Growth and biomass of mycorrhizal mycelia in coniferous forests along short natural nutrient gradients

Hybridization may lead to unique phytochemical expression in plant individuals. Hybrids may express novel combinations or extreme concentrations of secondary metabolites or, in some cases, produce metabolites novel to both parental species. Here we test whether there is evidence for extreme metabolite expression or novelty in F1 hybrids between Senecio aquaticus and Senecio jacobaea. Hybridization is thought to occur frequently within Senecio, and hybridization might facilitate secondary metabolite diversification within this genus. Parental species express different quantities of several classes of compounds known to be involved in antiherbivore defence, including pyrrolizidine alkaloids, chlorogenic acid, flavonoids and benzoquinoids. Hybrids demonstrate differential expression of some metabolites, producing lower concentrations of amino acids, and perhaps flavonoids, than either parental species. Despite evidence for quantitative hybrid novelty in this system, NMR profiling did not detect any novel compounds among the plant groups studied. Metabolomic profiling is a useful technique for identifying qualitative changes in major metabolites according to plant species and/or genotype, but is less useful for identifying small differences between plant groups, or differences in compounds expressed in low concentrations.     

Evolution of secondary metabolites in legumes (Fabaceae)

Legumes produce a high diversity of secondary metabolites which serve as defence compounds against herbivores and microbes, but also as signal compounds to attract pollinating and fruit-dispersing animals. As nitrogen-fixing organisms, legumes produce more nitrogen containing secondary metabolites than other plant families. Compounds with nitrogen include alkaloids and amines (quinolizidine, pyrrolizidine, indolizidine, piperidine, pyridine, pyrrolidine, simple indole, Erythrina, simple isoquinoline, and imidazole alkaloids; polyamines, phenylethylamine, tyramine, and tryptamine derivatives), non-protein amino acids (NPAA), cyanogenic glucosides, and peptides (lectins, trypsin inhibitors, antimicrobial peptides, cyclotides). Secondary metabolites without nitrogen are phenolics (phenylpropanoids, flavonoids, isoflavones, catechins, anthocyanins, tannins, lignans, coumarins and furanocoumarins), polyketides (anthraquinones), and terpenoids (especially triterpenoid, steroidal saponins, tetraterpenes). While some secondary metabolites have a wide distribution (flavonoids, triterpenes, pinitol), however, others occur in a limited number of taxa. The distributions of secondary metabolites with an irregular occurrence are mapped on a molecular phylogeny of the Fabaceae, reconstructed from a combined data set of nucleotide sequences from rbcL, matK and ITS genes. In most cases, the distribution patterns of secondary metabolites do not agree with the phylogeny of the plants producing them. In contrary, the distribution of many secondary metabolites is patchy and irregular. Thus, the use of phytochemical data to reconstruct a phylogeny of plants is often not informative and can be misleading. The patchy distribution may be due to convergent evolution, a contribution of endophytic fungi or more likely, to an early acquisition of the key genes of secondary metabolism in the evolution of land plants among others by horizontal gene transfer from bacteria. Thus it would be a matter of gene regulation whether these genes are active in some but not all taxa.     

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

Background and AimsSecondary 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.MethodsWe 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 ResultsFor 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.ConclusionsSecondary 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.     

Withanolide A is inherently de novo biosynthesized in roots of the medicinal plant Ashwagandha (Withania somnifera)

Ashwagandha ( Withania somnifera Dunal., Solanaceae) is one of the most reputed medicinal plants of Ayurveda, the traditional medical system. Several of its traditionally proclaimed medicinal properties have been corroborated by recent molecular pharmacological investigations and have been shown to be associated with its specific secondary metabolites known as withanolides, the novel group of ergostane skeletal phytosteroids named after the plant. Withanolides are structurally distinct from tropane/nortropane alkaloids (usually found in Solanaceae plants) and are produced only by a few genera within Solanaceae. W. somnifera contains many structurally diverse withanolides in its leaves as well as roots. To date, there has been little biosynthetic or metabolism-related research on withanolides. It is thought that withanolides are synthesized in leaves and transported to roots like the tropane alkaloids, a group of bioactive secondary metabolites in Solanaceae members known to be synthesized in roots and transported to leaves for storage. To examine this, we have studied incorporation of ¹⁴C from [2-¹⁴C]-acetate and [U-¹⁴C]-glucose into withanolide A in the in vitro cultured normal roots as well as native/orphan roots of W. somnifera . Analysis of products by thin layer chromatography revealed that these primary metabolites were incorporated into withanolide A, demonstrating that root-contained withanolide A is de novo synthesized within roots from primary isoprenogenic precursors. Therefore, withanolides are synthesized in different parts of the plant (through operation of the complete metabolic pathway) rather than imported.     

Role of Secondary Metabolites and Brassinosteroids in Plant Defense Against Environmental Stresses

Being sessile, plants are subjected to a diverse array of environmental stresses during their life span. Exposure of plants to environmental stresses results in the generation of reactive oxygen species (ROS). These activated oxygen species tend to oxidize various cellular biomolecules like proteins, nucleic acids, and lipids, a process that challenges the core existence of the cell. To prevent the accumulation of these ROS and to sustain their own survival, plants have developed an intricate antioxidative defence system. The antioxidative defence system comprises various enzymatic and nonenzymatic molecules, produced to counter the adverse effect of environmental stresses. A sizable number of these molecules belong to the category of compounds called secondary metabolites. Secondary metabolites are organic compounds that are not directly involved in the growth and development of plants but perform specialized functions under a given set of conditions. Absence of secondary metabolites results in long-term impairment of the plant’s survivability. Such compounds generally include pigments, phenolics, and so on. Plant phenolic compounds such as flavonoids and lignin precursors have been reported to accumulate in response to various biotic and abiotic stresses and are regarded as crucial defence compounds that can scavenge harmful ROS. Another important category of plant metabolites, called brassinosteroids, exhibit stress regulatory and growth-promoting activity and are classified as phytohormones. Elucidation of the physiological and molecular effects of secondary metabolites and brassinosteroids have catapulted them as highly promising and environment-friendly natural substances, suitable for wider application in plant protection and crop yield promotion. The present review focuses on our current understanding of how plants respond to the generation of excessive ROS and the role of secondary metabolites and brassinosteroids in countering the adverse effects of environmental stresses.     

