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The Role of Arbuscular Mycorrhizal and Dark Septate Fungi in an Invasive Plant of Patagonian Wetlands
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Arbuscular mycorrhizal and dark septate fungi are common plant symbionts, but their role in promoting host plant fitness depends on environmental variables. Particularly in wetland plants, these associations are less understood. We analysed the role of arbuscular mycorrhizal fungi (AMF) and dark septate fungi (DSF) in the roots of Potentilla anserina (Rosaceae), an invasive species of Patagonia, widely distributed in wetlands. We tested three hypotheses: that fungi colonization varies according to soil moisture and nutrient content (nitrogen and phosphorus), that they enhance P. anserina nutrient content, and benefit plant growth. We measured the percentage of colonization in plants from five wetlands across a moisture gradient with different nutrient content, and performed a growth experiment with soil from these wetlands to evaluate changes in mycorrhizal and endophytic fungal colonization, aerial nutrient content and biomass production. In the field, root colonization by AMF was high in all sites (~90%), whereas DSF was less abundant (~20%), positively related to soil organic matter, and negatively related to soil phosphorus. In the experiment, DSF colonization was inversely related to increasing tissue N and P content. Potentilla anserina grew similarly in all the treatments, but biomass was positively related to DSF colonization. Our results provide evidence that DSF, rather than AMF, confer to this invasive species the ability to grow in soils with different water and nutrient content and may help to explain the wide distribution of this alien species in Patagonian wetlands.
Societal Impact Statement
The world faces major changes in rainfall patterns and water availability, posing a significant threat to plant productions systems and food security. The arbuscular mycorrhizal (AM) fungi associate with most major crops and can support plant nutrient and water uptake. Here, AM fungi were shown to mitigate the negative effects of low water availability on sorghum growth and phosphorus uptake, an effect that was associated with shifts in the fungal community structure. To realise the potential of AM fungi in sustainable agriculture requires more examination of their interactions with edaphic stresses in crop systems.
Most plants form mycorrhizal associations with mutualistic soil fungi. Through these partnerships, resources are exchanged including photosynthetically fixed carbon for fungal‐acquired nutrients. Recently, it was shown that the diversity of associated fungi is greater than previously assumed, extending to Mucoromycotina fungi. These Mucoromycotina ‘fine root endophytes’ (MFRE) are widespread and generally co‐colonise plant roots together with Glomeromycotina ‘coarse’ arbuscular mycorrhizal fungi (AMF). Until now, this co‐occurrence has hindered the determination of the direct function of MFRE symbiosis.
To overcome this major barrier, we developed new techniques for fungal isolation and culture and established the first monoxenic in vitro cultures of MFRE colonising a flowering plant, clover. Using radio‐ and stable‐isotope tracers in these in vitro systems, we measured the transfer of ³³P, ¹⁵N and ¹⁴C between MFRE hyphae and the host plant.
Our results provide the first unequivocal evidence that MFRE fungi are nutritional mutualists with a flowering plant by showing that clover gained both ¹⁵N and ³³P tracers directly from fungus in exchange for plant‐fixed C in the absence of other micro‐organisms.
Our findings and methods pave the way for a new era in mycorrhizal research, firmly establishing MFRE as both mycorrhizal and functionally important in terrestrial ecosystems.
Introducción y objetivos
: Los humedales son ecosistemas naturales relevantes para la biodiversidad y aseguran una variedad de servicios ecosistémicos. Sin embargo, son ambientes susceptibles a las invasiones de especies. La especie introducida e invasora Potentilla anserina se encontró en humedales de la Patagonia. Los objetivos de este estudio fueron determinar la distribución de esta especie en el Parque Nacional Nahuel Huapi y conocer cómo se relaciona con los tipos de hábitats y usos del suelo.
M&M
: Estudiamos la distribución de P. anserina en 28 humedales a lo largo de un gradiente de precipitaciones (55,4 km Este-Oeste), en el Parque Nacional Nahuel Huapi, en relación con el tipo de ambiente, hábitat, grado de urbanización y presencia de ganado. También, relacionamos su cobertura con la riqueza de especies en 8 de los mallines analizados.
Resultados
: P. anserina estuvo correlacionada con humedales someros y temporarios y con la presencia de ganado. La cobertura de P. anserina no superó el 10% (con una excepción) y no estuvo relacionada con la riqueza de especies.
Conclusión
: Debido a la alta capacidad de dispersión de P. anserina en humedales patagónicos y la alta cobertura observada en uno de los mallines estudiados, nuestros resultados alertan sobre el potencial invasor de esta especie introducida.
Background
Soils harbour a remarkable diversity of interacting fungi, bacteria, and other microbes: together these perform a wide variety of ecological roles from nutrient cycling and organic matter breakdown, to pathogenic and symbiotic interactions with plants. Many studies demonstrate the role of microbes in plant-soil feedbacks and their interactions with plants. However, interactions among microbes are seldom addressed, and there is no consensus regarding the nature and outcomes of interactions among microbial functional guilds.
Scope
Here, we critically review what is known about microbe-microbe interactions among functional guilds within the plant-soil system, with the aim to initiate a path to disentangling the “microbe black-box”. Our review confirms that the nature of microbial interactions among major functional guilds is explained by niche theory. This means that, among microbes, a competitive relationship is likely when their benefits to plants, source of carbon and nutrients, or nutrient scavenging mechanisms overlap, while a neutral-to-facilitative relationship is likely when these microbial traits differ or complement each other.
Conclusions
We highlight the numerous knowledge gaps and provide a framework to characterise microbe-microbe interactions that offers insight into the contributions of microbes to key ecosystem functions such as carbon sequestration and nutrient cycling.
This study determined the influence of soil nitrogen and phosphorus on the colonisation dynamics of arbuscular mycorrhizal fungi (AMF) and Dark Septate Endophytes (DSE) in plant species adapted to nutrient poor soils. Plant colonisation dynamics were investigated using a field experiment and a manipulative greenhouse experiment on Australian native seedlings. Tubestock of Poa labillardierei were transplanted into the field to large tracts of land of high and low nutrient availability, near Sydney, Australia, and root colonisation was determined from monthly plant collections over three months, between late austral autumn and through winter. In the greenhouse experiment, two native shrubs and one grass, respectively, Rhagodia candolleana, Dodonaea triquetra and P. labillardierei, were germinated from seeds and grown under a full factorial nitrogen and phosphorus fertilisation experiment for three months before root colonisation was determined. The selected study species were of varied mycotrophy. AMF colonisation in both experiments was lowest in plants from high nutrient treatments. Removal of either nitrogen or phosphorus from the fertiliser resulted in the same patterns of colonisation, with AMF being greater when either nutrient was removed. DSE colonisation was greatest in high nutrient soils. However, colonisation did not vary with nutrient treatments in the glasshouse experiment, suggesting that nitrogen and phosphorus minimally affect DSE colonisation. The difference in AMF and DSE colonisation patterns demonstrates a distinction in how different fungal symbionts are affected by abiotic factors. Both nitrogen and phosphorus are important drivers of colonisation dynamics of AMF. The results indicate the potential for disparate controls of AMF and DSE root associations.
