Plant allelochemical interference or soil chemical ecology?

While allelopathy has been defined as plant-plant chemical interference, there has been much confusion about what the concept encompasses and how important it is in nature. We distinguish between (1) direct plant-plant interference mediated by allelochemicals, and (2) the effects of secondary compounds released by plants on abiotic and biotic soil processes that affect other plants.It very difficult to demonstrate direct effects of chemicals released by a plant on nearby plants. Although soil ecology-mediated effects of secondary plant compounds do not fit the classical concept of allelopathy, we find support in the literature for the hypothesis that the most important effects of compounds released into the soil environment by plants on other plants occur through such indirect effects. The emphasis on, and skepticism of, direct plant-plant allelopathic interference has led some researchers to demand unreasonably high standards of evidence for establishing even the existence of allelopathic interactions, standards that are not demanded for other plant-plant interactions such as resource competition. While the complete elucidation of the mechanisms by which allelochemicals function in the field is many years away, such elucidation is not necessary to establish the existence of allelopathic interactions.We propose that most of the phenomena broadly referred to as allelopathic interference are better conceptualized and investigated in terms of soil chemical ecology. Even when direct plant-plant allelochemical interference occur, the levels of allelochemicals in the environment and their effects on plants are heavily influenced by abiotic and biotic components of the soil ecosystem. Putting allelopathy in the context of soil ecology can further research and reduce some of the less fruitful controversy surrounding the phenomenon.     

Root flavonoids

Informations on root flavonoids have been extensively reviewed and their functional diversity critically appraised. Root flavonoids play significant roles in protecting the plants against pests and diseases, regulating root growth and functions, influencing different aspects of nitrogen cycle and exerting allelopathic growth effects. They also constitute an essential source of Pharmaceuticals. An exhaustive list of flavonoids which are of significance in relation to these properties has been compiled. A thorough understanding of the flavonoid composition, level, metabolism and regulation in the roots of various plants may help us in developing several applied topics. Safe and specific chemicals against root pests; chemicals regulating root growth and mineral nutrition; plant varieties resistant to root pests and adverse allelopathic effects; an improved nitrogen economy in agroecosystems; desirable varieties of medicinal plants, in whose roots flavonoids are the active principles; in vitro systems for flavonoid production obtained from root cultures, at a commercial scale.     

Roles of Allelopathy in Plant Biodiversity and Sustainable Agriculture

Allelopathic compounds are metabolites released from plants that might be beneficial or detrimental to the growth of receptor plants. These compounds are involved in the environmental complex of managed or natural ecosystems. Allelopathic compounds have been shown to play important roles in the determination of plant diversity, dominance, succession, and climax of natural vegetation and in the plant productivity of agroecosystems. The overuse of synthetic agrochemicals often causes environmental hazards, an imbalance of soil microorganisms, nutrient deficiency, and change of soil physicochemical properties, resulting in a decrease of crop productivity. The incorporation of allelopathic substances into agricultural management may reduce the use of synthetic herbicides, fungicides, and insecticides and lessen environmental deterioration. Scientists in many different habitats around the world have demonstrated the above examples previously. It is known that most volatile compounds, such as terpenoids, are released from plants in drought areas. In contrast, water-borne phytotoxins, such as phenolics, flavonoids, or alkaloids, are released from plants in humid zone areas. Both allelopathy and autointoxication play an important mechanism in regulating plant biodiversity and plant productivity. A unique case study of a pasture-forest intercropping system, which is particularly emphasized here, could be used as a model for forest management. After the deforestation of coniferous or hardwood forests, a pasture grass, kikuyu grass (Pennisetum clandestinum), was transplanted onto the land. The grass was quickly established within 6 months. Significant suppression of weed growth by the kikuyu grass was found; however, the growth of coniferous or hardwood plants was not suppressed but stimulated. This example as well as others described in this text clearly indicate that allelopathy plays a significant role in sustainable agriculture. Nevertheless, room for allelopathic research in the next century is available for biologists, biochemists, biotechnologists, and chemists. Future allelopathic research should focus on the following tasks: (1) a continuous survey of potential allelochemicals from natural vegetation or microorganisms, (2) the establishment of practical ways of using allelochemicals in the field, (3) to understand the mode of action of allelopathic chemicals in receptor organisms, (4) to understand the role of allelopathic chemicals in biodiversity and ecosystem function, (5) to explore advanced biotechnology for allocating allelopathic chemical genes in plants or microorganisms for biological control, and (6) to challenge the natural product chemists to develop a better methodology for isolating allelopathic compounds or their degraded compounds from the environment, particularly the soil environment.     

