Flavonoids induce germination of basidiospores of the ectomycorrhizal fungus Suillus bovinus

Morphogenesis and metabolomics reveal the compatible relationship among Suillus bovinus, Phialocephala fortinii, and their co-host, Pinus massoniana

Ectomycorrhizal (ECM) fungi and dark septate endophytes (DSEs) can both form a symbiotic relationship with the same host plant. However, the interactions that occur among these two types of fungi and their co-hosts are largely unknown. Here, we investigated interactions that occur among the ECM fungus Suillus bovinus, the DSE Phialocephala fortinii, and their co-host Pinus massoniana. We used both scanning electron microscopy and optical microscopy to characterize the morphogenesis of the two symbionts and employed the ultra-high-performance liquid chromatography-tandem mass spectrometry technique to assess the effects of fungal inoculation on the root metabolome. Under pure culture conditions, no synergistic or antagonistic effects were observed between Phi. fortinii and S. bovinus. Generally, S. bovinus and Phi. fortinii can simultaneously colonize P. massoniana roots without affecting each other's symbiotic processes. S. bovinus can colonize the root locus where Phi. fortinii has already invaded but not vice versa, which may be due to the physical barrier effect of the mantle. Both fungi can significantly promote the growth of P. massoniana, and they have a synergistic effect on host N and K uptake. Metabolite accumulation patterns in roots inoculated with Phi. fortinii and/or S. bovinus were greatly altered, especially with respect to organic acids, flavonoids, lipids, and phenolic acids. S. bovinus inoculation significantly enhanced root flavonoid biosynthesis, whereas Phi. fortinii and dual-inoculation treatments mainly induced phenylpropanoid biosynthesis. These findings reveal compatible relationships among P. massoniana, S. bovinus, and Phi. fortinii, and suggest a theoretical basis for ECM fungi and DSE co-application when cultivating seedlings. IMPORTANCE The prevalence of both ectomycorrhizal fungi and dark septate endophytes in the roots of a wide spectrum of tree species is well recognized. In this study, we investigated the interactions that occur among the ECM fungus S. bovinus, the DSE Phi. fortinii, and their co-host, P. massoniana. The two fungi can simultaneously colonize P. massoniana roots without affecting each other's symbiotic processes. S. bovinus appears to be superior to Phi. fortinii in microniche competition, which may be due to the physical barrier effect of the mantle. The two fungi have different effects on root metabolite accumulation patterns. S. bovinus inoculation significantly enhanced root flavonoid biosynthesis, whereas Phi. fortinii and dual-inoculation treatments mainly induced phenylpropanoid biosynthesis. This is the first study revealing the morphological and metabolic mechanisms that contribute to the compatible relationship among ECM fungi, DSEs, and their co-host.

Molecular characterization of chilli leaf curl Ahmedabad virus: homology modelling and evaluation of viral proteins interacting with host protein SnRK1 and docking against flavonoids-an in silico approach

Chilli leaf curl Ahmedabad virus (ChiLCAV), a begomovirus belonging to the family Geminiviridae, has been reported for its occurrence in India, infecting chilli and tomato plants. The viral proteins associated with ChiLCAV involves in the primary pathogenesis and transmission of the virus by whitefly. Viral protein interactions with host proteins show the dynamics of structural binding and interaction in their infection cycle. At the same time, plants have multiple defence mechanisms against bacterial and viral infections. Secondary metabolites play a significant role in the inborne defence mechanism of plants. Host proteins are also the prime producers of secondary metabolites. In the present study, we evaluated the host protein SnRK1 interaction with all six viral proteins (V1, V2, C1, C2, C3 and C4). Apart from C4, all the other viral proteins showed appreciable binding and interaction with SnRK1. SnRK1 has the regulation mechanism for the accumulation of diterpenoids, secondary metabolites. Flavonoids are secondary metabolites produced by the plant under stress conditions. Further, we studied the binding and interaction of six selected flavonoids produced by Solanaceae family members with all the ChiLCAV proteins. All six selected flavonoids showed considerable binding energy with all viral proteins. Each flavonoid showed high binding energy with different viral proteins. Molecular docking is carried out for both flavonoids and the host protein SnRK1. These in silico interactions and docking studies could be useful for understanding the plants defence mechanism against viral infections at the molecular level.

