Evolution of secondary metabolites from an ecological and molecular phylogenetic perspective

Evolution of sesquiterpene hydrocarbons in virgin olive oil during fruit ripening

Despite the potential of sesquiterpene hydrocarbons in olive oil authentication, their metabolism in Olea europaea is poorly understood, and little is known about their biochemical regulation in olives as a function of ripening. To ascertain some metabolic relationships between sesquiterpene hydrocarbons and olive ripening, the content of sesquiterpene hydrocarbons was assessed in virgin olive oils from two olive varieties grown in the same geographical area and produced at different harvesting periods. During the ripening, the accumulation of sesquiterpenes in the olive itself, and thus in the oil, differed according to their molecular structure: bicyclic sesquiterpenes, showed decreasing concentrations the later the harvest, while acyclic farnesene-like compounds progressively increased through the olive ripening process. This is first evidence that the accumulation of sesquiterpene hydrocarbons in olive, and hence in olive oil, is modulated during ripening. Therefore, the degree of ripening of olives should be taken into consideration when considering the sesquiterpenic profile of virgin olive oil for their authentication.

Metabolism and function of phenazines in bacteria: impacts on the behavior of bacteria in the environment and biotechnological processes

Phenazines constitute a large group of nitrogen-containing heterocyclic compounds produced by a diverse range of bacteria. Both natural and synthetic phenazine derivatives are studied due their impacts on bacterial interactions and biotechnological processes. Phenazines serve as electron shuttles to alternate terminal acceptors, modify cellular redox states, act as cell signals that regulate patterns of gene expression, contribute to biofilm formation and architecture, and enhance bacterial survival. Phenazines have diverse effects on eukaryotic hosts and host tissues, including the modification of multiple host cellular responses. In plants, phenazines also may influence growth and elicit induced systemic resistance. Here, we discuss emerging evidence that phenazines play multiple roles for the producing organism and contribute to their behavior and ecological fitness.

Metabolomics: the chemistry between ecology and genetics

Metabolomics is a fast developing field of comprehensive untargeted chemical analyses. It has many applications and can in principle be used on any organism without prior knowledge of the metabolome or genome. The amount of functional information that is acquired with metabolomics largely depends on whether a metabolome database has been developed for the focal species. Metabolomics is a level downstream from transcriptomics and proteomics and has been widely advertised as a functional genomics and systems biology tool. Indeed, it has been successfully applied to link phenotypes to genotypes in the model plant Arabidopsis thaliana. Metabolomics is also increasingly being used in ecology (ecological metabolomics) and environmental sciences (environmental metabolomics). In ecology, the technique has led to novel insights into the mechanisms of plant resistance to herbivores. Some of the most commonly used analytical metabolomic platforms are briefly discussed in this review, as well as their limitations. We will mainly focus on the application of metabolomics in plant ecology and genetics.

Biological elicitors of plant secondary metabolites: mode of action and use in the production of nutraceutics

Many secondary metabolites of interest for human health and nutrition are produced by plants when they are under attack of microbial pathogens or insects. Treatment with elicitors derived from phytopathogens can be an effective strategy to increase the yield of specific metabolites obtained from plant cell cultures. Understanding how plant cells perceive microbial elicitors and how this perception leads to the accumulation of secondary metabolites, may help us improve the production of nutraceutics in terms of quantity and of quality of the compounds. The knowledge gathered in the past decades on elicitor perception and transduction is now being combined to high-throughput methodologies, such as transcriptomics and metabolomics, to engineer plant cells that produce compounds of interest at industrial scale.

To speciate, or not to speciate? Resource heterogeneity, the subjectivity of similarity, and the macroevolutionary consequences of niche-width shifts in plant-feeding insects

