Identification of defense compounds in Barbarea vulgaris against the herbivore Phyllotreta nemorum by an ecometabolomic approach

Climate-Affected Australian Tropical Montane Cloud Forest Plants: Metabolomic Profiles, Isolated Phytochemicals, and Bioactivities

The Australian Wet Tropics World Heritage Area (WTWHA) in northeast Queensland is home to approximately 18 percent of the nation's total vascular plant species. Over the past century, human activity and industrial development have caused global climate changes, posing a severe and irreversible danger to the entire land-based ecosystem, and the WTWHA is no exception. The current average annual temperature of WTWHA in northeast Queensland is 24 °C. However, in the coming years (by 2030), the average annual temperature increase is estimated to be between 0.5 and 1.4 °C compared to the climate observed between 1986 and 2005. Looking further ahead to 2070, the anticipated temperature rise is projected to be between 1.0 and 3.2 °C, with the exact range depending on future emissions. We identified 84 plant species, endemic to tropical montane cloud forests (TMCF) within the WTWHA, which are already experiencing climate change threats. Some of these plants are used in herbal medicines. This study comprehensively reviewed the metabolomics studies conducted on these 84 plant species until now toward understanding their physiological and metabolomics responses to global climate change. This review also discusses the following: (i) recent developments in plant metabolomics studies that can be applied to study and better understand the interactions of wet tropics plants with climatic stress, (ii) medicinal plants and isolated phytochemicals with structural diversity, and (iii) reported biological activities of crude extracts and isolated compounds.

Recent advances in triterpenoid pathway elucidation and engineering

Triterpenoids are among the most assorted class of specialized metabolites found in all the taxa of living organisms. Triterpenoids are the leading active ingredients sourced from plant species and are utilized in pharmaceutical and cosmetic industries. The triterpenoid precursor 2,3-oxidosqualene, which is biosynthesized via the mevalonate (MVA) pathway is structurally diversified by the oxidosqualene cyclases (OSCs) and other scaffold-decorating enzymes such as cytochrome P450 monooxygenases (P450s), UDP-glycosyltransferases (UGTs) and acyltransferases (ATs). A majority of the bioactive triterpenoids are harvested from the native hosts using the traditional methods of extraction and occasionally semi-synthesized. These methods of supply are time-consuming and do not often align with sustainability goals. Recent advancements in metabolic engineering and synthetic biology have shown prospects for the green routes of triterpenoid pathway reconstruction in heterologous hosts such as Escherichia coli, Saccharomyces cerevisiae and Nicotiana benthamiana, which appear to be quite promising and might lead to the development of alternative source of triterpenoids. The present review describes the biotechnological strategies used to elucidate complex biosynthetic pathways and to understand their regulation and also discusses how the advances in triterpenoid pathway engineering might aid in the scale-up of triterpenoid production in engineered hosts.

Reciprocal mutations of two multifunctional β-amyrin synthases from Barbarea vulgaris shift α/β-amyrin ratios

In the wild cruciferous wintercress (Barbarea vulgaris), β-amyrin-derived saponins are involved in resistance against insect herbivores like the major agricultural pest diamondback moth (Plutella xylostella). Enzymes belonging to the 2,3-oxidosqualene cyclase family have been identified and characterized in B. vulgaris G-type and P-type plants that differ in their natural habitat, insect resistance and saponin content. Both G-type and P-type plants possess highly similar 2,3-oxidosqualene cyclase enzymes that mainly produce β-amyrin (Barbarea vulgaris Lupeol synthase 5 G-Type; BvLUP5-G) or α-amyrin (Barbarea vulgaris Lupeol synthase 5 P-Type; BvLUP5-P), respectively. Despite the difference in product formation, the two BvLUP5 enzymes are 98% identical at the amino acid level. This provides a unique opportunity to investigate determinants of product formation, using the B. vulgaris 2,3-oxidosqualene cyclase enzymes as a model for studying amino acid residues that determine differences in product formation. In this study, we identified two amino acid residues at position 121 and 735 that are responsible for the dominant changes in generated product ratios of β-amyrin and α-amyrin in both BvLUP5 enzymes. These amino acid residues have not previously been highlighted as directly involved in 2,3-oxidosqualene cyclase product specificity. Our results highlight the functional diversity and promiscuity of 2,3-oxidosqualene cyclase enzymes. These enzymes serve as important mediators of metabolic plasticity throughout plant evolution.

Comparative Transcriptome and Phytochemical Analysis Provides Insight into Triterpene Saponin Biosynthesis in Seeds and Flowers of the Tea Plant (Camellia sinensis)

Triterpene saponins exhibit various biological and pharmacological activities. However, the knowledge on saponin biosynthesis in tea plants (Camellia sinensis L.) is still limited. In this work, tea flower and seed samples at different developmental stages and leaves were collected and analyzed with UPLC-PDA-MS and RNA sequencing for saponin determination and transcriptome comparison. The saponin content reached around 19% in the freshly mature seeds and 7% in the green flower buds, and decreased with the fruit ripeness and flower blooming. Almost no saponins were detected in leaf samples. PCA and KEGG analysis suggested that the gene expression pattern and secondary metabolism in TF1 and TS2 vs. leaf samples were significantly different. Weighted gene coexpression network analysis (WGCNA) uncovered two modules related to saponin content. The mevalonate (MVA) instead of 2-C-methyl-d-erythritol-4-phospate (MEP) pathway was responsible for saponin accumulation in tea plants, and 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS), diphosphomevalonate decarboxylase (MVD) and isopentenyl diphosphate isomerase (IDI) may be the key enzymes involved in saponin biosynthesis in tea seeds and flowers. Moreover, ten transcription factors (TFs) were predicted to regulate saponin biosynthesis in the tea plant. Taken together, our study provides a global insight into the saponin biosynthesis and accumulation in the tea plant.

Characterized constituents of insect herbivore oral secretions and their influence on the regulation of plant defenses

For more than 350 million years, there have been ongoing dynamic interactions between plants and insects. In several cases, insects cause-specific feeding damage with ensuing herbivore-associated molecular patterns that invoke characteristic defense responses. During feeding on plant tissue, insects release oral secretions (OSs) containing a repertoire of molecules affecting plant defense (effectors). Some of these OS components might elicit a defense response to combat insect attacks (elicitors), while some might curb the plant defenses (suppressors). Few reports suggest that the synthesis and function of OS components might depend on the host plant and associated microorganisms. We review these intricate plant-insect interactions, during which there is a continuous exchange of molecules between plants and feeding insects along with the associated microorganisms. We further provide a list of commonly identified inducible plant produced defensive molecules released upon insect attack as well as in response to OS treatments of the plants. Thus, we describe how plants specialized and defense-related metabolism is modulated at innumerable phases by OS during plant-insect interactions. A molecular understanding of these complex interactions will provide a means to design eco-friendly crop protection strategies.

