Frequent gain and loss of pyrrolizidine alkaloids in the evolution of Senecio section Jacobaea (Asteraceae)

Alkaloids in food: a review of toxicity, analytical methods, occurrence and risk assessments

Alkaloids have been utilized by humans for years. They have diverse applications in pharmaceuticals. They have been proven to be effective in treating a number of diseases. They also form an important part of regular human diets, as they are present in food items, food supplements, diet ingredients and food contaminants. Despite their obvious importance, these alkaloids are toxic to humans. Their toxicity is dependent on a range of factors, such as specific dosage, exposure time and individual properties. Mild toxic effects include nausea, itching and vomiting while chronic effects include paralysis, teratogenicity and death. This review summarizes the published studies on the toxicity, analytical methods, occurrence and risk assessments of six major alkaloid groups that are present in food, namely, ergot, glycoalkaloids, purine, pyrrolizidine, quinolizidine and tropane alkaloids.

The chemical composition of the aerial parts essential oil of Jacobaea delphiniifolia (Asteraceae) growing in Sicily (Italy)

Jacobaea is a small genus of plants belonging to Asteraceae family, formerly joint to Senecio genus, that includes more than 60 species and sub-species. Several taxa of this genus have been extensively studied both for their non-volatile and volatile metabolites. In the present study the chemical composition of the essential oil (EO) from aerial parts of Jacobaea delphiniifolia (Vahl) Pelser & Veldkamp, collected in Sicily, was analyzed by GC-MS. No one report has been previously published on the EO of this species. The result showed the presence of a large quantity of two metabolites, namely 1-undecene (63.57%) and thymol methyl ether (13.65%). Chemotaxonomic considerations were carried out by comparing the other oils of Jacobaea taxa studied so far.

Analysis of the Amine Submetabolome Using Novel Isotope-Coded Pyrylium Salt Derivatization and LC-MS: Herbs and Cancer Tissues as Cases

The amine submetabolome, including amino acids (AAs) and biogenic amines (BAs), is a class of small molecular compounds exhibiting important physiological activities. Here, a new pyrylium salt named 6,7-dimethoxy-3-methyl isochromenylium tetrafluoroborate ([d₀]-DMMIC) with stable isotope-labeled reagents ([d₃]-/[d₆]-DMMIC) was designed and synthesized for amino compounds. [d₀]-/[d₃]-/[d₆]-DMMIC-derivatized had a charged tag and formed a set of molecular ions with an increase of 3.02 m/z and the characteristic fragment ions of m/z 204.1:207.1:210.1. When DMMIC coupled with liquid chromatography-mass spectrometry (LC-MS), a systematic methodology evaluation for quantitation proved to have good linearity (R² between 0.9904 and 0.9998), precision (interday: 2.2-21.9%; intraday: 1.0-19.7%), and accuracy (recovery: 71.8-108.8%) through the test AAs. Finally, the methods based on DMMIC and LC-MS demonstrated the advantaged application by the nontargeted screening of BAs in a common medicinal herb Senecio scandens and an analysis of metabolic differences among the amine submetabolomes between the carcinoma and paracarcinoma tissues of esophageal squamous cell carcinoma (ESCC). A total of 20 BA candidates were discovered in S. scandens as well as the finding of 13 amine metabolites might be the highest-potential differential metabolites in ESCC. The results showed the ability of DMMIC coupled with LC-MS to analyze the amine submetabolome in herbs and clinical tissues.

The Role of Interspecific Hybridisation in Adaptation and Speciation: Insights From Studies in Senecio

Hybridisation is well documented in many species, especially plants. Although hybrid populations might be short-lived and do not evolve into new lineages, hybridisaiton could lead to evolutionary novelty, promoting adaptation and speciation. The genus Senecio (Asteraceae) has been actively used to unravel the role of hybridisation in adaptation and speciation. In this article, we first briefly describe the process of hybridisation and the state of hybridisation research over the years. We then discuss various roles of hybridisation in plant adaptation and speciation illustrated with examples from different Senecio species, but also mention other groups of organisms whenever necessary. In particular, we focus on the genomic and transcriptomic consequences of hybridisation, as well as the ecological and physiological aspects from the hybrids' point of view. Overall, this article aims to showcase the roles of hybridisation in speciation and adaptation, and the research potential of Senecio, which is part of the ecologically and economically important family, Asteraceae.

Metabolic Toxification of 1,2-Unsaturated Pyrrolizidine Alkaloids Causes Human Hepatic Sinusoidal Obstruction Syndrome: The Update

Saturated and unsaturated pyrrolizidine alkaloids (PAs) are present in more than 6000 plant species growing in countries all over the world. They have a typical heterocyclic structure in common, but differ in their potential toxicity, depending on the presence or absence of a double bond between C1 and C2. Fortunately, most plants contain saturated PAs without this double bond and are therefore not toxic for consumption by humans or animals. In a minority of plants, however, PAs with this double bond between C1 and C2 exhibit strong hepatotoxic, genotoxic, cytotoxic, neurotoxic, and tumorigenic potentials. If consumed in error and in large emouns, plants with 1,2-unsaturated PAs induce metabolic breaking-off of the double bonds of the unsaturated PAs, generating PA radicals that may trigger severe liver injury through a process involving microsomal P450 (CYP), with preference of its isoforms CYP 2A6, CYP 3A4, and CYP 3A5. This toxifying CYP-dependent conversion occurs primarily in the endoplasmic reticulum of the hepatocytes equivalent to the microsomal fraction. Toxified PAs injure the protein membranes of hepatocytes, and after passing their plasma membranes, more so the liver sinusoidal endothelial cells (LSECs), leading to life-threatening hepatic sinusoidal obstruction syndrome (HSOS). This injury is easily diagnosed by blood pyrrolizidine protein adducts, which are perfect diagnostic biomarkers, supporting causality evaluation using the updated RUCAM (Roussel Uclaf Causality Assessment Method). HSOS is clinically characterized by weight gain due to fluid accumulation (ascites, pleural effusion, and edema), and may lead to acute liver failure, liver transplantation, or death. In conclusion, plant-derived PAs with a double bond between C1 and C2 are potentially hepatotoxic after metabolic removal of the double bond, and may cause PA-HSOS with a potential lethal outcome, even if PA consumption is stopped.

