Does responsiveness to arbuscular mycorrhizal fungi depend on plant invasive status?

Differences in the direction and degree to which invasive alien and native plants are influenced by mycorrhizal associations could indicate a general mechanism of plant invasion, but whether or not such differences exist is unclear. Here, we tested whether mycorrhizal responsiveness varies by plant invasive status while controlling for phylogenetic relatedness among plants with two large grassland datasets. Mycorrhizal responsiveness was measured for 68 taxa from the Northern Plains, and data for 95 taxa from the Central Plains were included. Nineteen percent of taxa from the Northern Plains had greater total biomass with mycorrhizas while 61% of taxa from the Central Plains responded positively. For the Northern Plains taxa, measurable effects often depended on the response variable (i.e., total biomass, shoot biomass, and root mass ratio) suggesting varied resource allocation strategies when roots are colonized by arbuscular mycorrhizal fungi. In both datasets, invasive status was nonrandomly distributed on the phylogeny. Invasive taxa were mainly from two clades, that is, Poaceae and Asteraceae families. In contrast, mycorrhizal responsiveness was randomly distributed over the phylogeny for taxa from the Northern Plains, but nonrandomly distributed for taxa from the Central Plains. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, we find no evidence that invasiveness in general is associated with the degree of mycorrhizal responsiveness. However, mycorrhizal responsiveness among species grown under common conditions was highly variable, and more work is needed to determine the causes of this variation.     

Do arbuscular mycorrhizal fungi recolonize revegetated grasslands?

Fifteen native and common exotic herbaceous species from four functional groups (C4 grass, C3 grass, chamaephyte and hemicryptophyte) occurring within remnant and revegetated grassland and grassy woodlands were sampled for evidence of structures associated with functioning arbuscular mycorrhizal fungi (AMF) from across a broad geographical range of central and south‐western Victoria, Australia. Revegetated communities had been established on ex‐agricultural land by direct seeding. They included sites that had been kept fallow with herbicide for up to 3 years prior to seeding and those from which topsoil had been removed (scalped) to a depth of 100 mm prior to seeding. Structures associated with AMF (external and internal aseptate hyphae, arbuscules and vesicles) were observed in root samples from all native and exotic species, regardless of site history (remnant or revegetated; fallowed or scalped). These findings indicate that AMF are ubiquitous in the herbaceous flora of this region (native and exotic), even in situations where sites had been intensively disturbed prior to revegetation treatment. However, while there was evidence of AMF in all revegetated communities, only sites which had been scalped prior to direct seeding supported species‐rich native herbaceous communities.     

Do arbuscular mycorrhizal fungi affect the allometric partition of host plant biomass to shoots and roots? A meta-analysis of studies from 1990 to 2010

Arbuscular mycorrhizas (AM) are ubiquitous root symbioses with often pervasive effects on the plant host, one of which may be above- and belowground biomass allocation. A meta-analysis was conducted on 516 trials that were described in 90 available articles to examine whether AM colonization could result in a modification of partitioning of plant biomass in shoots and roots. It was hypothesized that alleviating plant nutrient limitations could result in a decrease of root to shoot (R/S) ratio in AM plants or, alternatively, the direction of shifts in the R/S ratio would be determined by the changes in total dry biomass. In our analysis, we considered four types of stresses: drought stress, single heavy metal stress, multiple heavy metal stress, and other potential abiotic plant stress factors. When disregarding any factors that could regulate effects, including stress status and mode of propagation, the overall AM effect was a significant modification of biomass towards shoot growth. However, the responses of stressed and clonally propagated plants differed from those of seed-grown unstressed plants. Our meta-analysis detected a considerable decline in the R/S ratio when plants were grown from seeds in the absence of abiotic stresses. Moreover, we demonstrate that additional regulators of the AM-mediated impact on R/S ratio were presence of competition from other plants, plant growth outcome of the symbiosis, growth substrate volume, experimental duration, and the identities of both plant and AM fungus. Our results indicate that a prediction of AM effects on R/S allocation becomes more accurate when considering regulators, most notably propagation mode and stress. We discuss possible mechanisms through which stress and other regulators may operate.     

