Allelochemical Interactions Research Articles

Food as a factor determining the physiological state of populations of the phytopagous pests of agricultural crops

Relevance. Trophic relationships along with competition and mutualism are the most basic and significant interactions in ecosystems. To develop, survive, and multiply, insects need to consume a certain amount of nutrients at a certain ratio. Food products are complex mixes of nutrients and non-nutritive substances (sometimes toxic ones): macronutrients (proteins, carbohydrates, and lipids), micronutrients (vitamins and minerals), and water. Some nutrients are essential; insects lack the ability to synthesize them in their bodies and must obtain them from their diet or through symbiosis with beneficial organisms. Identification mechanisms being well developed in the system “phytophagous insect – plant” allow insects to successfully spread, multiply, and feed on certain plant species. The complex of hydrolytic enzymes in the insect intestine is one of the main targets for plant defense responses because these enzymes determine the availability of structural compounds for phytophagous insects. For this reason, enzymes in the insect intestine play a key role in the adaptation of insects to the pest resistance of fodder plants. For instance, when proteinase inhibitors are synthesized in a fodder plant as the result of induced insect resistance the complex of enzymes in an insect intestine might change negating the effect of these inhibitors. The development of co-adaptations due to interactions among species in food chains depends on a complicated constellation of conflicting relationships between consumers and food objects. The mechanisms of this influence may be rooted in the allelochemical interactions in the system “phytophagous insect – plant recipient”. Allelopathic interactions are among the most complex interactions because they are constituted by direct and indirect effects. Plants when damaged by phytophagous insects activate defense responses, which incorporate several mechanisms, including an increase in the concentration of secondary metabolites, many of which are phenolic compounds.The aim of the work is to describe the mechanisms of relationships in the system “phytophage-plant”. Conclusion. Management of processes of intra-water divergence of insect-phytophages in agrobiocoenoses in order to prevent the emergence of races and populations of pests adapted to live on initially resistant to them plant forms is possible in compliance with the transition to a targeted selection of agricultural crops for resistance to a complex of pests.

Allelopathic Effects of Caffeic Acid and Its Derivatives on Seed Germination and Growth Competitiveness of Native Plants (Lantana indica) and Invasive Plants (Solidago canadensis)

Allelopathy has garnered considerable attention, but the effects of different allelochemicals on invasive plants remain unclear. This study addressed the knowledge gap surrounding allelopathy and its impact on native and invasive plant species. We focused on the impact of caffeic acid and its derivatives on the growth and competitiveness of the native Lantana indica and the invasive plant Solidago canadensis. We selected three allelochemicals, caffeic acid, methyl caffeic acid, and ethyl caffeic acid, for evaluation at two concentrations (0.1 mM and 1.0 mM). Three planting methods were employed: (1) a single species of S. canadensis, (2) a single species of L. indica, and (3) a combination of S. canadensis and L. indica. In addition, a control group was also included. Results revealed that high concentrations (1 mM) of methyl caffeate (MC) and ethyl caffeate (EC) significantly reduced seed germination rate, seed germination index, and seed germination speed index of L. indica compared to a low concentration (0.1 mM). Plant height, stem diameter, biomass, and root length in the control group (CK) of S. canadensis were significantly higher than those in the treated groups. However, with increasing allelochemical concentration, L. indica’s relative competitiveness gradually decreased. These findings provide insights into the concentration-dependent effects of allelopathic compounds on the growth of L. indica and S. canadensis. By analyzing how these allelochemicals influence the growth and competitiveness of native and invasive plants, the study sheds light on the dynamics of allelochemical interactions between these species. This knowledge can be pivotal for understanding plant competition dynamics in ecosystems and could inform strategies to control invasive species or promote native plant growth.

Exploring miRNA-mRNA regulatory modules responding to tannic acid stress in Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae) via small RNA sequencing.