There is evidence that the distribution of ecotypes of plants and their symbiotic arbuscular mycorrhizal (AM) fungi and other associated soil biota may be structured by the availability of essential soil nutrients; and that locally adapted partnerships most successfully acquire limiting nutrients. This study tests the hypotheses that plant genotypes are adapted to the water availability of their local environment, and this adaptation involves associations with local soil biota, including AM fungi.
We grew semi‐arid Bouteloua gracilis ecotypes from relatively wet and dry sites, with either sympatric or allopatric soil inoculum under moderate and extreme soil drying treatments to examine (a) how varying degrees of water limitation influence grass responses to soil biota and (b) the relationship between AM fungal structures and the responses.
Under extreme soil drying, the dry site ecotype tended to perform better than the wet site ecotype. Both ecotypes performed best in either drying treatment when inoculated with their sympatric soil biota. Sympatric pairings produced more AM fungal hyphae, arbuscules and dark septate fungi. Extreme soil drying tended to accentuate these apparent benefits of sympatry to both plants and fungal symbionts, relative to the moderate drying treatment.
Our findings support the hypothesis that AM symbioses help Bouteloua gracilis ecotypes adapt to local water availability. This conclusion is based on the observations that as water became increasingly limited, sympatric partnerships produced more AM fungal hyphae and arbuscules and fewer vesicles. The abundances of hyphae and arbuscules were positively correlated with plant growth, suggesting that in sympatric pairs of plants and AM fungi, allocation to fungal structures is optimized to maximize benefits and minimize the costs of the symbioses. This provides strong evidence that co‐adaptation among plants and their associated AM fungi can ameliorate drought stress.
Synthesis. Our study documents the role of locally adapted soil borne plant symbionts in ameliorating water stress. We found a relationship between AM fungal structures in roots and plant performance. Generally, plants and fungi from the same site resulted in more positive effects on plant growth.
Background:
Wetlands in Neotropics harbor high fungal diversity, including arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE). This study describes the interaction of plant roots with AMF and DSE in a freshwater wetland belonging to a hotspot of biodiversity.
Hypothesis:
Differential root colonization between arbuscular mycorrhizal and dark septate endophyte fungi is influenced by plant species and abiotic conditions in a freshwater wetland.
Studied species:
Plant species colonized by arbuscular mycorrhizal and dark septate endophyte fungi.
Methods:
Properties of soils and the water column, floristic composition, root colonization by AMF and DSE, and molecular identification of AMF inside roots were studied.
Results:
Soils were Gleysol and flooded during the rainy season. Most of identified plant species were herbaceous, with Cyperus articulatus and Mimosa pigra as the dominant species. Seven of 8 analyzed plant species exhibited differential co-colonization between AMF and DSE. Repeated sampling for one year under flooding/dry conditions demonstrated that C. articulatus and M. pigra were mainly associated with DSE and AMF, respectively. A positive correlation between dissolved O2 in the water column and fungal colonization was observed in C. articulatus. Glomerales and Archaeosporales were molecularly identified inside roots containing arbuscules of M. pigra.
Conclusions:
Findings highlight differential coexistence between AMF and DSE in plant roots; fungal colonization was influenced by flooding/dry conditions in a neotropical wetland; the community of AMF inside arbusculated roots of M. pigra includes at least four clades.
La vegetación de los humedales desempeña un papel crucial en los ecosistemas andinos, fundamentales por su biodiversidad, por ofrecer numerosos servicios ecosistémicos y por mitigar el cambio climático. A pesar de esto, se conoce muy poco sobre la riqueza y la diversidad de las especies presentes en los humedales y, en particular, en los mallines urbanos. El objetivo de este trabajo fue caracterizar y cuantificar la composición florística de cuatro mallines con distinto grado de urbanización y uso de la tierra (e.g., ganadero, recreativo, turístico, industrial, etc.) dentro del ejido de la ciudad de San Carlos de Bariloche. Se encontró una riqueza total de 81 especies, de las cuales 50% fueron de origen exótico. Además, nuestros resultados muestran una relación negativa entre la distancia al centro de la ciudad y el porcentaje de especies exóticas. Se señala la presencia de especies nativas exclusivas en cada mallín y se alerta sobre la posible invasión de la especie exótica Potentilla anserina, hallada en los cuatro mallines, lo cual pone en evidencia la necesidad de proteger estos ambientes vulnerables.
Anthropogenic landscape modification can disrupt mutualistic interactions between native plants and soil microbes. Restoration of native vegetation in disturbed habitats may depend upon reconnecting plants with their fungal symbionts, such as arbuscular mycorrhizal fungi (AMF). We compared levels of root colonisation by AMF (arbuscules, vesicles, aseptate hyphae) and dark septate endophytes (DSE; septate hyphae, microsclerotia) between reconstructed and remnant dunes along the southern coastline of New South Wales (Australia) for two native plants: Lomandra longifolia and Carpobrotus glaucescens. Reconstruction was undertaken approximately 30 years ago to reinstate native vegetation and reduce erosion on dunes deforested by European settlers during the 1800s. Fungal colonisation was assessed using the point-intercept method on stained root sections under a light microscope. Root colonisation by AMF did not differ significantly between reconstructed and remnant dune habitats, but did vary significantly amongst sites. In contrast, DSE was two times lower for L. longifolia plants growing in reconstructed compared with remnant fore dunes. Our finding of reduced DSE colonisation in reconstructed dunes may indicate that impacts of land clearing on plant–fungal associations may persist over long time periods for some key plant species. Reduced DSE colonisation may be associated with limited restoration potential and functioning of reconstructed fore dune ecosystems. Future research will be needed to assess the scale of reduced DSE across reconstructed coastal habitats, the role of plant–DSE relationships in vegetation community function, and implications of reduced DSE for ecosystem restoration.
Arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) are two fungal groups that colonize plant roots and can benefit plant growth, but little is known about their landscape distributions. We performed sequencing and microscopy on a variety of plants across a high-elevation landscape featuring plant density, snowpack, and nutrient gradients. Percent colonization by both AMF and DSE varied significantly among plant species, and DSE colonized forbs and grasses more than sedges. AMF were more abundant in roots at lower elevation areas with lower snowpack and lower phosphorus and nitrogen content, suggesting increased hyphal recruitment by plants to aid in nutrient uptake. DSE colonization was highest in areas with less snowpack and higher inorganic nitrogen levels, suggesting an important role for these fungi in mineralizing organic nitrogen. Both of these groups of fungi are likely to be important for plant fitness and establishment in areas limited by phosphorus and nitrogen.