Interaction of 8-Hydroxyquinoline with Soil Environment Mediates Its Ecological Function

Background

Allelopathic functions of plant-released chemicals are often studied through growth bioassays assuming that these chemicals will directly impact plant growth. This overlooks the role of soil factors in mediating allelopathic activities of chemicals, particularly non-volatiles. Here we examined the allelopathic potential of 8-hydroxyquinoline (HQ), a chemical reported to be exuded from the roots of Centaurea diffusa.

Methodology/Principal Findings

Growth bioassays and HQ recovery experiments were performed in HQ-treated soils (non-sterile, sterile, organic matter-enriched and glucose-amended) and untreated control soil. Root growth of either Brassica campestris or Phalaris minor was not affected in HQ-treated non-sterile soil. Soil modifications (organic matter and glucose amendments) could not enhance the recovery of HQ in soil, which further supports the observation that HQ is not likely to be an allelopathic compound. Hydroxyquinoline-treated soil had lower values for the CO₂ release compared to untreated non-sterile soil. Soil sterilization significantly influenced the organic matter content, PO₄-P and total organic nitrogen levels.

Conclusion/Significance

Here, we concluded that evaluation of the effect of a chemical on plant growth is not enough in evaluating the ecological role of a chemical in plant-plant interactions. Interaction of the chemical with soil factors largely determines the impact of HQ on plant growth.     

Soil Microorganisms: An Important Determinant of Allelopathic Activity

Current evidence illustrates the significance of soil microbes in influencing the bioavailability of allelochemicals. This review discusses (i) the significance of soil microorganisms in influencing allelopathic expression, (ii) different ways of avoiding microbial degradation of putative allelochemicals, and (iii) the need of incorporating experiments on microbial modification of allelochemicals in laboratory bioassays for allelopathy. Several climatic and edaphic factors affect the soil microflora; therefore, allelopathy should be assessed in a range of soil types. Allelopathy can be better understood in terms of soil microbial ecology, and appropriate methodologies are needed to evaluate the roles of soil microorganisms in chemically-mediated interactions between plants.     

Plant phenolics in allelopathy

Phenolics are one of the many secondary metabolites implicated in allelopathy. To establish that allelopathy functions in a natural ecosystem, the allelopathic bioassay must be ecologically realistic so that responses of appropriate bioassay species are determined at relevant concentrations. It is important to isolate, identify, and characterize phenolic compounds from the soil. However, since it is essentially impossible to simulate exact field conditions, experiments must be designed with conditions resembling those found in natural systems. It is argued that allelopathic potential of phenolics can be appreciated only when we have a good understanding of 1) species responses to phenolic allelochemicals, 2) methods for extraction and isolation of phenolic allelochemicals, and 3) how abiotic and biotic factors affect phenolic toxicity.     

陆地生态系统中马陆的生态功能

马陆是陆地生态系统中物种多样性极高的大型土壤无脊椎动物类群。作为营腐生动物, 马陆在陆地生态系统中具有不可替代的重要功能。通过大量取食及随后的肠道过程, 马陆在很大程度上决定着陆地生态系统凋落物的破碎、转化和分解过程, 从而驱动碳和关键养分元素的循环周转。然而, 目前对马陆生态功能的研究还非常有限, 远远落后于其他土壤动物类群(如蚯蚓等)。本文初步总结了马陆的生态功能: (1)通过破碎、取食凋落物来加速凋落物的分解。马陆偏好取食半分解的凋落物, 其同化效率受到凋落物来源、温度和凋落物中微生物含量的影响。(2)主要通过取食和排泄等活动影响养分循环。但对于马陆如何影响土壤碳循环, 存在两种不同的观点: 一是马陆粪球的分解速率比凋落物更快, 加速了碳的循环; 二是马陆粪球更难分解, 有助于碳的固存和稳定。马陆破碎凋落物后, 凋落物释放氮素进入土壤。此外, 马陆的活动也影响土壤磷的循环, 提高土壤中有效磷的含量。(3)调控微生物特性, 与蚯蚓也有互作关系。通过以上三个方面的总结, 展望了未来马陆的主要研究方向, 以期引起更多思考和研究。     