Coevolutionary studies on plants and plant-feeding insects have significantly improved our understanding of the role of niche shifts in the generation of new species. Evolving plant lineages essentially constitute moving islands and archipelagoes in resource space, and host shifts by insects are usually preceded by colonizations of novel resources. Critical to hypotheses concerning ecological speciation is what happens immediately before and after colonization attempts: if an available plant is too similar to the current host(s), it simply will be incorporated into the existing diet, but if it is too different, it will not be colonized in the first place. It thus seems that the probability of speciation is maximized when alternative hosts are at an 'intermediate' distance in resource space. In this review, I wish to highlight the possibility that resource similarity and, thus, the definition of 'intermediate', are subjective concepts that depend on the herbivore lineage's tolerance to dietary variation. This subjectivity of similarity means that changes in tolerance can either decrease or increase speciation probabilities depending on the distribution of plants in resource space: insect lineages with narrow tolerances are likely to speciate by 'island-hopping' on young, species-rich plant groups, whereas more generalized lineages could speciate by shifting among resource archipelagoes formed by higher plant taxa. Repeated and convergent origins of traits known to broaden or to restrict host-plant use in multiple different insect groups provide opportunities for studying how tolerance and resource heterogeneity may interact to determine speciation rates.

Phylogenetic methods in natural product research

Natural products researchers are increasingly employing evolutionary analyses of genes and gene products that rely on phylogenetic trees. The field of phylogenetic inference and of evolutionary analyses based on phylogenies is growing at an amazing rate, making it difficult to keep up with the latest methodologies. Here, we summarize phylogenetic applications in natural products research, and review methods and software useful for carrying out analyses inferring or using phylogenetic trees. We include an updated overview of available alignment methods and programs, as well as a selection of some useful phylogenetic analysis tools. This review covers primarily the period 2000-2009 for applications of phylogenetic methods in natural product research, and 1990-2009 for phylogenetic methods, with some references going back to the 1960s.

A survival strategy: the coevolution of the camptothecin biosynthetic pathway and self-resistance mechanism

A diverse array of secondary metabolites in plants represents the process of coevolution between the plants and their natural enemies including herbivores and pathogens. For defense, plants produce many toxic compounds that harm other organisms. However, if the target of these compounds is a fundamental biological process then the producing plant may also be harmed. In such cases self-resistance strategies must coevolve with the biosynthetic pathway of toxic metabolites. In this review, we discuss the recent elucidation of the self-resistance mechanism of camptothecin (CPT)-producing plants. In this case the target protein of CPT, topoisomerase (Top) 1, has been mutated in order to overcome the toxicity of the compound. Similar mechanisms might also be used by other plants producing different toxic compounds which target fundamental metabolism.

The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga

Plants and their herbivores constitute more than half of the organisms in tropical forests. Therefore, a better understanding of the evolution of plant defenses against their herbivores may be central for our understanding of tropical biodiversity. Here, we address the evolution of antiherbivore defenses and their possible contribution to coexistence in the Neotropical tree genus Inga (Fabaceae). Inga has >300 species, has radiated recently, and is frequently one of the most diverse and abundant genera at a given site. For 37 species from Panama and Peru we characterized developmental, ant, and chemical defenses against herbivores. We found extensive variation in defenses, but little evidence of phylogenetic signal. Furthermore, in a multivariate analysis, developmental, ant, and chemical defenses varied independently (were orthogonal) and appear to have evolved independently of each other. Our results are consistent with strong selection for divergent defensive traits, presumably mediated by herbivores. In an analysis of community assembly, we found that Inga species co-occurring as neighbors are more different in antiherbivore defenses than random, suggesting that possessing a rare defense phenotype increases fitness. These results imply that interactions with herbivores may be an important axis of niche differentiation that permits the coexistence of many species of Inga within a single site. Interactions between plants and their herbivores likely play a key role in the generation and maintenance of the conspicuously high plant diversity in the tropics.

Environmental regulation of floral anthocyanin synthesis in Ipomoea purpurea

Responses of metabolites to environmental fluctuations may play large roles in biological adaptation, yet how these responses initiate in the natural environment and the molecular mechanisms remain unclear. Synthesis of floral anthocyanins, as typical examples of secondary metabolites, is known to respond to the physical environment and therefore an ideal system for understanding the process of the environmental regulation. Here, by simultaneous monitoring of six natural environmental variables and anthocyanin content of daily opening flowers throughout a natural flowering season ( approximately 50 days) of Ipomoea purpurea, we have identified significant and positive correlations of temperature (3-days ago) and ultraviolet (UV) light intensity (5-days ago) with the floral anthocyanin content. We sequenced all known (seven structural and three regulatory) anthocyanin genes in I. purpurea flowers and examined their transcript quantities in the natural environment across eight floral developmental stages (covering 0-96 h before anthesis). The anthocyanin gene expression patterns corroborated with the inferred effects from the time-series data, and further showed that the positive UV effect became negative on transcript levels about 36 h before anthesis. With falling natural temperature, content of the principal anthocyanin declined, whereas that of an alternative anthocyanin with fewer glucose and caffeic acid moieties increased. Our data suggest that environmental regulation of the anthocyanin pathway may account for more than half of the flux variation in the floral limb, and is influenced mainly by daily average temperature and UV light intensity that modulate anthocyanin transcript levels (most likely via myb1) at floral developmental stages.