Identification and Simultaneous Determination of the Main Toxical Pyrrolizidine Alkaloids in a Compound Prescription of Traditional Chinese Medicine: Qianbai Biyan Tablet

Qianbai biyan tablet (QT) is a compound prescription of traditional Chinese medicine which is used to treat nasal congestion, rhinitis, and nasosinusitis, with Senecio scandens as its main plant material. Several pyrrolizidine alkaloids (PAs) were reported in Senecio scandens and others of Senecio species. Although Senecio scandens is assigned as the legal plant material of QT, whether replaced use of it by other Senecio plants can bring toxicity is unknown because of the lack of quantitative data about toxic PAs between different Senecio species. In the present study, adonifoline, senkirkine, and another PA presumed as emiline have been identified in QT; however, there was no senecionine detected in all tablets. PA contents in QTs varied in different companies and different batches. Adonifoline existed only in Senecio scandens, and senecionine was detected in all eight Senecio plants investigated in the present study. Data showed that replaced use of Senecio scandens with a low level of senecionine by other Senecio plants such as Senecio vulgaris containing a high level of senecionine is advertised to be forbidden. Data of the present study may be used as a reference to make new drug quality regularity and recommendation guideline for the safety of QT.

Next-Generation Mass Spectrometry Metabolomics Revives the Functional Analysis of Plant Metabolic Diversity

The remarkable diversity of specialized metabolites produced by plants has inspired several decades of research and nucleated a long list of theories to guide empirical ecological studies. However, analytical constraints and the lack of untargeted processing workflows have long precluded comprehensive metabolite profiling and, consequently, the collection of the critical currencies to test theory predictions for the ecological functions of plant metabolic diversity. Developments in mass spectrometry (MS) metabolomics have revolutionized the large-scale inventory and annotation of chemicals from biospecimens. Hence, the next generation of MS metabolomics propelled by new bioinformatics developments provides a long-awaited framework to revisit metabolism-centered ecological questions, much like the advances in next-generation sequencing of the last two decades impacted all research horizons in genomics. Here, we review advances in plant (computational) metabolomics to foster hypothesis formulation from complex metabolome data. Additionally, we reflect on how next-generation metabolomics could reinvigorate the testing of long-standing theories on plant metabolic diversity.

Development and phylogenetic utilities of a new set of single-/low-copy nuclear genes in Senecioneae (Asteraceae), with new insights into the tribal position and the relationships within subtribe Tussilagininae

The tribe Senecioneae is one of the largest tribes in Asteraceae, with a nearly cosmopolitan distribution. Despite great efforts devoted to elucidate the evolution of Senecioneae, many questions still remain concerning the systematics of this group, from the tribal circumscription and position to species relationships in many genera. The hybridization-based target enrichment method of next-generation sequencing has been accepted as a promising approach to resolve phylogenetic problems. We herein develop a set of single-/low-copy genes for Senecioneae, and test their phylogenetic utilities. Our results demonstrate that these genes work highly efficiently for Senecioneae, with a high average gene recovery of 98.8% across the tribe and recovering robust phylogenetic hypotheses at different levels. In particular, the delimitation of the Senecioneae has been confirmed to include Abrotanella and exclude Doronicum, with the former sister to core Senecioneae and the latter shown to be more closely related to Calenduleae. Moreover, Doronicum and Calenduleae are inferred to be the closest relatives of Senecioneae, which is a new hypothesis well supported by statistical topology tests, morphological evidence, and the profile of pyrrolizidine alkaloids, a special kind of chemical characters generally used to define Senecioneae. Furthermore, this study suggests a complex reticulation history in the diversification of Senecioneae, accounting for the prevalence of polyploid groups in the tribe. With subtribe Tussilagininae s.str. as a case study showing a more evident pattern of gene duplication, we further explored reconstructing the phylogeny in the groups with high ploidy levels. Our results also demonstrate that tree topologies based on sorted paralogous copies are stable across different methods of phylogenetic inference, and more congruent with the morphological evidence and the results of previous phylogenetic studies.

Diversity and evolution of cytochrome P450s of Jacobaea vulgaris and Jacobaea aquatica

BACKGROUND

Collectively, plants produce a huge variety of secondary metabolites (SMs) which are involved in the adaptation of plants to biotic and abiotic stresses. The most characteristic feature of SMs is their striking inter- and intraspecific chemical diversity. Cytochrome P450 monooxygenases (CYPs) often play an important role in the biosynthesis of SMs and thus in the evolution of chemical diversity. Here we studied the diversity and evolution of CYPs of two Jacobaea species which contain a characteristic group of SMs namely the pyrrolizidine alkaloids (PAs).

RESULTS

We retrieved CYPs from RNA-seq data of J. vulgaris and J. aquatica, resulting in 221 and 157 full-length CYP genes, respectively. The analyses of conserved motifs confirmed that Jacobaea CYP proteins share conserved motifs including the heme-binding signature, the PERF motif, the K-helix and the I-helix. KEGG annotation revealed that the CYPs assigned as being SM metabolic pathway genes were all from the CYP71 clan but no CYPs were assigned as being involved in alkaloid pathways. Phylogenetic analyses of full-length CYPs were conducted for the six largest CYP families of Jacobaea (CYP71, CYP76, CYP706, CYP82, CYP93 and CYP72) and were compared with CYPs of two other members of the Asteraceae, Helianthus annuus and Lactuca sativa, and with Arabidopsis thaliana. The phylogenetic trees showed strong lineage specific diversification of CYPs, implying that the evolution of CYPs has been very fast even within the Asteraceae family. Only in the closely related species J. vulgaris and J. aquatica, CYPs were found often in pairs, confirming a close relationship in the evolutionary history.

CONCLUSIONS

This study discovered 378 full-length CYPs in Jacobaea species, which can be used for future exploration of their functions, including possible involvement in PA biosynthesis and PA diversity.