The presence of arbuscular mycorrhizal fungi (AMF) in wetlands is widespread. Wetlands are transition ecosystems between aquatic and terrestrial systems, where shallow water stands or moves over the land surface. The presence of AMF in wetlands suggests that they are ecologically significant; however, their function is not yet clearly understood. With the aim of determining the overall magnitude and direction of AMF effect on wetland plants associated with them in pot assays, we conducted a meta-analysis of data extracted from 48 published studies. The AMF effect on their wetland hosts was estimated through different plant attributes reported in the studies including nutrient acquisition, photosynthetic activity, biomass production, and saline stress reduction. As the common metric, we calculated the standardized unbiased mean difference (Hedges’ d) of wetland plant performance attributes in AMF-inoculated plants versus non-AMF-inoculated plants. Also, we examined a series of moderator variables regarding symbiont identity and experimental procedures that could influence the magnitude and direction of an AMF effect. Response patterns indicate that wetland plants significantly benefit from their association with AMF, even under flooded conditions. The beneficial AMF effect differed in magnitude depending on the plant attribute selected to estimate it in the published studies. The nature of these benefits depends on the identity of the host plant, phosphorus addition, and water availability in the soil where both symbionts develop. Our meta-analysis synthetizes the relationship of AMF with wetland plants in pot assays and suggests that AMF may be of comparable importance to wetland plants as to terrestrial plants.
The majority of vascular plants are mycorrhizal: 72% are arbuscular mycorrhizal (AM), 2.0% are ectomycorrhizal (EcM), 1.5% are ericoid mycorrhizal and 10% are orchid mycorrhizal. Just 8% are completely nonmycorrhizal (NM), whereas 7% have inconsistent NM–AM associations. Most NM and NM–AM plants are nutritional specialists (e.g. carnivores and parasites) or habitat specialists (e.g. hydrophytes and epiphytes). Mycorrhizal associations are consistent in most families, but there are exceptions with complex roots (e.g. both EcM and AM). We recognize three waves of mycorrhizal evolution, starting with AM in early land plants, continuing in the Cretaceous with multiple new NM or EcM linages, ericoid and orchid mycorrhizas. The third wave, which is recent and ongoing, has resulted in root complexity linked to rapid plant diversification in biodiversity hotspots.
There have been several reports of symbionts in the roots of plants that live in aquatic environments. Arbuscular mycorrhizal fungi (AMF) are the most common microsymbionts and possibly recolonized the aquatic environment together with plants; however, their functions and the extent of their benefits are unclear. Furthermore, the presence of other groups of fungi, such as dark septate fungi (DSF), with functions supposedly analogous to those of mycorrhizal fungi, has also been reported. The present work provides a compilation of data regarding the presence of arbuscular mycorrhizae in plants from, or under the influence of, aquatic environments, and co-colonization by AMF and DSF. Forty species of non-vascular plants, ferns, fern allies, and gymnosperms from 15 families, and 659 species of angiosperms from 87 families were investigated. From the first group (non-flowering plants) 57 % of the species showed arbuscular mycorrhizal structures in their tissues or roots, whereas among the second group (flowering plants) 71% had such structures. Among the mycorrhizal angiosperms, 52 % showed arbuscules in their roots. DSF were found in 1% of non-flowering plants and 5 % of angiosperms. All of these are discussed in this review.
We studied the arbuscular mycorrhizal (AM) and dark septate endophyte (DSE) fungal associations in five species of pot grown Asparagus (A. aethiopicus, A. densiflorus, A. setaceus, A. racemosus, and A. umbellatus). Root colonization by AM and DSE fungi and AM spore numbers in the soil were assessed. AM fungal diversity indices like species richness, Shannon–Weiner index, Simpson’s index, evenness, and Jaccard’s index were determined. Relationships among the fungal and soil variables were also examined. All the species of Asparagus examined were colonized by both AM and DSE fungi, and these associations have been reported for the first time in four species. Root colonization by both the fungi varied significantly among Asparagus species and was related to each other and soil factors. The AM colonization patterns reported in four Asparagus species for the first time were characterized by either intermediate or Arum–Paris type. The AM fungal community was dominated by members belonging to the Glomerales and was significantly influenced by Asparagus species and soil factors. We conclude that Asparagus species could support a diverse AM fungal community even under pot conditions; however, certain AM fungal species are sensitive to changes in soil factors and host species.
The hypothesis of this study is that the variable hydrological regime of intermittent lakes and wetlands affects the primary productivity, decomposition and root fungal colonisation of Phragmites australis, with effects on the whole ecosystem metabolism. The above-ground biomass of reed stands was monitored in littoral and riparian stands of Lake Cerknica, under different water levels. Leaf and culm litter decomposition was also studied at three locations, from predominantly dry to permanently submerged. Root colonisation by fungi was monitored across two seasons in the littoral reed stand. The primary productivity of the littoral stands was related to a variable degree with water levels and air temperature in July in the current year. In the riparian reeds, only monthly temperatures in October of the previous year were important. The submersion duration of the litter enhanced the decomposition rates. The frequency of fungal colonisation of reed roots was decreasing with the submersion time length. Overall, roots were colonised by arbuscules, hyphae, vesicles, hyphal coils and microsclerotia. The main outcomes of this study deals with the role of intermittent hydrological conditions in determining the structure and functioning of reed-dominated lacustrine wetlands, which are exposed to climate threats.
In this article we review the main characteristics of the Patagonian climate, the spatial and temporal patterns of the most important climatic variables, and the influence of climate on ecosystem processes. The winter distribution of precipitation determines an asynchrony between the wet and the growing season in Patagonia. The amount of water that can be transferred from the wet season to the growing season depends mainly on the physical characteristics of the soil. In the semiarid steppe of Chubut, drainage accounted for 10% of annual precipitation. Winter distribution of precipitation determines also an asynchronic dynamics of evaporation and transpiration fluxes. The ENSO phenomenon have a significant impact on regional precipitation. In central-west Patagonia, spring precipitation (September to November) was lower than normal during La Nina events and greater than normal during El Nino events. From December to February the opposite pattern can be observed: higher than normal precipitation during La Nina events and lower than normal precipitation during El Nino events. The impact of this phenomenon on the seasonal temperature was not as clear as for precipitation. We did not detect any temporal trends in annual precipitation for the period 1961-1996. The phenology of carbon gains is quite homogeneous in Patagonia. Most of the region showed a peak of production in November, when, simultaneously, water availability and temperature are high. Toward the west, production peaked later (December). Deciduous forests showed the peak in January and February.
