Distribution of naphthoquinones, plumbagin, droserone, and 5-O-methyl droserone in chitin-induced and uninduced Nepenthes khasiana: molecular events in prey capture

Structural variety and pharmacological potential of naphthylisoquinoline alkaloids

Naphthylisoquinoline alkaloids are a fascinating class of natural biaryl compounds. They show characteristic mono- and dimeric scaffolds, with chiral axes and stereogenic centers. Since the appearance of the last comprehensive overview on these secondary plant metabolites in this series in 1995, the number of discovered representatives has tremendously increased to more than 280 examples known today. Many novel-type compounds have meanwhile been discovered, among them naphthylisoquinoline-related follow-up products like e.g., the first seco-type (i.e., ring-opened) and ring-contracted analogues. As highlighted in this review, the knowledge on the broad structural chemodiversity of naphthylisoquinoline alkaloids has been decisively driven forward by extensive phytochemical studies on the metabolite pattern of Ancistrocladus abbreviatus from Coastal West Africa, which is a particularly "creative" plant. These investigations furnished a considerable number of more than 80-mostly new-natural products from this single species, with promising antiplasmodial activities and with pronounced cytotoxic effects against human leukemia, pancreatic, cervical, and breast cancer cells. Another unique feature of naphthylisoquinoline alkaloids is their unprecedented biosynthetic origin from polyketidic precursors and not, as usual for isoquinoline alkaloids, from aromatic amino acids-a striking example of biosynthetic convergence in nature. Furthermore, remarkable botanical results are presented on the natural producers of naphthylisoquinoline alkaloids, the paleotropical Dioncophyllaceae and Ancistrocladaceae lianas, including first investigations on the chemoecological role of these plant metabolites and their storage and accumulation in particular plant organs.

Do Reactive Oxygen and Nitrogen Species Have a Similar Effect on Digestive Processes in Carnivorous Nepenthes Plants and Humans?

Carnivorous plants attract animals, trap and kill them, and absorb nutrients from the digested bodies. This unusual (for autotrophs) type of nutrient acquisition evolved through the conversion of photosynthetically active leaves into specialised organs commonly called traps. The genus Nepenthes (pitcher plants) consists of approximately 169 species belonging to the group of carnivorous plants. Pitcher plants are characterised by specialised passive traps filled with a digestive fluid. The digestion that occurs inside the traps of carnivorous plants depends on the activities of many enzymes. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) also participate in the digestive process, but their action is poorly recognised. ROS and RNS, named together as RONS, exhibit concentration-dependent bimodal functions (toxic or signalling). They act as antimicrobial agents, participate in protein modification, and are components of signal transduction cascades. In the human stomach, ROS are considered as the cause of different diseases. RNS have multifaceted functions in the gastrointestinal tract, with both positive and negative impacts on digestion. This review describes the documented and potential impacts of RONS on the digestion in pitcher plant traps, which may be considered as an external stomach.

Carnivorous Plants from Nepenthaceae and Droseraceae as a Source of Secondary Metabolites

Carnivorous plants are able to attract small animals or protozoa and retain them in their specialized traps. Later, the captured organisms are killed and digested. The nutrients contained in the prey bodies are absorbed by the plants to use for growth and reproduction. These plants produce many secondary metabolites involved in the carnivorous syndrome. The main purpose of this review was to provide an overview of the secondary metabolites in the family Nepenthaceae and Droseraceae, which were studied using modern identification techniques, i.e., high-performance liquid chromatography or ultra-high-performance liquid chromatography with mass spectrometry and nuclear magnetic resonance spectroscopy. After literature screening, there is no doubt that tissues of species from the genera Nepenthes, Drosera, and Dionaea are rich sources of secondary metabolites that can be used in pharmacy and for medical purposes. The main types of the identified compounds include phenolic acids and their derivatives (gallic, protocatechuic, chlorogenic, ferulic, p-coumaric acids, gallic, hydroxybenzoic, vanillic, syringic caffeic acids, and vanillin), flavonoids (myricetin, quercetin, and kaempferol derivatives), including anthocyanins (delphinidin-3-O-glucoside, cyanidin-3-O-glucoside, and cyanidin), naphthoquinones (e.g., plumbagin, droserone, and 5-O-methyl droserone), and volatile organic compounds. Due to the biological activity of most of these substances, the importance of the carnivorous plant as a pharmaceutical crop will increase.