Specialized metabolites as versatile tools in shaping plant-microbe associations

Plants are rich repository of a large number of chemical compounds collectively referred to as specialized metabolites. These compounds are of importance for adaptive processes including responses against changing abiotic conditions and interactions with various co-existing organisms. One of the strikingly affirmed functions of these specialized metabolites is their involvement in plants' life-long interactions with complex multi-kingdom microbiomes including both beneficial and harmful microorganisms. Recent developments in genomic and molecular biology tools not only help to generate well-curated information about regulatory and structural components of biosynthetic pathways of plant specialized metabolites but also to create and screen mutant lines defective in their synthesis. In this review, we have comprehensively surveyed the function of these specialized metabolites and discussed recent research findings demonstrating the responses of various microbes on tested mutant lines having defective biosynthesis of particular metabolites. In addition, we attempt to provide key clues about the impact of these metabolites on the assembly of the plant microbiome by summarizing the major findings of recent comparative metagenomic analyses of available mutant lines under customized and natural microbial niches. Subsequently, we delineate benchmark initiatives that aim to engineer or manipulate the biosynthetic pathways to produce specialized metabolites in heterologous systems but also to diversify their immune function. While denoting the function of these metabolites, we also discuss the critical bottlenecks associated with understanding and exploiting their function in improving plant adaptation to the environment.

Morphological and Transcriptional Characteristics of the Symbiotic Interaction between Pinus massoniana and Suillus bovinus

Ectomycorrhiza (ECM) function has been well studied; however, there is little detailed information regarding the establishment of ECM symbioses. We investigated the morphological and transcriptional changes that occur during the establishment of the Pinus massoniana-Suillus bovinus ECM. S. bovinus promoted the growth of P. massoniana via the release of volatile organic compounds and exudates during the pre-symbiotic stage. Exudate-induced effects showed host plant specificity. At seven days post-inoculation (dpi), the mycelium started to penetrate P. massoniana roots. At 28 dpi, the Hartig net and mantle formed. At the pre-symbiotic stage, most differentially expressed genes in P. massoniana roots were mapped to the biosynthesis of secondary metabolites, signal transduction, and carbohydrate metabolism. At the symbiotic stage, S. bovinus colonization induced the reprogramming of pathways involved in genetic information processing in P. massoniana, particularly at the Hartig net and mantle formation stage. Phenylpropanoid biosynthesis was present at all stages and was regulated via S. bovinus colonization. Enzyme inhibitor tests suggested that hydroxycinnamoyl-CoA shikimate/quinate transferase is involved in the development of the Hartig net. Our findings outline the mechanism involved in the P. massoniana-S. bovinus ECM. Further studies are needed to clarify the role of phenylpropanoid biosynthesis in ECM formation.

Soil propagule banks of ectomycorrhizal fungi associated with <i>Larix cajanderi</i> above the treeline in the Siberian Arctic

Microbial symbionts are essential for plant niche expansion into novel habitats. Dormant propagules of ectomycorrhizal (EM) fungi are thought to play an important role in seedling establishment in invasion fronts; however, propagule bank communities above the treeline are poorly understood in the Eurasian Arctic, where treelines are expected to advance under rapid climate change. To investigate the availability of EM fungal propagules, we collected 100 soil samples from Arctic tundra sites and applied bioassay experiments using Larix cajanderi as bait seedlings. We detected 11 EM fungal operational taxonomic units (OTUs) by obtaining entire ITS regions. Suillus clintonianus was the most frequently observed OTU, followed by Cenococcum geophilum and Sebacinales OTU1. Three Suillus and one Rhizopogon species were detected in the bioassay seedlings, indicating the availability of Larix-specific suilloid spores at least 30 km from the contemporary treeline. Spores of S. clintonianus and S. spectabilis remained infective after preservation for 14 mo and heat treatment at 60 °C, implying the durability of the spores. Long-distance dispersal capability and spore resistance to adverse conditions may represent ecological strategies employed by suilloid fungi to quickly associate with emerging seedlings of compatible hosts in treeless habitats.