Isolation and sequence analysis of a β-tubulin gene from arbuscular mycorrhizal fungi

A full-length β-tubulin gene has been cloned and sequenced from Gigaspora gigantea and Glomus clarum, two arbuscular mycorrhizal fungi (AMF) species in the phylum Glomeromyota. The gene in both species is organized into five exons and four introns. Both genes are 94.9% similar and encode a 447 amino acid protein. In comparison with other fungal groups, the amino acid sequence is most similar to that of fungi in the Chytridiomycota. The codon usage of the gene in both AMF species is broad and biased in favor of an A or a T in the third position. The four introns varied in length from 87 to 168 bp for G. gigantea and from 90 to 136 bp for G. clarum. Of all fungi in which full-length sequences have been published, only AMF do not have an intron before codon 174. The introns positioned at codons 174 and 257 in AMF match the position of different introns in β-tubulin genes of some Zygomycete, Basidiomycete, and Ascomycete fungi. The 5′ and 3′ splice site consensus sequences are similar to those found in introns of most fungi. Sequence analysis from single-strand conformation polymorphism analysis confirmed the presence of two β-tubulin gene copies in G. clarum, but only one copy was evident in G. gigantea based on Southern hybridization analysis.     

Is there a role for arbuscular mycorrhizal fungi in production agriculture?

This review presents the point of view that arbuscular mycorrhizal fungi (AMF) do not play a vital role in the nutrition and growth of plants in many production-orientated agricultural systems. Highly available soil P often limits AM colonisation and causes the C-costs to the host to outweigh any benefits from colonisation. Even when P availability is low and AM colonisation levels are high, as may occur in organic and biodynamic agricultural systems, AMF may not always contribute to plant growth for reasons not yet understood. AM fungal activity may also be greatly limited by soil fumigation, non-responsive plant varieties, or rotations based primarily on non-mycorrhizal crops or crops of low AM dependency. Thus, profitability may sometimes be enhanced by management practices, such as tillage and P-fertilisation, which limit AM colonisation. Manipulation of agricultural systems to favour AMF must occur only if there is clear evidence that AMF make a positive contribution to yield or are vital for maintenance of ecosystem health and sustainability. A crucial role for AMF in soil structural stability or in enhancing micronutrient concentrations in produce may be sufficient evidence and may eventually compel consideration of AMF responsiveness when breeding new crop varieties.     

Are there benefits of simultaneous root colonization by different arbuscular mycorrhizal fungi?

Arbuscular mycorrhizal fungal (AMF) communities were established in pots using fungal isolates from a single field in Switzerland. It was tested whether multispecies mixtures provided more phosphorus and supported greater plant growth than single AMF species. Two host plants, medic (Medicago truncatula) and leek (Allium porrum), were inoculated with three AMF species (Glomus mosseae, G. claroideum and G. intraradices), either separately or in mixtures. The composition of the AMF communities in the roots was assessed using real-time PCR to determine the copy number of large ribosomal subunit genes. Fungal communities in the roots were usually dominated by one AMF species (G. mosseae). The composition of the communities depended on both plant identity and the time of harvest. Leek colonized by a mixture of G. claroideum and G. intraradices acquired more P than with either of the two AMF separately. Direct evidence is provided for functional complementarity among species within the AMF community colonizing a single root system. Competition among the species poses a major challenge in interpreting experiments with mixed inoculations, but this is greatly facilitated by use of real-time PCR.     

Do arbuscular mycorrhizal fungi play a role in the ability of rare plant species to colonize abandoned fields?