Carnivorous Nepenthes x ventrata plants use a naphthoquinone as phytoanticipin against herbivory

Carnivorous plants feed on animal prey, mainly insects, to get additional nutrients. This carnivorous syndrome is widely investigated and reported. In contrast, reports on herbivores feeding on carnivorous plants and related defenses of the plants under attack are rare. Here, we studied the interaction of a pitcher plant, Nepenthes x ventrata, with a generalist lepidopteran herbivore, Spodoptera littoralis, using a combination of LC/MS-based chemical analytics, choice and feeding assays. Chemical defenses in N. x ventrata leaves were analyzed upon S. littoralis feeding. A naphthoquinone, plumbagin, was identified in Nepenthes defense against herbivores and as the compound mainly responsible for the finding that S. littoralis larvae gained almost no weight when feeding on Nepenthes leaves. Plumbagin is constitutively present but further 3-fold increased upon long-term (> 1 day) feeding. Moreover, in parallel de novo induced trypsin protease inhibitor (TI) activity was identified. In contrast to TI activity, enhanced plumbagin levels were not phytohormone inducible, not even by defense-related jasmonates although upon herbivory their level increased more than 50-fold in the case of the bioactive jasmonic acid-isoleucine. We conclude that Nepenthes is efficiently protected against insect herbivores by naphthoquinones acting as phytoanticipins, which is supported by additional inducible defenses. The regulation of these defenses remains to be investigated.

Metabolomic analysis reveals reliance on secondary plant metabolites to facilitate carnivory in the Cape sundew, Drosera capensis

BACKGROUND AND AIMS

Secondary metabolites are integral to multiple key plant processes (growth regulation, pollinator attraction and interactions with conspecifics, competitors and symbionts) yet their role in plant adaptation remains an underexplored area of research. Carnivorous plants use secondary metabolites to acquire nutrients from prey, but the extent of the role of secondary metabolites in plant carnivory is not known. We aimed to determine the extent of the role of secondary metabolites in facilitating carnivory of the Cape sundew, Drosera capensis.

METHODS

We conducted metabolomic analysis of 72 plants in a time-series experiment before and after simulated prey capture. We used ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) and the retention time index to identify compounds in the leaf trap tissue that changed up to 72 h following simulated prey capture. We identified associated metabolic pathways, and cross-compared these compounds with metabolites previously known to be involved in carnivorous plants across taxa.

KEY RESULTS

For the first time in a carnivorous plant, we have profiled the whole-leaf metabolome response to prey capture. Reliance on secondary plant metabolites was higher than previously thought - 2383 out of 3257 compounds in fed leaves had statistically significant concentration changes in comparison with unfed controls. Of these, ~34 compounds are also associated with carnivory in other species; 11 are unique to Nepenthales. At least 20 compounds had 10-fold changes in concentration, 12 of which had 30-fold changes and are typically associated with defence or attraction in non-carnivorous plants.

CONCLUSIONS

Secondary plant metabolites are utilized in plant carnivory to an extent greater than previously thought - we found a whole-metabolome response to prey capture. Plant carnivory, at the metabolic level, likely evolved from at least two distinct functions: attraction and defence. Findings of this study support the hypothesis that secondary metabolites play an important role in plant diversification and adaptation to new environments.

UPLC-TOF-MS/MS-Based Metabolomics Analysis Reveals Species-Specific Metabolite Compositions in Pitchers of Nepenthes ampullaria, Nepenthes rafflesiana, and Their Hybrid Nepenthes × hookeriana

Hybridization is key to the evolution and diversity of plants in nature. Nepenthaceae comprises a family of diverse tropical carnivorous pitcher plant species with extensive hybridization. However, there is no study to date on the metabolite expression of hybrids in this family. We performed a non-targeted metabolomics analysis of the pitchers of two Nepenthes species with different dietary habits, namely, the semi-detritivorous N. ampullaria and carnivorous N. rafflesiana with their hybrid (N. × hookeriana) for a comparative study. The whole-pitcher samples were extracted in methanol:chloroform:water (3:1:1) via sonication-assisted extraction and analyzed using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOF-MS) followed by data analysis to profile chemical compositions. A total of 1,441 metabolite features were profiled from the three species in which 43.3% of features in the hybrid samples were not found in either of its parents. The partial least squares discriminant analysis (PLS-DA) found 324 metabolite features with variable in projection (VIP) values greater than one in which 55 features were statistically significant. This showed that the hybrid is closer to N. rafflesiana, which is consistent to the previous study on gene and protein expressions. A total of 105 metabolites were putatively identified with manual searches using public metabolite databases. Phenols were detected to be the most abundant secondary metabolites due to a high flavonoid content, especially in N. rafflesiana. The most abundant feature 476.3s:449.102 was found to be the most significant VIP for distinguishing between the three species as a chemical marker. This is the first study comparing metabolites in the carnivory organs of different Nepenthes species with comprehensive profiling and putative identification. The differential metabolite compositions in the pitchers of different species might have ecological implications with the hybrid showing intermediate phenotype between the parents as well as manifesting unique metabolites. However, there is no clear evidence of metabolites related to the differences in dietary habits between the hybrid and the two parent species.