MicroRNAs (miRNAs) are small noncoding RNAs (sRNAs) that regulate gene expression by inhibiting translation or degrading mRNA. Although the functions of miRNAs in many biological processes have been reported, there is currently no research on the possible roles of miRNAs in Micromelalopha troglodyta (Graeser) involved in the response of plant allelochemicals. In this article, six sRNA libraries (three treated with tanic acid and three control) from M. troglodyta were constructed using Illumina sequencing. From the results, 312 known and 43 novel miRNAs were differentially expressed. Notably, some of the most abundant miRNAs, such as miR-432, miR-541-3p, and miR-4448, involved in important physiological processes were also identified. To better understand the function of the targeted genes, we performed Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. The results indicated that differentially expressed miRNA targets were involved in metabolism, development, hormone biosynthesis, and immunity. Finally, we visualized a miRNA-mRNA regulatory module that supports the role of miRNAs in host-allelochemical interactions. To our knowledge, this is the first report on miRNAs responding to tannic acid in M. troglodyta. This study provides indispensable information for understanding the potential roles of miRNAs in M. troglodyta and the applications of these miRNAs in M. troglodyta management.

Unknown Extracellular and Bioactive Metabolites of the Genus Alexandrium: A Review of Overlooked Toxins.

Various species of Alexandrium can produce a number of bioactive compounds, e.g., paralytic shellfish toxins (PSTs), spirolides, gymnodimines, goniodomins, and also uncharacterised bioactive extracellular compounds (BECs). The latter metabolites are released into the environment and affect a large range of organisms (from protists to fishes and mammalian cell lines). These compounds mediate allelochemical interactions, have anti-grazing and anti-parasitic activities, and have a potentially strong structuring role for the dynamic of Alexandrium blooms. In many studies evaluating the effects of Alexandrium on marine organisms, only the classical toxins were reported and the involvement of BECs was not considered. A lack of information on the presence/absence of BECs in experimental strains is likely the cause of contrasting results in the literature that render impossible a distinction between PSTs and BECs effects. We review the knowledge on Alexandrium BEC, (i.e., producing species, target cells, physiological effects, detection methods and molecular candidates). Overall, we highlight the need to identify the nature of Alexandrium BECs and urge further research on the chemical interactions according to their ecological importance in the planktonic chemical warfare and due to their potential collateral damage to a wide range of organisms.

Allelochemicals of Alexandrium minutum: Kinetics of membrane disruption and photosynthesis inhibition in a co-occurring diatom

Allelopathy is an efficient strategy by which some microalgae can outcompete other species. Allelochemicals from the toxic dinoflagellate Alexandrium minutum have deleterious effects on diatoms, inhibiting metabolism and photosynthesis and therefore give a competitive advantage to the dinoflagellate. The precise mechanisms of allelochemical interactions and the molecular target of allelochemicals remain however unknown. To understand the mechanisms, the short-term effects of A. minutum allelochemicals on the physiology of the diatom Chaetoceros muelleri were investigated. The effects of a culture filtrate were measured on the diatom cytoplasmic membrane integrity (polarity and permeability) using flow-cytometry and on the photosynthetic performance using fluorescence and absorption spectroscopy. Within 10 min, the unknown allelochemicals induced a depolarization of the cytoplasmic membranes and an impairment of photosynthesis through the inhibition of the plastoquinone-mediated electron transfer between photosystem II and cytochrome b6f. At longer time of exposure, the cytoplasmic membranes were permeable and the integrity of photosystems I, II and cytochrome b6f was compromised. Our demonstration of the essential role of membranes in this allelochemical interaction provides new insights for the elucidation of the nature of the allelochemicals. The relationship between cytoplasmic membranes and the inhibition of the photosynthetic electron transfer remains however unclear and warrants further investigation.

Allelopathic effects of native and invasive Brassica nigra do not support the novel-weapons hypothesis.