While some plant species colonize abandoned agricultural fields and dry grasslands with similar frequency (generalists), others are absent or underrepresented in abandoned fields (specialists). We tested if inoculation with dry grassland or abandoned field soil could improve specialist performance in an abandoned field and compared the effects of inoculation in the stage of sown seeds and transplanted seedlings. Arbuscular mycorrhizal fungi from abandoned field had higher root colonization potential. This could explain the higher performance of the sown specialists inoculated with the abandoned field inoculum compared to those inoculated with dry grassland inoculum. This difference disappeared when specialists were transplanted instead of sown. The results do not provide any support for higher performance of specialists inoculated with dry grassland inoculum. Transplantation, however, seems to be an efficient way to introduce specialists into the abandoned fields.     

Is stimulation of carotenoid biosynthesis in arbuscular mycorrhizal roots a general phenomenon?

The identification and quantification of cyclohexenone glycoside derivatives from the model legume Lotus japonicus revealed far higher levels than expected according to the stoichiometric relation to another, already determined carotenoid cleavage product, i.e., mycorradicin. Mycorradicin is responsible for the yellow coloration of many arbuscular mycorrhizal (AM) roots and is usually esterified in a complex way to other compounds. After liberation from such complexes it has been detected in AM roots of many, but not of all plants examined. The non-stoichiometric occurrence of this compound compared with other carotenoid cleavage products suggested that carotenoid biosynthesis might be activated upon mycorrhization even in plant species without detectable levels of mycorradicin. This assumption has been supported by inhibition of a key enzyme of carotenoid biosynthesis (phytoene desaturase) and quantification of the accumulating enzymic substrate (phytoene). Our observations suggest that the activation of carotenoid biosynthesis in AM roots is a general phenomenon and that quantification of mycorradicin is not always a good indicator for this activation.     

Fern-associated arbuscular mycorrhizal fungi are represented by multiple Glomus spp.: do environmental factors influence partner identity?

Symbioses involving arbuscular mycorrhizal fungi (AMF) are among the most important ecological associations for many plant species. The diversity of AMF associated with ferns, however, remains poorly studied. Using recently designed Glomus-specific primers, we surveyed the AMF community associated with ferns from deciduous, broad-leaved second-growth forest habitats at the eastern edge of the piedmont region of central Virginia, USA. Results indicate that this molecular approach may be a useful tool for detecting AMF in ferns compared to traditional techniques based on morphology. Over 30 potential fungal ribotypes were identified from eight fern species using denaturing gradient gel electrophoresis. Fungal ribotypes were found to differ widely in terms of (1) the number of fern partners with which they interact and (2) their relative frequency within each fern. Sequence analysis of fungal isolates from three species of fern indicated that the primers were generally highly specific for Glomus species but some non-target DNA was also amplified. Cloned polymerase chain reaction (PCR) products from Polystichum acrostichoides and Osmunda regalis revealed several phylogenetically distinct Glomus species. A single Glomus species was identified in the cloned PCR products from Botrychium virginianum. These findings challenge the hypothesis that the extent or degree of fern–fungal symbiosis is somehow tied to root complexity. Environmental factors appear to influence the suite of AMF that form partnerships with ferns. Some species of fern from similar habitats associated with dissimilar fungal partners (e.g., P. acrostichoides and Athyrium filix-femina var. asplenioides), whereas others harbored uniform fungal communities (e.g., Asplenium platyneuron). The significance of these data in terms of ecological and evolutionary dynamics of the AMF–fern symbiosis is discussed. Brittany West, Jessica Brandt, and Kay Holstien contributed equally to this work.     

Can orchid mycorrhizal fungi be persistently harbored by the plant host?

The environmental distribution of non-obligate orchid mycorrhizal (OM) symbionts belonging to the ‘rhizoctonia’ complex remains elusive. Some of these fungi, indeed, are undetectable in soil outside the host rhizosphere. A manipulation experiment was performed to assess the importance of neighbouring non-orchid plants and soil as possible reservoirs of OM fungi for Spiranthes spiralis, a widespread photosynthetic European terrestrial orchid species. Fungi of S. spiralis roots were identified by DNA metabarcoding before and 4 months after the removal of the surrounding vegetation and soil. Although such a treatment significantly affected fungal colonization of newly-formed orchid roots, most OM fungi were consistently associated with the host roots. Frequency patterns in differently aged roots suggest that these fungi colonize new orchid roots from either older roots or other parts of the same plant, which may thus represent an environmental source for the subsequent establishment of the OM symbiosis.     