Information theory tests critical predictions of plant defense theory for specialized metabolism

Different plant defense theories have provided important theoretical guidance in explaining patterns in plant specialized metabolism, but their critical predictions remain to be tested. Here, we systematically explored the metabolomes of Nicotiana attenuata, from single plants to populations, as well as of closely related species, using unbiased tandem mass spectrometry (MS/MS) analyses and processed the abundances of compound spectrum-based MS features within an information theory framework to test critical predictions of optimal defense (OD) and moving target (MT) theories. Information components of plant metabolomes were consistent with the OD theory but contradicted the main prediction of the MT theory for herbivory-induced dynamics of metabolome compositions. From micro- to macroevolutionary scales, jasmonate signaling was confirmed as the master determinant of OD, while ethylene signaling provided fine-tuning for herbivore-specific responses annotated via MS/MS molecular networks.

Extracting and Analyzing Pyrrolizidine Alkaloids in Medicinal Plants: A Review

Pyrrolizidine alkaloids (PAs) are distributed in plant families of Asteraceae, Boraginaceae, and Fabaceae and serve in the chemical defense mechanism against herbivores. However, they became a matter of concern due to their toxicity associated with the high risk of intake within herbal preparations, e.g., phytopharmaceutical formulations, medicinal teas, or other plant-derived drug products. In 1992, the German Federal Ministry of Health established the first limits of PA content for fourteen medicinal plants. Because of the toxic effects of PAs, the Federal Institute of Risk Assessment (BfR) established more stringent limits in 2011, whereby a daily intake <0.007 µg/kg body weight was recommended and valid until 2018. A threefold higher limit was then advised by BfR. To address consumer safety, there is the need for more efficient extraction procedures along with robust, selective, and sensitive analytical methods to address these concerns. With the increased prevalence of, e.g., phytopharmaceutical formulations, this timely review comprehensively focuses on the most relevant extraction and analysis strategies for each of those fourteen plant genera. While a variety of extraction procedures has been reported, differences in PA content of up to 1110 ppm (0.11% (w/w)) were obtained dependent on the nature of the solvent and the applied extraction technique. It is evident that the efficient extraction of PAs requires further improvements or at least standardization of the extraction conditions. Comparing the various analytical techniques applied regarding selectivity and sensitivity, LC-MS methods appear most suited. This review shows that both standardized extraction and sensitive determination of PAs is required for achieving appropriate safety levels concerning public health in future.

Pyrrolizidine Alkaloids: Biosynthesis, Biological Activities and Occurrence in Crop Plants

Pyrrolizidine alkaloids (PAs) are heterocyclic secondary metabolites with a typical pyrrolizidine motif predominantly produced by plants as defense chemicals against herbivores. They display a wide structural diversity and occur in a vast number of species with novel structures and occurrences continuously being discovered. These alkaloids exhibit strong hepatotoxic, genotoxic, cytotoxic, tumorigenic, and neurotoxic activities, and thereby pose a serious threat to the health of humans since they are known contaminants of foods including grain, milk, honey, and eggs, as well as plant derived pharmaceuticals and food supplements. Livestock and fodder can be affected due to PA-containing plants on pastures and fields. Despite their importance as toxic contaminants of agricultural products, there is limited knowledge about their biosynthesis. While the intermediates were well defined by feeding experiments, only one enzyme involved in PA biosynthesis has been characterized so far, the homospermidine synthase catalyzing the first committed step in PA biosynthesis. This review gives an overview about structural diversity of PAs, biosynthetic pathways of necine base, and necic acid formation and how PA accumulation is regulated. Furthermore, we discuss their role in plant ecology and their modes of toxicity towards humans and animals. Finally, several examples of PA-producing crop plants are discussed.

Evolution of Increased Photosynthetic Capacity and Its Underlying Traits in Invasive Jacobaea vulgaris

The evolution of increased competitive ability (EICA) hypothesis and the shifting defense hypothesis (SDH) predict that evolutionary changes occur in a suite of traits related to defense and growth in invasive plant species as result of the absence of specialist herbivores. We tested how this suite of traits changed due to the absence of specialist herbivores in multiple invasive regions that differ in climatic conditions with native and invasive Jacobaea vulgaris in a controlled environment. We hypothesized that invasive J. vulgaris in all invasive regions have i) a higher plant growth and underlying traits, such as photosynthetic capacity, ii) lower regrowth-related traits, such as carbohydrate storage, and iii) an increased plant qualitative defense, such as pyrrolizidine alkaloids (PAs). Our results show that invasive J. vulgaris genotypes have evolved a higher photosynthetic rate and total PA concentration but a lower investment in root carbohydrates, which supports the SDH hypothesis. All the traits changed consistently and significantly in the same direction in all four invasive regions, indicative of a parallel evolution. Climatic and soil variables did differ between ranges but explained only a very small part of the variation in trait values. The latter suggests that climate and soil changes were not the main selective forces on these traits.

HPLC-MS detection of pyrrolizidine alkaloids and their N-oxides in herbarium specimens dating back to the 1850s

PREMISE OF THE STUDY

Understanding the phylogenetic distribution of defensive plant secondary metabolites is essential to the macroevolutionary study of chemically mediated plant-animal interactions. The chemical ecology of pyrrolizidine alkaloids (PAs) has been extensively studied in a number of plant-herbivore systems, including Apocynaceae (the milkweed and dogbane family) and Danainae (the milkweed and clearwing butterflies). A systematic survey is necessary to establish a detailed understanding of their occurrence across Apocynaceae. A survey of this species-rich, mainly tropical and subtropical family will rely heavily on small tissue samples removed from herbarium specimens, some of which will be very old and/or preserved with alcohols or mercuric chloride.

METHODS

We optimized PA extraction methods from small leaf fragments of recently collected silica-dried leaves of the PA-positive Echites umbellatus, varying crushing and extraction time. We then applied our optimized method to leaf fragments from 70-167-year-old herbarium specimens of E. umbellatus. To simulate the effect of alcohol treatment on PA detectability in herbarium specimens, we incubated freshly collected leaves of the PA-positive Parsonsia alboflavescens in three different alcohols before drying and compared PA recovery to freshly dried controls. PAs were quantified using high-performance liquid chromatography-mass spectrometry analysis. X-ray fluorescence was used to identify mercury-containing specimens.