The novel-weapons hypothesis predicts that some plants are successful invaders because they release allelopathic compounds that are highly suppressive to naïve competitors in invaded ranges but are relatively ineffective against competitors in the native range. For its part, the evolution of enhanced weaponry hypothesis predicts that invasive populations may evolve increased expression of the allelopathic compounds. However, these predictions have rarely been tested empirically. Here, we made aqueous extracts of roots and shoots of invasive (North American) and native (European) Brassica nigra plants. Seeds of nine species from North America and nine species from Europe were exposed to these extracts. As control solutions, we used pure distilled water and distilled water with the osmotic potential adjusted with polyethylene glycol (PEG) to match that of root and shoot extracts of B.nigra. The extracts had a strong negative effect on germination rates and seedling root lengths of target species compared to the water-control. Compared to the osmolality-adjusted controls, the extracts had a negative effect on seedling root length. We found no differences between the effects of B.nigra plant extracts from the invasive vs. native populations on germination rates and seedling root growth of target plant species. Responses were largely independent of whether the target plant species were from the invaded or native range of B.nigra. The results show that B.nigra can interfere with other species through allelochemical interactions, but do not support predictions of the novel-weapons hypothesis and evolution of increased allelopathy.

Associated Bacteria and Their Effects on Growth and Toxigenicity of the Dinoflagellate Prorocentrum lima Species Complex From Epibenthic Substrates Along Mexican Coasts

The dinoflagellate genus Prorocentrum is globally represented by a wide variety of species found upon benthic and/or epiphytic substrates. Many epibenthic Prorocentrum species produce lipophilic polyether toxins, some of which act as potent protein phosphatase inhibitors and tumor-promoters associated with Diarrheic Shellfish Poisoning (DSP). Most members of the P. lima species complex (PLSC) commonly found in the tropics and sub-tropics are toxigenic. Epiphytic and planktonic bacteria co-occur with toxigenic Prorocentrum but reciprocal allelochemical interactions are under-investigated. The aim of the present study was to identify the culturable bacteria collected together with isolates of the PLSC from seagrass (Thalassia testudinum) and macroalgae (Laurencia sp. and Sargassum sp.) along tropical Mexican coasts, and to explore potential species interactions with selected isolates. Twenty-one bacterial genera belonging to Proteobacteria, Actinobacteria and Bacteroidetes were identified by amplification of the 16S rRNA gene marker from nine clonal Prorocentrum cultures, with γ-proteobacteria comprising the dominant class. A positive correlation was found between the bacterial genera associated with two Prorocentrum clones and the esterified toxin analog DTX1a-D8, but there was no apparent correlation between the other PLSC clones and their associated bacteria with the other five DSP toxins detected. No bacteriostatic or allelochemical response was found for cell- and culture medium extracts of five Prorocentrum isolates assayed for bioactivity against Staphylococcus sp. DMBS2 and Vibrio sp. HEL66. Bulk cell-washing of Prorocentrum PA1, followed by growth with antibiotics, was only effective in reducing bacterial load in the initial growth stages, but did not yield axenic cultures or lower bacterial cell densities throughout the culture cycle. Antibiotic treatment did not impair growth or survival of the dinoflagellate, or apparently affect DSP toxin production. There was no significant correlation between Prorocentrum cell volume, growth rate, bacterial cell counts or cellular toxin concentration over the entire time-series culture cycle. Benthic Prorocentrum and associated bacterial communities comprise highly diverse and characteristic microbiomes upon substrates, and among compartments in culture, but this study provides little evidence that allelochemical interactions among Prorocentrum cells and associated bacteria originating from epibenthic substrates play a definable role in growth and toxigenicity.

Characterisation of allelochemical compounds signature in two mangrove forest species of Rhizophora apiculata and Acrostichum aureum and potential in suppressing weed growth

Rhizophora apiculata and Acrostichum aureum are a common mangrove species in Malaysia. Allelochemical interaction of the mangrove species was speculated to play an important role in dominance in the harsh mangrove environment. This study assessed the quantitative and qualitative determination for total phenolic content and individual phenolic compounds as allelochemical content for R. apiculata and A. aureum leaves as a potential allelopathic substance. The second objective is to investigate the allelopathic effect towards weed germination and growth through in vitro model system. Two types of maceration extraction comprising water extraction and sequential alkaline extract were used for allelochemical screening analysis. Both extractions were separated by ethyl acetate. Acrostichum aureum had the highest total phenolic content (1129.52 µg GAE/g DW) in comparison with R. apiculata. Meanwhile, the quantitative analysis by HPLC, revealed that different extraction detected different individual phenolic acid, Caffeic acid detected in both mangrove species in sequential alkaline extraction by A. aureum. The allelopathic effect of Chloris barbata for A. aureum extract indicated that during pre-emergent development, the suppression of weed growth was higher compared to post-emergent. Therefore, A. aureum may work as an allelochemical producer and can contribute to the establishment of pure colonies of A. aureum in the mangrove ecosystem.