Is plant performance limited by abundance of arbuscular mycorrhizal fungi? A meta-analysis of studies published between 1988 and 2003

We conducted meta-analyses of 290 published field and glasshouse trials to determine the effects of various agricultural practices on mycorrhizal colonization in nonsterile soils, and the consequence of those effects on yield, biomass, and phosphorus (P) concentration. Mycorrhizal colonization was increased most by inoculation (29% increase), followed by shortened fallow (20%) and reduced soil disturbance (7%). The effect of crop rotation depended on whether the crop was mycorrhizal. Increased colonization resulted in a yield increase in the field of 23% across all management practices. Biomass at harvest and shoot P concentration in early season were increased by inoculation (57 and 33%, respectively) and shortened fallow (55 and 24%). Reduced disturbance increased shoot P concentration by 27%, but biomass was not significantly affected. Biomass was significantly reduced in 2% of all trials in which there was a significant increase in colonization. Irrespective of management practice, an increased mycorrhizal colonization was less likely to increase biomass if either soil P or indigenous inoculum potential was high.     

The carbon origin of arbuscular mycorrhizal fungi estimated from δ13C values of individual spores

 The origin of carbon in the spores of arbuscular mycorrhizal (AM) fungi was quantified based on their obligate symbiosis with C₃ and C₄ plants showing clearly different δ¹³C values. The δ¹³C values of individual spores of the AM fungus Gigaspora margarita were analyzed. In monoculture pots of a C₃ or a C₄ plant species, spore δ¹³C values were ca. 3.5‰ lower than those of host roots. In coculture pots of a C₃ and a C₄ plant species, spore δ¹³C values varied between those of the roots of C₃ and C₄ plants, and increased linearly from the C₃ to the proximity of the C₄ plant (P<0.01). This reflects the higher δ¹³C values in C₄ plants than in C₃ plants. Thus the carbon origin of G. margarita spores changed with growth state and combination of host plants. In the presence of fresh plant residue instead of living host plants, spore δ¹³C values did not vary with distance from the residue. This finding supports the current view that AM fungi are obligate symbionts. Accepted: 12 February 1999     

Impact of growth and uptake patterns of arbuscular mycorrhizal fungi on plant phosphorus uptake—a modelling study

In this paper we present a mathematical model for estimating external mycelium growth of arbuscular mycorrhizal fungi and its effect on root uptake of phosphate (P). The model describes P transport in soil and P uptake by both root and fungi on the single root scale. We investigate differences in soil P depletion and overall P influx into a mycorrhizal root by assuming that different spatial regions of mycelia are active in P uptake. When all external hyphae contribute to P uptake, overall uptake is dominated by the fungus and the most effective growth pattern appears to be the one using a high level of anastomosis. The same is true when only the proportion of external hyphae assumed to be active contributes to uptake. When uptake is restricted to the tips, hyphal contribution to overall P uptake is less dominant; the most effective growth pattern appears to be the one characterised by nonlinear branching where branching stops at a given maximal hyphal tip density. Comparison to measured P depletion in the literature suggests that the scenario where active hyphae are contributing to P uptake is likely to fit the data best. These quantitative predictions promote our understanding of the mycorrhizal symbiosis and its role in plant P nutrition.     