European oak chemical diversity - from ecotypes to herbivore resistance

Climate change is increasing insect pressure and forcing plants to adapt. Although chemotypic differentiation and phenotypic plasticity in spatially separated tree populations are known for decades, understanding their importance in herbivory resistance across forests remains challenging. We studied four oak forest stands in Germany using nontarget metabolomics, elemental analysis, and chemometrics and mapped the leaf metabolome of herbivore-resistant (T-) and herbivore-susceptible (S-) European oaks (Quercus robur) to Tortrix viridana, an oak pest that causes severe forest defoliation. Among the detected metabolites, we identified reliable metabolic biomarkers to distinguish S- and T-oak trees. Chemotypic differentiation resulted in metabolic shifts of primary and secondary leaf metabolism. Across forests, T-oaks allocate resources towards constitutive chemical defense enriched of polyphenolic compounds, e.g. the flavonoids kaempferol, kaempferol and quercetin glucosides, while S-oaks towards growth-promoting substances such as carbohydrates and amino-acid derivatives. This extensive work across natural forests shows that oaks' resistance and susceptibility to herbivory are linked to growth-defense trade-offs of leaf metabolism. The discovery of biomarkers and the developed predictive model pave the way to understand Quercus robur's susceptibility to herbivore attack and to support forest management, contributing to the preservation of oak forests in Europe.

Morphological and Chemical Factors Related to Western Flower Thrips Resistance in the Ornamental Gladiolus

Understanding the mechanisms involved in host plant resistance opens the way for improved resistance breeding programs by using the traits involved as markers. Pest management is a major problem in cultivation of ornamentals. Gladiolus (Gladiolus hybridus L.) is an economically important ornamental in the Netherlands. Gladiolus is especially sensitive to attack by western flower thrips (Frankliniella occidentalis (Pergande) (Thysanoptera:Thripidae)). The objective of this study was, therefore, to investigate morphological and chemical markers for resistance breeding to western flower thrips in Gladiolus varieties. We measured thrips damage of 14 Gladiolus varieties in a whole-plant thrips bioassay and related this to morphological traits with a focus on papillae density. Moreover, we studied chemical host plant resistance to using an eco-metabolomic approach comparing the 1H NMR profiles of thrips resistant and susceptible varieties representing a broad range of papillae densities. Thrips damage varied strongly among varieties: the most susceptible variety showed 130 times more damage than the most resistant one. Varieties with low thrips damage had shorter mesophylls and epidermal cells, as well as a higher density of epicuticular papillae. All three traits related to thrips damage were highly correlated with each other. We observed a number of metabolites related to resistance against thrips: two unidentified triterpenoid saponins and the amino acids alanine and threonine. All these compounds were highly correlated amongst each other as well as to the density of papillae. These correlations suggest that papillae are involved in resistance to thrips by producing and/or storing compounds causing thrips resistance. Although it is not possible to distinguish the individual effects of morphological and chemical traits statistically, our results show that papillae density is an easy marker in Gladiolus-breeding programs targeted at increased resistance to thrips.

Comparative TMT Proteomic Analysis Unveils Unique Insights into Helicoverpa armigera (Hübner) Resistance in Cajanus scarabaeoides (L.) Thouars

Pigeonpea [Cajanus cajan (L.) Millspaugh] is an economically important legume playing a crucial role in the semi-arid tropics. Pigeonpea is susceptible to Helicoverpa armigera (Hübner), which causes devastating yield losses. This pest is developing resistance to many commercially available insecticides. Therefore, crop wild relatives of pigeonpea, are being considered as potential sources of genes to expand the genetic base of cultivated pigeonpea to improve traits such as host plant resistance to pests and pathogens. Quantitative proteomic analysis was conducted using the tandem mass tag platform to identify differentially abundant proteins between IBS 3471 and ICPL 87 tolerant accession and susceptible variety to H. armigera, respectively. Leaf proteome were analysed at the vegetative and flowering/podding growth stages. H. armigera tolerance in IBS 3471 appeared to be related to enhanced defence responses, such as changes in secondary metabolite precursors, antioxidants, and the phenylpropanoid pathway. The development of larvae fed on an artificial diet with IBS 3471 lyophilised leaves showed similar inhibition with those fed on an artificial diet with quercetin concentrations with 32 mg/25 g of artificial diet. DAB staining (3,3′-diaminobenzidine) revealed a rapid accumulation of reactive oxygen species in IBS 3471. We conclude that IBS 3471 is an ideal candidate for improving the genetic base of cultivated pigeonpea, including traits for host plant resistance.

Chemical defence in Brassicaceae against pollen beetles revealed by metabolomics and flower bud manipulation approaches

Divergence of chemical plant defence mechanisms within the Brassicaceae can be utilized to identify means against specialized pest insects. Using a bioassay-driven approach, we (a) screened 24 different Brassica napus cultivars, B. napus resyntheses and related brassicaceous species for natural plant resistance against feeding adults of the pollen beetle (Brassicogethes aeneus), (b) tested for gender-specific feeding resistance, (c) analysed the flower bud metabolomes by a non-targeted approach and (d) tested single candidate compounds for their antifeedant activity. (a) In no-choice assays, beetles were allowed to feed on intact plants. Reduced feeding activity was mainly observed on Sinapis alba and Barbarea vulgaris but not on B. napus cultivars. (b) Males fed less and discriminated more in feeding than females. (c) Correlation of the metabolite abundances with the beetles' feeding activity revealed several glucosinolates, phenylpropanoids, flavonoids and saponins as potential antifeedants. (d) These were tested in dual-bud-choice assays developed for medium-throughput compound screening. Application of standard compounds on single oilseed rape flower buds revealed highly deterrent effects of glucobarbarin, oleanolic acid and hederagenin. These results help to understand chemical plant defence in the Brassicaceae and are of key importance for further breeding strategies for insect-resistant oilseed rape cultivars.

High Concentrations of Very Long Chain Leaf Wax Alkanes of Thrips Susceptible Pepper Accessions (Capsicum spp)

The cuticular wax layer can be important for plant resistance to insects. Thrips (Frankliniella occidentalis) damage was assessed on 11 pepper accessions of Capsicum annuum and C. chinense in leaf disc and whole plant assays. Thrips damage differed among the accessions. We analyzed the composition of leaf cuticular waxes of these accessions by GC-MS. The leaf wax composition was different between the two Capsicum species. In C. annuum, 1-octacosanol (C28 alcohol) was the most abundant component, whereas in C. chinense 1-triacotanol (C30 alcohol) was the prominent. Thrips susceptible accessions had significantly higher concentrations of C25-C29 n-alkanes and iso-alkanes compared to relatively resistant pepper accessions. The triterpenoids α- and ß-amyrin tended to be more abundant in resistant accessions. Our study suggests a role for very long chain wax alkanes in thrips susceptibility of pepper.

Diamondback Moth Larvae Trigger Host Plant Volatiles that Lure Its Adult Females for Oviposition

The diamondback moth (DBM) is a destructive pest of crucifer crops. In this study, DBM larvae shown to herbivore induced plant volatiles (HIPVs) that were attractive to adult females exposed in a Y-tube olfactometer. Our results showed that olfactory responses of adult females to HIPVs induced by third instar larvae feeding on Barbarea vulgaris were significantly higher (20.40 ± 1.78; mean moths (%) ± SD) than those induced by first instar larvae (14.80 ± 1.86; mean moths (%) ± SD). Meanwhile, a significant concentration of Sulphur-containing isothiocyanate, 3-methylsulfinylpropyl isothiocyanate, and 4-methylsulfinyl-3-butenyl isothiocyanate were detected in HIPVs released by third instar larvae compared to those released by first instar larvae while feeding on B. vulgaris. When the DBM females were exposed to synthetic chemicals, singly and in blend form, a similar response was observed as to natural HIPVs. Our study demonstrated that the relationship between isothiocyanates acting as plant defense compounds, host plant cues emission and regulation of the DBM adult female behavior due to key volatile triggered by the DBM larvae feeding on B. vulgaris.