Effects of toxic cyanobacterium Microcystis aeruginosa on the morphology of green alga Chlorella vulgaris

In eutrophic freshwater lakes, cyanobacteria and green algae are in succession due to abiotic factors. Allelochemical interaction also exists. In this study, we indicated that filtrates of Microcystis aeruginosa can influence the growth rate and photosynthetic pigments of the target alga named Chlorella vulgaris. We also determined the morphological appearance and colony formation of green alga C. vulgaris that were induced by chemicals associated with a competitor cyanobacterium named M. aeruginosa. However, microcystins (MCs) were not the active substances in this study. The morphology changes and growth of C. vulgaris affected by M. aeruginosa filtrates were dependent on the initial cultivation density of the target algae and the cultivation phase of M. aeruginosa. We also assumed that the morphology changes were defensive strategies utilised by C. vulgaris to resist M. aeruginosa. The temporary stress of M. aeruginosa was favourable to the growth of C. vulgaris. By contrast, the continuous induction of colony formation by M. aeruginosa in the field inhibited the growth of C. vulgaris. The present results provided new insights into the interaction between algae and theoretical basis for algae succession in the field.

Allelochemicals from Alexandrium minutum induce rapid inhibition of metabolism and modify the membranes from Chaetoceros muelleri

Allelochemical interactions are likely to be a contributing factor explaining the success of large blooms of the harmful marine dinoflagellate Alexandrium, however, the physiological mechanisms of allelochemical interactions remain poorly described. Here we investigated the sub-lethal effects (on an hourly scale) of a filtrate containing allelochemicals from Alexandrium minutum on the physiology of the common diatom Chaetoceros muelleri. The filtrate induced deleterious effects to the diatom physiology within only 30 min of exposure. Esterase activity and photosynthesis were drastically inhibited, with up to 34% of the population being metabolically inactive and up to 30% reduction in photosystem II quantum yield when exposed to the filtrate. In addition, intracellular reactive oxygen species increased by 26% in response to allelochemical exposure. C. muelleri pigment and lipid analyses indicated that the photosystem II was inhibited, with photoinhibition-like responses (activation of xanthophyll cycles, and changes in associated lipids) upregulated to mitigate the toxic effects of allelochemicals. Changes in the proportions of membrane lipid classes and increased membrane fatty acids saturation by 9% may be an attempt to maintain membrane integrity and associated enzyme activity, or could be the result of deleterious effects on membranes. An 8% decrease in cellular storage lipids (triglycerides) revealed a mobilization of energy suggesting an energetic cost for the diatom to counteract the allelochemical effects. We hypothesize that the rapid alteration of physiological functions such as photosynthesis and some enzymatic activities may result from direct damage on external membranes. Overall this study describes the sub-lethal mechanisms and provides useful biomarkers to understand the role of allelochemical interactions and associated ecological processes in structuring plankton communities.

A rapid quantitative fluorescence-based bioassay to study allelochemical interactions from Alexandrium minutum