Plumbagin, a Potent Naphthoquinone from Nepenthes Plants with Growth Inhibiting and Larvicidal Activities

Some plant species are less susceptible to herbivore infestation than others. The reason for this is often unknown in detail but is very likely due to an efficient composition of secondary plant metabolites. Strikingly, carnivorous plants of the genus Nepenthes show extremely less herbivory both in the field and in green house. In order to identify the basis for the efficient defense against herbivorous insects in Nepenthes, we performed bioassays using larvae of the generalist lepidopteran herbivore, Spodoptera littoralis. Larvae fed with different tissues from Nepenthes x ventrata grew significantly less when feeding on a diet containing leaf tissue compared with pitcher-trap tissue. As dominating metabolite in Nepenthes tissues, we identified a naphthoquinone, plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone). When plumbagin was added at different concentrations to the diet of S. littoralis larvae, an EC₅₀ value for larval growth inhibition was determined with 226.5 µg g^-1 diet. To further determine the concentration causing higher larval mortality, sweet potato leaf discs were covered with increasing plumbagin concentrations in no-choice-assays; a higher mortality of the larvae was found beyond 60 µg plumbagin per leaf, corresponding to 750 µg g^-1. Plant-derived insecticides have long been proposed as alternatives for pest management; plumbagin and derivatives might be such promising environmentally friendly candidates.

Metabolomics Analysis Reveals Tissue-Specific Metabolite Compositions in Leaf Blade and Traps of Carnivorous Nepenthes Plants

Nepenthes is a genus of carnivorous plants that evolved a pitfall trap, the pitcher, to catch and digest insect prey to obtain additional nutrients. Each pitcher is part of the whole leaf, together with a leaf blade. These two completely different parts of the same organ were studied separately in a non-targeted metabolomics approach in Nepenthes x ventrata, a robust natural hybrid. The first aim was the analysis and profiling of small (50–1000 m/z) polar and non-polar molecules to find a characteristic metabolite pattern for the particular tissues. Second, the impact of insect feeding on the metabolome of the pitcher and leaf blade was studied. Using UPLC-ESI-qTOF and cheminformatics, about 2000 features (MS/MS events) were detected in the two tissues. They showed a huge chemical diversity, harboring classes of chemical substances that significantly discriminate these tissues. Among the common constituents of N. x ventrata are phenolics, flavonoids and naphthoquinones, namely plumbagin, a characteristic compound for carnivorous Nepenthales, and many yet-unknown compounds. Upon insect feeding, only in pitchers in the polar compounds fraction, small but significant differences could be detected. By further integrating information with cheminformatics approaches, we provide and discuss evidence that the metabolite composition of the tissues can point to their function.

Transcriptome-wide shift from photosynthesis and energy metabolism upon endogenous fluid protein depletion in young Nepenthes ampullaria pitchers

Carnivorous pitcher plants produce specialised pitcher organs containing secretory glands, which secrete acidic fluids with hydrolytic enzymes for prey digestion and nutrient absorption. The content of pitcher fluids has been the focus of many fluid protein profiling studies. These studies suggest an evolutionary convergence of a conserved group of similar enzymes in diverse families of pitcher plants. A recent study showed that endogenous proteins were replenished in the pitcher fluid, which indicates a feedback mechanism in protein secretion. This poses an interesting question on the physiological effect of plant protein loss. However, there is no study to date that describes the pitcher response to endogenous protein depletion. To address this gap of knowledge, we previously performed a comparative RNA-sequencing experiment of newly opened pitchers (D0) against pitchers after 3 days of opening (D3C) and pitchers with filtered endogenous proteins (>10 kDa) upon pitcher opening (D3L). Nepenthes ampullaria was chosen as a model study species due to their abundance and unique feeding behaviour on leaf litters. The analysis of unigenes with top 1% abundance found protein translation and stress response to be overrepresented in D0, compared to cell wall related, transport, and signalling for D3L. Differentially expressed gene (DEG) analysis identified DEGs with functional enrichment in protein regulation, secondary metabolism, intracellular trafficking, secretion, and vesicular transport. The transcriptomic landscape of the pitcher dramatically shifted towards intracellular transport and defence response at the expense of energy metabolism and photosynthesis upon endogenous protein depletion. This is supported by secretome, transportome, and transcription factor analysis with RT-qPCR validation based on independent samples. This study provides the first glimpse into the molecular responses of pitchers to protein loss with implications to future cost/benefit analysis of carnivorous pitcher plant energetics and resource allocation for adaptation in stochastic environments.