Ecometabolomics for a Better Understanding of Plant Responses and Acclimation to Abiotic Factors Linked to Global Change

The number of ecometabolomic studies, which use metabolomic analyses to disentangle organisms' metabolic responses and acclimation to a changing environment, has grown exponentially in recent years. Here, we review the results and conclusions of ecometabolomic studies on the impacts of four main drivers of global change (increasing frequencies of drought episodes, heat stress, increasing atmospheric carbon dioxide (CO2) concentrations and increasing nitrogen (N) loads) on plant metabolism. Ecometabolomic studies of drought effects confirmed findings of previous target studies, in which most changes in metabolism are characterized by increased concentrations of soluble sugars and carbohydrate derivatives and frequently also by elevated concentrations of free amino acids. Secondary metabolites, especially flavonoids and terpenes, also commonly exhibited increased concentrations when drought intensified. Under heat and increasing N loads, soluble amino acids derived from glutamate and glutamine were the most responsive metabolites. Foliar metabolic responses to elevated atmospheric CO2 concentrations were dominated by greater production of monosaccharides and associated synthesis of secondary metabolites, such as terpenes, rather than secondary metabolites synthesized along longer sugar pathways involving N-rich precursor molecules, such as those formed from cyclic amino acids and along the shikimate pathway. We suggest that breeding for crop genotypes tolerant to drought and heat stress should be based on their capacity to increase the concentrations of C-rich compounds more than the concentrations of smaller N-rich molecules, such as amino acids. This could facilitate rapid and efficient stress response by reducing protein catabolism without compromising enzymatic capacity or increasing the requirement for re-transcription and de novo biosynthesis of proteins.

Evolution of Structural Diversity of Triterpenoids

Plants have evolved to produce a blend of specialized metabolites that serve functional roles in plant adaptation. Among them, triterpenoids are one of the largest subclasses of such specialized metabolites, with more than 14,000 known structures. They play a role in plant defense and development and have potential applications within food and pharma. Triterpenoids are cyclized from oxidized squalene precursors by oxidosqualene cyclases, creating more than 100 different cyclical triterpene scaffolds. This limited number of scaffolds is the first step towards creating the vast structural diversity of triterpenoids followed by extensive diversification, in particular, by oxygenation and glycosylation. Gene duplication, divergence, and selection are major forces that drive triterpenoid structural diversification. The triterpenoid biosynthetic genes can be organized in non-homologous gene clusters, such as in Avena spp., Cucurbitaceae and Solanum spp., or scattered along plant chromosomes as in Barbarea vulgaris. Paralogous genes organized as tandem repeats reflect the extended gene duplication activities in the evolutionary history of the triterpenoid saponin pathways, as seen in B. vulgaris. We review and discuss examples of convergent and divergent evolution in triterpenoid biosynthesis, and the apparent mechanisms occurring in plants that drive their increasing structural diversity within and across species. Using B. vulgaris' saponins as examples, we discuss the impact a single structural modification can have on the structure of a triterpenoid and how this affect its biological properties. These examples provide insight into how plants continuously evolve their specialized metabolome, opening the way to study uncharacterized triterpenoid biosynthetic pathways.

Molecular Basis of C-30 Product Regioselectivity of Legume Oxidases Involved in High-Value Triterpenoid Biosynthesis

The triterpenes are structurally diverse group of specialized metabolites with important roles in plant defense and human health. Glycyrrhizin, with a carboxyl group at C-30 of its aglycone moiety, is a valuable triterpene glycoside, the production of which is restricted to legume medicinal plants belonging to the Glycyrrhiza species. Cytochrome P450 monooxygenases (P450s) are important for generating triterpene chemodiversity by catalyzing site-specific oxidation of the triterpene scaffold. CYP72A154 was previously identified from the glycyrrhizin-producing plant Glycyrrhiza uralensis as a C-30 oxidase in glycyrrhizin biosynthesis, but its regioselectivity is rather low. In contrast, CYP72A63 from Medicago truncatula showed superior regioselectivity in C-30 oxidation, improving the production of glycyrrhizin aglycone in engineered yeast. The underlying molecular basis of C-30 product regioselectivity is not well understood. Here, we identified two amino acid residues that control C-30 product regioselectivity and contribute to the chemodiversity of triterpenes accumulated in legumes. Amino acid sequence comparison combined with structural analysis of the protein model identified Leu149 and Leu398 as important amino acid residues for C-30 product regioselectivity. These results were further confirmed by mutagenesis of CYP72A154 homologs from glycyrrhizin-producing species, functional phylogenomics analyses, and comparison of corresponding residues of C-30 oxidase homologs in other legumes. These findings could be combined with metabolic engineering to further enhance the production of high-value triterpene compounds.

Adaptation of flea beetles to Brassicaceae: host plant associations and geographic distribution of Psylliodes Latreille and Phyllotreta Chevrolat (Coleoptera, Chrysomelidae)

The cosmopolitan flea beetle genera Phyllotreta and Psylliodes (Galerucinae, Alticini) are mainly associated with host plants in the family Brassicaceae and include economically important pests of crucifer crops. In this review, the host plant associations and geographical distributions of known species in these genera are summarised from the literature, and their proposed phylogenetic relationships to other Alticini analysed from published molecular phylogenetic studies of Galerucinae. Almost all Phyllotreta species are specialised on Brassicaceae and related plant families in the order Brassicales, whereas Psylliodes species are associated with host plants in approximately 24 different plant families, and 50% are specialised to feed on Brassicaceae. The current knowledge on how Phyllotreta and Psylliodes are adapted to the characteristic chemical defence in Brassicaceae is reviewed. Based on our findings we postulate that Phyllotreta and Psylliodes colonised Brassicaceae independently from each other.

Role of Saponins in Plant Defense Against Specialist Herbivores

The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae) is a very destructive crucifer-specialized pest that has resulted in significant crop losses worldwide. DBM is well attracted to glucosinolates (which act as fingerprints and essential for herbivores in host plant recognition) containing crucifers such as wintercress, Barbarea vulgaris (Brassicaceae) despite poor larval survival on it due to high-to-low concentration of saponins and generally to other plants in the genus Barbarea. B. vulgaris build up resistance against DBM and other herbivorous insects using glucosinulates which are used in plant defense. Aside glucosinolates, Barbarea genus also contains triterpenoid saponins, which are toxic to insects and act as feeding deterrents for plant specialist herbivores (such as DBM). Previous studies have found interesting relationship between the host plant and secondary metabolite contents, which indicate that attraction or resistance to specialist herbivore DBM, is due to higher concentrations of glucosinolates and saponins in younger leaves in contrast to the older leaves of Barbarea genus. As a response to this phenomenon, herbivores as DBM has developed a strategy of defense against these plant biochemicals. Because there is a lack of full knowledge in understanding bioactive molecules (such as saponins) role in plant defense against plant herbivores. Thus, in this review, we discuss the role of secondary plant metabolites in plant defense mechanisms against the specialist herbivores. In the future, trials by plant breeders could aim at transferring these bioactive molecules against herbivore to cash crops.