Harmful microalgal blooms are a threat to aquatic organisms, ecosystems and human health. Toxic dinoflagellates of the genus Alexandrium are known to produce paralytic shellfish toxins and to release bioactive extracellular compounds (BECs) with potent cytotoxic, hemolytic, ichtyotoxic and allelopathic activity. Negative allelochemical interactions refer to the chemicals that are released by the genus Alexandrium and that induce adverse effects on the physiology of co-occurring protists and predators. Releasing BECs gives the donor a competitive advantage that may help to form dense toxic blooms of phytoplankton. However BECs released by Alexandrium minutum are uncharacterized and it is impossible to quantify them using classical chemical methods. Allelochemical interactions are usually quantified through population growth inhibition or lytic-activity based bioassays using a secondary target organism. However these bioassays require time (for growth or microalgal counts) and/or are based on lethal effects. The use of pulse amplitude modulation (PAM) fluorometry has been widely used to assess the impact of environmental stressors on phytoplankton but rarely for allelochemical interactions. Here we evaluated the use of PAM and propose a rapid chlorophyll fluorescence based bioassay to quantify allelochemical BECs released from Alexandrium minutum. We used the ubiquitous diatom Chaetoceros muelleri as a target species. The bioassay, based on sub-lethal effects, quantifies allelochemical activity from different samples (filtrates, extracts in seawater) within a short period of time (2 h). This rapid bioassay will help investigate the role of allelochemical interactions in Alexandrium bloom establishment. It will also further our understanding of the potential relationship between allelochemical activities and other cytotoxic activities from BECs. While this bioassay was developed for the species A. minutum, it may be applicable to other species producing allelochemicals and may provide further insights into the role and impact of allelochemical interactions in forming dense algal blooms and structuring marine ecosystems.

Species composition, functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test.

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of 'invasion success' in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non-native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant-soil fungi and plant allelochemical interactions on invasion success were species-specific.

Allelopathic interactions between Skeletonema costatum and Alexandrium minutum

ABSTRACTPotential allelopathic interactions between Skeletonema costatum and Alexandrium minutum were investigated using mixed cultures and culture filtrate in nutrient-replete medium. A. minutum growth was inhibited when grown in S. costatum filtrate, with the inhibitory effect directly proportional to the percentage of filtrate added. This demonstrates that the release of allelopathic compounds caused the growth inhibition. In contrast, the filtrate of A. minutum exerted no allelopathic activity on S. costatum. An autoinhibitory compound (15(S)-HEPE) extracted and purified from S. costatum culture was added to cultures of both S. costatum and A. mintum. This substance could depress S. costatum growth, but showed no significant inhibitory activity on A. minutum. This documented a second type of allelochemical interaction, termed auto-allelopathy, caused by a different compound from the one or ones that affected A. minutum in the co-cultures with added crude filtrate. Further studies are needed to explore the relative importance of these two types of allelopathy as factors influencing the competition between S. costatum and A. minutum in the field. Furthermore, given the observed decrease in diatom dominance relative to dinoflagellates with increasing eutrophication, one can predict that toxic species like A. minutum might become more prevalent in the future in the East China Sea if the trend of increasing pollution of coastal waters continues.

Soil microbial communities alter leaf chemistry and influence allelopathic potential among coexisting plant species.

While both plant-soil feedbacks and allelochemical interactions are key drivers of plant community dynamics, the potential for these two drivers to interact with each other remains largely unexplored. If soil microbes influence allelochemical production, this would represent a novel dimension of heterogeneity in plant-soil feedbacks. To explore the linkage between soil microbial communities and plant chemistry, we experimentally generated soil microbial communities and evaluated their impact on leaf chemical composition and allelopathic potential. Four native perennial old-field species (two each of Aster and Solidago) were grown in pairwise combination with each species' soil microbial community as well as a sterilized inoculum. We demonstrated unequivocally that variation in soil microbial communities altered leaf chemical fingerprints for all focal plant species and also changed their allelopathic potential. Soil microbes reduced allelopathic potential in bioassays by increasing germination 25-54% relative to sterile control soils in all four species. Plants grown with their own microbial communities had the lowest allelopathic potential, suggesting that allelochemical production may be lessened when growing with microbes from conspecifics. The allelopathic potential of plants grown in congener and confamilial soils was indistinguishable from each other, indicating an equivalent response to all non-conspecific microbial communities within these closely related genera. Our results clearly demonstrated that soil microbial communities cause changes in leaf tissue chemistry that altered their allelopathic properties. These findings represent a new mechanism of plant-soil feedbacks that may structure perennial plant communities over very small spatial scales that must be explored in much more detail.