RESULTS

Fifteen seconds of leaf crushing followed by 24 h of extraction were optimal for PA free-base and N-oxide recovery. This method yielded ~50-fold greater PA recovery than prior methods. Half of the herbarium specimens (13 of 23), including the oldest, tested positive for PAs; leaf age did not correlate with success in PA extraction. Treatment of fresh leaves with alcohol before drying did not diminish PA recovery; mercury was observed in both PA-positive and PA-negative specimens.

CONCLUSIONS

PAs can be reliably detected in small tissue samples from herbarium specimens up to 167 years old, including specimens that had been treated with alcohol or mercury salts. The variability of PA presence among herbarium specimens of E. umbellatus indicates that multiple specimens will need to be tested before a particular species is determined to lack PAs.

The effect of structurally related metabolites on insect herbivores: A case study on pyrrolizidine alkaloids and western flower thrips

Plant specialised metabolites (SMs) are very diverse in terms of both their number and chemical structures with more than 200,000 estimated compounds. This chemical diversity occurs not only among different groups of compounds but also within the groups themselves. In the context of plant-insect interactions, the chemical diversity within a class of structurally related metabolites is generally also related to their bioactivity. In this study, we tested firstly whether individual SMs within the group of pyrrolizidine alkaloids (PAs) differ in their effects on insect herbivores (western flower thrips, Frankliniella occidentalis). Secondly, we tested combinations of PA N-oxides to determine whether they are more active than their individual components. We also evaluated the bioactivity of six PA free bases and their corresponding N-oxides. At concentrations similar to that in plants, several PAs reduced thrip's survival but the effect also differed strongly among PAs. In general, PA free bases caused a lower survival than their corresponding N-oxides. Among the tested PA free bases, we found jacobine and retrorsine to be the most active against second instar larvae of thrips, followed by erucifoline and seneciphylline, while senecionine and monocrotaline did not exhibit significant dose-dependent effects on thrip's survival. In the case of PA N-oxides, we found that only senecionine N-oxide and jacobine N-oxide reduced thrip's survival, although the effect of senecionine N-oxide was weak. Combinations of PA N-oxides showed no synergistic effects. These findings indicate the differences observed in the effect of structurally related SMs on insect herbivores. It is of limited value to study the bioactivity of combined groups, such as PAs, without taking their composition into account.

Determination of pyrrolizidine alkaloids in selected feed materials with gas chromatography-mass spectrometry

1,2-Dehydropyrrolizidine alkaloids are known to be toxic to many animals and humans. To provide safety of feeds a method based on gas chromatography-mass spectrometry enabling the determination of a content of 1,2-unsaturated PAs in feed materials was developed. After extraction with aqueous solution of HCl and purification of the extract, 1,2-unsaturated alkaloids are reduced to their common backbone structures and subsequently derivatised with heptafluorobutyric anhydride (HFBA). The method was validated according to SANTE/11945/2015. All received parameters are consistent with the document requirements as recovery of a final compound retronecine derivative was from 81.8% to 94.4% when retrosine was used for spiking and from 72.7% to 85.5% when retrorsine N-oxide was spiked. The repeatability was calculated as relative standards deviation and ranged from 7.5% to 14.4%, for N-oxide was from 7.9% to 15.4%. The reproducibility was in the range from 14.2% to 16.3% and from 17.0% to 18.1% for free base and N-oxide respectively. The limit of quantification was determined as 10 µg kg^-1. Good linearity of the method was obtained with coefficient of determination R² > 0.99. The method was applied to 35 silage and two hay samples analysis.

(Un)targeted metabolomics in Asteraceae: probing the applicability of essential-oil profiles of senecio L. (Senecioneae) taxa in chemotaxonomy

The possible applicability of (un)targeted metabolomics (volatile metabolites) for revealing taxonomic/evolutionary relationships among Senecio L. species (Asteraceae; tribe Senecioneae) was explored. Essential-oil compositional data of selected Senecio/Senecioneae/Asteraceae taxa (93 samples in total) were mutually compared by means of multivariate statistical analysis (MVA), i.e., agglomerative hierarchical clustering and principal component analysis. The MVA input data set included the very first compositional data on the essential oil extracted from the aerial parts of S. viscosus L. as well as on four different Serbian populations of S. vernalis Waldst. & Kit. (oils from aerial parts and roots; eight samples in total). This metabolomic screening of Senecio/Senecioneae/Asteraceae species (herein presented results and data from the literature) pointed to short-chain alk-1-enes (e.g., oct-1-ene, non-1-ene, and undec-1-ene), with up to now restricted general occurrence in Plantae, as characteristic chemotaxonomic markers/targets for future metabolomic studies of Senecio/Senecioneae taxa. The MVA additionally showed that the evolution of the terpene metabolism (volatile mono- and sesquiterpenoids) within the Asteraceae tribe Senecioneae was not genera specific. However, the MVA did confirm plant-organ specific production/accumulation of volatiles within S. vernalis and suggested the existence of at least two volatile chemotypes for this species.

Explaining intraspecific diversity in plant secondary metabolites in an ecological context

Plant secondary metabolites (PSMs) are ubiquitous in plants and play many ecological roles. Each compound can vary in presence and/or quantity, and the composition of the mixture of chemicals can vary, such that chemodiversity can be partitioned within and among individuals. Plant ontogeny and environmental and genetic variation are recognized as sources of chemical variation, but recent advances in understanding the molecular basis of variation may allow the future deployment of isogenic mutants to test the specific adaptive function of variation in PSMs. An important consequence of high intraspecific variation is the capacity to evolve rapidly. It is becoming increasingly clear that trait variance linked to both macro- and micro-environmental variation can also evolve and may respond more strongly to selection than mean trait values. This research, which is in its infancy in plants, highlights what could be a missing piece of the picture of PSM evolution. PSM polymorphisms are probably maintained by multiple selective forces acting across many spatial and temporal scales, but convincing examples that recognize the diversity of plant population structures are rare. We describe how diversity can be inherently beneficial for plants and suggest fruitful avenues for future research to untangle the causes and consequences of intraspecific variation.