Novel Microdialysis Technique Reveals a Dramatic Shift in Metabolite Secretion during the Early Stages of the Interaction between the Ectomycorrhizal Fungus Pisolithus microcarpus and Its Host Eucalyptus grandis

The colonisation of tree roots by ectomycorrhizal (ECM) fungi is the result of numerous signalling exchanges between organisms, many of which occur before physical contact. However, information is lacking about these exchanges and the compounds that are secreted by each organism before contact. This is in part due to a lack of low disturbance sampling methods with sufficient temporal and spatial resolution to capture these exchanges. Using a novel in situ microdialysis approach, we sampled metabolites released from Eucalyptus grandis and Pisolithus microcarpus independently and during indirect contact over a 48-h time-course using UPLC-MS. A total of 560 and 1530 molecular features (MFs; ESI- and ESI+ respectively) were identified with significant differential abundance from control treatments. We observed that indirect contact between organisms altered the secretion of MFs to produce a distinct metabolomic profile compared to either organism independently. Many of these MFs were produced within the first hour of contact and included several phenylpropanoids, fatty acids and organic acids. These findings show that the secreted metabolome, particularly of the ECM fungus, can rapidly shift during the early stages of pre-symbiotic contact and highlight the importance of observing these early interactions in greater detail. We present microdialysis as a useful tool for examining plant–fungal signalling with high temporal resolution and with minimal experimental disturbance.

The Significance of Flavonoids in the Process of Biological Nitrogen Fixation

Nitrogen is essential for the growth of plants. The ability of some plant species to obtain all or part of their requirement for nitrogen by interacting with microbial symbionts has conferred a major competitive advantage over those plants unable to do so. The function of certain flavonoids (a group of secondary metabolites produced by the plant phenylpropanoid pathway) within the process of biological nitrogen fixation carried out by Rhizobium spp. has been thoroughly researched. However, their significance to biological nitrogen fixation carried out during the actinorhizal and arbuscular mycorrhiza-Rhizobium-legume interaction remains unclear. This review catalogs and contextualizes the role of flavonoids in the three major types of root endosymbiosis responsible for biological nitrogen fixation. The importance of gaining an understanding of the molecular basis of endosymbiosis signaling, as well as the potential of and challenges facing modifying flavonoids either quantitatively and/or qualitatively are discussed, along with proposed strategies for both optimizing the process of nodulation and widening the plant species base, which can support nodulation.

Root flavonoids are related to enhanced AMF colonization of an invasive tree

Arbuscular mycorrhizal fungi (AMF) are important mutualistic microbes in soil, which have capacity to form mutualistic associations with most land plants. Arbuscular mycorrhizal fungi play an important role in plant invasions and their interactions with invasive plants have received increasing attention. However, the chemical mechanisms underlying the interactions of AMF and invasive plants are still poorly understood. In this study we aim to test whether root secondary chemicals are related to enhanced AMF colonization and rapid growth in an invasive tree. We conducted a common garden experiment in China with Chinese tallow tree (Triadica sebifera) to examine the relationships among AMF colonization and secondary metabolites in roots of plants from introduced (USA) and native (China) populations. We found that AMF colonization rate was higher in introduced populations compared to native populations. Roots of plants from introduced populations had lower levels of phenolics and tannins, but higher levels of flavonoids than those of plants from native populations. Flavonoids were positively correlated with AMF colonization, and this relationship was especially strong for introduced populations. Besides, AMF colonization was positively correlated with plant biomass suggesting that higher root flavonoids and AMF colonization may impact plant performance. This suggests that higher root flavonoids in plants from introduced populations may promote AMF spore germination and/or attract hyphae to their roots, which may subsequently increase plant growth. Overall, our results support a scenario in which invasive plants enhance their AMF association and invasion success via genetic changes in their root flavonoid metabolism. These findings advance our understanding of the mechanisms underlying plant invasion success and the evolutionary interactions between plants and AMF. Understanding such mechanisms of invasive plant success is critical for predicting and managing plant invasions in addition to providing important insights into the chemical mechanism of AMF-plant interactions.