Can seasonal dynamics of allelochemicals play a role in plant invasions? A case study with Helianthus tuberosus L.

Helianthus tuberosus L. (Jerusalem artichoke), a perennial plant native to North America, is one of the most dangerous invasive species in Europe. The fast spread of the plant may be enhanced by its allelopathic effects, through the release of chemical compounds into the environment. The goal of this work was to determine the allelopathic effect of different plant parts of H. tuberosus and to study the temporal changes of allelopathic factors throughout the vegetation period, based on germination bioassays, chemical analysis of allelochemicals, and greenhouse experiments. Bioassay results demonstrated differences in the effect of H. tuberosus extracts, depending on the concentration, the test species, the plant part used for extraction, and the time of the treatment. From the bioactive compounds salicylic acid, 2-OH-cinnamic acid and 4-OH-benzaldehyde showed characteristic distribution patterns throughout the vegetation period. In the competition experiment, the presence of H. tuberosus exerted a strong negative effect on all tested species, independently of their treatment with activated carbon. Based on our results, we concluded that H. tuberosus can interfere with other species through allelochemical interactions. Moreover, seasonal dynamics of allelochemicals could be more important than suspected in plant competition and is likely to play an important role in the spread of the invasive H. tuberosus into new areas.

Bioactive compounds of marine dinoflagellate isolates from western Greenland and their phylogenetic association within the genus Alexandrium

The diversity and biogeography of populations of the toxigenic marine dinoflagellate genus Alexandrium, a major global cause of paralytic shellfish poisoning (PSP), are represented by only a few studies based upon a low number of cultured isolates and remain poorly described in Arctic and sub-Arctic waters. Multiple clonal isolates (n=22) of the Alexandrium tamarense species complex, and a single isolate of A. tamutum, were collected from the water column while on board an oceanographic expedition to the west coast of Greenland. After culturing of these isolates under controlled conditions, their phylogenetic affinities within the genus Alexandrium were characterized by sequence analysis of nuclear large sub-unit (LSU) rDNA. Based upon morphological and molecular genetic criteria, all isolates of the A. tamarense species complex were consistent with membership in the Group I ribotype (previously known as the North American ribotype). Phenotypic signatures were also analyzed based upon their respective profiles of paralytic shellfish toxins (PST) and allelochemical interactions against a target cryptophyte Rhodomonas, as determined by lytic potency. All isolates conforming to the A. tamarense Group I produced PST, but no toxins were detected in A. tamutum P2E2. Unusually, only carbamoyl toxins were produced among the A. tamarense Group I isolates from Greenland; sulfocarbamoyl derivatives, generally present in A. tamarense population from other locations, including the Arctic, North Pacific and North Atlantic, were absent from all isolates. Allelochemical activity, causing cell lysis of Rhodomonas, but generally being unrelated to cellular PST, was expressed by all A. tamarense isolates and also by A. tamutum, but varied widely in potency. Comparison of the genotypic (rDNA) and phenotypic (PST profile, allelochemical activity) characteristics of Greenland isolates with those of other Arctic populations reveals a complex pattern of intra-specific diversity. Estimation of diversity relationships is problematic because of the distinct patterns of divergence and lack of evidence of linkage among the alternative biomarkers and morphology. Nevertheless, such studies are necessary as the basis for constructing hindcasting scenarios and predicting changes in Alexandrium species distribution in the Arctic from the regional to the global scale.

Gut microbes may facilitate insect herbivory of chemically defended plants.