Unravelling the beneficial role of microbial contributors in reducing the allelopathic effects of weeds

The field of allelopathy is one of the most fascinating but controversial processes in plant ecology that offers an exciting, interdisciplinary, complex, and challenging study. In spite of the established role of soil microbes in plant health, their role has also been consolidated in studies of allelopathy. Moreover, allelopathy can be better understood by incorporating soil microbial ecology that determines the relevance of allelopathy phenomenon. Therefore, while discussing the role of allelochemicals in plant–plant interactions, the dynamic nature of soil microbes should not be overlooked. The occurrence and toxicity of allelochemicals in soil depend on various factors, but the type of microflora in the surroundings plays a crucial role because it can interfere with its allelopathic nature. Such microbes could be of prime importance for biological control management of weeds reducing the cost and ill effects of chemical herbicides. Among microbes, our main focus is on bacteria—as they are dominant among other microbes and are being used for enhancing crop production for decades—and fungi. Hence, to refer to both bacteria and fungi, we have used the term microbes. This review discusses the beneficial role of microbes in reducing the allelopathic effects of weeds. The review is mainly focused on various functions of bacteria in (1) reducing allelopathic inhibition caused by weeds to reduce crop yield loss, (2) building inherent defense capacity in plants against allelopathic weed, and (3) deciphering beneficial rhizospheric process such as chemotaxis/biofilm, degradation of toxic allelochemicals, and induced gene expression.     

植物根系分泌物的生态效应

根系分泌物是在一定的生长条件下 ,活的且未被扰动的根系释放到根际环境中的有机物质的总称 ,在植物主动适应和抵御不良环境中具有重要作用 :(1)通过化感作用影响根际微生物和周围其它植物的生长 ,并进一步改善植物的生态环境 (生物因素 ) ;(2 )通过对土壤中矿质元素的溶解、螯合作用、迁移和活化等作用 ,不仅在营养缺乏的情况下提高矿质营养元素的有效性 ,而且在面临重金属胁迫时能降低根际中金属污染物的活性 ,减少植物对金属的吸收。根系分泌物在植物与环境的相互作用中起着传递信息的作用     

An ecosystem-level perspective of allelopathy

Allelopathy is an interference mechanism by which plants release chemicals which affect other plants; while it has often been proposed as a mechanism for influencing plant populations and communities, its acceptance by plant ecologists has been limited because of methodological problems as well as difficulties of relating the results of bioassays used for testing allelopathy to vegetation patterns in the field. Here we argue that the concept of allelopathy is more appropriately applied at the ecosystem-level, rather than the traditional population/community level of resolution. Firstly, we consider the wide ranging effects of secondary metabolites (widely regarded as allelochemicals) on organisms and processes which regulate ecosystem function, including herbivory, decomposition and nutrient mineralization. It is apparent that plants with allelopathic potential against other organisms induce net changes in ecosystem properties, which may in turn impact upon the plant community in the longer term. We then illustrate these concepts using two contrasting examples of how invasive plant species with allelopathic potential may alter ecosystem properties through the production of secondary metabolites, i.e. Carduus nutans (nodding thistle) in New Zealand pastures and Empetrum hermaphroditum (crowberry) in Swedish boreal forests. In both cases the production of secondary metabolites by the invasive species induces important effects on other organisms and key processes, which help determine how the ecosystem functions and ultimately the structure of the plant community. These examples help demonstrate that the concept of allelopathy is most effectively applied at the ecosystem-level of resolution, rather than at the population-level (i.e. plant-plant interference).     

Collection and Identification of Allelopathic Compounds from the Undisturbed Root System of Bigalta Limpograss (Hemarthria altissima)

Collection of allelopathic chemicals from the undisturbed plant root system is difficult because of their low concentrations and the high level of contaminants in growth media such as soil. A new approach for the continuous trapping of quantities of extracellular chemicals from donor plants is described. Bigalta limpograss (Hemarthria altissima), a tropical forage with allelopathic activities, was established in sand culture. Nutrient solution was circulated continuously through the root system and a column containing XAD-4 resin. Extracellular hydrophobic metabolites were selectively adsorbed by the resin, while inorganic nutrients were recycled to sustain plant growth. Columns were eluted with methanol and the eluate separated into neutral, acidic, and basic fractions. Bioassays of trapped root exudates using lettuce seed combined with paper and thin layer chromatography showed that the inhibitors were mainly phenolic compounds. The active neutral fraction was methylated and analyzed by gas chromatography-mass spectrometry. Twelve compounds were identified, with two additional compounds tentatively identified. 3-Hydroxyhydrocinnamic, benzoic, phenylacetic, and hydrocinnamic acids were the major rhizospheric compounds with known growth regulatory activities.     