Fifty years of alkaloid biosynthesis in Phytochemistry

An overview is presented of the studies related to the biosynthesis of alkaloids published in Phytochemistry in the past 50 years.

Analysis of Pyrrolizidine Alkaloids and Evaluation of Some Biological Activities of Algerian Senecio Delphinifolius (Asteraceae)

Although Senecio species are known as sources of potentially toxic pyrrolizidine alkaloids (PAs), some species of this genus are traditionally used as remedies, notably in Algeria. In this paper, the evaluation of biological activities and the analysis of PAs of Algerian specimens of Senecio delphinifolius Vahl are reported. The n-butanolic extract of the herb showed a weak antibacterial effect against Escherichia coli with a MIC of 1 mg/mL, but was inactive against Staphylococcus aureus and Pseudomonas aeruginosa. The n-butanolic extracts of the roots, stems and herb showed a modest antioxidant activity, scavenging the free radical DPPH with respective IC50 values of 55.3, 50.2 and 13.3 μg/mL. A cytotoxic effect against a series of human tumor cell lines was observed with the n-butanolic extract from stems (IC50 ranging between 34 and 88 μg/mL). The herb of the evaluated sample contains 140 ppm of PAs (senecionine, seneciphylline, integerrimine, senkirkine) and PA-related alkaloids (dehydrosenkirkine and neosenkirkine). As the major PAs belong to the toxic series (1,2-unsaturation in the pyrrolizidine cycle and macrocyclic diester), the use of S. delphinifolius should be discouraged in traditional medicine.

Differential tissue distribution of metabolites in Jacobaea vulgaris, Jacobaea aquatica and their crosses

Plants are attacked by many different herbivores. Some will consume whole leaves or roots, while others will attack specific types of tissue. Thus, insight into the metabolite profiles of different types of leaf tissues is necessary to understand plant resistance against herbivores. Jacobaea vulgaris, J. aquatica and three genotypes of their crossings were used to study the variation in metabolomic profiles between epidermis and mesophyll tissues. Extracts of epidermis and mesophyll tissues were obtained using carborundum abrasion (CA). Subsequently, (1)H nuclear magnetic resonance (NMR) spectroscopy and multivariate data analyses were applied to compare the metabolome profiles. Orthogonal partial least-squares-discriminant analysis (OPLS-DA) resulted in a clear separation of epidermis and mesophyll extracts. The epidermis contained significantly higher amounts of jacaranone and phenylpropanoids, specifically chlorogenic (5-O-CQA) and feruloyl quinic (FQA) acids compared to the mesophyll. In contrast, the mesophyll showed significantly higher concentrations of pyrrolizidine alkaloids (PAs), specifically jacobine and jaconine. The tissue specific distribution of these compounds was constant over all genotypes tested. Phenylpropanoids, 5-O-CQA and FQA, as well as PAs are known for their inhibitory effect on herbivores, especially against thrips. Thrips feeding commences with the penetration of the epidermis, followed by ingestion of sub-epidermal or mesophyll. Thrips thus may have to encounter phenylpropanoids in the epidermis as the first line of defence, before encountering the PAs as the ultimate defence in the mesophyll. The finding of tissue specific defense may have a major impact on studies of plant resistance. We cannot judge resistance using analyses of a whole roots, leafs or flowers. In such a whole-organism approach, the levels of potential defense compounds are far below the real ones encountered in tissues involved in the first line of defense. Instead, it is of great importance to study the defence compounds in the specific tissue to which the herbivore is confined.

Specialist versus generalist insect herbivores and plant defense

There has been a long-standing hypothesis that specialist and generalist insects interact with plants in distinct ways. Although many tests exist, they typically compare only one species of each, they sometimes confound specialization and feeding guild, and often do not link chemical or transcriptional measures of the plant to actual resistance. In this review, we synthesize current data on whether specialists and generalists actually differ, with special attention to comparisons of their differential elicitation of plant responses. Although we find few consistencies in plant induction by specialists versus generalists, feeding guilds are predictive of differential plant responses. We outline a novel set of predictions based on current coevolutionary hypotheses and make methodological suggestions for improved comparisons of specialists and generalists.

Pyrrolizidine alkaloid variation in shoots and roots of segregating hybrids between Jacobaea vulgaris and Jacobaea aquatica

Hybridization can lead to novel qualitative or quantitative variation of secondary metabolite (SM) expression that can have ecological and evolutionary consequences. We measured pyrrolizidine alkaloid (PA) expression in the shoots and roots of a family including one Jacobaea vulgaris genotype and one Jacobaea aquatica genotype (parental genotypes), two F(1) hybrid genotypes, and 102 F(2) hybrid genotypes using liquid chromatography-tandem mass spectrometry (LC-MS/MS). We detected 37 PAs in the roots and shoots of J. vulgaris, J. aquatica and the hybrids. PA concentrations and compositions differed between genotypes, and between roots and shoots. Three otosenine-like PAs that only occurred in the shoots of parental genotypes were present in the roots of F(2) hybrids; PA compositions were sometimes novel in F(2) hybrids compared with parental genotypes, and in some cases transgressive PA expression occurred. We also found that PAs from within structural groups covaried both in the roots and in the shoots, and that PA expression was correlated between shoots and roots. Considerable and novel variation present among F(2) hybrids indicates that hybridization has a potential role in the evolution of PA diversity in the genus Jacobaea, and this hybrid system is useful for studying the genetic control of PA expression.

Can plant resistance to specialist herbivores be explained by plant chemistry or resource use strategy?