Tobacco mosaic virus (TMV) inhibitors from Picrasma quassioides Benn

To investigate natural inhibitors against tobacco mosaic virus (TMV) from plants, 10 known beta-carboline alkaloids and one quassinoid have been isolated from MeOH extract of the wood of Picrasma quassioides Benn. These compounds were screened for their inhibitory activities against tobacco mosaic virus (TMV). The activity of each compound against TMV infection and replication was tested using a half-leaf assay method, a leaf-disk method, and Western blotting analyses. All of the beta-carboline alkaloids showed moderate anti-TMV activities and exhibited synergistic effects when combined with the quassinoid nigakilactone B (11). To our knowledge, this is the first report on anti-TMV activity of beta-carbolines and their synergistic effects against TMV when combined with a quassinoid.

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.

The Flavonoids and Phenolic Acids of the Genus Silphium and Their Chemosystematic Value

The current review outlines the distribution of flavonoids and phenolic acids in eleven species of Silphium (Asteraceae). The chemosystematic significance of these findings is discussed and compared with DNA and morphological-based assignments for eleven species. Special attention is given to the five flavonol triglycosides that were previously isolated and characterized from Silphium species: isorhamnetin 3- O-α-L-rhamnosyl (1″′→6″)- O-β-D-galactopyranoside 7- O-β-L-apiofuranoside (1), quercetin 3- O-α-L-rhamnosyl (1″′→6″)- O-β-D-galactopyranoside 7- O-β-L-apiofuranoside (2), quercetin 3- O-β-L-galactosyl (1″′→6″)- O-β-D-rhamnopyranoside 7- O-α-L-apiofuranoside (3), kaempferol 3- O-β-D-apiofuranoside 7- O-αL-rhamnosyl (1″″→6″′)- O-β-D-galactopyranoside (4) and kaempferol 3- O-β-D-apiofuranoside 7- O-α-L-rhamnosyl (1″″→6″′)- O-βD (2″′- O- E-caffeoylgalactopyranoside) (5). In addition, several other known mono and diglycosidic flavonoids, along with 18 phenolic acids, were also identified and the structures of all of the compounds were established by LC/MS and NMR spectral investigations.

Convergent antifouling activities of structurally distinct bioactive compounds synthesized within two sympatric Haliclona demosponges

A wide range of sessile and sedentary marine invertebrates synthesize secondary metabolites that have potential as industrial antifoulants. These antifoulants tend to differ in structure, even between closely related species. Here, we determine if structurally divergent secondary metabolites produced within two sympatric haliclonid demosponges have similar effects on the larvae of a wide range of benthic competitors and potential fouling metazoans (ascidians, molluscs, bryozoans, polychaetes, and sponges). The sponges Haliclona sp. 628 and sp. 1031 synthesize the tetracyclic alkaloid, haliclonacyclamine A (HA), and the long chain alkyl amino alcohol, halaminol A (LA), respectively. Despite structural differences, HA and LA have identical effects on phylogenetically disparate ascidian larvae, inducing rapid larval settlement but preventing subsequent metamorphosis at precisely the same stage. HA and LA also have similar effects on sponge, polychaete, gastropod and bryozoan larvae, inhibiting both settlement and metamorphosis. Despite having identical roles in preventing fouling and colonisation, HA and LA differentially affect the physiology of cultured HeLa human cells, indicating they have different molecular targets. From these data, we infer that the secondary metabolites within marine sponges may emerge by varying evolutionary and biosynthetic trajectories that converge on specific ecological roles.