Homology modeling and docking analysis of ßC1 protein encoded by Cotton leaf curl Multan betasatellite with different plant flavonoids

Cotton leaf curl Multan virus (CLCuMuV) belonging to begomoviruses (Family Geminiviridae) can infect cotton and many other agricultural crops. Betasatellite associated with CLCuMuV i.e., cotton leaf curl Multan betasatellite (CLCuMuB) is a small circular single-stranded deoxyribose nucleic acid (ssDNA) molecule that is essential for CLCuMuV to induce disease symptoms. Betasatellite molecule contains a ßC1 gene encoding for a pathogenicity determinant multifunctional protein, which extensively interacts with host plant machinery to cause virus infection. In this study the interaction of ßC1 with selected plant flavonoids has been studied. The study was focused on sequence analysis, three-dimensional structural modeling and docking analysis of ßC1 protein of CLCuMuB. Sequence analysis and physicochemical properties showed that ßC1 is negatively charged protein having more hydrophilic regions and is not very stable. Three-dimensional model of this protein revealed three helical, four beta pleated sheets and four coiled regions. The score of docking experiments using flavonoids as ligand indicated that plant flavonoids robinetinidol-(4alpha,8)-gallocatechin, quercetin 7-O-beta-D-glucoside, swertianolin, 3′,4′,5-trihydroxy-3-methoxyflavon-7-olate, agathisflavone, catiguanin B, 3′,4′,5,6-tetrahydroxy-3,7-dimethoxyflavone, quercetin-7-O-[alpha-L-rhamnopyranosyl(1->6)-beta-D-galactopyranoside], prunin 6″-O-gallate and luteolin 7-O-beta-D-glucosiduronic acid have strong binding with active site of ßC1 protein. The results obtained from this study clearly indicate that flavonoids are involved in defense against the virus infection, as these molecules binds to the active site of ßC1 protein. This information might be interesting to study plant defense mechanism based on the special compounds produced by the plants.

Cinnamic acid as an inhibitor of growth, flavonoids exudation and endophytic fungus colonization in maize root

Cinnamic acid (CA) is an allelochemical that inhibits the growth of root promoting soil microorganisms. To prevent the growth of soil microbes, CA modulates several metabolic pathways in host plants and soil microbes. The aim of the current study was to investigate the effect of CA on maize root growth, exudation of secondary metabolites and its interaction with beneficial endophyte Pz11. The endophyte Pz11 was isolated from the roots of drought stressed Asphodelus tenuifolius (wild onion). The Pz11 strain was identified as Fusarium culmorum by homology of the internal transcribed spacer (ITS) region of 18 S rDNA sequence. The F. culmorum Pz11 produced phytostimulants and signaling compounds, such as indole-3-acetic acid (IAA), flavonoids and sugars. Moreover, the strain have effectively colonized the roots of maize and subsequently enhanced the growth of its host plants. On the contrary, application of CA has reduced root growth in maize seedlings as well as root colonization ability of F. culmorum Pz11. Also, maize seedlings exposed to CA exude low quantities of flavonoids and polyphenols. In conclusion, CA reduces the maize root growth and exudation of secondary metabolites, which may affects its ability to attract plant growth promoting endophytic fungi.