The cytochrome P450 CYP72A552 is key to production of hederagenin-based saponins that mediate plant defense against herbivores

Plants continuously evolve new defense compounds. One class of such compounds is triterpenoid saponins. A few species in the Barbarea genus produce saponins as the only ones in the large crucifer family. However, the molecular mechanism behind saponin biosynthesis and their role in plant defense remains unclear. We used pathway reconstitution in planta, enzymatic production of saponins in vitro, insect feeding assays, and bioinformatics to identify a missing gene involved in saponin biosynthesis and saponin-based herbivore defense. A tandem repeat of eight CYP72A cytochromes P450 colocalise with a quantitative trait locus (QTL) for saponin accumulation and flea beetle resistance in Barbarea vulgaris. We found that CYP72A552 oxidises oleanolic acid at position C-23 to hederagenin. In vitro-produced hederagenin monoglucosides reduced larval feeding by up to 90% and caused 75% larval mortality of the major crucifer pest diamondback moth and the tobacco hornworm. Sequence analysis indicated that CYP72A552 evolved through gene duplication and has been under strong selection pressure. In conclusion, CYP72A552 has evolved to catalyse the formation of hederagenin-based saponins that mediate plant defense against herbivores. Our study highlights the evolution of chemical novelties by gene duplication and selection for enzyme innovations, and the importance of chemical modification in plant defense evolution.

High-Throughput Screening and Characterization of a High-Density Soybean Mutant Library Elucidate the Biosynthesis Pathway of Triterpenoid Saponins

Triterpenes (C30) constitute one of the diverse class of natural products with potential applications in food, cosmetic and pharmaceutical industries. Soyasaponins are oleanane-type triterpenoids widespread among legumes and particularly abundant in soybean seeds. They have associated with various pharmacological implications and undesirable taste properties of soybean-based food products. Uncovering the biosynthetic genes of soyasaponins will provide new opportunities to control the pathway for human benefits. However, the pathway of soyasaponin biosynthesis has not been fully elucidated in part because of a paucity of natural mutants. Here, we applied a structured high-density soybean mutant library for the forward genetic screening of triterpenoid biosynthesis. The seed soyasaponin polymorphism in the mutant library was evaluated using a high-throughput thin-layer chromatography and liquid chromatography tandem mass spectrometry analysis. This screening identified 35 mutants (3.85% of 909 mutant lines) with seven unusual soyasaponin phenotypes (Categories 1-7), which was greater than the number of natural mutants reported previously (22 mutants, 0.18% of ∼12,428 accessions). Nine unique intermediates of soyasaponin biosynthesis were identified and their chemical structures were estimated based on their MS/MS fragment patterns. Based on published information, 19 mutants could be associated with loss of function of four individual soyasaponin biosynthesis genes identified through expressed sequence tag mining or positional cloning, whereas the remaining 16 mutants were novel and may facilitate discovery of the unknown biosynthetic genes of soyasaponins. Our approach and library may help to identify new phenotype materials and causative genes associated with specialized metabolite production and other traits.

Multi-omics data-driven investigations of metabolic diversity of plant triterpenoids

The vast majority of structurally diverse metabolites play essential roles in mediating the interactions between plant and environment, and constitute a valuable resource for industrial applications. Recent breakthroughs in sequencing technology have greatly accelerated metabolic studies of natural plant products, providing opportunities to investigate the molecular basis underlying the diversity of specialized plant metabolites through large-scale analysis. Here, we focus on the biosynthesis of plant triterpenoids, especially the three diversifying reactions (cyclization, oxidation and glycosylation) that largely contribute to the structural diversity of triterpenoids. Gene mining through large-scale omics data and functional characterization of metabolic genes including enzymes, transcription factors and transporters could provide important insights into the evolution of specialized plant metabolism and pave the way for the production of high-value metabolites or derivatives using synthetic biology approaches.

NMR-based metabolomic profiling of Peganum harmala L. reveals dynamic variations between different growth stages

Xerophytes play an active role in preventing soil denudation and desertification in arid and semi-arid areas. Peganum harmala L. (Zygophyllaceae family), a seasonally growing, poisonous and drought-tolerant plant, is widely distributed in the Xinjiang Uygur Autonomous Region and used as a traditional herbal medicine as well as, in winter, a fodder source. Previous research has focused on the pharmacological activity of isolated compounds and stress responses to growth environments. However, the metabolic profile of P. harmala and variations in its metabolites, including medicinally active and stress resistance components, have not been illustrated during different growth stages. Here, we collected plant samples in May, August, October and December. We determined the metabolic composition of methanol extracts using NMR spectroscopy, and comparisons of four growth stages were accomplished by applying statistical analysis. The results showed that vasicine, choline and sucrose were significantly elevated in samples harvested in May. Significantly higher amounts of betaine, lysine, 4-hydroxyisoleucine and proline were found in samples collected in August than in samples collected in other months, and the concentrations of phosphorylcholine, glucose, acetic acid and vasicinone were highest in December. The relationships between differential biomarkers and plant physiological states affected by diverse growth environmental factors were discussed. Our result deepened the understanding of metabolic mechanisms in plant development and confirmed the advantage of using NMR-based metabolomic treatments in quality evaluation when P. harmala is used for different purposes.

De novo sequencing, assembly and characterisation of Aloe vera transcriptome and analysis of expression profiles of genes related to saponin and anthraquinone metabolism

BACKGROUND

Aloe vera is a perennial, succulent, drought-resistant plant that exhibits many pharmacological characteristics such as wound healing ability against skin burns, anti-ulcer, anti-inflammatory, anti-tumor, anti-viral, anti-hypercholesterolemic, anti-hyperglycemic, anti-asthmatic and much more. Despite great medicinal worth, little genomic information is available on Aloe vera. This study is an initiative to explore the full-scale functional genomics of Aloe vera by generating whole transcriptome sequence database, using Illumina HiSeq technology and its progressive annotation specifically with respect to the metabolic specificity of the plant.

RESULTS

Transcriptome sequencing of root and leaf tissue of Aloe vera was performed using Illumina paired-end sequencing technology. De novo assembly of high quality paired-end reads, resulted into 1,61,733 and 2,21,792 transcripts with mean length of 709 and 714 nucleotides for root and leaf respectively. The non-redundant transcripts were clustered using CD-HIT-EST, yielding a total of 1,13,063 and 1,41,310 unigenes for root and leaf respectively. A total of 6114 and 6527 CDS for root and leaf tissue were enriched into 24 different biological pathway categories using KEGG pathway database. DGE profile prepared by calculating FPKM values was analyzed for differential expression of specific gene encoding enzymes involved in secondary metabolite biosynthesis. Sixteen putative genes related to saponin, lignin, anthraquinone, and carotenoid biosynthesis were selected for quantitative expression by real-time PCR. DGE as well as qRT PCR expression analysis represented up-regulation of secondary metabolic genes in root as compared to leaf. Furthermore maximum number of genes was found to be up-regulated after the induction of methyl jasmonate, which stipulates the association of secondary metabolite synthesis with the plant's defense mechanism during stress. Various transcription factors including bHLH, NAC, MYB were identified by searching predicted CDS against PlantTFdb.