Essential oil composition and internal transcribed spacer (ITS) sequence variability of four South-Croatian Satureja species (Lamiaceae)

The purpose of this study was to compare the essential oil profiles of four South-Croatian Satureja species, as determined by GC/FID and GC/MS, with their DNA sequences for an internal transcribed spacer (ITS1-5.8S-ITS2) of the nuclear ribosomal DNA. A phylogenetic analysis showed that S. montana and S. cuneifolia, characterized by a similar essential oil composition, rich in the monoterpene hydrocarbon carvacrol, clustered together with high and moderate bootstrap support. On the contrary, S. subspicata and S. visianii, characterized by quite unique essential oil compositions, clustered together with the moderate bootstrap support. All four Croatian Satureja species clustered in one clade, separately from Macaronesian S. hortensis, although it had essential oil composition similar to that of S. montana and S. cuneifolia. This is the first report on the comparison between the phytochemical and DNA sequence data in Satureja species and useful contribution to the better understanding of interspecies relationships in this genus.

Bioactivity and structure of biophenols as mediators of chronic diseases

Biophenols and their associated activity have generated intense interest. Current topics of debate are their bioavailability and bioactivity. It is generally assumed that their plasma concentrations are insufficient to produce the health benefits previously attributed to their consumption. However, data on localized in vivo concentrations are not available and many questions remain unanswered. Potential mechanisms by which they may exert significant bioactivity are discussed together with structure activity relationships. Biophenols are highly reactive species and they can react with a range of other compounds. Products of their reaction when functioning as antioxidants are examined.

Two new Daphniphyllum alkaloids from Daphniphyllum macropodum Miq

Two new Daphniphyllum alkaloids, 4,21-deacetyl-deoxyyuzurimine (1) and macropodumine L (2), together with the two known related alkaloids, have been isolated from the bark of Daphniphyllum macropodum Miq. The structures of the new compounds were elucidated on the basis of the detailed analysis of spectroscopic data, chemical method, and by comparison of the spectroscopic data with those of known compounds. Compounds 2 and 4 exhibited weak cytotoxicity against human carcinoma cell lines SMMC-7721 and HO-8910.

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.

Transcription factors for predictive plant metabolic engineering: are we there yet?

Transcription factors (TFs) are considered viable alternatives to 'single enzyme' approaches for the manipulation of plant metabolic pathways. Because of the ability to control multiple, if not all steps in a particular metabolic pathway, TFs provide attractive tools for overcoming flux bottlenecks involving multiple enzymatic steps, or for deploying pathway genes in specific organs, cell types or even plants where they normally do not express. The potential of a TF for the predictive manipulation of plant metabolism is intimately linked to understanding how it fits in the gene regulatory organization. The knowledge gained over the past decade on how plant pathways are controlled together with increasing efforts aimed at deciphering the overall architecture of plant gene regulatory networks are starting to realize the potential of TFs for predictive plant metabolic engineering.

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.

Plant Secondary Metabolism: Diversity, Function and its Evolution

A typical character of plants is the production and storage of usually complex mixtures of secondary metabolites (SM). The main function of secondary metabolites is defense against herbivores and microbes; some SM are signal compounds to attract pollinating and seed dispersing animals or play a role in the symbiotic relationships with plants and microbes. The distribution of SM in the plant kingdom shows an interesting pattern. A specific SM is often confined to a particular systematic unit, but isolated occurrences can occur in widely unrelated taxonomic groups. This review tries to explain the patchy occurrence of SM in plants. It could be due to convergent evolution, but evidence is provided that the genes that encode the biosynthesis of SM appear to have a much wider distribution than the actual secondary metabolite. It seems to be rather a matter of differential gene regulation whether a pathway is active and expressed in a given taxonomic unit or not. It is speculated that the genes of some pathways derived from an early horizontal gene transfer from bacteria, which later became mitochondria and chloroplasts. These genes/pathways should be present in most if not all land plants. About 80% of plants live in close symbiotic relationships with symbiotic fungi (ectomycorrhiza, endophytes). Recent evidence is presented that these fungi can either directly produce SM, which were formerly considered as plant SM or that these fungi have transferred the corresponding pathway gene to the host plant. The fungal contribution could also explain part of the patchy occurrence patterns of several secondary metabolites.