Plant Host-Associated Mechanisms for Microbial Selection

Plants serve as host to numerous microorganisms. The members of these microbial communities interact among each other and with the plant, and there is increasing evidence to suggest that the microbial community may promote plant growth, improve drought tolerance, facilitate pathogen defense and even assist in environmental remediation. Therefore, it is important to better understand the mechanisms that influence the composition and structure of microbial communities, and what role the host may play in the recruitment and control of its microbiome. In particular, there is a growing body of research to suggest that plant defense systems not only provide a layer of protection against pathogens but may also actively manage the composition of the overall microbiome. In this review, we provide an overview of the current research into mechanisms employed by the plant host to select for and control its microbiome. We specifically review recent research that expands upon the role of keystone microbial species, phytohormones, and abiotic stress, and in how they relate to plant driven dynamic microbial structuring.

Ectomycorrhizal host specificity in a changing world: can legacy effects explain anomalous current associations?

Despite the importance of ectomycorrhizal (ECM) fungi in forest ecosystems, knowledge about the ecological and co-evolutionary mechanisms underlying ECM host associations remains limited. Using a widely distributed group of ECM fungi known to form tight associations with trees in the family Pinaceae, we characterized host specificity among three unique Suillus-host species pairs using a combination of field root tip sampling and experimental bioassays. We demonstrate that the ECM fungus S. subaureus can successfully colonize Quercus hosts in both field and glasshouse settings, making this species unique in an otherwise Pinaceae-specific clade. Importantly, however, we found that the colonization of Quercus by S. subaureus required co-planting with a Pinaceae host. While our experimental results indicate that gymnosperms are required for the establishment of new S. subaureus colonies, Pineaceae hosts are locally absent at both our field sites. Given the historical presence of Pineaceae hosts before human alteration, it appears the current S. subaureus-Quercus associations represent carryover from past host presence. Collectively, our results suggest that patterns of ECM specificity should be viewed not only in light of current forest community composition, but also as a legacy effect of host community change over time.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

Soil propagule banks of ectomycorrhizal fungi share many common species along an elevation gradient

We conducted bioassay experiments to investigate the soil propagule banks of ectomycorrhizal (EM) fungi in old-growth forests along an elevation gradient and compared the elevation pattern with the composition of EM fungi on existing roots in the field. In total, 150 soil cores were collected from three forests on Mt. Ishizuchi, western Japan, and subjected to bioassays using Pinus densiflora and Betula maximowicziana. Using molecular analyses, we recorded 23 EM fungal species in the assayed propagule banks. Eight species (34.8 %) were shared across the three sites, which ranged from a warm-temperate evergreen mixed forest to a subalpine conifer forest. The elevation pattern of the assayed propagule banks differed dramatically from that of EM fungi on existing roots along the same gradient, where only a small proportion of EM fungal species (3.5 %) were shared across sites. The EM fungal species found in the assayed propagule banks included many pioneer fungal species and composition differed significantly from that on existing roots. Furthermore, only 4 of 23 species were shared between the two host species, indicating a strong effect of bioassay host identity in determining the propagule banks of EM fungi. These results imply that the assayed propagule bank is less affected by climate compared to EM fungal communities on existing roots. The dominance of disturbance-dependent fungal species in the assayed propagule banks may result in higher ecosystem resilience to disturbance even in old-growth temperate forests.

Rice Seed Priming with Picomolar Rutin Enhances Rhizospheric Bacillus subtilis CIM Colonization and Plant Growth

The effect of rutin, a bioflavonoid on the growth and biofilm formation of Bacillus subtilis strain CIM was investigated. In addition to swimming, swarming, and twitching potentials of B. subtilis CIM (BS), one picomolar (1 pM) of rutin was also observed to boost the biofilm forming ability of the bacterium. Bio-priming of rice seeds with BS and rutin not only augmented root and shoot lengths but also the photosynthetic pigments like chlorophyll and carotenoid. Similarly, high accumulation of phenolic and flavonoid contents was observed in the leaves. Fluorescent microscopic images revealed that BS plus rutin enhanced callose deposition in the leaves. It was also established that the least formation of reactive oxygen species in BS plus rutin treated rice plants was due to higher free radicals scavenging activity and total antioxidant potential. The results highlight chemo attractant nature of BS towards rutin, which by enhancing biofilm formation and root colonization indirectly strengthened the plants' defensive state.