CONCLUSIONS

This is the first transcriptome database of Aloe vera and can be potentially utilized to characterize the genes involved in the biosynthesis of important secondary metabolites, metabolic regulation, signal transduction mechanism, understanding function of a particular gene in the biology and physiology of plant of this species as well as other species of Aloe genus.

Glycoalkaloid composition explains variation in slug resistance in Solanum dulcamara

In natural environments, plants have to deal with a wide range of different herbivores whose communities vary in time and space. It is believed that the chemical diversity within plant species has mainly arisen from selection pressures exerted by herbivores. So far, the effects of chemical diversity on plant resistance have mostly been assessed for arthropod herbivores. However, also gastropods, such as slugs, can cause extensive damage to plants. Here we investigate to what extent individual Solanum dulcamara plants differ in their resistance to slug herbivory and whether this variation can be explained by differences in secondary metabolites. We performed a series of preference assays using the grey field slug (Deroceras reticulatum) and S. dulcamara accessions from eight geographically distinct populations from the Netherlands. Significant and consistent variation in slug preference was found for individual accessions within and among populations. Metabolomic analyses showed that variation in steroidal glycoalkaloids (GAs) correlated with slug preference; accessions with high GA levels were consistently less damaged by slugs. One, strongly preferred, accession with particularly low GA levels contained high levels of structurally related steroidal compounds. These were conjugated with uronic acid instead of the glycoside moieties common for Solanum GAs. Our results illustrate how intraspecific variation in steroidal glycoside profiles affects resistance to slug feeding. This suggests that also slugs should be considered as important drivers in the co-evolution between plants and herbivores.

Current Challenges in Plant Eco-Metabolomics

The relatively new research discipline of Eco-Metabolomics is the application of metabolomics techniques to ecology with the aim to characterise biochemical interactions of organisms across different spatial and temporal scales. Metabolomics is an untargeted biochemical approach to measure many thousands of metabolites in different species, including plants and animals. Changes in metabolite concentrations can provide mechanistic evidence for biochemical processes that are relevant at ecological scales. These include physiological, phenotypic and morphological responses of plants and communities to environmental changes and also interactions with other organisms. Traditionally, research in biochemistry and ecology comes from two different directions and is performed at distinct spatiotemporal scales. Biochemical studies most often focus on intrinsic processes in individuals at physiological and cellular scales. Generally, they take a bottom-up approach scaling up cellular processes from spatiotemporally fine to coarser scales. Ecological studies usually focus on extrinsic processes acting upon organisms at population and community scales and typically study top-down and bottom-up processes in combination. Eco-Metabolomics is a transdisciplinary research discipline that links biochemistry and ecology and connects the distinct spatiotemporal scales. In this review, we focus on approaches to study chemical and biochemical interactions of plants at various ecological levels, mainly plant⁻organismal interactions, and discuss related examples from other domains. We present recent developments and highlight advancements in Eco-Metabolomics over the last decade from various angles. We further address the five key challenges: (1) complex experimental designs and large variation of metabolite profiles; (2) feature extraction; (3) metabolite identification; (4) statistical analyses; and (5) bioinformatics software tools and workflows. The presented solutions to these challenges will advance connecting the distinct spatiotemporal scales and bridging biochemistry and ecology.

A tandem array of UDP-glycosyltransferases from the UGT73C subfamily glycosylate sapogenins, forming a spectrum of mono- and bisdesmosidic saponins

KEY MESSAGE

This study identifies six UGT73Cs all able to glucosylate sapogenins at positions 3 and/or 28 which demonstrates that B. vulgaris has a much richer arsenal of UGTs involved in saponin biosynthesis than initially anticipated. The wild cruciferous plant Barbarea vulgaris is resistant to some insects due to accumulation of two monodesmosidic triterpenoid saponins, oleanolic acid 3-O-β-cellobioside and hederagenin 3-O-β-cellobioside. Insect resistance depends on the structure of the sapogenin aglycone and the glycosylation pattern. The B. vulgaris saponin profile is complex with at least 49 saponin-like metabolites, derived from eight sapogenins and including up to five monosaccharide units. Two B. vulgaris UDP-glycosyltransferases, UGT73C11 and UGT73C13, O-glucosylate sapogenins at positions 3 and 28, forming mainly 3-O-β-D-glucosides. The aim of this study was to identify UGTs responsible for the diverse saponin oligoglycoside moieties observed in B. vulgaris. Twenty UGT genes from the insect resistant genotype were selected and heterologously expressed in Nicotiana benthamiana and/or Escherichia coli. The extracts were screened for their ability to glycosylate sapogenins (oleanolic acid, hederagenin), the hormone 24-epibrassinolide and sapogenin monoglucosides (hederagenin and oleanolic acid 3-O-β-D-glucosides). Six UGTs from the UGT73C subfamily were able to glucosylate both sapogenins and both monoglucosides at positions 3 and/or 28. Some UGTs formed bisdesmosidic saponins efficiently. At least four UGT73C genes were localized in a tandem array with UGT73C11 and possibly UGT73C13. This organization most likely reflects duplication events followed by sub- and neofunctionalization. Indeed, signs of positive selection on several amino acid sites were identified and modelled to be localized on the UGT protein surface. This tandem array is proposed to initiate higher order bisdesmosidic glycosylation of B. vulgaris saponins, leading to the recently discovered saponin structural diversity, however, not directly to known cellobiosidic saponins.

Integration of Omics Approaches toward Understanding Whitefly Transmission of Viruses

Viruses transmitted by whiteflies are predominantly classified as having either persistent circulative or semipersistent transmission, and the majority of studies have addressed transmission of viruses in the genera Begomovirus (family Geminiviridae) and Crinivirus (family Closteroviridae), respectively. Early studies on vector transmission primarily addressed individual aspects of transmission; however, with the breadth of new technology now available, an increasingly greater number of studies involve coordinated research that is beginning to assemble a more complete picture of how whiteflies and viruses have coevolved to facilitate transmission. In particular the integration of gene expression and metabolomic studies into broader research topics is providing knowledge of changes within the whitefly vector in response to the presence of viruses that would have been impossible to identify previously. Examples include comparative studies on the response of Bemisia tabaci to begomovirus and crinivirus infection of common host plants, evolution of whitefly endosymbiont relationships, and opportunities to evaluate responses to specific transmission-related events. Integration of metabolomics, as well as the application of electrical penetration graphing, can lead to an ability to monitor the changes that occur in vector insects associated with specific aspects of virus transmission. Through gaining more complete knowledge of the mechanisms behind whitefly transmission of viruses new control strategies will undoubtedly emerge for control of whiteflies and the viruses they transmit.