At both a macro- and micro-evolutionary level, selection of and performance on host plants by specialist herbivores are thought to be governed partially by host plant chemistry. Thus far, there is little evidence to suggest that specialists can detect small structural differences in secondary metabolites of their hosts, or that such differences affect host choice or performance of specialists. We tested whether phytochemical differences between closely related plant species are correlated with specialist host choice. We conducted no-choice feeding trials using 17 plant species of three genera of tribe Senecioneae (Jacobaea, Packera, and Senecio; Asteraceae) and a more distantly related species (Cynoglossum officinale; Boraginaceae) containing pyrrolizidine alkaloids (PAs), and four PA-sequestering specialist herbivores of the genus Longitarsus (Chrysomelidae). We also assessed whether variation in feeding by specialist herbivores is attributable to different resource use strategies of the tested plant species. Plant resource use strategy was quantified by measuring leaf dry matter content, which is related to both plant nutritive value and to plant investment in quantitative defences. We found no evidence that intra-generic differences in PA profiles affect feeding by specialist herbivores. Instead, our results indicate that decisions to begin feeding are related to plant resource use strategy, while decisions to continue feeding are not based on any plant characteristics measured in this study. These findings imply that PA composition does not significantly affect host choice by these specialist herbivores. Leaf dry matter content is somewhat phylogenetically conserved, indicating that plants may have difficulty altering resource use strategy in response to selection pressure by herbivores and other environmental factors on an evolutionary time scale.

The relationship between structurally different pyrrolizidine alkaloids and western flower thrips resistance in F(2) hybrids of Jacobaea vulgaris and Jacobaea aquatica

Segregating plant hybrids often have more ecological and molecular variability compared to parental species, and are therefore useful for studying relationships between different traits, and the adaptive significance of trait variation. Hybrid systems have been used to study the relationship between the expression of plant defense compounds and herbivore susceptibility. We conducted a western flower thrips (WFT) bioassay using a hybrid family and investigated the relationship between WFT resistance and pyrrolizidine alkaloid (PA) variation. The hybrid family consisted of two parental (Jacobaea vulgaris and Jacobaea aquatica) genotypes, two F₁ genotypes, and 94 F₂ hybrid lines. The J. aquatica genotype was more susceptible to thrips attack than the J. vulgaris genotype, the two F₁ hybrids were as susceptible as J. aquatica, and susceptibility to WFT differed among F₂ hybrid lines: 69 F₂ lines were equally susceptible compared to J. aquatica, 10 F₂ lines were more susceptible than J. aquatica and 15 F₂ lines were as resistant as J. vulgaris or were intermediate to the two parental genotypes. Among 37 individual PAs that were derived from four structural groups (senecionine-, jacobine-, erucifoline- and otosenine-like PAs), the N-oxides of jacobine, jaconine, and jacoline were negatively correlated with feeding damage caused by WFT, and the tertiary amines of jacobine, jaconine, jacoline, and other PAs did not relate to feeding damage. Total PA concentration was negatively correlated with feeding damage. Among the four PA groups, only the total concentration of the jacobine-like PAs was negatively correlated with feeding damage. Multiple regression tests suggested that jacobine-like PAs play a greater role in WFT resistance than PAs from other structural groups. We found no evidence for synergistic effects of different PAs on WFT resistance. The relationship between PA variation and WFT feeding damage in the Jacobaea hybrids suggests a role for PAs in resistance to generalist insects.

Evolutionary recruitment of a flavin-dependent monooxygenase for stabilization of sequestered pyrrolizidine alkaloids in arctiids

Pyrrolizidine alkaloids are secondary metabolites that are produced by certain plants as a chemical defense against herbivores. They represent a promising system to study the evolution of pathways in plant secondary metabolism. Recently, a specific gene of this pathway has been shown to have originated by duplication of a gene involved in primary metabolism followed by diversification and optimization for its specific function in the defense machinery of these plants. Furthermore, pyrrolizidine alkaloids are one of the best-studied examples of a plant defense system that has been recruited by several insect lineages for their own chemical defense. In each case, this recruitment requires sophisticated mechanisms of adaptations, e.g., efficient excretion, transport, suppression of toxification, or detoxification. In this review, we briefly summarize detoxification mechanism known for pyrrolizidine alkaloids and focus on pyrrolizidine alkaloid N-oxidation as one of the mechanisms allowing insects to accumulate the sequestered toxins in an inactivated protoxic form. Recent research into the evolution of pyrrolizidine alkaloid N-oxygenases of adapted arctiid moths (Lepidoptera) has shown that this enzyme originated by the duplication of a gene encoding a flavin-dependent monooxygenase of unknown function early in the arctiid lineage. The available data suggest several similarities in the molecular evolution of this adaptation strategy of insects to the mechanisms described previously for the evolution of the respective pathway in plants.

The effect of hybridization on secondary metabolites and herbivore resistance: implications for the evolution of chemical diversity in plants

The diversity of secondary metabolites (SMs) has been poorly understood from both a mechanistic and a functional perspective. Hybridization is suggested to contribute to the evolution of diversity of SMs. In this paper we discuss the effects of hybridization on SMs and herbivore resistance by evaluating the literature and with special reference to our own research results from the hybrids between Jacobaea vulgaris (syn. Senecio jacobaea) and Jacobaea aquatica (syn. Senecio aquaticus). We also review the possible genetic mechanism which causes the variation of SMs and herbivore resistance in hybrids. Most SMs in hybrids are present in the parents as well. But hybrids may miss some parental SMs or have novel SMs. The concentration of parental SMs in hybrids generally is constrained by that in parental plants, but transgressive expression was present in some hybrids. Hybrids may be as susceptible (resistant) as the parents or more susceptible than the parents, but rarely more resistant than the parents. However, different hybrid classes (F1, F2, backcrossing and mixed genotypes) show different patterns in relation to herbivore resistance. The variation in SMs and herbivore resistance occurring in hybrids could be explained by complicated genetic mechanisms rather than a simple one-gene model. Most previous work in this field only reported mean trait values for hybrid classes and few studies focused on genotype differences within hybrid classes. Our study in Jacobaea hybrids showed transgressive segregation in most SMs and herbivore resistance. To summarize, our article shows that hybridization may increase the variation of SMs and affect herbivore resistance, which may partially explain the evolution of chemical diversity in plants.