Nature's assembly line: biosynthesis of simple phenylpropanoids and polyketides

Plants produce large amounts of phenylpropanoids, both in terms of molecular diversity and absolute quantity of these compounds. The phenylpropanoids, and the related plant polyketides, have multiple biological functions. They serve to attract pollinators, support secondary cell-wall growth, provide protection against various plant diseases, and interact with beneficial soil microbes. Their basic chemical properties also make them useful in the biofuel and biomaterial industries. Phenylpropanoid metabolism begins with the amino acid phenylalanine, which feeds into various biosynthetic pathways that generate a wide range of structurally related polyphenolic compounds. This review focuses on four sub-groups of these polyphenolic compounds - polyketides, stilbenes, isoflavones and catechins. We discuss the biosynthesis of these molecules, their physiological role in plants, and their striking pharmacological and physiological effects on humans. This review also highlights metabolic engineering efforts aimed at increasing or decreasing the amounts of each class of compound in various model plants and crops.

A systems biology investigation of the MEP/terpenoid and shikimate/phenylpropanoid pathways points to multiple levels of metabolic control in sweet basil glandular trichomes

The glandular trichome is an excellent model system for investigating plant metabolic processes and their regulation within a single cell type. We utilized a proteomics-based approach with isolated trichomes of four different sweet basil (Ocimum basilicum L.) lines possessing very different metabolite profiles to clarify the regulation of metabolism in this single cell type. Significant differences in the distribution and accumulation of the 881 highly abundant and non-redundant protein entries demonstrated that although the proteomes of the glandular trichomes of the four basil lines shared many similarities they were also each quite distinct. Correspondence between proteomic, expressed sequence tag, and metabolic profiling data demonstrated that differential gene expression at major metabolic branch points appears to be responsible for controlling the overall production of phenylpropanoid versus terpenoid constituents in the glandular trichomes of the different basil lines. In contrast, post-transcriptional and post-translational regulation of some enzymes appears to contribute significantly to the chemical diversity observed within compound classes for the different basil lines. Differential phosphorylation of enzymes in the 2-C-methyl-d-erythritol 4-phosphate (MEP)/terpenoid and shikimate/phenylpropanoid pathways appears to play an important role in regulating metabolism in this single cell type. Additionally, precursors for different classes of terpenoids, including mono- and sesquiterpenoids, appear to be almost exclusively supplied by the MEP pathway, and not the mevalonate pathway, in basil glandular trichomes.

Genotype, age, tissue, and environment regulate the structural outcome of glucosinolate activation

Glucosinolates are the inert storage form of a two-part phytochemical defense system in which the enzyme myrosinase generates an unstable intermediate that rapidly rearranges into the biologically active product. This rearrangement step generates simple nitriles, epithionitriles, or isothiocyanates, depending on the structure of the parent glucosinolate and the presence of proteins that promote specific structural outcomes. Glucosinolate accumulation and myrosinase activity differ by plant age and tissue type and respond to environmental stimuli such as planting density and herbivory; however, the influence of these factors on the structural outcome of the rearrangement step remains unknown. We show that the structural outcome of glucosinolate activation is controlled by interactions among plant age, planting density, and natural genetic variation in Arabidopsis (Arabidopsis thaliana) rosette leaves using six well-studied accessions. We identified a similarly complex interaction between tissue type and the natural genetic variation present within these accessions. This raises questions about the relative importance of these novel levels of regulation in the evolution of plant defense. Using mutants in the structural specifier and glucosinolate activation genes identified previously in Arabidopsis rosette leaves, we demonstrate the requirement for additional myrosinases and structural specifiers controlling these processes in the roots and seedlings. Finally, we present evidence for a novel EPITHIOSPECIFIER PROTEIN-independent, simple nitrile-specifying activity that promotes the formation of simple nitriles but not epithionitriles from all glucosinolates tested.

Characterization of the RND-type multidrug efflux pump MexAB-OprM of the plant pathogen Pseudomonas syringae

In gram-negative bacteria, transporters belonging to the RND family are the transporters most relevant for resistance to antimicrobial compounds. In Pseudomonas aeruginosa, a clinically important pathogen, the RND-type pump MexAB-OprM has been recognized as one of the major multidrug efflux systems. Here, homologues of MexAB-OprM in the plant pathogens Pseudomonas syringae pv. phaseolicola 1448A, P. syringae pv. syringae B728a, and P. syringae pv. tomato DC3000 were identified, and mexAB-oprM-deficient mutants were generated. Determination of MICs revealed that mutation of MexAB-OprM dramatically reduced the tolerance to a broad range of antimicrobials. Moreover, the ability of the mexAB-oprM-deficient mutants to multiply in planta was reduced. RNA dot blot hybridization revealed growth-dependent regulation of the mexAB-oprM operon in P. syringae; the expression of this operon was maximal in early exponential phase and decreased gradually during further growth.