Molecular signals required for the establishment and maintenance of ectomycorrhizal symbioses

Ectomycorrhizal (ECM) symbioses are among the most widespread associations between roots of woody plants and soil fungi in forest ecosystems. These associations contribute significantly to the sustainability and sustainagility of these ecosystems through nutrient cycling and carbon sequestration. Unfortunately, the molecular mechanisms controlling the mutual recognition between both partners are still poorly understood. Elegant work has demonstrated that effector proteins from ECM and arbuscular mycorrhizal (AM) fungi regulate host defenses by manipulating plant hormonal pathways. In parallel, genetic and evolutionary studies in legumes showed that a 'common symbiosis pathway' is required for the establishment of the ancient AM symbiosis and has been recruited for the rhizobia-legume association. Given that genes of this pathway are present in many angiosperm trees that develop ectomycorrhizas, we propose their potential involvement in some but not all ECM associations. The maintenance of a successful long-term relationship seems strongly regulated by resource allocation between symbiotic partners, suggesting that nutrients themselves may serve as signals. This review summarizes our current knowledge on the early and late signal exchanges between woody plants and ECM fungi, and we suggest future directions for decoding the molecular basis of the underground dance between trees and their favorite fungal partners.

Novel findings on the role of signal exchange in arbuscular and ectomycorrhizal symbioses

The availability of genome sequences from both arbuscular and ectomycorrhizal fungi and their hosts has, together with elegant biochemical and molecular biological analyses, provided new information on signal exchange between the partners in mycorrhizal associations. The progress in understanding cellular processes has been more rapid in arbuscular than ectomycorrhizal symbiosis due to its similarities of early processes with Rhizobium-legume symbiosis. In ectomycorrhiza, the role of auxin and ethylene produced by both fungus and host plant is becoming understood at the molecular level, although the actual ligands and receptors leading to ectomycorrhizal symbiosis have not yet been discovered. For both systems, the functions of small effector proteins secreted from the respective fungus and taken up into the plant cell may be pivotal in understanding the attenuation of host defense. We review the subject by comparing cross-talk between fungal and plant partners during formation and establishment of arbuscular and ectomycorrhizal symbioses.

The fungal leaf endophyte Paraconiothyrium variabile specifically metabolizes the host-plant metabolome for its own benefit

Fungal endophytes live inside plant tissues and some have been found to provide benefits to their host. Nevertheless, their ecological impact is not adequately understood. Considering the fact that endophytes are continuously interacting with their hosts, it is conceivable that both partners have substantial influence on each other's metabolic processes. In this context, we have investigated the action of the endophytic fungus Paraconiothyrium variabile, isolated from the leaves of Cephalotaxus harringtonia, on the secondary metabolome of the host-plant. The alteration of the leaf compounds by the fungus was monitored through metabolomic approaches followed by structural characterization of the altered products. Out of more than a thousand molecules present in the crude extract of the plant leaf, we have observed a specific biotransformation of glycosylated flavonoids by the endophyte. In all cases it led to the production of the corresponding aglycone via deglycosylation. The deglycosylated flavonoids turned out to display significant beneficial effects on the hyphal growth of germinated spores. Our finding, along with the known allelopathic role of flavonoids, illustrates the chemical cooperation underlying the mutualistic relationship between the plant and the endophyte.