Tropical pitcher plants (Nepenthes) act as ecological filters by altering properties of their fluid microenvironments

Characteristics of host species can alter how other, interacting species assemble into communities by acting as ecological filters. Pitchers of tropical pitcher plants (Nepenthes) host diverse communities of aquatic arthropods and microbes in nature. This plant genus exhibits considerable interspecific diversity in morphology and physiology; for example, different species can actively control the pH of their pitcher fluids and some species produce viscoelastic fluids. Our study investigated the extent to which Nepenthes species differentially regulate pitcher fluid traits under common garden conditions, and the effects that these trait differences had on their associated communities. Sixteen species of Nepenthes were reared together in the controlled environment of a glasshouse using commonly-sourced pH 6.5 water. We analyzed their bacterial and eukaryotic communities using metabarcoding techniques, and found that different plant species differentially altered fluid pH, viscosity, and color, and these had strong effects on the community structure of their microbiota. Nepenthes species can therefore act as ecological filters, cultivating distinctive microbial communities despite similar external conditions, and blurring the conceptual line between biotic and abiotic filters.

The function of secondary metabolites in plant carnivory

BACKGROUND

Carnivorous plants are an ideal model system for evaluating the role of secondary metabolites in plant ecology and evolution. Carnivory is a striking example of convergent evolution to attract, capture and digest prey for nutrients to enhance growth and reproduction and has evolved independently at least ten times. Though the roles of many traits in plant carnivory have been well studied, the role of secondary metabolites in the carnivorous habit is considerably less understood.

SCOPE

This review provides the first synthesis of research in which secondary plant metabolites have been demonstrated to have a functional role in plant carnivory. From these studies we identify key metabolites for plant carnivory and their functional role, and highlight biochemical similarities across taxa. From this synthesis we provide new research directions for integrating secondary metabolites into understanding of the ecology and evolution of plant carnivory.

CONCLUSIONS

Carnivorous plants use secondary metabolites to facilitate prey attraction, capture, digestion and assimilation. We found ~170 metabolites for which a functional role in carnivory has been demonstrated. Of these, 26 compounds are present across genera that independently evolved a carnivorous habit, suggesting convergent evolution. Some secondary metabolites have been co-opted from other processes, such as defence or pollinator attraction. Secondary metabolites in carnivorous plants provide a potentially powerful model system for exploring the role of metabolites in plant evolution. They also show promise for elucidating how the generation of novel compounds, as well as the co-option of pre-existing metabolites, provides a strategy for plants to occupy different environments.

Enhanced plumbagin production in Plumbago indica root culture by simultaneous and sequential dual elicitations using chitosan with ʟ-alanine and methyl-β-cyclodextrin

The simultaneous and sequential dual elicitation effect on plumbagin production in Plumbago indica L. root cultures, revealed that combination of chitosan (150 mg L−1) with ʟ-alanine (5 mM) or methyl-β-cyclodextrin (MCD; 2 mM) significantly increased plumbagin production, but in the different treatment manners. The simultaneous treatment using chitosan + ʟ-alanine on a 14-day-old culture enhanced plumbagin production to 14.62 mg g−1 DW, while the sequential additions of MCD to a 12-day-old culture followed by chitosan after 48 h enhanced production of plumbagin to 14.33 mg g−1 DW. The plumbagin productivity from both treatments were up to 1.3- and 8-fold higher than the chitosan treated (10.93 mg g−1 DW) and untreated root cultures (1.76 mg g−1 DW). Moreover, the present studies provided new information on the effect of simultaneous and sequential elicitation on plumbagin-producing P. indica root cultures using chitosan in combinations with MCD or ʟ-alanine.

The scent gland chemistry of neogoveid cyphophthalmids (Opiliones): an unusual methyljuglone from Metasiro savannahensis