Towards eco-friendly crop protection: natural deep eutectic solvents and defensive secondary metabolites

With mounting concerns over health and environmental effects of pesticides, the search for environmentally acceptable substitutes has amplified. Plant secondary metabolites appear in the horizon as an attractive solution for green crop protection. This paper reviews the need for changes in the techniques and compounds that, until recently, have been the mainstay for dealing with pest insects. Here we describe and discuss main strategies for selecting plant-derived metabolites as candidates for sustainable agriculture. The second part surveys ten important insecticidal compounds, with special emphasis on those involved in human health. Many of these insecticidal metabolites, however, are crystalline solids with limited solubility which might potentially hamper commercial formulation. As such, we introduce the concept of natural deep eutectic solvents for enhancing solubility and stability of such compounds. The concept, principles and examples of green pest control discussed here offer a new suite of environmental-friendly tools designed to promote and adopt sustainable agriculture.

Screening for Triterpenoid Saponins in Plants Using Hyphenated Analytical Platforms

Recently the number of studies investigating triterpenoid saponins has drastically increased due to their diverse and potentially attractive biological activities. Currently the literature contains chemical structures of few hundreds of triterpenoid saponins of plant and animal origin. Triterpenoid saponins consist of a triterpene aglycone with one or more sugar moieties attached to it. However, due to similar physico-chemical properties, isolation and identification of a large diversity of triterpenoid saponins remain challenging. This study demonstrates a methodology to screen saponins using hyphenated analytical platforms, GC-MS, LC-MS/MS, and LC-SPE-NMR/MS, in the example of two different phenotypes of the model plant Barbarea vulgaris (winter cress), glabrous (G) and pubescent (P) type that are known to differ by their insect resistance. The proposed methodology allows for detailed comparison of saponin profiles from intact plant extracts as well as saponin aglycone profiles from hydrolysed samples. Continuously measured 1D proton NMR data during LC separation along with mass spectrometry data revealed significant differences, including contents of saponins, types of aglycones and numbers of sugar moieties attached to the aglycone. A total of 49 peaks were tentatively identified as saponins from both plants; they are derived from eight types of aglycones and with 2–5 sugar moieties. Identification of two previously known insect-deterrent saponins, hederagenin cellobioside and oleanolic acid cellobioside, demonstrated the applicability of the methodology for relatively rapid screening of bioactive compounds.

Aromatic Glucosinolate Biosynthesis Pathway in Barbarea vulgaris and its Response to Plutella xylostella Infestation

The inducibility of the glucosinolate resistance mechanism is an energy-saving strategy for plants, but whether induction would still be triggered by glucosinolate-tolerant Plutella xylostella (diamondback moth, DBM) after a plant had evolved a new resistance mechanism (e.g., saponins in Barbara vulgaris) was unknown. In B. vulgaris, aromatic glucosinolates derived from homo-phenylalanine are the dominant glucosinolates, but their biosynthesis pathway was unclear. In this study, we used G-type (pest-resistant) and P-type (pest-susceptible) B. vulgaris to compare glucosinolate levels and the expression profiles of their biosynthesis genes before and after infestation by DBM larvae. Two different stereoisomers of hydroxylated aromatic glucosinolates are dominant in G- and P-type B. vulgaris, respectively, and are induced by DBM. The transcripts of genes in the glucosinolate biosynthesis pathway and their corresponding transcription factors were identified from an Illumina dataset of G- and P-type B. vulgaris. Many genes involved or potentially involved in glucosinolate biosynthesis were induced in both plant types. The expression patterns of six DBM induced genes were validated by quantitative PCR (qPCR), while six long-fragment genes were validated by molecular cloning. The core structure biosynthetic genes showed high sequence similarities between the two genotypes. In contrast, the sequence identity of two apparent side chain modification genes, the SHO gene in the G-type and the RHO in P-type plants, showed only 77.50% identity in coding DNA sequences and 65.48% identity in deduced amino acid sequences. The homology to GS-OH in Arabidopsis, DBM induction of the transcript and a series of qPCR and glucosinolate analyses of G-type, P-type and F1 plants indicated that these genes control the production of S and R isomers of 2-hydroxy-2-phenylethyl glucosinolate. These glucosinolates were significantly induced by P. xylostella larvae in both the susceptiple P-type and the resistant G-type, even though saponins are the main DBM-resistance causing metabolites in G-type plants. Indol-3-ylmethylglucosinolate was induced in the G-type only. These data will aid our understanding of the biosynthesis and induction of aromatic glucosinolates at the molecular level and also increase our knowledge of the complex mechanisms underpinning defense induction in plants.

Identification and genome organization of saponin pathway genes from a wild crucifer, and their use for transient production of saponins in Nicotiana benthamiana

The ability to evolve novel metabolites has been instrumental for the defence of plants against antagonists. A few species in the Barbarea genus are the only crucifers known to produce saponins, some of which make plants resistant to specialist herbivores, like Plutella xylostella, the diamondback moth. Genetic mapping in Barbarea vulgaris revealed that genes for saponin biosynthesis are not clustered but are located in different linkage groups. Using co-location with quantitative trait loci (QTLs) for resistance, transcriptome and genome sequences, we identified two 2,3-oxidosqualene cyclases that form the major triterpenoid backbones. LUP2 mainly produces lupeol, and is preferentially expressed in insect-susceptible B. vulgaris plants, whereas LUP5 produces β-amyrin and α-amyrin, and is preferentially expressed in resistant plants; β-amyrin is the backbone for the resistance-conferring saponins in Barbarea. Two loci for cytochromes P450, predicted to add functional groups to the saponin backbone, were identified: CYP72As co-localized with insect resistance, whereas CYP716As did not. When B. vulgaris sapogenin biosynthesis genes were transiently expressed by CPMV-HT technology in Nicotiana benthamiana, high levels of hydroxylated and carboxylated triterpenoid structures accumulated, including oleanolic acid, which is a precursor of the major resistance-conferring saponins. When the B. vulgaris gene for sapogenin 3-O-glucosylation was co-expressed, the insect deterrent 3-O-oleanolic acid monoglucoside accumulated, as well as triterpene structures with up to six hexoses, demonstrating that N. benthamiana further decorates the monoglucosides. We argue that saponin biosynthesis in the Barbarea genus evolved by a neofunctionalized glucosyl transferase, whereas the difference between resistant and susceptible B. vulgaris chemotypes evolved by different expression of oxidosqualene cyclases (OSCs).

Expanding the nasturlexin family: Nasturlexins C and D and their sulfoxides are phytoalexins of the crucifers Barbarea vulgaris and B. verna

The metabolites produced in leaves of the crucifers winter cress (Barbarea vulgaris) and upland cress (Barbarea verna) abiotically elicited were investigated and their chemical structures were elucidated by analyses of spectroscopic data and confirmed by syntheses. Nasturlexins C and D and their sulfoxides are cruciferous phytoalexins displaying antifungal activity against the crucifer pathogens Alternaria brassicicola, Leptosphaeria maculans and Sclerotinia sclerotiorum. The biosynthesis of these metabolites is proposed based on pathways of cruciferous indolyl phytoalexins. This work indicates that B. vulgaris and B. verna have great potential as sources of defense pathways transferable to agriculturally important crops within the Brassica species.