Altering the ratio of phenazines in Pseudomonas chlororaphis (aureofaciens) strain 30-84: effects on biofilm formation and pathogen inhibition

Pseudomonas chlororaphis strain 30-84 is a plant-beneficial bacterium that is able to control take-all disease of wheat caused by the fungal pathogen Gaeumannomyces graminis var. tritici. The production of phenazines (PZs) by strain 30-84 is the primary mechanism of pathogen inhibition and contributes to the persistence of strain 30-84 in the rhizosphere. PZ production is regulated in part by the PhzR/PhzI quorum-sensing (QS) system. Previous flow cell analyses demonstrated that QS and PZs are involved in biofilm formation in P. chlororaphis (V. S. R. K. Maddula, Z. Zhang, E. A. Pierson, and L. S. Pierson III, Microb. Ecol. 52:289-301, 2006). P. chlororaphis produces mainly two PZs, phenazine-1-carboxylic acid (PCA) and 2-hydroxy-PCA (2-OH-PCA). In the present study, we examined the effect of altering the ratio of PZs produced by P. chlororaphis on biofilm formation and pathogen inhibition. As part of this study, we generated derivatives of strain 30-84 that produced only PCA or overproduced 2-OH-PCA. Using flow cell assays, we found that these PZ-altered derivatives of strain 30-84 differed from the wild type in initial attachment, mature biofilm architecture, and dispersal from biofilms. For example, increased 2-OH-PCA production promoted initial attachment and altered the three-dimensional structure of the mature biofilm relative to the wild type. Additionally, both alterations promoted thicker biofilm development and lowered dispersal rates compared to the wild type. The PZ-altered derivatives of strain 30-84 also differed in their ability to inhibit the fungal pathogen G. graminis var. tritici. Loss of 2-OH-PCA resulted in a significant reduction in the inhibition of G. graminis var. tritici. Our findings suggest that alterations in the ratios of antibiotic secondary metabolites synthesized by an organism may have complex and wide-ranging effects on its biology.

A molecular perspective on terpene variation in Australian Myrtaceae

The terpenoid-dominated essential oils in Australian Myrtaceae mediate many ecological interactions and are important industrially. Of all the significant essential oil-producing families, Myrtaceae is the only one for which there is no molecular information on terpene biosynthesis. Here we summarise available knowledge on terpene biosynthesis and its relevance to the Myrtaceae to provide a foundation for ecological and genetic studies of chemical diversity. There are several steps in the terpene biosynthesis pathway that have potential for influencing the oil yield, profile and composition of leaf oils in Myrtaceae. The biochemical steps that influence oil yield in Myrtaceae probably occur in the steps of the pathway leading up to the synthesis of the terpene backbone. Qualitative differences in oil profiles are more likely to be due to variation in terpene synthases and terpene-modifying enzymes. Most of the information on molecular variation in terpene biosynthesis is based on the analysis of artificially derived mutants but Australian Myrtaceae can provide examples of the same mechanisms in an ecological context.

In vitro antimicrobial activity and the mode of action of indole-3-carbinol against human pathogenic microorganisms

Indole-3-carbinol (I3C) is a naturally occurring constituent of cruciferous vegetables. The aim of this study was to assess the in vitro antimicrobial activity of I3C and its mode of action. By using an NCCLS broth microdilution assay, the activity of I3C was evaluated against human pathogenic microorganisms including clinically isolated antibiotic-resistant bacterial strains. The results indicated that I3C exhibited broad spectrum antimicrobial activities. To elucidate the physiological changes of the fungal cells induced by I3C, we performed a flow cytometric analysis for a cell cycle. The results showed that I3C arrested the cell cycle at the G(2)/M phase in Candida albicans. To understand the antifungal mode of action of I3C, the change in the membrane dynamics was monitored by using fluorescence changing experiments against C. albicans. The results suggest that I3C may exert antifungal activity by disrupting the structure of the cell membrane. The present study indicates that I3C has considerable antimicrobial activity, deserving further investigation for clinical applications.