Soil biota reduce allelopathic effects of the invasive Eupatorium adenophorum

Allelopathy has been hypothesized to play a role in exotic plant invasions, and study of this process can improve our understanding of how direct and indirect plant interactions influence plant community organization and ecosystem functioning. However, allelopathic effects can be highly conditional. For example allelopathic effects demonstrated in vivo can be difficult to demonstrate in field soils. Here we tested phytotoxicity of Eupatorium adenophorum (croftonweed), one of the most destructive exotic species in China, to a native plant species Brassica rapa both in sand and in native soil. Our results suggested that natural soils from different invaded habitats alleviated or eliminated the efficacy of potential allelochemicals relative to sand cultures. When that soil is sterilized, the allelopathic effects returned; suggesting that soil biota were responsible for the reduced phytotoxicity in natural soils. Neither of the two allelopathic compounds (9-Oxo-10,11-dehydroageraphorone and 9b-Hydroxyageraphorone) of E. adenophorum could be found in natural soils infested by the invader, and when those compounds were added to the soils as leachates, they showed substantial degradation after 24 hours in natural soils but not in sand. Our findings emphasize that soil biota can reduce the allelopathic effects of invaders on other plants, and therefore can reduce community invasibility. These results also suggest that soil biota may have stronger or weaker effects on allelopathic interactions depending on how allelochemicals are delivered.     

Allelopathy and plant invasions: traditional,congeneric, and bio-geographical approaches

A relatively small subset of exotic plant species competitively exclude their neighbors in invaded “recipient” communities but coexist with neighbors in their native habitat. Allelopathy has been argued as one of the mechanisms by which such exotics may become successful invaders. Three approaches have been used to examine allelopathy as a mechanism for invasion. The traditional approach examines exotic invasives in the same way that other native plants also suspected of allelopathic activities are studied. In this approach dose, fate, and replenishment of chemicals can provide powerful evidence for allelopathic processes. The bio-geographical approach often does not provide as much mechanistic evidence for allelopathy, but comparing the allelopathic effects of exotic invasives on species from their native and invaded communities yields stronger evidence than the traditional approach for whether or not allelopathy actually contributes to invasive success. The congeneric, or phylogenetic, approach involves comparative studies of exotic species with natives in the same genus or that are as closely related as possible. Congeneric approaches are limited in inference and have been used to study the role of natural enemies in exotic invasion, but this approach has not been widely used to study allelopathy and invasion. We discuss these three approaches and present a data set for congeneric Lantana and Prosopis to illustrate how the congeneric approach can be used, and use Centaurea maculosa and (±)-catechin to demonstrate experimentally how traditional and bio-geographic approaches can be integrated to shed light on allelopathy in exotic plant invasions.     

The role of root exudates and allelochemicals in the rhizosphere

Plant roots serve a multitude of functions in the plant including anchorage, provision of nutrients and water, and production of exudates with growth regulatory properties. The root–soil interface, or rhizosphere, is the site of greatest activity within the soil matrix. Within this matrix, roots affect soil structure, aeration and biological activity as they are the major source of organic inputs into the rhizosphere, and are also responsible for depletion of large supplies of inorganic compounds. Roots are very complicated morphologically and physiologically, and their metabolites are often released in large quantities into the soil rhizosphere from living root hairs or fibrous root systems. Root exudates containing root-specific metabolites have critical ecological impacts on soil macro and microbiota as well as on the whole plant itself. Through the exudation of a wide variety of compounds, roots impact the soil microbial community in their immediate vicinity, influence resistance to pests, support beneficial symbioses, alter the chemical and physical properties of the soil, and inhibit the growth of competing plant species. In this review, we outline recent research on root exudation and the role of allelochemicals in the rhizosphere by studying the case of three plants that have been shown to produce allelopathic root exudates: black walnut, wheat and sorghum     