Glucosinolate hydrolysis products in the crucifer Barbarea vulgaris include a thiazolidine-2-one from a specific phenolic isomer as well as oxazolidine-2-thiones

Two isomeric phenolic glucosinolates, m- and p-hydroxyl derivatives of epiglucobarbarin [(R)-2-hydroxy-2-phenylethylglucosinolate], co-occur in an eastern chemotype (P-type) of the crucifer Barbarea vulgaris along with epiglucobarbarin itself. Levels of the phenolic derivatives in B. vulgaris were low in summer but higher during fall and winter, allowing isolation of all three glucosinolates. Hydrolysis in vitro, catalyzed by Sinapis alba myrosinase at near neutral pH, resulted in expectable oxazolidine-2-thione type hydrolysis products of epiglucobarbarin and its m-hydroxyl derivative. In contrast, a thiazolidine-2-one type product was formed in vitro from p-hydroxy epiglucobarbarin and characterized by UV, IR, MS/MS and 2D NMR. Maceration of leaf material resulted in disappearance of the glucosinolates and formation of the same oxazolidine-2-thione and thiazolidine-2-one products as found in vitro. The detected amounts were comparable to initial amounts of precursor glucosinolates. The corresponding oxazolidine-2-thione type product was also detected quantitatively from glucobarbarin in foliage of a western genotype (G-type). We suggest that p-hydroxy epiglucobarbarin is initially converted into the conventional oxazolidine-2-thione, which would further rearrange to a thiazolidine-2-one due to the activating effect of the p-hydroxyl group. We conclude that a subtle difference between isomeric phenolic glucosinolates results in significantly different natural hydrolysis products.

Multiple hydroxyphenethyl glucosinolate isomers and their tandem mass spectrometric distinction in a geographically structured polymorphism in the crucifer Barbarea vulgaris

Two distinct glucosinolate (GSL) chemotypes (P and G-types) of Barbarea vulgaris (Brassicaceae) were known from southern Scandinavia, but whether the types were consistent in a wider geographic area was not known. Populations (26) from Eastern and Central Europe were analyzed for GSLs in order to investigate whether the two types were consistent in this area. Most (21) could be attributed to one of the previously described GSL profile types, the P-type (13 populations) and the G-type (8 populations), based on differences in the stereochemistry of 2-hydroxylation, presence or absence of phenolic glucobarbarin derivatives, and qualitative differences in indole GSL decoration (tested for a subset of 8+6 populations only). The distinction agreed with previous molecular genetic analysis of the same individuals. Geographically, the P-type typically occurred in Eastern Europe while the G-type mainly occurred in Central Europe. Of the remaining five populations, minor deviations were observed in some individuals from two populations genetically assigned to the G-type, and a hybrid population from Finland contained an additional dihydroxyphenethyl GSL isomer attributed to a combinatorial effect of P-type and G-type genes. Major exceptions to the typical GSL profiles were observed in two populations: (1) A G-type population from Slovenia deviated by a high frequency of a known variant in glucobarbarin biosynthesis ('NAS form') co-occurring with usual G-type individuals. (2) A population from Caucasus exhibited a highly deviating GSL profile dominated by p-hydroxyphenethyl GSL that was insignificant in other accessions, as well as two GSLs investigated by NMR, m-hydroxyphenethylGSL and a partially identified m,p disubstituted hydroxy-methoxy derivative of phenethylGSL. Tandem HPLC-MS of seven NMR-identified desulfoGSLs was carried out and interpreted for increased certainty in peak identification and as a tool for partial structure elucidation. The distinct, geographically separated chemotypes and rare variants are discussed in relation to future taxonomic revision and the genetics and ecology of GSLs in B. vulgaris.

Multidimensional approaches for studying plant defence against insects: from ecology to omics and synthetic biology

The biggest challenge for modern biology is to integrate multidisciplinary approaches towards understanding the organizational and functional complexity of biological systems at different hierarchies, starting from the subcellular molecular mechanisms (microscopic) to the functional interactions of ecological communities (macroscopic). The plant-insect interaction is a good model for this purpose with the availability of an enormous amount of information at the molecular and the ecosystem levels. Changing global climatic conditions are abruptly resetting plant-insect interactions. Integration of discretely located heterogeneous information from the ecosystem to genes and pathways will be an advantage to understand the complexity of plant-insect interactions. This review will present the recent developments in omics-based high-throughput experimental approaches, with particular emphasis on studying plant defence responses against insect attack. The review highlights the importance of using integrative systems approaches to study plant-insect interactions from the macroscopic to the microscopic level. We analyse the current efforts in generating, integrating and modelling multiomics data to understand plant-insect interaction at a systems level. As a future prospect, we highlight the growing interest in utilizing the synthetic biology platform for engineering insect-resistant plants.

Biosynthesis and biological functions of terpenoids in plants

Terpenoids (isoprenoids) represent the largest and most diverse class of chemicals among the myriad compounds produced by plants. Plants employ terpenoid metabolites for a variety of basic functions in growth and development but use the majority of terpenoids for more specialized chemical interactions and protection in the abiotic and biotic environment. Traditionally, plant-based terpenoids have been used by humans in the food, pharmaceutical, and chemical industries, and more recently have been exploited in the development of biofuel products. Genomic resources and emerging tools in synthetic biology facilitate the metabolic engineering of high-value terpenoid products in plants and microbes. Moreover, the ecological importance of terpenoids has gained increased attention to develop strategies for sustainable pest control and abiotic stress protection. Together, these efforts require a continuous growth in knowledge of the complex metabolic and molecular regulatory networks in terpenoid biosynthesis. This chapter gives an overview and highlights recent advances in our understanding of the organization, regulation, and diversification of core and specialized terpenoid metabolic pathways, and addresses the most important functions of volatile and nonvolatile terpenoid specialized metabolites in plants.

The effect of drought stress on the expression of key genes involved in the biosynthesis of triterpenoid saponins in liquorice (Glycyrrhiza glabra)

Glycyrrhiza glabra is an important medicinal plant throughout the world. Glycyrrhizin is a triterpenoid that is among the most important secondary metabolites produced by liquorice. Drought stress is proposed to enhance the levels of secondary metabolites. In this study, the effect of drought stress on the expression of important genes involved in the glycyrrhizin biosynthetic pathway was examined. Drought stress at the seedling stage was applied to 8-day-old plants using polyethylene glycol. Subsequently, the samples were collected 0, 4, 8 or 24 h post-treatment. At the adult plant stage, 10-month-old plants were subjected to drought stress by discontinuing irrigation. Subsequently, samples were collected at 2, 16 and 28 days after drought imposition (S(2d), S(16d) and S(28d), respectively). We performed semi-quantitative RT-PCR assays to evaluate the gene expression levels of sequalene synthase (SQS), β-amyrin synthase (bAS), lupeol synthase (LUS) and cycloartenol synthase (CAS) during stress. Finally, the glycyrrhizin content of stolons was determined via HPLC. The results revealed that due to osmotic stress, the gene expression levels of SQS and bAS were increased, whereas those of CAS were relatively unchanged at the seedling stage. At the adult plant stage, the expression levels of SQS and bAS were increased under drought stress conditions, whereas the gene expression level of CAS remained relatively constant. The glycyrrhizin content in stolons was increased only under severe drought stress conditions (S(28d)). Our results indicate that application of controlled drought stress up-regulates the expression of key genes involved in the biosynthesis of triterpenoid saponins and directly enhances the production of secondary metabolites, including glycyrrhizin, in liquorice plants.