While the chemistries of scent gland secretions from a few selected species of three families of Cyphophthalmi, namely Sironidae, Pettalidae, and Stylocellidae, have already been reported and found to consist of complex blends of naphthoquinones and methyl ketones, nothing is known about the other families. We here report on the secretions of Metasiro savannahensis Clouse and Wheeler (Zootaxa 3814:177–201, 2014), a first representative of the family Neogoveidae. The secretions from males, females and one juvenile were extracted and analyzed by gas chromatography–mass spectrometry. Twenty-five compounds were identified, all of which belong to the chemical classes of naphthoquinones and methyl ketones, confirming a hypothesized chemical uniformity of cyphophthalmid exudates. One major naphthoquinone compound, however, was new for cyphophthalmids and for arthropod exocrine secretions in general: a methyljuglone isomer, 6-methyljuglone (= 6-MJ; iupac name: 5-hydroxy-6-methyl-1,4-naphthoquinone), amounted for about 20% of the secretion and was eventually identified by synthesis. Hydroxy-naphthoquinones and their derivatives are known to possess a variety of antibiotic effects, probably enhancing the antimicrobial/antifungal potential of the Metasiro-secretion. Currently, without further data on neogoveids, the compound represents a chemical autapomorphy of M. savannahensis, and—just as the strange chloro-naphthoquinones of Sironidae and Pettalidae—adds to the repertoire of unusual naphthoquinone compounds across the Cyphophthalmi.

Specialized naphthoquinones present in Impatiens glandulifera nectaries inhibit the growth of fungal nectar microbes

The invasion success of Impatiens glandulifera (Himalayan balsam) in certain parts of Europe and North America has been partially attributed to its ability to compete for bee pollinators with its rich nectar and due to its capacity to produce and release allelopathic 1,4-naphthoquinones (1,4-NQs) from its roots and leaves. Given that other 1,4-NQs present in the digestive fluids of certain carnivorous plants are proposed to control microbial colonization, we investigated the potential for the 1,4-NQs, 2-methoxy-1,4-naphthoquinone (2-MNQ) and lawsone, to fulfill an analogous role in the nectaries of I. glandulifera. Both 2-MNQ and lawsone were detected in the floral nectaries of I. glandulifera at levels comparable to leaves and roots, but were discovered to be at significantly higher levels in its extra-floral nectaries (EFNs) and to be present in EFN nectar itself. Nectar microbe inhibition assays revealed that the common nectar bacteria Gluconobacter oxydans and Asaia prunellae are not inhibited by 2-MNQ or lawsone, although both compounds were found to inhibit the growth of the common fungal nectar microbes Metschnikowia reukaufii and Aureobasidium pullulans. Taken together, these findings suggest that 2-MNQ and lawsone could serve to protect the rich nectar of I. glandulifera against fungal growth. The high abundance of 2-MNQ and lawsone in I. glandulifera EFNs may also point to an unsuspected mechanism for how allelopathic 1,4-NQs are leached into the soil where they exhibit their known allelopathic effects.

Plumbagin from a tropical pitcher plant (Nepenthes alata Blanco) induces apoptotic cell death via a p53-dependent pathway in MCF-7 human breast cancer cells

Plumbagin (5-hydroxy-2-methyl-1,4-naphthaquinone) has displayed antitumor activity in vitro and in animal models; however, the underlying molecular mechanisms have not been fully explored. The aim of this study was to investigate the anticancer effects of plumbagin isolated from Nepenthes alata against MCF-7 breast cancer cells. We examined the cytotoxicity, cell cycle regulation, apoptotic cell death, and generation of intracellular reactive oxygen species (ROS) in MCF-7 cells. Plumbagin exhibited potent cytotoxicity in MCF-7 cells (wild-type p53) compared to that in SK-OV-3 (null-type) human epithelial ovarian cancer cells. Specifically, plumbagin upregulated the expression of p21^CIP1/WAF1 in MCF-7 cells, causing cell cycle arrest in the G2/M phase through inhibition of cyclin B1 levels. Plumbagin also significantly increased the ratio of Bax/Bcl-2 and release of cytochrome c, resulting in apoptotic cell death in MCF-7 cells. Furthermore, plumbagin dramatically increased the intracellular ROS level, whereas pretreatment with the ROS scavenger N-acetyl cysteine protected against plumbagin-induced cytotoxicity, suggesting that ROS formation plays a pivotal role in antitumor activity in MCF-7 cells. In mice bearing MCF-7 cell xenografts, plumbagin significantly reduced tumor growth and weight without apparent side effects. We therefore concluded that plumbagin exerts anticancer activity against MCF-7 cells through the generation of intracellular ROS, resulting in the induction of apoptosis via a p53-dependent pathway. This study thus identifies a new anticancer mechanism of plumbagin against p53-dependent breast cancer cells and suggests a novel strategy for overcoming of breast cancer therapy.

Nepenthes: State of the art of an inspiring plant for biotechnologists

Plant carnivory results from the adaptation of plants to their environment. The capture and digestion of preys, followed by their assimilation by the plant is a source of additional nutrients to overcome scarce nutrient in poor soils. Nepenthes are highly studied carnivorous plants and have developed a number of ecological traits which have attracted the attention of plant biologists. Multiple adaptive strategies developed by these plants make them a source of inspiration for many applications ranging from therapeutic treatments to biocontrol solution in agriculture. The outstanding tissue organization of the digestive pitcher can help to create new and original materials usable in everyday life. In this review article, we propose a state of the art of the latest studies carried out on these particular plants and we establish a list of potential tracks for their exploitation.