The xenobiotic receptors PXR and CAR in liver physiology,an update

Pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are two nuclear receptors that are well-known for their roles in xenobiotic detoxification by regulating the expression of drug-metabolizing enzymes and transporters. In addition to metabolizing drugs and other xenobiotics, the same enzymes and transporters are also responsible for the production and elimination of numerous endogenous chemicals, or endobiotics. Moreover, both PXR and CAR are highly expressed in the liver. As such, it is conceivable that PXR and CAR have major potentials to affect the pathophysiology of the liver by regulating the homeostasis of endobiotics. In recent years, the physiological functions of PXR and CAR in the liver have been extensively studied. Emerging evidence has suggested the roles of PXR and CAR in energy metabolism, bile acid homeostasis, cell proliferation, to name a few. This review summarizes the recent progress in our understanding of the roles of PXR and CAR in liver physiology.     

The role of flavonoids in root-rhizosphere signalling: opportunities and challenges for improving plant-microbe interactions

The flavonoid pathway produces a diverse array of plant compounds with functions in UV protection, as antioxidants, pigments, auxin transport regulators, defence compounds against pathogens and during signalling in symbiosis. This review highlights some of the known function of flavonoids in the rhizosphere, in particular for the interaction of roots with microorganisms. Depending on their structure, flavonoids have been shown to stimulate or inhibit rhizobial nod gene expression, cause chemoattraction of rhizobia towards the root, inhibit root pathogens, stimulate mycorrhizal spore germination and hyphal branching, mediate allelopathic interactions between plants, affect quorum sensing, and chelate soil nutrients. Therefore, the manipulation of the flavonoid pathway to synthesize specifically certain products has been suggested as an avenue to improve root-rhizosphere interactions. Possible strategies to alter flavonoid exudation to the rhizosphere are discussed. Possible challenges in that endeavour include limited knowledge of the mechanisms that regulate flavonoid transport and exudation, unforeseen effects of altering parts of the flavonoid synthesis pathway on fluxes elsewhere in the pathway, spatial heterogeneity of flavonoid exudation along the root, as well as alteration of flavonoid products by microorganisms in the soil. In addition, the overlapping functions of many flavonoids as stimulators of functions in one organism and inhibitors of another suggests caution in attempts to manipulate flavonoid rhizosphere signals.     

水稻和稗草共生土壤微生物生物量碳及酶活性的变化

以稻田稗草、化感水稻PI312777和普通水稻辽粳9为试材,研究了田间稗草和水稻1∶1共生条件下,土壤微生物生物量碳及脱氢酶、脲酶和转化酶活性的变化.结果表明：在稗草 的干扰下,化感水稻PI312777根区土壤微生物生物量碳含量比单作减少了 50.52％（P<0.01）,而行间土壤微生物生物量碳含量增加;普通水稻辽粳9根区土壤 微生物生物量碳含量比单作减少了38.99％（P<0.01）,但其行间土壤微生物生物量碳含量无明显变化.两个水稻品种根区土壤脱氢酶活性均被显著抑制（P<0.05）,下降率都在20％以上;PI312777根区土壤脲酶和转化酶活性均被显著促进（P<0.01）;而辽粳9根区土壤转化酶活性也被显著抑制（P<0.01）,但脲酶活性无明显变化.化感水稻根区土壤微生物生物量碳含量的显著减少及脲酶、转化酶活性的增加是其化感特性的表现,表明土壤微生物和酶均参与了水稻和稗草的种间作用,化感水稻具有抗稗草干扰的明显优势.     

Srs2: The “Odd-Job Man” in DNA repair

Homologous recombination plays a key role in the maintenance of genome integrity, especially during DNA replication and the repair of double-stranded DNA breaks (DSBs). Just a single un-repaired break can lead to aneuploidy, genetic aberrations or cell death. DSBs are caused by a vast number of both endogenous and exogenous agents including genotoxic chemicals or ionizing radiation, as well as through replication of a damaged template DNA or the replication fork collapse. It is essential for cell survival to recognise and process DSBs as well as other toxic intermediates and launch most appropriate repair mechanism. Many helicases have been implicated to play role in these processes, however their detail roles, specificities and co-operativity in the complex protein-protein interaction networks remain unclear. In this review we summarize the current knowledge about Saccharomyces cerevisiae helicase Srs2 and its effect on multiple DNA metabolic processes that generally affect genome stability. It would appear that Srs2 functions as an “Odd-Job Man” in these processes to make sure that the jobs proceed when and where they are needed.