Degradation of yew, ragwort and rhododendron toxins during composting

Recent concerns have been raised that plants such as ragwort (Senecio jacobaea), yew (Taxus baccata) and rhododendron (Rhododendron ponticum) that are toxic to livestock may be included in compost windrows but may not be fully detoxified by the composting process. This study investigates the decomposition during composting of toxic pyrrolizidine alkaloids present in ragwort, taxines (A and B) present in yew, and grayanotoxins (GTX I, II, and III) present in rhododendron during composting. Plant samples were contained within microporous bags either towards the edge or within the centre of a pilot-scale compost heap. They were destructively harvested at regular intervals over 1200 degrees C cumulative temperature (about three months). Samples were analysed for levels of toxins by liquid chromatography time of flight mass spectrometry (LC-TOF-MS). Pyrrolizidine alkaloids and taxines were shown to degrade completely during the composting process. While GTX I showed significant reductions, concentrations of GTX III remained unchanged after 1200 degrees C cumulative temperature. However, estimates of exposure to grazing livestock coming into contact with source-segregated green waste compost containing up to 7% rhododendron suggest that GTX III poses no appreciable risk.

Species by environment interactions affect pyrrolizidine alkaloid expression in Senecio jacobaea, Senecio aquaticus, and their hybrids

We examined the effects of water and nutrient availability on the expression of the defense pyrrolizidine alkaloids (PAs) in Senecio jacobaea and S. aquaticus. Senecio jacobaea, and S. aquaticus are adapted to different natural habitats, characterized by differing abiotic conditions and different selection pressures from natural enemies. We tested if PA concentration and diversity are plastic over a range of water and nutrient treatments, and also whether such plasticity is dependent on plant species. We also tested the hypothesis that hybridization may contribute to PA diversity within plants, by comparing PA expression in parental species to that in artificially generated F₁ hybrids, and also in later generation natural hybrids between S. jacobaea and S. aquaticus. We showed that total PA concentration in roots and shoots is not dependent on species, but that species determines the pattern of PA diversification. Pyrrolizidine alkaloid diversity and concentration are both dependent on environmental factors. Hybrids produce a putatively novel PA, and this PA is conserved in natural hybrids, that are backcrossed to S. jacobaea. Natural hybrids that are backcrossed several times to S. jacobaea are with regard to PA diversity significantly different from S. jacobaea but not from S. aquaticus, while F₁ hybrids are in all cases more similar to S. jacobaea. These results collectively suggest that PA diversity is under the influence of natural selection.

Phylogenetic ecology of leaf surface traits in the milkweeds (Asclepias spp.): chemistry, ecophysiology, and insect behavior

The leaf surface is the contact point between plants and the environment and plays a crucial role in mediating biotic and abiotic interactions. Here, we took a phylogenetic approach to investigate the function, trade-offs, and evolution of leaf surface traits in the milkweeds (Asclepias). Across 47 species, we found trichome densities of up to 3000 trichomes cm(-2) and epicuticular wax crystals (glaucousness) on 10 species. Glaucous species had a characteristic wax composition dominated by very-long-chain aldehydes. The ancestor of the milkweeds was probably a glaucous species, from which there have been several independent origins of glabrous and pubescent types. Trichomes and wax crystals showed negatively correlated evolution, with both surface types showing an affinity for arid habitats. Pubescent and glaucous milkweeds had a higher maximum photosynthetic rate and lower stomatal density than glabrous species. Pubescent and glaucous leaf surfaces impeded settling behavior of monarch caterpillars and aphids compared with glabrous species, although surface types did not show consistent differentiation in secondary chemistry. We hypothesize that pubescence and glaucousness have evolved as alternative mechanisms with similar functions. The glaucous type, however, appears to be ancestral, lost repeatedly, and never regained; we propose that trichomes are a more evolutionarily titratable strategy.

Loss of pyrrolizidine alkaloids on decomposition of ragwort (Senecio jacobaea) as measured by LC-TOF-MS

The decomposition of toxic pyrrolizidine alkaloids in ragwort (Senecio jacobaea) on storage in waste bags has been evaluated by a new time-of-flight mass spectrometric detection method. The method makes progress in meeting the clear need for modern analytical methods for pyrrolizidine alkaloids and for studies into factors affecting the stability of the toxins in the uprooted plant, which might still be accessible to animals. The experiments demonstrated a rapid decomposition of the toxins in ragwort stored in bags, from 340 mg/kg to less than 40 mg/kg in four weeks and virtually complete loss after 10 weeks. The information obtained can guide effective ragwort removal procedures to safeguard grazing animals.

Simultaneous characterization of pyrrolizidine alkaloids and N-oxides in Gynura segetum by liquid chromatography/ion trap mass spectrometry

Gynura segetum (Lour.) Merr. (Jusanqi) is a traditional herbal product used for hemostasis and detumescence in Chinese folk medicine. However, its hepatic toxicity should not be ignored. In this study, pyrrolizidine alkaloids (PAs) and their corresponding N-oxides (PANOs) were extracted from the whole plant of G. segetum and analyzed by high-performance liquid chromatography coupled to ion trap mass spectrometry (ITMS). Identification of eluted peaks as PANOs was indicated by virtue of their MS and MS(n) analysis, in addition to the [M+H](+) adduct ion, characteristically showed a significant (usually 100% abundance) dimer adduct [2M+H](+) that is not observed in the MS of the parent PAs. A total of 20 compounds were identified or tentatively characterized based on their mass spectra and possible biosynthetic pathways, of which three PAs and one PANO, namely seneciphylline, senecionine, seneciphylline and seneciphyllinine N-oxide, were unequivocally characterized, while other PAs and PANOs were tentatively assigned. Sixteen constituents were reported for the first time from G. segetum and tetrahydrosenecionine has not been previously reported as a natural product. Our results are the first comprehensive analysis of PAs and PANOs in G. segetum constituents and will be helpful for the quality control of the herb of G. segetum and its related preparations.