The majority of insect species consume plants, many of which produce chemical toxins that defend their tissues from attack. How then are herbivorous insects able to develop on a potentially poisonous diet? While numerous studies have focused on the biochemical counter-adaptations to plant toxins rooted in the insect genome, a separate body of research has recently emphasized the role of microbial symbionts, particularly those inhabiting the gut, in plant-insect interactions. Here we outline the "gut microbial facilitation hypothesis," which proposes that variation among herbivores in their ability to consume chemically defended plants can be due, in part, to variation in their associated microbial communities. More specifically, different microbes may be differentially able to detoxify compounds toxic to the insect, or be differentially resistant to the potential antimicrobial effects of some compounds. Studies directly addressing this hypothesis are relatively few, but microbe-plant allelochemical interactions have been frequently documented from non-insect systems-such as soil and the human gut-and thus illustrate their potential importance for insect herbivory. We discuss the implications of this hypothesis for insect diversification and coevolution with plants; for example, evolutionary transitions to host plant groups with novel allelochemicals could be initiated by heritable changes to the insect microbiome. Furthermore, the ecological implications extend beyond the plant and insect herbivore to higher trophic levels. Although the hidden nature of microbes and plant allelochemicals make their interactions difficult to detect, recent molecular and experimental techniques should enable research on this neglected, but likely important, aspect of insect-plant biology.

Invasive species, disrupted chemical community dynamics and future adaptations: Commentary on Chabaane et al. 2015

Chabaane et al. (2015) describe the subtle disruptive chemical changes that an alien herbivore (a moth caterpillar, Spodoptera littoralis) may cause in a native community. This disruption is described across trophic links from the plant it eats (a mustard family species, Brassica rapa) to native herbivores (the cabbage white butterfly, Pieris brassicae) and their native natural enemies (parasitic wasps, Cotesia glomerata). This paper reinforces the concern that new insect species arriving in natural communities may potentially have a detrimental impact: inducing unexpected changes in plant chemistry; altering competitive interactions with other herbivores; and changing the dynamics of the natural enemies of such herbivores. Extreme disruptions by ‘invasive’ Lepidoptera include pest population outbreaks that may cause large-scale defoliations which affect the entire ecosystem by cascading chemical, physiological and ecological changes across all trophic levels. In consequence altering the functional dynamics in various ways (Redman & Scriber 2000; Kenis et al. 2009; Gandi & Herms 2010). The study by Chabaane et al. (2015) shows how subtle changes in chemical cues from alien herbivores may result in major changes with cascading responses of the native plant–herbivore–parasite community. Whether such chemically based disruptions such as those described by Chabaane et al. (2015) result in local population outbreaks, and whether such changes become part of the future community dynamics, depends on the intensity and variability of natural selection that occurs. All of these biotic interactions resulting from invasive species are shaped by a combination of ecological and evolutionary processes (Moran & Alexander 2014). The fundamental component at all levels (organism, population, community and ecosystem) is the underlying genetic variability, which both interacts with and shapes all aspects of the phenotypes (phytochemical, biochemical, behavioural, physiological and morphological traits) and the potential for rapid adaptation and introgressive gene flow (Brasier 2001; Scriber 2011; Franks & Hoffmann 2012; Pauls et al. 2013). Plant genotypes can shape communities and drive evolutionary change (‘community genetics’; Smith et al. 2011). Similarly, underlying genetics shape interactions and natural selection across different environmental gradients at the landscape and ecosystem levels (‘geographic mosaic of co-evolution’; Thompson 2005). Understanding the underlying chemical, ecological and genetic sources of evolutionary responses to invasive species in the face of rapid environmental changes (such as global warming) will become one of the most significant challenges in the next decade (Moran & Alexander 2014; Scriber 2014). There is clearly a need for additional studies such as Chabaane et al. (2015) because the impacts of introduced species may run contrary to expectation. Illustrated by the fact that the alien (potentially invasive) herbivore species caused disruptive impacts on the community via induced changes in plant chemistry. This occurred despite the fact that exotic herbivores themselves were not actually used as hosts by the native parasitoids. Chabaane et al. (2015) show that exotic herbivores may invade new environments where they can significantly alter the native communities. This is done via both direct (induced changes in plant phytochemistry or nutritional ecology; Scriber 2002; Gandi & Herms 2010) and indirect (cascading) impacts on species across trophic levels in the ecosystem (Redman & Scriber 2000; Dicke & Baldwin 2012; Stam et al. 2014). The chemical basis of these plant/herbivore/enemy interactions has historically focused on host plant resistance, herbivore counter-adaptations and chemical defenses derived from the plants or herbivores in a relatively recent field of chemical ecology (Price et al. 1980; Feeny 1992; Gols 2014). Changes in other abiotic factors (e.g. climate and local drought conditions, as well as stoichiochemistry; Mattson & Scriber 1987; Mattson & Haack 1987; Elser et al. 2000; Chunco 2014) can also greatly alter the chemical ecology of the community, including interactions of plants, herbivores and their enemies (predators, parasites, and pathogens: Stamp 1993; Ayres & Lombardero 2000; Singer & Stireman 2005). However, Chabaane et al. (2015) show that changes in the chemical environment are not always ecologically predictable. More ground-breaking research as described in the interactions of chemical components among native and exotics across local populations and trophic levels will be required in the future (Chabaane et al. 2015). However, the elucidation of the genetic basis of ecological ripples caused by allelochemical interactions will likely be the next major step in better understanding the autecological (organism & population) causes of synecological responses in the community and ecosystem (Whitham et al. 2003; Wheat et al. 2007; Stam et al. *Correspondence author: E-mail: scriber@msu.edu