The capacity of dark septate endophytes (DSE; Phialocephala turiciensis, Acephala applanata, P. glacialis and Phaeomollisia piceae) to solubilize inorganic phosphate (P) and to mineralize the organic form was studied. We analysed the effect of DSE strains on P uptake by Trifolium repens in the presence or absence of arbuscular mycorrhizal fungi (AMF). Phosphatases were observed both in the absence of the host plant and the organic resource, showing that the P mineralization process is not induced by the enzyme substrate or the host. DSE were more efficient at mineralizing organic P. Independently of the presence of AMF, DSE increased the pool of P in the soil, with significant differences being found in P levels among the different DSE. In contrast, plant P uptake was increased by AMF. The P content of plants increased with the co-inoculation of AMF and P. turiciensis or P. piceae. We hypothesize a close relationship between DSE and AMF in relation to P availability and uptake in plants. Whereas DSE increase the pool of P in the rhizosphere, AMF are responsible for P transfer to the host, with co-colonization of plants by DSE and AMF showing a synergistic outcome.
For the first time, hybrids of Spartina alterniflora x foliosa are reported to form mycorrhizal associations. This is important in light of the invasion dynamics within San Francisco Bay-where Spartina hybrids are invading tidal habitats and causing functional changes in the ecosystem. Mycorrhizal associations can positively influence biomass production in invasive Spartina and may contribute to increased invasion success. Of the Spartina hybrids investigated, 83% were mycorrhizal. During hybridization, the ability to be mycorrhizal may be contributed by the native S. foliosa, also found to be mycorrhizal, whereas the introduced S. alterniflora is non-mycorrhizal in its native habitat. Seedlings of Spartina hybrids inoculated with a commercial mycorrhizal mix showed greater above-ground growth and total biomass compared to control plants in the greenhouse. Mycorrhizal associations have the po-tential to influence the invasion trajectory of hybrid Spartina in San Francisco Bay, but additional research is needed.
Utilization of major forms of carbon, nitrogen and phosphorus commonly present in plant litter and detritus was determined for cultures of Phialophora finlandia, Phialocephala fortinii and five dark-septate, root endophyte isolates from alpine plant communities. All cultures utilized cellulose, laminarin, starch and xylan as sole carbon source. Protein and ribonucleic acids were hydrolyzed by all cultures as sole nitrogen and phosphorus sources, respectively. The fatty acid ester, Tween 40, was hydrolyzed by all cultures. None of the cultures decolorized two polymeric dyes used as presumptive tests for lignolytic activity. These hydrolytic capabilities suggest that these dark-septate root endophytes, either as biotrophs or saprotrophs, are able to access major organic detrital nutrient pools.
Plant tissues host a variety of fungi. One important group is the dark septate endophytes (DSEs) that colonize plant roots and form characteristic intracellular structures - melanized hyphae and microsclerotia. The DSE associations are common and frequently observed in various biomes and plant taxa. Reviews suggest that the proportion of plant species colonized by DSE equal that colonized by AM and microscopic studies show that the proportion of the root system colonized by fungi DSE can equal, or even exceed, the colonization by AM fungi. Despite the high frequency and suspected ecological importance, the effects of DSE colonization on plant growth and performance have remained unclear. Here, we draw from over a decade of experimentation with the obscure DSE symbiosis and synthesize across large bodies of published and unpublished data from Arabidopsis thaliana and Allium porrum model systems as well as from experiments that use native plants to better resolve the host responses to DSE colonization. The data indicate similar distribution of host responses in model and native plant studies, validating the use of model plants for tractable dissection of DSE symbioses. The available data also permit empirical testing of the environmental modulation of host responses to DSE colonization and refining the "mutualism-parasitism-continuum" paradigm for DSE symbioses. These data highlight the context dependency of the DSE symbioses: not only plant species but also ecotypes vary in their responses to populations of conspecific DSE fungi - environmental conditions further shift the host responses similar to those predicted based on the mutualism-parasitism-continuum paradigm. The model systems provide several established avenues of inquiry that permit more detailed molecular and functional dissection of fungal endophyte symbioses, identifying thus likely mechanisms that may underlie the observed host responses to endophyte colonization.
Investigations on the prevalence of arbuscular mycorrhizal (AM) and dark septate endophyte (DSE) fungal symbioses are limited for plants growing intropical aquatic andwetlandhabitats compared to those growingon terrestrialmoist or dryhabitats.Therefore, we assessed the incidence of AM and DSE symbiosis in 8 hydrophytes and 50 wetland plants from four sites in south India. Of the 58 plant species examined, we found AM and DSE fungal symbiosis in 21 and five species, respectively. We reported for the first time AM and DSE fungal symbiosis in seven and five species, respectively. Intermediate-type AM morphology was common, and AM morphology is reported for the first time in 16 plant species. Both AM and DSE fungal colonization varied significantly across plant species and sites. Intact and identifiable AM fungal spores occurred in root zones of nine plant species, but AM fungal species richness was low. Though no clear relationship between AM and DSE fungal colonization was recognized, a significant negative correlation between AM colonization and spore numbers was established. Our study suggests that the occurrence of AM and DSE fungal symbiosis in plants growing in hydrophytic and wetland habitats is not as common as in terrestrial habitats.
The knowledge on nutrient cycling in Patagonian ecosystems of Argentina is scarce. However, studies not directly focused on nutrient cycling provide relevant information about the mechanisms of nutrient conservation in the climatically different ecosystems of the region. Here, we identified indicators of litter decomposition and soil N mineralization rates of some representative species of the dominant plant functional groups along the wide precipitation gradient of Patagonia. Senescent leaves and litter of forest trees have higher C concentration and C/N ratio and lower N concentration than steppe shrubs. Within the tree life form, evergreen species have higher C/N ratio than deciduous species. Differences in N concentration between green and senescent leaves suggest a higher N use efficiency in forest trees than in steppe shrubs. Within the steppe, grasses have higher nutrient use efficiency than shrubs due to their higher C and much lower N, P and K content in senescent leaves and litter. Thus, we hypothesize the occurrence of (i) a gradient from nutrient-conserving ecosystems in wetter sites (Andean-Patagonian forest) to relatively nutrient-rich (low nutrient use efficient) ecosystems in drier sites (Patagonian steppe), and (ii) differences in nutrient conservation mechanisms among different functional groups: in the Andean-Patagonian forests N conservation and N use efficiency is greater in evergreens than in deciduous woody species and in conifers than in broad-leaf species, whereas in the Patagonian steppe, grasses have higher nutrient resorption than shrubs. Likely as a consequence of these differences in litter quality, potential N mineralization is greater in deciduous than in evergreen Patagonian forests. Within the steppe, N inineralization seems to depend on grass and shrub cover, which in turn is regulated by disturbance, largely grazing. Since nutrient conservation in vegetation, specially N, is associated to the rates of litter decomposition and soil N mineralization, the confirmation of these patterns would allow to predict ecosystem resilience and resistance to nutrient losses, and contribute to understand and predict the response of the different Patagonian species or functional groups to interannual climatic variability and natural or anthropogenic disturbances. There is a special need for further research on P cycling, nutrient allocation in vegetation, and field measurements of litter decomposition and N mineralization.