Phylogenetic trends in phenolic metabolism of milkweeds (Asclepias): evidence for escalation

Although plant-defense theory has long predicted patterns of chemical defense across taxa, we know remarkably little about the evolution of defense, especially in the context of directional phylogenetic trends. Here we contrast the production of phenolics and cardenolides in 35 species of milkweeds (Asclepias and Gomphocarpus). Maximum-likelihood analyses of character evolution revealed three major patterns. First, consistent with the defense-escalation hypothesis, the diversification of the milkweeds was associated with a trend for increasing phenolic production; this pattern was reversed (a declining evolutionary trend) for cardenolides, toxins sequestered by specialist herbivores. Second, phylogenetically independent correlations existed among phenolic classes across species. For example, coumaric acid derivatives showed negatively correlated evolution with caffeic acid derivatives, and this was likely driven by the fact that the former are used as precursors for the latter. In contrast, coumaric acid derivatives were positively correlated with flavonoids, consistent with competition for the precursor p-coumaric acid. Finally, of the phenolic classes, only flavonoids showed correlated evolution (positive) with cardenolides, consistent with a physiological and evolutionary link between the two via malonate. Thus, this study presents a rigorous test of the defense-escalation hypothesis and a novel phylogenetic approach to understanding the long-term persistence of physiological constraints on secondary metabolism.

Pyrrolizidine alkaloids of the endemic Mexican genus Pittocaulon and assignment of stereoisomeric 1,2-saturated necine bases

The endemic Mexican genus Pittocaulon (subtribe Tussilagininae, tribe Senecioneae, Asteraceae) belongs to a monophyletic group of genera distributed in Mexico and North America. The five Pittocaulon species represent shrubs with broom-like succulent branches. All species were found to contain pyrrolizidine alkaloids (PAs). With one exception (i.e., stems of Pittocaulon velatum are devoid of PAs) PAs were found in all plant organs with the highest levels (up to 0.3% of dry weight) in the flower heads. Three structural types of PAs were found: (1) macrocyclic otonecine esters, e.g. senkirkine and acetylpetasitenine; (2) macrocyclic retronecine esters, e.g. senecionine, only found in roots, and (3) monoesters of 1,2-saturated necines with angelic acid. For an unambiguous assignment of the different stereoisomeric 1,2-saturated necine bases a GC-MS method was established that allows the separation and identification of the four stereoisomers as their diacetyl or trimethylsilyl derivatives. All otonecine esters that generally do not form N-oxides and the 1,2-saturated PAs were exclusively found as free bases, while the 1,2-unsaturated 7-angeloylheliotridine occurring in P. velatum was found only as its N-oxide. In a comparative study the 1H and 13C NMR spectra of the four stereoisomeric necine bases were completely assigned by the use of DEPT-135, H,H-COSY, H,C-HSQC and H,H-NOESY experiments and by iterative analysis of the 1H NMR spectra. Based on these methods the PA monoesters occurring in Pittocaulon praecox and P. velatum were assigned as 7-O-angeloyl ester respectively 9-O-angeloyl ester of dihydroxyheliotridane which could be identified for the first time as naturally occurring necine base. Unexpectedly, in the monoesters isolated from the three other Pittocaulon species dihydroxyheliotridane is replaced by the necine base turneforcidine with opposite configuration at C-1 and C-7. The species-specific and organ-typical PA profiles of the five Pittocaulon species are discussed in a biogenetic context.

Tissue distribution, core biosynthesis and diversification of pyrrolizidine alkaloids of the lycopsamine type in three Boraginaceae species

Three species of the Boraginaceae were studied: greenhouse-grown plants of Heliotropium indicum and Agrobacterium rhizogenes transformed roots cultures (hairy roots) of Cynoglossum officinale and Symphytum officinale. The species-specific pyrrolizidine alkaloid (PA) profiles of the three systems were established by GC-MS. All PAs are genuinely present as N-oxides. In H. indicum the tissue-specific PA distribution revealed the presence of PAs in all tissues with the highest levels in the inflorescences which in a flowering plant may account for more than 70% of total plant alkaloid. The sites of PA biosynthesis vary among species. In H. indicum PAs are synthesized in the shoot but not roots whereas they are only made in shoots for C. officinale and in roots of S. officinale. Classical tracer studies with radioactively labelled precursor amines (e.g., putrescine, spermidine and homospermidine) and various necine bases (trachelanthamidine, supinidine, retronecine, heliotridine) and potential ester alkaloid intermediates (e.g., trachelanthamine, supinine) were performed to evaluate the biosynthetic sequences. It was relevant to perform these comparative studies since the key enzyme of the core pathway, homospermidine synthase, evolved independently in the Boraginaceae and, for instance, in the Asteraceae [Reimann, A., Nurhayati, N., Backenkohler, A., Ober, D., 2004. Repeated evolution of the pyrrolizidine alkaloid-mediated defense system in separate angiosperm lineages. Plant Cell 16, 2772-2784.]. These studies showed that the core pathway for the formation of trachelanthamidine from putrescine and spermidine via homospermidine is common to the pathway in Senecio ssp. (Asteraceae). In both pathways homospermidine is further processed by a beta-hydroxyethylhydrazine sensitive diamine oxidase. Further steps of PA biosynthesis starting with trachelanthamidine as common precursor occur in two successive stages. Firstly, the necine bases are structurally modified and either before or after this modification are converted into their O(9)-esters by esterification with one of the stereoisomers of 2,3-dihydroxy-2-isopropylbutyric acid, the unique necic acid of PAs of the lycopsamine type. Secondly, the necine O(9)-esters may be further diversified by O(7)- and/or O(3')-acylation.

Chemical variation within and among six northern willow species

Plant tissues typically contain a diverse complement of secondary metabolites that serve as protection against various biotic and abiotic hazards. Chemical similarities are commonly used to infer phylogenetic relationships among plant taxa, but the studies are typically based on the mean concentration of each compound in each study species, thus overlooking within-species variability. In order to investigate patterns of intra- and interspecific chemical variation in plants, we measured the concentrations of condensed tannins and 36 other phenolic compounds in 120 leaf samples representing six northern Salix species. Multivariate clustering and ordination analyses of the data show that: (1) Despite considerable within-species variation in chemical profiles, intraspecific variability is on average lower than the variation among species. (2) Interspecific similarities are sensitive to the data analysis methods used, and different chemical classes produce partly contradictory results. (3) Compounds within each biosynthetic class tend to behave in a correlated manner and, consequently, overall chemical similarities are weakly correlated with the phylogeny of the studied species. The conclusion is that chemical data are poorly suited for phylogenetic inference, unless methods for data analysis are improved to take into account the biosynthetic routes by which the compounds are produced.