Nepenthes pitchers are CO2-enriched cavities, emit CO2 to attract preys

Carnivorous plants of the genus Nepenthes supplement their nutrient deficiency by capturing arthropods or by mutualistic interactions, through their leaf-evolved biological traps (pitchers). Though there are numerous studies on these traps, mostly on their prey capture mechanisms, the gas composition inside them remains unknown. Here we show that, Nepenthes unopened pitchers are CO2-enriched 'cavities', when open they emit CO2, and the CO2 gradient around open pitchers acts as a cue attracting preys towards them. CO2 contents in near mature, unopened Nepenthes pitchers were in the range 2500-5000 ppm. Gas collected from inside open N. khasiana pitchers showed CO2 at 476.75 ± 59.83 ppm. CO2-enriched air-streaming through N. khasiana pitchers (at 619.83 ± 4.53 ppm) attracted (captured) substantially higher number of aerial preys compared to air-streamed pitchers (CO2 at 412.76 ± 4.51 ppm). High levels of CO2 dissolved in acidic Nepenthes pitcher fluids were also detected. We demonstrate respiration as the source of elevated CO2 within Nepenthes pitchers. Most unique features of Nepenthes pitchers, viz., high growth rate, enhanced carbohydrate levels, declined protein levels, low photosynthetic capacity, high respiration rate and evolved stomata, are influenced by the CO2-enriched environment within them.

Dipteran larvae and microbes facilitate nutrient sequestration in the Nepenthes gracilis pitcher plant host

The fluid-containing traps of Nepenthes carnivorous pitcher plants (Nepenthaceae) are often inhabited by organisms known as inquilines. Dipteran larvae are key components of such communities and are thought to facilitate pitcher nitrogen sequestration by converting prey protein into inorganic nitrogen, although this has never been demonstrated in Nepenthes Pitcher fluids are also inhabited by microbes, although the relationship(s) between these and the plant is still unclear. In this study, we examined the hypothesis of digestive mutualism between N. gracilis pitchers and both dipteran larvae and fluid microbes. Using dipteran larvae, prey and fluid volumes mimicking in situ pitcher conditions, we conducted in vitro experiments and measured changes in available fluid nitrogen in response to dipteran larvae and microbe presence. We showed that the presence of dipteran larvae resulted in significantly higher and faster releases of ammonium and soluble protein into fluids in artificial pitchers, and that the presence of fluid microbes did likewise for ammonium. We showed also that niche segregation occurs between phorid and culicid larvae, with the former fragmenting prey carcasses and the latter suppressing fluid microbe levels. These results clarify the relationships between several key pitcher-dwelling organisms, and show that pitcher communities facilitate nutrient sequestration in their host.

Slow food: insect prey and chitin induce phytohormone accumulation and gene expression in carnivorous Nepenthes plants

BACKGROUND AND AIMS

Carnivorous Nepenthes plants use modified leaves forming pitfall traps to capture and digest prey, mainly insects, for additional nutrient supply. These traps, so called pitchers, contain a plant-derived fluid composed of many hydrolytic enzymes and defence-related proteins. In this study, the prey-induced induction of corresponding genes of those proteins and a role for phytohormones in this process was analysed.

METHODS

Tissue from insect prey-fed, chitin- and phytohormone-challenged pitchers was harvested and analysed for selected gene expressions by a quantitative PCR technique. Phytohormone levels were determined by LC-MS/MS. Nepenthesin proteolytic activities were measured in the digestive fluid using a fluorescence substrate.

KEY RESULTS

Insect prey in the pitchers induced the accumulation of phytohormones such as jasmonates as well as the transcription of studied genes encoding a chitinase 3 and a protease (nepenthesin I), whereas a defence-related protein (PR-1) gene was not induced. Treatment with chitin as a component of the insects' exoskeleton triggered the accumulation of jasmonates, the expression of nepenthesin I and chitinase 3 genes similar to jasmonic acid treatment, and induced protease activity in the fluid. All detectable responses were slowly induced.

CONCLUSIONS

The results suggest that upon insect prey catch a sequence of signals is initiated: (1) insect-derived chitin, (2) jasmonate as endogenous phytohormone signal, (3) the induction of digestive gene expression and (4) protein expression. This resembles a similar hierarchy of events as described from plant pathogen/herbivore interactions, supporting the idea that carnivory evolved from plant defences.

