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 β-carotene to β-cyclocitral (βCC). 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. βCC inhibited both DXS transcript accumulation and DXS activity. Molecular models suggested that βCC binds to DXS at the binding site for the thymine pyrophosphate cofactor and blocks catalysis, which was confirmed by direct assays of βCC 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 βCC 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. Significance Plants are known to respond to insect herbivory in multiple ways. Under herbivore attack, plants increase their commitment to defense but decrease their commitment to growth and carbon assimilation. Unfortunately, this second aspect is less studied. Here we report the discovery of an herbivore-triggered signal (β-cyclocitral) that enhances plant resistance and down-regulates the methylerythritol-4-phosphate (MEP) pathway of terpenoid biosynthesis. The MEP pathway supplies the precursors for many metabolites important in photosynthesis such as chlorophylls, carotenoids, plastoquinones, phylloquinones, and tocopherols. We also elucidated the mechanism responsible for the reduction of MEP pathway flux under herbivory and found that β-cyclocitral inhibits the activity of the first step of the pathway, which was also shown to control flux under normal growth conditions.

β-cyclocitral (βCC), a major apocarotenoid of β-carotene, enhances plants’ defense against environmental stresses. However, the knowledge of βCC’s involvement in the complex stress-signaling network is limited. Here we demonstrate how βCC reprograms the transcriptional responses that enable Solanum lycopersicum L. (tomato) plants to endure a plethora of environmental stresses. Comparative transcriptome analysis of control and βCC-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 βCC treatment upregulates multiple stress-responsive genes without eliciting JA and ABA biosynthesis.

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.

β-cyclocitral (βCC) is one of the significant oxidative products of β-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 βCC-mediated changes in a plant’s metabolic network are unknown. Here, we demonstrate how βCC-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 βCC-treatment using liquid chromatography and tandem mass spectrometry identified 57 compounds. A principal coordinate analysis suggested that βCC 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 βCC 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 βCC- and simulated-herbivory-treated plants confirmed βCC’s role against herbivores. We conclude that βCC steers a temporal separation in amino acids and defense metabolite accumulation that optimizes resource allocation to growth and defense.