Aims
The most common metric of arbuscular mycorrhizal fungal (AMF) abundance is percent root length colonized (PRLC) by mycorrhizal structures. Frequently, plants with greater PRLC are assumed to receive more nutrients (such as phosphorus, P) from their mycorrhizal symbionts, leading to greater plant growth. Nevertheless, the functional significance of this metric remains controversial. In this review, I discuss whether manipulations of PRLC generally led to changes in plant biomass and P content, and whether AMF taxa and plant functional groups influence these relationships.
Methods
I conducted a meta-analysis of laboratory- and field-based trials in which mycorrhizal colonization was directly altered compared to unmanipulated controls. For each trial, I calculated (1) the difference in PRLC (ΔPRLC) between the treatments, and (2) the response ratio of plant biomass. In a subset of these studies, the response ratio of P content of host plants could also be calculated.
Results
The response ratio of plant biomass and P content rose significantly and exponentially as ΔPRLC increased. Nevertheless, ΔPRLC explained only a fraction of the variation in response ratios in each case. Moreover, AMF taxa varied in their effects on biomass per unit ΔPRLC. In addition, plant functional groups differed in effects on plant P content per unit ΔPRLC, with C4 grasses responding most strongly.
Conclusions
It appears that as the extent to which plant roots are colonized by AMF increases, plant growth and P content often increase, although substantial variability exists among trials. As others have found, a likely mechanism for this relationship is increased transfer of P (and perhaps other nutrients) through the more-prevalent mycorrhizal structures.
Arbuscular mycorrhizal fungi (AMF) were investigated in roots of 18 host plant species in a salinized south coastal plain of Laizhou Bay, China. From 18 clone libraries of 18S rRNA genes, all of the 22 AMF phylotypes were identified into Glomus, of which 18 and 4 were classified in group A and B in the phylogenetic tree, respectively. The phylotypes related to morphologically defined Glomus species occurred generally in soil with higher salinity. AMF phylotype richness, Shannon index, and evenness were not significantly different between root samples from halophytes vs. non-halophytes, invades vs. natives, or annuals vs. perennials. However, AMF diversity estimates frequently differed along the saline gradient or among locations, but not among pH gradients. Moreover, UniFrac tests showed that both plant traits (salt tolerance, life style or origin) and abiotic factors (salinity, pH, or location) significantly affected the community composition of AMF colonizers. Redundancy and variation partitioning analyses revealed that soil salinity and pH, which respectively explained 6.9 and 4.2 % of the variation, were the most influential abiotic variables in shaping the AMF community structure. The presented data indicate that salt tolerance, life style, and origin traits of host species may not significantly affect the AMF diversity in roots, but do influence the community composition in this salinized ecosystem. The findings also highlight the importance of soil salinity and pH in driving the distribution of AMF in plant and soil systems.
Vascular plants have been considered as autonomous organisms especially when their performance has been interpreted at the genome and cellular level. In reality, vascular plants provide a unique ecological niche for diverse communities of cryptic symbiotic microbes which often contribute multiple benefits, such as enhanced photosynthetic efficiency, nutrient and water use and tolerance to abiotic and biotic stress. These benefits are similar to improvements sought by plant scientists working to develop ecologically sustainable crops for food, fiber and biofuels. Native desert plants include a community of indigenous endosymbiotic fungi that are structural components with cells, tissues, cell cultures, and regenerated plants. These fungi regulate plant growth and development and contribute genes and natural products that enable plants to adapt to changing environments. A method developed for transferring these endophytes from cell cultures to non-host plants promises to be a revolutionary approach for the development of novel plant germplasm and has application in the field of plant biotechnology.
Many arbuscular mycorrhizal (AM) fungal species have worldwide distributions. However, it is not clear whether such species have adapted to local conditions. We compared the responses of mesic temperate and semi-arid tropical isolates of Glomus mosseae and Glomus etunicatum to extremes of temperature and moisture in a pot experiment. Treatments (warmmoist, warmdry, freeze/thawmoist, freeze/thawdry) were applied to whole soil mycorrhizal inoculum, and their effects were evaluated as both the change in viability of extraradical hyphae and mycorrhizal colonization of bait plants. Moist soil decreased hyphal viability compared with dry soil, irrespective of temperature, but mycorrhizal colonization of bait plants was lower in moist soil only when warm. Frost-heave could have physically ruptured hyphae in the freezingmoist soil without an effect on spores, but parasitism and (or) respiratory depletion of carbon reserves may have reduced survival of all propagules in the warmmoist soil. Hyphae of semi-arid tropical isolates survived all treatments better than hyphae of mesic temperate isolates, but these differences were not reflected in mycorrhizal colonization of bait plants. We found no evidence that these isolates have adapted to local conditions of moisture and temperature. Instead, wide environmental tolerances seem to be present within both populations of these AM fungal species.
Wetlands are subject to invasion by exotic plant species, especially during the dry season when they resemble terrestrial systems; therefore, terrestrial plants could exploit this situation to colonize this environment. We analyzed P. anserina invading Patagonian wetlands in terms of elemental ratios that would modify wetland stoichiometry due to organic matter inputs. We studied the elemental relationship (carbon/nitrogen/phosphorus) of P. anserina in comparison with native emergent macrophytes ( Eleocharis pachicarpa and Carex aematorrhyncha ). These plant species are common and dominant in the wetland. Additionally, we analyzed the presence of mycorrhizal fungi in the roots and their proportion of root infection. Our study reveals that the invasive species presented nutrient (especially phosphorus) allocation in roots and differences in mycorrhizal infection, with a predominance of arbuscular mycorrhiza, compared with native species. During flooded periods with the decay of aerial parts, P. anserina stores phosphorus in the roots and releases dissolved organic matter of high molecular weight molecules, high color, and a high C-to-nutrient ratio in comparison with native macrophytes. These results show the strategy of an invasive terrestrial plant in temporary aquatic systems, and how the elemental relationships of the invasive plant can modify the stoichiometry of the environment.
Arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) are two types of root symbiotic fungi that enhance nutrient uptake by host plants and their resistance to biotic and abiotic stresses. However, it remains unclear whether AMF and DSE are synergistic or antagonistic in the presence of host plants to environmental gradients, especially on large geographical scales. To determine the relationships between AMF and DSE and their adaptability on a regional scale, we measured AMF and DSE colonization in the roots of 1023 plants of different species within the Artemisia genus collected from 81 sites across central and eastern China. We used general linear mixed models to analyze the relationships between colonization, and temperature and precipitation conditions. We found no significant correlation between AMF and DSE. The AMF colonization rate followed a significant longitudinal trend, but there was no latitudinal pattern. DSE colonization did not follow any geographical pattern. The AMF colonization rate was positively correlated with temperature and precipitation, whereas it was not significantly correlated with soil. There was no significant correlation between DSE colonization and climate or soil. Our results suggest that AMF and DSE play independent roles in the response of Artemisia to the regional environment. Therefore, studies on mycorrhizal symbiosis should discern the differential responses between AMF and DSE to climate and soil when evaluating the adaptability of the two types of symbiosis on large geographical scales.
Arbuscular mycorrhizal fungi (AMF) have been widely reported to occur in the association with wetland plants. However, the factors that affect AMF colonization in wetland plants and physiological functions in AMF inoculated wetland plants are poorly studied. This study investigated the effects of four water regimes (below the surface of sands: water levels of 5 cm, 9 cm, 11 cm, and fluctuating water depth (9–11 cm)) on AMF root colonization in two wetland plants (Phalaris arundinacea and Scirpus sylvaticus) which are commonly used in constructed wetland. Results showed that two lower water regimes were the most suitable for the formation of root colonization by AMF. Plant species did not show any significant difference in AMF colonization. The AMF colonization of 15.6–23.3% in the roots of both wetland plants were determined under the water regimes of 11 cm and 9–11 cm. In comparison to the non-inoculated plants, root length, shoot height, biomass, shoot total phosphorus and chlorophyll contents of both wetland plants under the fluctuating water regimes (9–11 cm) were increased by 35.4–46.2%, 13.1–26.6%, 33.3–114.3%, 25.7–80% and 14.3–24%, respectively. Although malondialdehyde (MDA) contents in both AMF inoculated wetland plants were decreased under the lower water levels, the MDA contents under the water regime of 11 cm were still high. Therefore, these results indicated that the physiological functions in wetland plants with high AMF colonization might be improved under a specific water regime condition (e.g. depth of fluctuating water regime).
Urban wetlands play an important role in improving urban environment and microclimate, while they are suffering degradation and destruction during urbanization process. Arbuscular mycorrhizae may enhance plant tolerance to environmental stresses and increase vegetation restoration in wetlands. However, presently little is known how about arbuscular mycorrhizal (AM) symbiosis in urban wetland plants and the factors affecting AM formation. Here, a survey for AM status in urban wetland plants was done in Beijing area, and the factors affecting AM formation were also discussed, such as water and sediment characteristics. Results showed that 87.5% of plants (49 of 56 species) were colonized by AM fungi. Mycorrhizal colonization rates (MCRs) ranged from 2% to 72%, while most of them were low level (<25%). The highest mycorrhizal colonization rate (MCR) was observed in Tephroseris palustris. Relationships between MCRs and water properties as well as sediment properties were analyzed by Pearson's correlation analysis. MCRs in Phragmites australi were negatively correlated with water ammonium nitrogen and a total dissolved phosphorus, while were positively correlated with nitrate nitrogen, nitrite nitrogen and total organic carbon (TOC) in sediment. MCRs in Typha orientalis were negatively correlated with water oxidation-reduction potential while were positively correlated with sediment TOC. MCRs in Glyceria maxima were positively correlated with sediment nitrate nitrogen. MCRs showed seasonal and temporal shift, while the variation was related to plant species. This study indicates that AM symbiosis widely exists in urban wetland plants, while AM formation in various plants is affected by different water and sediment properties.
Plants that live in aquatic habitats are frequently subjected to oxygen limitation and many of them modify their anatomy and physiology to counteract hypoxia. In these habitats, the role of plant associations with arbuscular mycorrhizal fungi (AMF), which are widespread in terrestrial environments and frequently confer benefits to the associated plant, is still debated. Starting from data taken from 34 selected papers, this study focuses on the occurrence of AMF in the roots of wetland and aquatic plants, taking into account the hydrological conditions of the sites, the plant wetland indicators and life forms, plant taxonomy and colonization by dark septate endophytes. The results have demonstrated the importance of hydrology in controlling the frequency and intensity of AMF root colonization, which tends to be low in obligate wetland plants. Moreover, colonization is generally lower and, possibly, less functional in monocots than in dicots. We suggest that the hydrological conditions, by filtering species according to their water tolerance, shape plant community composition, and that although AMF colonization is one of the traits that may increase plant fitness, it is not the most important one. In fact, a range of nutritional and growth strategies, which are more variegated than in terrestrial habitats, exists in wetland/aquatic habitats, and these strategies may rely, or not, on AMF colonization, as a consequence of the habitat and species.
Wetlands are ecosystems where the water regime is the main factor that shapes the physical, chemical and biological characteristics. Wetland plants are rooted in water-saturated soils that are frequently anoxic. In spite of this, the rhizosphere can be oxygenated due to the aerenchyma of the wetland plants, which enable active ventilation of roots, rhizomes and the nearby rhizosphere. Some wetland species have an amphibious character, whereby they can thrive both in water and on dry land, with the development of structurally different aquatic and terrestrial forms. Studies of fungal colonisation in wetlands have revealed the presence of fungal endophytes and mycorrhizal fungi. These colonisers are affected by the hydrological regime of the specific wetland. The availability of oxygen also alters the morphology and density of the individual fungal structures. It has been shown that occurrence of arbuscular mycorrhiza is negatively correlated with water depth and duration of flooding. In wetlands, the availability of nutrients depends on a variety of factors, which can mask the role of these fungi. This is particularly the case for phosphorus, which is the main plant benefit from mycorrhizal symbiosis. The same holds true for the potentially positive role of aerenchyma, as the conditions that induce their development inhibit colonisation by arbuscular mycorrhiza. Studies carried out in an intermittent lake, Lake Cerknica, have revealed relatively high arbuscular mycorrhizal colonisation of amphibious species. This appears to be due to the low organic matter content and the low level of plant-available phosphorus in the rhizosphere. At the same time, the frequency of colonisation is lower in aquatic specimens. The impact of water level fluctuations and season on fungal root colonisation of the common reed Phragmites australis is reflected in an altered frequency and intensity of fungal colonisation. The structures of dark septate endophytes that might have a similar role in plants as arbuscular mycorrhiza under stress conditions are relatively frequent in this species.