In vitro responses of Fusarium oxysporum f.sp.niveum to phenolic acids in decaying watermelon tissues

To establish the relationships among wilt disease and pathogens and the chemical environment during the allelochemical interactions of plant–microbe–soil, extracts from decaying watermelon tissues were added to media to investigate the effect of allelochemical decaying watermelon tissues on Fusarium oxysporum f.sp.niveum (FON) in vitro. Twelve types of phenolic acids from various decaying watermelon cultivars (Zaojia 82-24 and Jingxing 3) with different levels of resistance to FON were analysed using HPLC. The results demonstrated that there were fewer types and quantities of chemical components released from decaying susceptible watermelon tissues (Zaojia 84-24) than there were from decaying resistant watermelon tissues (Jingxing 3). More antifungal phenolic acids in decaying resistant watermelon tissues (with a ratio of 29.2:1 of antifungal phenolic acids to fungus-stimulation phenolic acids) were observed than in decaying susceptible tissues (with a ratio of 8.4:1 of antifungal phenolic acids to fungus-stimulation phenolic acids). Decaying resistant watermelon tissue extracts inhibited the growth and conidia germination and formation of FON, while decaying susceptible extracts stimulated growth and conidia germination and formation.Extracts from decaying resistant watermelon tissues stimulated virulence factors considerably less than did those from decaying susceptible tissues. We conclude that the decaying plant extracts from susceptible watermelon cultivars stimulated FON because of their higher content of fungus-stimulation phenolic acids, while the decaying resistant watermelon plants inhibited FON because of their higher content of antifungal phenolic acids. These results imply that phenolic acids containing decaying plant residues in soil would induce or inhibit pathogens in plant–microbe interactions.

Bioavailability of Allelochemicals As Affected by Companion Compounds in Soil Matrices

Multicompound allelochemical interactions were studied using Centaurea maculosa as a model source to understand how the bioavailability of complex allelochemical mixtures is modified in soil-microbial systems. Litter decomposition of C. maculosa in sandy loam soil yielded five phenolic acids, namely, hydroxybenzoic, vanillic, protocatechuic, p-coumaric, and ferulic acids. The degradation studies were conducted by exogenous application of catechin, the primary allelochemical exuded by C. maculosa, and the phenolic acid cosolutes in a sandy loam and silt loam soil. Compared to a single-solute system, in a multisolute system the persistence of individual allelochemicals was significantly increased in both soils. Oxidation and sorption were primarily involved in the disappearance of allelochemicals. Mass spectrometric data showed that catechin rapidly underwent polymerization to form procyanidin dimer both in soil and in bioassay medium, resulting in reduced persistence and phytotoxicity. Hence, catechin phytotoxicity could occur only under conditions that would inhibit these condensation reactions. This study clearly demonstrates that various soil mechanisms including competitive sorption and preferential degradation would increase the persistence of allelochemical mixtures in a soil matrix.