Can Epichloë endophytes enhance direct and indirect plant defence?

Induced or constitutive production of secondary metabolites is a successful plant defence strategy against herbivores which can be mediated by plant associated micro-organisms. Several grass species can be associated with an endophytic fungus of the genus Epichloë which produces herbivore toxic or deterring alkaloids. Besides these direct defences, herbivorous insects are controlled via indirect plant defence mechanisms by attracting predators. Recent studies indicate that Epichloë endophytes can improve the grass emitted volatile organic compounds towards herbivore deterrence. Due to their defensive mutualistic function, we hypothesize that Epichloë altered plant volatiles can attract aphid predators and contribute to an increased indirect plant defence. With a common garden study, we show that hoverfly (Syrphidae) larvae and pupae were more abundant on endophyte-infected plants compared to uninfected plants. Our results indicate that the Epichloë endophyte provides, besides direct defence (alkaloid), indirect plant defence by improving the plant odor attracting more olfactory foraging aphid predators. Future research is needed in order to understand: (I) whether endophyte-mediated changes in plant volatiles are induced herbivore specific, (II) whether there is a trade-off between endophyte-mediated direct and indirect plant defence, (III) whether the endophyte produces volatiles or induces a change in plant-derived volatiles, (IV) the role of plant signals in endophyte-mediated plant defence.     

Structure, function, and evolution of plant O-methyltransferases.

Plant O-methyltransferases (OMTs) constitute a large family of enzymes that methylate the oxygen atom of a variety of secondary metabolites including phenylpropanoids, flavonoids, and alkaloids. O-Methylation plays a key role in lignin biosynthesis, stress tolerance, and disease resistance in plants. To gain insights into the evolution of the extraordinary diversity of plant O-methyltransferases, and to develop a framework phylogenetic tree for improved prediction of the putative function of newly identified OMT-like gene sequences, we performed a comparative and phylogenetic analysis of 61 biochemically characterized plant OMT protein sequences. The resulting phylogenetic tree revealed two major groups. One of the groups included two sister clades, one comprising the caffeoyl CoA OMTs (CCoA OMTs) that methylate phenolic hydroxyl groups of hydroxycinnamoyl CoA esters, and the other containing the carboxylic acid OMTs that methylate aliphatic carboxyl groups. The other group comprised the remaining OMTs, which act on a diverse group of metabolites including hydroxycinnamic acids, flavonoids, and alkaloids. The results suggest that some OMTs may have undergone convergent evolution, while others show divergent evolution. The high number of unique conserved regions within the CCoA OMTs and carboxylic acid OMTs provide an opportunity to design oligonucleotide primers to selectively amplify and characterize similar OMT genes from many plant species.     

Chemotaxonomy and geographical distribution of tropane alkaloids

This review illustrates the distribution of tropane alkaloids within the families Solanaceae, Erythroxylaceae, Proteaceae, Euphorbiaceae, Rhizophoraceae, Convolvulaceae and Cruciferae. Whereas tropane alkaloids are characteristic of the genera Datura, Brugmansia (tree datura) and Duboisia of the Solanaceae, the distribution is more widespread with novel tropane derivatives in families not traditionally associated with these bases. The chemical nature of more recently discovered water-soluble calystegines and the di- and trimeric forms from the Convolvulaceae (e.g. schizanthines from Schizanthus spp.), truxillines from Bolivian coca leaves and moonines of Erythroxylum moonii are highlighted. Where possible and appropriate, links between the phytochemistry and taxonomy are discussed.     

Chemical warfare or modulators of defence responses - the function of secondary metabolites in plant immunity

In plants, a host's responses to an attempted infection include activation of various secondary metabolite pathways, some of which are specific for particular plant phylogenetic clades. Phytochemicals that represent respective end products in plant immunity have been stereotypically linked to antimicrobial properties. However, in many cases, owing to the lack of unequivocal evidence for direct antibiotic action in planta, alternative functions of secondary metabolites should be considered. Correspondingly, recent findings have identified novel, and rather unexpected, functions of phytochemicals in plant immunity that mediate regulatory pathways for conserved defence responses. It also seems likely that these conserved responses can be regulated by clade-specific phytochemicals.     

Spatial organization of the glucosinolate-myrosinase system in brassica specialist aphids is similar to that of the host plant.

Secondary metabolites are important in plant defence against pests and diseases. Similarly, insects can use plant secondary metabolites in defence and, in some cases, synthesize their own products. The paper describes how two specialist brassica feeders, Brevicoryne brassicae (cabbage aphid) and Lipaphis erysimi (turnip aphid) can sequester glucosinolates (thioglucosides) from their host plants, yet avoid the generation of toxic degradation products by compartmentalizing myrosinase (thioglucosidase) into crystalline microbodies. We propose that death, or damage, to the insect by predators or disease causes disruption of compartmentalized myrosinase, which results in the release of isothiocyanate that acts as a synergist for the alarm pheromone E-beta-farnesene.