Bacterial Diversity and Community Structure in Two Bornean Nepenthes Species with Differences in Nitrogen Acquisition Strategies

Carnivorous plants of the genus Nepenthes have been studied for over a century, but surprisingly little is known about associations with microorganisms. The two species Nepenthes rafflesiana and Nepenthes hemsleyana differ in their pitcher-mediated nutrient sources, sequestering nitrogen from arthropod prey and arthropods as well as bat faeces, respectively. We expected bacterial communities living in the pitchers to resemble this diet difference. Samples were taken from different parts of the pitchers (leaf, peristome, inside, outside, digestive fluid) of both species. Bacterial communities were determined using culture-independent high-throughput amplicon sequencing. Bacterial richness and community structure were similar in leaves, peristomes, inside and outside walls of both plant species. Regarding digestive fluids, bacterial richness was higher in N. hemsleyana than in N. rafflesiana. Additionally, digestive fluid communities were highly variable in structure, with strain-specific differences in community composition between replicates. Acidophilic taxa were mostly of low abundance, except the genus Acidocella, which strikingly reached extremely high levels in two N. rafflesiana fluids. In N. hemsleyana fluid, some taxa classified as vertebrate gut symbionts as well as saprophytes were enriched compared to N. rafflesiana, with saprophytes constituting potential competitors for nutrients. The high variation in community structure might be caused by a number of biotic and abiotic factors. Nitrogen-fixing bacteria were present in both study species, which might provide essential nutrients to the plant at times of low prey capture and/or rare encounters with bats.

Biosynthesis and molecular actions of specialized 1,4-naphthoquinone natural products produced by horticultural plants

The 1,4-naphthoquinones (1,4-NQs) are a diverse group of natural products found in every kingdom of life. Plants, including many horticultural species, collectively synthesize hundreds of specialized 1,4-NQs with ecological roles in plant-plant (allelopathy), plant-insect and plant-microbe interactions. Numerous horticultural plants producing 1,4-NQs have also served as sources of traditional medicines for hundreds of years. As a result, horticultural species have been at the forefront of many basic studies conducted to understand the metabolism and function of specialized plant 1,4-NQs. Several 1,4-NQ natural products derived from horticultural plants have also emerged as promising scaffolds for developing new drugs. In this review, the current understanding of the core metabolic pathways leading to plant 1,4-NQs is provided with additional emphasis on downstream natural products originating from horticultural species. An overview on the biochemical mechanisms of action, both from an ecological and pharmacological perspective, of 1,4-NQs derived from horticultural plants is also provided. In addition, future directions for improving basic knowledge about plant 1,4-NQ metabolism are discussed.

Fluid physico-chemical properties influence capture and diet in Nepenthes pitcher plants

BACKGROUND AND AIMS

Nepenthes pitcher plants have evolved modified leaves with slippery surfaces and enzymatic fluids that trap and digest prey, faeces and/or plant detritus. Although the fluid's contribution to insect capture is recognized, the physico-chemical properties involved remain underexplored and may vary among species, influencing their diet type. This study investigates the contributions of acidity and viscoelasticity in the fluid's capture efficiency of two ant and two fly species in four Nepenthes species with different nutrition strategies.

METHODS

Four Nepenthes species were studied, namely N. rafflesiana, N. gracilis, N. hemsleyana and N. ampullaria. Fluid was collected from pitchers of varying ages from plants growing in the field and immediately transferred to glass vials, and individual ants (tribe Campotini, Fomicinae) and flies (Calliphora vomitoria and Drosophila melanogaster) were dropped in and observed for 5 min. Water-filled vials were used as controls. Survival and lifetime data were analysed using models applied to right-censored observations. Additional laboratory experiments were carried out in which C. vomitoria flies were immersed in pH-controlled aqueous solutions and observed for 5 min.

KEY RESULTS

Pitcher fluid differed among Nepenthes species as regards insect retention capacity and time-to-kill, with differences observed between prey types. Only the fluids of the reputedly insectivorous species were very acidic and/or viscoelastic and retained significantly more insects than the water controls. Viscoelastic fluids were fatal to flies and were able to trap the broadest diversity of insects. Younger viscoelastic fluids showed a better retention ability than older fluids, although with less rapid killing ability, suggesting that a chemical action follows a mechanical one. Insect retention increased exponentially with fluid viscoelasticity, and this happened more abruptly and at a lower threshold for flies compared with ants. Flies were more often retained if they fell into the traps on their backs, thus wetting their wings. Insect retention and death rate increased with fluid acidity, with a lower threshold for ants than for flies, and the time-to-kill decreased with increasing acidity. The laboratory experiments showed that fewer flies escaped from acidic solutions compared with water.