Consequences of combined herbivore feeding and pathogen infection for fitness of Barbarea vulgaris plants

Plants are often attacked by pathogens and insects. Their combined impact on plant performance and fitness depends on complicated three-way interactions and the plant's ability to compensate for resource losses. Here, we evaluate the response of Barbarea vulgaris, a wild crucifer, to combined attack by an oomycete Albugo sp., a plant pathogen causing white rust, and a flea beetle, Phyllotreta nemorum. Plants from two B. vulgaris types that differ in resistance to P. nemorum were exposed to Albugo and P. nemorum alone and in combination and then monitored for pathogen infection, herbivore damage, defence compounds, nutritional quality, biomass and seed production. Albugo developed infections in the insect-resistant plants, whereas insect-susceptible plants were scarcely infected. Concentrations of Albugo DNA were higher in plants also exposed to herbivory; similarly, flea beetle larvae caused more damage on Albugo-infected plants. Concentrations of saponins and glucosinolates strongly increased when the plants were exposed to P. nemorum and when the insect-susceptible plants were exposed to Albugo, and some of these compounds increased even more in the combined treatment. The biomass of young insect-susceptible plants was lower following exposure to flea beetles, and the number of leaves of both plant types was negatively affected by combined exposure. After flowering, however, adult plants produced similar numbers of viable seeds, irrespective of treatment. Our findings support the concept that pathogens and herbivores can affect each other's performance on a host plant and that the plant reacts by inducing specific and general defences. However, plants may be able to compensate for biomass loss from single and combined attacks over time.

Acylated glucosinolates with diverse acyl groups investigated by high resolution mass spectrometry and infrared multiphoton dissociation

With the aim of developing a procedure for detecting and identifying intact acylated glucosinolates (a-GLSs) found in trace quantities in natural plant samples, extracts of Barbarea vulgaris seeds were analyzed by reversed-phase liquid chromatography coupled with electrospray ionization and Fourier-transform ion cyclotron resonance mass spectrometry (RPLC-ESI FTICR MS). After a preliminary optimization of fragmentation conditions, based on a non-acylated parent glucosinolate (glucobarbarin) and three previously identified a-GLSs (the 6'-isoferuloyl esters of glucobarbarin, gluconasturtiin and glucobrassicin), infrared multiphoton dissociation (IRMPD) was employed for a tandem MS-based elucidation of the molecular structures of novel a-GLSs. As a result, three acylated derivatives of glucobarbarin, esterified at the thioglucose moiety with a coumaric acid isomer, sinapic acid or an isomer and a dimethoxycinnamic acid isomer, were identified. In addition, a further acylated glucosinolate was tentatively identified as the isoferuloyl ester of an unidentified hydroxylic derivative of glucobarbarin. This is the first demonstration of diversity in the acyl moieties of thioglucose-acylated glucosinolates, which may reflect the substrate specificity of the endogenous acyl transferase. As expected, 6'-isoferuloyl-glucobarbarin was detected as the main acylated GLS in extracts of B. vulgaris seeds. A quantitative estimate suggested that non-isoferuloyl substituted glucobarbarins correspond to ca. 0.026% of the level of 6'-isoferuloyl glucobarbarin. The formation of an uncommon distonic radical anion, most likely generated in the gas phase upon methyl radical (CH3·) loss from the isoferuloyl anion, is demonstrated.

Metabolic responses of Quercus ilex seedlings to wounding analysed with nuclear magnetic resonance profiling

Plants defend themselves against herbivory at several levels. One of these is the synthesis of inducible chemical defences. Using NMR metabolomic techniques, we studied the metabolic changes of plant leaves after a wounding treatment simulating herbivore attack in the Mediterranean sclerophyllous tree Quercus ilex. First, an increase in glucose content was observed in wounded plants. There was also an increase in the content of C-rich secondary metabolites such as quinic acid and quercitol, both related to the shikimic acid pathway and linked to defence against biotic stress. There was also a shift in N-storing amino acids, from leucine and isoleucine to asparagine and choline. The observed higher content of asparagine is related to the higher content of choline through serine that was proved to be the precursor of choline. Choline is a general anti-herbivore and pathogen deterrent. The study shows the rapid metabolic response of Q. ilex in defending its leaves, based on a rapid increase in the production of quinic acid, quercitol and choline. The results also confirm the suitability of (1)H NMR-based metabolomic profiling studies to detect global metabolome shifts after wounding stress in tree leaves, and therefore its suitability in ecometabolomic studies.

Using plant chemistry and insect preference to study the potential of Barbarea (Brassicaceae) as a dead-end trap crop for diamondback moth (Lepidoptera: Plutellidae)

Barbarea vulgaris R. Br. has been proposed as a dead-end trap crop for diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), because its larvae do not survive on this plant species despite being highly preferred for oviposition. We compared plants of several species, varieties, and types in the genus Barbarea (Brassicaceae) to study their potential as trap crops for P. xylostella. In terms of insect behavior, Barbarea plants were assessed based on the criteria of high oviposition preference by P. xylostella moths (compared to other Barbarea plants and to three Brassica oleracea L. crop varieties) and low survival of P. xylostella larvae. Barbarea plants were also assessed based on the criteria of high content of glucosinolates, which stimulate adult oviposition and larval feeding in P. xylostella, and high content of saponins, which are detrimental to survival of P. xylostella larvae. All Barbarea plants tested were preferred over cabbage by ovipositing P. xylostella. Among Barbarea plants, few significant differences in oviposition preference by P. xylostella were found. Ovipositing P. xylostella preferred B. vulgaris plants containing mainly 2-phenylethylglucosinolate over B. vulgaris plants containing mainly (S)-2-hydroxy-2-phenylethylglucosinolate, and P-type B. vulgaris var. arcuata plants over Barbarea rupicola and B. vulgaris var. variegata plants. Despite containing a lower content of saponins than other Barbarea plants tested, Barbarea verna did not allow survival of P. xylostella larvae. Our studies show that, except for B. rupicola and P-type B. vulgaris var. arcuata, which allowed survival of P. xylostella larvae, all Barbarea plants tested have potential as dead-end trap crops for P. xylostella.

Triterpene biosynthesis in plants

The triterpenes are one of the most numerous and diverse groups of plant natural products. They are complex molecules that are, for the most part, beyond the reach of chemical synthesis. Simple triterpenes are components of surface waxes and specialized membranes and may potentially act as signaling molecules, whereas complex glycosylated triterpenes (saponins) provide protection against pathogens and pests. Simple and conjugated triterpenes have a wide range of applications in the food, health, and industrial biotechnology sectors. Here, we review recent developments in the field of triterpene biosynthesis, give an overview of the genes and enzymes that have been identified to date, and discuss strategies for discovering new triterpene biosynthetic pathways.