Oak root response to ectomycorrhizal symbiosis establishment: RNA-Seq derived transcript identification and expression profiling

Ectomycorrhizal symbiosis is essential for the life and health of trees in temperate and boreal forests where it plays a major role in nutrient cycling and in functioning of the forest ecosystem. Trees with ectomycorrhizal root tips are more tolerant to environmental stresses, such as drought, and biotic stresses such as root pathogens. Detailed information on these molecular processes is essential for the understanding of symbiotic tissue development in order to optimize the benefits of this natural phenomenon. Next generation sequencing tools allow the analysis of non model ectomycorrhizal plant-fungal interactions that can contribute to find the "symbiosis toolkits" and better define the role of each partner in the mutualistic interaction. By using 454 pyrosequencing we compared ectomycorrhizal cork oak roots with non-symbiotic roots. From the two cDNA libraries sequenced, over 2 million reads were obtained that generated 19,552 cork oak root unique transcripts. A total of 2238 transcripts were found to be differentially expressed when ECM roots were compared with non-symbiotic roots. Identification of up- and down-regulated gens in ectomycorrhizal roots lead to a number of insights into the molecular mechanisms governing this important symbiosis. In cork oak roots, ectomycorrhizal colonization resulted in extensive cell wall remodelling, activation of the secretory pathway, alterations in flavonoid biosynthesis, and expression of genes involved in the recognition of fungal effectors. In addition, we identified genes with putative roles in symbiotic processes such as nutrient exchange with the fungal partner, lateral root formation or root hair decay. These findings provide a global overview of the transcriptome of an ectomycorrhizal host root, and constitute a foundation for future studies on the molecular events controlling this important symbiosis.

Soil nutrient content influences the abundance of soil microbes but not plant biomass at the small-scale

Small-scale heterogeneity of abiotic and biotic factors is expected to play a crucial role in species coexistence. It is known that plants are able to concentrate their root biomass into areas with high nutrient content and also acquire nutrients via symbiotic microorganisms such as arbuscular mycorrhizal (AM) fungi. At the same time, little is known about the small-scale distribution of soil nutrients, microbes and plant biomass occurring in the same area. We examined small-scale temporal and spatial variation as well as covariation of soil nutrients, microbial biomass (using soil fatty acid biomarker content) and above- and belowground biomass of herbaceous plants in a natural herb-rich boreonemoral spruce forest. The abundance of AM fungi and bacteria decreased during the plant growing season while soil nutrient content rather increased. The abundance of all microbes studied also varied in space and was affected by soil nutrient content. In particular, the abundance of AM fungi was negatively related to soil phosphorus and positively influenced by soil nitrogen content. Neither shoot nor root biomass of herbaceous plants showed any significant relationship with variation in soil nutrient content or the abundance of soil microbes. Our study suggests that plants can compensate for low soil phosphorus concentration via interactions with soil microbes, most probably due to a more efficient symbiosis with AM fungi. This compensation results in relatively constant plant biomass despite variation in soil phosphorous content and in the abundance of AM fungi. Hence, it is crucial to consider both soil nutrient content and the abundance of soil microbes when exploring the mechanisms driving vegetation patterns.

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.

Germination and infectivity of ectomycorrhizal fungal spores in relation to their ecological traits during primary succession

The spores of ectomycorrhizal fungi (EMF) play critical roles in the population and community development of EMF. Here, the germination and infectivity of EMF spores are examined with reference to the ecological traits of the EMF species. Spores were collected from 12 EMF species, whose successional patterns have been studied in the volcanic desert on Mount Fuji, Japan. Spore germination experiments were conducted with host plants (Salix reinii), with nonhost plants (Polygonum cuspidatum), and without plants. The mycorrhizal formation ability of spores was also examined in seven EMF using spore inoculation experiments. To determine the effects of the spore preservation period, both experiments were repeated up to 1 yr after spore collection. Spore germination was very low in the absence of host plants. In the presence of hosts, even 30 d after spore collection, spore germination was significantly enhanced in all pioneer EMF (c. 20%) but less so in late-stage EMF (< 5%), except in Hebeloma species. Mycorrhizal formation from spores was also greater in pioneer EMF but was significantly reduced by 1 yr of spore preservation. High spore germination and infectivity of pioneer EMF should enable these species to colonize disturbed and isolated areas in accordance with their ecological traits.