CONCLUSIONS

In addition to viscoelasticity, the pitcher's fluid acidity and wetting ability influence the fate of insects and hence the diet of Nepenthes. The plants might select the prey that they retain by manipulating the secretion of H(+) ions and polysaccharides in their pitcher fluid. This in turn might participate in possible adaptive radiation of this genus with regard to nutrient sequestration strategy. These plants might even structurally influence insect fall-orientation and capture-probability, inspiring biomimetic designs for pest control.

Venus Flytrap (Dionaea muscipula Solander ex Ellis) Contains Powerful Compounds that Prevent and Cure Cancer

Chemoprevention uses natural or synthetic molecules without toxic effects to prevent and/or block emergence and development of diseases including cancer. Many of these natural molecules modulate mitogenic signals involved in cell survival, apoptosis, cell cycle regulation, angiogenesis, or on processes involved in the development of metastases occur naturally, especially in fruits and vegetables bur also in non-comestible plants. Carnivorous plants including the Venus flytrap (Dionaea muscipula Solander ex Ellis) are much less investigated, but appear to contain a wealth of potent bioactive secondary metabolites. Aim of this review is to give insight into molecular mechanisms triggered by compounds isolated from these interesting plants with either therapeutic or chemopreventive potential.

Fluorescent prey traps in carnivorous plants

Carnivorous plants acquire most of their nutrients by capturing ants, insects and other arthropods through their leaf-evolved biological traps. So far, the best-known attractants in carnivorous prey traps are nectar, colour and olfactory cues. Here, fresh prey traps of 14 Nepenthes, five Sarracenia, five Drosera, two Pinguicula species/hybrids, Dionaea muscipula and Utricularia stellaris were scanned at UV 366 nm. Fluorescence emissions of major isolates of fresh Nepenthes khasiana pitcher peristomes were recorded at an excitation wavelength of 366 nm. N. khasiana field pitcher peristomes were masked by its slippery zone extract, and prey capture rates were compared with control pitchers. We found the existence of distinct blue fluorescence emissions at the capture spots of Nepenthes, Sarracenia and Dionaea prey traps at UV 366 nm. These alluring blue emissions gradually developed with the growth of the prey traps and diminished towards their death. On excitation at 366 nm, N. khasiana peristome 3:1 CHCl3–MeOH extract and its two major blue bands showed strong fluorescence emissions at 430–480 nm. Masking of blue emissions on peristomes drastically reduced prey capture in N. khasiana pitchers. We propose these molecular emissions as a critical factor attracting arthropods and other visitors to these carnivorous traps. Drosera, Pinguicula and Utricularia prey traps showed only red chlorophyll emissions at 366 nm.

Secreted pitfall-trap fluid of carnivorous Nepenthes plants is unsuitable for microbial growth

BACKGROUND AND AIMS

Carnivorous plants of the genus Nepenthes possess modified leaves that form pitfall traps in order to capture prey, mainly arthropods, to make additional nutrients available for the plant. These pitchers contain a digestive fluid due to the presence of hydrolytic enzymes. In this study, the composition of the digestive fluid was further analysed with regard to mineral nutrients and low molecular-weight compounds. A potential contribution of microbes to the composition of pitcher fluid was investigated.

METHODS

Fluids from closed pitchers were harvested and analysed for mineral nutrients using analytical techniques based on ion-chromatography and inductively coupled plasma-optical emission spectroscopy. Secondary metabolites were identified by a combination of LC-MS and NMR. The presence of bacteria in the pitcher fluid was investigated by PCR of 16S-rRNA genes. Growth analyses of bacteria and yeast were performed in vitro with harvested pitcher fluid and in vivo within pitchers with injected microbes.

KEY RESULTS

The pitcher fluid from closed pitchers was found to be primarily an approx. 25-mm KCl solution, which is free of bacteria and unsuitable for microbial growth probably due to the lack of essential mineral nutrients such as phosphate and inorganic nitrogen. The fluid also contained antimicrobial naphthoquinones, plumbagin and 7-methyl-juglone, and defensive proteins such as the thaumatin-like protein. Challenging with bacteria or yeast caused bactericide as well as fungistatic properties in the fluid. Our results reveal that Nepenthes pitcher fluids represent a dynamic system that is able to react to the presence of microbes.

CONCLUSIONS

The secreted liquid of closed and freshly opened Nepenthes pitchers is exclusively plant-derived. It is unsuitable to serve as an environment for microbial growth. Thus, Nepenthes plants can avoid and control, at least to some extent, the microbial colonization of their pitfall traps and, thereby, reduce the need to vie with microbes for the prey-derived nutrients.