduca sexta against its hostplant Nicotiana attenuata's major phytotoxins, 17-hydroxygeranyllinalool diterpene glycoside, lyciumoside IV and its malonylated forms. We show that alkalinity of larval regurgitant non-enzymatically demalonylates the malonylated forms to lyciumoside IV. Lyciumoside IV is then detoxified in the midgut by ß-glucosidase 1-catalysed deglycosylation, which is unusual, as typically the deglycosylation of glycosylated phytochemicals by insects results in the opposite: toxin activation. Suppression of deglucosylation by silencing larval ß-glucosidase 1 by plant-mediated RNAi causes moulting impairments and mortality. In the native habitat of N. attenuata, ß-glucosidase 1 silencing also increases larval unpalatability to native predatory spiders, suggesting that the defensive co-option of lyciumoside IV may be ecologically advantageous. We infer that M. sexta detoxifies this allelochemical to avoid its deleterious effects, rather than co-opting it against predators.

Independently silencing two photosynthetic proteins in Nicotiana attenuata has different effects on herbivore resistance.

Insect attack frequently down-regulates photosynthetic proteins. To understand how this influences the plant-insect interaction, we transformed Nicotiana attenuata to independently silence ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) activase (RCA) and RuBPCase and selected lines whose photosynthetic capacity was similarly reduced. Decreases in plant growth mirrored the decreases in photosynthesis, but the effects on herbivore performance differed. Both generalist (Spodoptera littoralis) and specialist (Manduca sexta) larvae grew larger on RCA-silenced plants, which was consistent with decreased levels of trypsin protease inhibitors and diterpene glycosides and increased levels of RuBPCase, the larvae's main dietary protein. RCA-silenced plants were impaired in their attack-elicited jasmonate (JA)-isoleucine (Ile)/leucine levels, but RuBPCase-silenced plants were not, a deficiency that could not be restored by supplementation with Ile or attributed to lower transcript levels of JAR4/6, the key enzyme for JA-Ile conjugation. From these results, we infer that JA-Ile/leucine signaling and the herbivore resistance traits elicited by JA-Ile are influenced by adenylate charge, or more generally, carbon availability in RCA- but not RuBPCase-silenced plants. Growth of generalist larvae on RuBPCase-silenced plants did not differ from growth on empty vector controls, but the specialist larvae grew faster on RuBPCase-silenced plants, which suggests that the specialist can better tolerate the protein deficiency resulting from RuBPCase silencing than the generalist can. We conclude that the plant-herbivore interaction is more influenced by the particular mechanisms that reduce photosynthetic capacity after herbivore attack than by the magnitude of the decrease, which highlights the value of understanding defense mechanisms in evaluating growth-defense tradeoffs.

Molecular interactions between the specialist herbivore Manduca sexta (Lepidoptera, Sphingidae) and its natural host Nicotiana attenuata. VII. Changes in the plant's proteome.

When Manduca sexta attacks Nicotiana attenuata, fatty acid-amino acid conjugates (FACs) in the larvae's oral secretions (OS) are introduced into feeding wounds. These FACs trigger a transcriptional response that is similar to the response induced by insect damage. Using two-dimensional gel electrophoresis, matrix-assisted laser desorption ionization-time of flight, and liquid chromatography-tandem mass spectrometry, we characterized the proteins in phenolic extracts and in a nuclear fraction of leaves elicited by larval attack, and/or in leaves wounded and treated with OS, FAC-free OS, and synthetic FACs. Phenolic extracts yielded approximately 600 protein spots, many of which were altered by elicitation, whereas nuclear protein fractions yielded approximately 100 spots, most of which were unchanged by elicitation. Reproducible elicitor-induced changes in 90 spots were characterized. In general, proteins that increased were involved in primary metabolism, defense, and transcriptional and translational regulation; those that decreased were involved in photosynthesis. Like the transcriptional defense responses, proteomic changes were strongly elicited by the FACs in OS. A semiquantitative reverse transcription-PCR approach based on peptide sequences was used to compare transcript and protein accumulation patterns for 17 candidate proteins. In six cases the patterns of elicited transcript accumulation were consistent with those of elicited protein accumulation. Functional analysis of one of the identified proteins involved in photosynthesis, RuBPCase activase, was accomplished by virus-induced gene silencing. Plants with decreased levels of RuBPCase activase protein had reduced photosynthetic rates and RuBPCase activity, and less biomass, responses consistent with those of herbivore-attacked plants. We conclude that the response of the plant's proteome to herbivore elicitation is complex, and integrated transcriptome-proteome-metabolome analysis is required to fully understand this ubiquitous ecological interaction.