The roots of most plants are colonized by symbiotic fungi to form mycorrhiza, which play a critical role in the capture of nutrients from the soil and therefore in plant nutrition. Mycorrhizal Symbiosis is recognized as the definitive work in this area. Since the last edition was published there have been major advances in the field, particularly in the area of molecular biology, and the new edition has been fully revised and updated to incorporate these exciting new developments. . Over 50% new material . Includes expanded color plate section . Covers all aspects of mycorrhiza . Presents new taxonomy . Discusses the impact of proteomics and genomics on research in this area.
The mycorrhizal fungi, especially those that are vesicular arbuscular (VA), are universally ubiquitous soil inhabitants, and form symbiotic relationships with roots of land plants from every phylum. This includes members of most families of angiosperms and gymnosperms, together with ferns, lycopods and bryophytes. A fossil record of VA mycorrhizas dates back to the earliest land plants from the Rhynie Chert (Pirozynski and Dalpe 1989), indicating a very long period of co-evolution between plants and these fungal symbionts (Trappe 1987; Morton 1990) through co-accommodation (Brooks 1979). Mycorrhizal fungi link host plants with host soil and their biota in the mycorrhizosphere and play an important role in plant health, productivity and soil structure.
Aim
Fungal symbionts are ubiquitous in plants and can mitigate abiotic stressors associated with climate change. Predicting fungal symbiont distributions under future climates first requires knowledge of current distributions and their potential drivers.
Location
We documented colonization by fungal symbionts in perennial, cool‐season grasses along altitudinal gradients in the Rocky Mountains of Colorado, USA .
Methods
Across seven replicate altitudinal gradients, spanning c . 1400 vertical meters, we scored fungal colonization for 46 grass species. We documented altitudinal clines in colonization by both above‐ground and below‐ground fungal symbionts for the first time, including localized foliar endophytes ( LFE ) and systemic endophytes (epichloae) in leaves and arbuscular mycorrhizal fungi ( AMF ) and dark septate endophytes ( DSE ) in roots. For a subset of 16 well‐sampled grass species, we used model selection procedures to evaluate the relative importance of geography, edaphic factors and host plant identity. We also assessed the influence of host phylogenetic relatedness and colonization by co‐infecting fungi.
Results
Levels of fungal colonization varied strongly with host plant identity, but the effects of particular host species were not consistent across fungal groups. In addition to the influence of host identity, epichloae colonization declined with elevation and varied with geography (latitude/longitude) and edaphic factors. Geography and collection date were important predictors of LFE colonization, with higher colonization later in the growing season. Colonization estimates for the obligately plant‐associated fungi (epichloae, AMF ) were phylogenetically conserved across the grass supertree. Positive correlations between AMF and DSE , which remained even after accounting for host plant relatedness, suggested possible synergisms between these fungal groups.
Main conclusions
Our survey showed greater host specificity in patterns of fungal colonization than prior reports and revealed that different fungal symbiont groups do not share similar drivers. Conserving plant–fungal symbioses under future climates may require unique strategies for different plant species and fungal symbiont types.
Organic matter (OM) plays a significant role in biogeochemical processes in soil and water systems. Water-soluble organic matter (WSOM), leached from soil samples, is often analyzed as representative of potentially mobile OM. However, there are many WSOM extraction methods in the literature with no clear guidelines for method selection. In this study, four common leaching solutions -- 0.5 M K2SO4, 0.01 M CaCl2, 2 M KCl, and H2O -- were used to extract WSOM from various locations within a forested catchment. Fluorescence spectroscopy was used to analyze the impact of extraction method on WSOM chemistry. While all four methods consistently identified chemical differences between WSOM from a north-facing slope, south-facing slope, and riparian zone, there were clear differences in fluorescence signal between the leaching methods. All three salt solutions contained WSOM with a higher fluorescence index (FI) and humification index (HIX) than WSOM leached with H2O, suggesting the presence of salts releases different fractions of the soil organic matter. A PARAFAC model developed from the leachates identified a distinctive soil humic fluorophore observed in all samples, as well as fluorescent artifacts present in H2O-leached samples.
Mycorrhizal phenotypes arise from interactions among plant and fungal genotypes and the environment. Differences in the stoichiometry and uptake capacity of fungi and plants make arbuscular mycorrhizal ( AM ) fungi inherently more nitrogen (N) limited and less phosphorus (P) limited than their host plants. Mutualistic phenotypes are most likely in P‐limited systems and commensal or parasitic phenotypes in N‐limited systems. Carbon (C) limitation is expected to cause phenotypes to shift from mutualism to commensalism and even parasitism.
Two experiments compared the influence of fertilizer and shade on mycorrhizas in A ndropogon gerardii across three naturally N‐limited or P‐limited grasslands. A third experiment examined the interactive effects of N and P enrichment and shade on A . gerardii mycorrhizas.
Our experiments generated the full spectrum of mycorrhizal phenotypes. These findings support the hypothesis that mutualism is likely in P‐limited systems and commensalism or parasitism is likely in N‐limited systems. Furthermore, shade decreased C‐assimilation and generated less mutualistic mycorrhizal phenotypes with reduced plant and fungal biomass.
Soil fertility is a key controller of mycorrhizal costs and benefits and the L aw of the M inimum is a useful predictor of mycorrhizal phenotype. In our experimental grasslands arbuscular mycorrhizas can ameliorate P‐limitation but not N‐limitation.
Multiple species of arbuscular mycorrhizal fungi (AMF) can colonize roots of an individual plant species but factors which determine the selection of a particular AMF species in a plant root are largely unknown. The present work analysed the effects of drought, flooding and optimal soil moisture (15-20 %) on AMF community composition and structure in Sorghum vulgare roots, using PCR-RFLP. Rhizophagus irregularis (isolate BEG 21), and rhizosphere soil (mixed inoculum) of Heteropogon contortus, a perennial C4 grass, collected from the semi-arid Delhi ridge, were used as AMF inocula. Soil moisture functioned as an abiotic filter and affected AMF community assembly inside plant roots by regulating AMF colonization and phylotype diversity. Roots of plants in flooded soils had lowest AMF diversity whilst root AMF diversity was highest under the soil moisture regime of 15-20 %. Although plant biomass was not affected, root P uptake was significantly influenced by soil moisture. Plants colonized with R. irregularis or mixed AMF inoculum showed higher root P uptake than non-mycorrhizal plants in drought and control treatments. No differences in root P levels were found in the flooded treatment between plants colonized with R. irregularis and non-mycorrhizal plants, whilst under the same treatment, root P uptake was lower in plants colonized with mixed AMF inoculum than in non-mycorrhizal plants.
The arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck and Smith was grown in symbiosis with Cucumis sativus L. ev. Aminex (Fl hybrid) in mesh bags surrounded by a sand-filled hyphal compartment (HC), allowing only the fungal hyphae to protrude into the HC. The hyphae in the HC were supplied with ¹⁵N-labelled NH4⁺ or NO3⁻ after 60 d (expt 1). Following a 48 h labelling period, the sand was removed from the HC and the hyphae extracted. In another experiment (expt 2), the hyphae were extracted from the sand before being incubated in vitro in a nutrient solution containing ¹⁵N-labelled NH4⁺ for 15 h. The hyphal material was incubated in a 0 or 2.5 mM solution of the GOGAT-inhibitor albizzine prior to labelling. In both experiments the hyphal content of free amino acids and fatty acids were measured as well as the ammo acid l5N enrichment.
Oxygen concentrations in the soil atmosphere influenced growth and mineral uptake in Eupatorium odoratum L., Sorghum bicolor (L.) Moench and Guizotia abyssinica (L.f.) Cass. inoculated with Glomus macrocarpus, Glomus mosseae and ‘white reticulate’. Shoot and root dry weights of mycorrhizal plants increased with O2 concentration up to 16%. Dry weights of roots and shoots of non-mycorrhizal plants were lower than those of mycorrhizal plants at all O2 concentrations except at 2 %. Mycorrhizal and non-mycorrhizal plants at 21 % O2, or non-aerated controls, were smaller than those at 16%. Generally, mycorrhizal plants contained higher quantities of mineral elements than did non-mycorrhizal plants and took up more nutrients as soil O2 concentrations increased. Phosphorus concentrations of mycorrhizal plants were mostly significantly higher than those in non-mycorrhizal plants. Soil O2 generally did not influence the P concentrations of plants. Inoculation increased the concentrations of K and Mg in E. odoratum, but only the concentration of Mg in S. bicolor and the concentrations of Ca and Mg in Guizotia abyssinica. Only the concentrations of K and Mg in E. ordoratum, the concentrations of Ca and Mg in S, bicolor and the concentration of Mg in Guizotia aybssinica showed positive responses to increase in soil O2. The development of Glomus macrocarpus, Glomus mosseae and ‘white reticulate’ exhibited quantitative and qualitative responses to different soil O2 concentrations.
S ummary
Assessment of infection is an essential part of many studies involving VA mycorrhiza. A summary is given of the range of techniques that have been used. We calculated the standard error of four methods of assessment based on observations of stained root samples either randomly arranged in a petri dish or mounted on microscope slides. The methods are based on presence or absence of infection at root/grid intersect points, on a visual estimate of percentage cortex occupied by fungus or on estimates of length, or presence or absence of infection in root pieces mounted on slides. The number of replicate observations required for a given standard error % infection can be read from the curves provided. The advantages of the different methods of assessment are discussed and reasons given why they all probably overestimate the true values.
Arbuscular mycorrhizae (AM) are the symbiotic fungi that predominate in the roots
and soils of agricultural crop plants. The most recognized beneficial effect of these
fungi is to enhance host plant uptake of relatively immobile nutrients, in particular
phosphorus (P), and several micronutrients. The AM fungi absorb inorganic P either
from the soluble P pools in the soil, or from insoluble forms such as rock phosphates
as well as from insoluble organic sources. Recent studies show that mycorrhizal fungi
would have access to rock phosphate through localized alterations of pH and/or by the
production of organic acid anions that may act as chelating agents. The AMcolonization
also improves plant N nutrition. Generally mycorrhizal symbiosis more influences on
nitrogen (N) uptake and translocation if ammonium (NH4 + ) rather than nitrate (NO3 − )
is the nitrogen source. However, under drought stress the role of mycorrhizae in NO3 −
transport to the root surfacemay be significant as the NO3 − mobility is severely restricted
due to its low concentration and diffusion rate under such circumstances. However, as
yet little is known about the mechanism of N uptake by the AM fungi. Uptake of
micronutrients is also influenced by mycorrhizal colonization.
Plant species that do not typically form mycorrhiza are most likely to be successful under conditions where mycorrhizal fungi are not important for plant coexistence or where the costs of symbiosis outweigh the benefits. The relative occurrence of non‐mycorrhizal species was investigated in relationship to exchangeable soil P in herbaceous vegetation, where arbuscular mycorrhizal (AM) associations are generally common. We investigated a total of 439 sites in rocky habitats with sparse vegetation and 110 meadows with dense vegetation.
In both rocky habitats and meadows, soil pH was inversely related to exchangeable P. In rocky habitats plant species richness increased greatly between pH 3 and 5 and was also inversely related to P.
Plant communities in rocky habitats contained a relatively larger proportion of non‐mycorrhizal species than those in meadows. More non‐mycorrhizal species occurred at high soil P in the rocky habitats, but no such relation was found in meadows.
Non‐mycorrhizal species in rocky habitats were most common at low soil pH (high P availability). Plant species that thrived at extreme soil pH were often non‐mycorrhizal.
The occurrence of fewer non‐mycorrhizal plants in meadows than in rocky habitats supports the hypothesis that AM associations are more important in ecosystems with intense competition among plants. In rocky habitats, where abiotic stress may restrict photosynthesis more than nutrient limitation does, it is adaptive for plant species to utilize strategies other than mycorrhiza, particularly at low pH sites where P availability is likely to be adequate.
Compensatory responses to herbivory by invasive weeds may foil attempts to arrest their spread with biological controls. We conducted an experiment to study the effects of defoliation and soil fungi on interactions between Centaurea melitensis, an invasive annual from Eurasia, and Nassella pulchra, a native Californian bunchgrass. Defoliation of C. melitensis reduced its final biomass in all species–fungicide treatments, except when C. melitensis was grown with both Nassella and non-treated soil fungi at the same time. In this treatment, the biomass of clipped C. melitensis plants was equal to that of unclipped plants, indicating that soil fungi and Nassella promoted a compensatory response in the weed. Overall, the biomass of C. melitensis was 44% lower when soil fungi were reduced. However, in soil not treated with fungicide, the total biomass of C. melitensis increased in the presence of Nassella, but decreased when it was grown alone. When stressed by defoliation, C. melitensis may benefit from a form of mycorrhizae-mediated parasitism through a common mycorrhizal network, or Nassella may alter the fungal community in a way that enhances the positive direct effects of soil fungi on Centaurea.
Exudates of a dark septate endophyte (DSE) identified as Dreschlera sp., a common endophyte isolated by the inner cortical cells of the grass Lolium multiflorum, were put in contact with the arbuscular mycorrhizal fungus (AMF) Gigaspora rosea. These exudates stimulated the hyphal length and the hyphal branching of the AMF. A negative effect on the extramatrical phase of the AMF was detected. This is the first report to show how exudates of DSE can affect the development of AMF. These results show that DSE could be modifying the mycorrhizal status of the plants, modulating a different symbiosis in the rhizosphere.