Differential RNA accumulation of two β-tubulin genes in arbuscular mycorrhizal fungi

RNA was isolated from spores of different arbuscular mycorrhizal (AM) fungi and used for RT-PCR with degenerate primers for beta-tubulin genes. PCR products were cloned and the sequence of several clones was analysed for each fragment. Comparison of sequences identified two loci for beta-tubulin genes with different GC content and codon usage. Btub1 sequences were most similar to beta-tubulin genes from the Oomycota, while Btub2 sequences showed highest similarity to sequences from the Zygomycota. RT-PCR experiments were carried out to monitor RNA accumulation patterns of Btub1 and Btub2 in asymbiotic germinating spores and in symbiotic extraradical hyphae of three different AM fungi. This indicated that Btub1 is constitutively expressed in Gigaspora rosea, but down-regulated during symbiosis in Glomus mosseae and Glomus intraradices. In contrast, Btub2 showed constitutive expression in the two Glomus species, but down-regulation in G. rosea. Further analysis of different fungi indicated that Btub2 primers could be used to specifically monitor RNA accumulation of AM fungi in environmental samples.     

Nested multiplex PCR——A feasible technique to study partial community of arbuscular mycorrhizal fungi in field-growing plant root

Arbuscular mycorrhizal fungi (AMF) is ubiquitous in Nature. About 85% of terrestrial plant families form arbuscular mycorrhiza (AM) symbiotic association. AMF are involved in plant nutrient uptake, especially improving phosphorus uptake, protection of pla…     

Is resource allocation and grain yield of rice altered by inoculation with arbuscular mycorrhizal fungi?

Aims Our study quantified the combined effects of fertilization and inoculation with arbuscular mycorrhizal fungi (AMF) on grain yield and allocation of biomass and nutrients in field-grown rice (Oryza sativa L.).Methods A two-factor experiment was conducted at a field site in northeast of China (in Shuangcheng, Heilongjiang Province, Songhua River basin): six nitrogen–phosphorus–potassium fertilizer levels were provided (0, 20, 40, 60, 80 and 100% of the local norm of fertilizer supply), with or without inoculation with Glomus mosseae. At maturity, we quantified the percentage of root length colonization by AMF, grain yield, shoot:root ratios, shoot N and P contents and nutrients allocated to panicles, leaves and stems.Important findings As expected, inoculation resulted in greatly increased AMF colonization, which in turn led to higher shoot:root ratios and greater shoot N contents. Shoot:root ratios of inoculated rice increased with increasing fertilization while there was a significant interaction between fertilization and inoculation on shoot:root ratio. Additionally, AMF inoculation increased panicle:shoot ratios, panicle N:shoot N ratios and panicle P:shoot P ratios, especially in plants grown at low fertilizer levels. Importantly, inoculated rice exhibited higher grain yield, with the maximum improvement (near 62%) at the lower fertilizer end. Our results showed that (i) AMF-inoculated plants conform to the functional equilibrium theory, albeit to a reduced extent compared to non-inoculated plants and (ii) AMF inoculation resulted in greater allocation of shoot biomass to panicles and increased grain yield by stimulating N and P redistribution to panicles.     

Nested multiplex PCR——A feasible technique to study partial community of arbuscular mycorrhizal fungi in field-growing plant root

Plant can be infected by different arbuscular mycorrhizal fungi, but little is known about the interaction between them within root tissues mainly because different species cannot be distinguished on the basis of fungal structure. Accurate species identification of Arbuscular mycorrhizal fungi (AMF) colonized in plant roots is the comerstone of mycorrhizal study, yet this fundamental step is impossible through its morphological character alone. For accurate, rapid and inexpensive detection of partial mycorrhizal fungal community in plant roots, a nested multiplex polymerase chain reaction (PCR) was developed in this study. Five discriminating primers designed based on the variable region of the 5′ end of the large ribosomal subunit were used in the experiment for testing their specificity and the sensitivity in nested PCR by using spores from Glomus mosseae (BEG12), Glomus intraradices (BEG141), Scutellospora castaneae (BEG1) and two unidentified Glomus sp. HAUO3 and HAUO4. The feasibility assay of nested multiplex PCR was conducted by use of spore mixture, Astragalus sinicum roots co-inoculated with 4 species of arbuscular mycorrhizal fungi from pot cultures and 15 different field-growing plant roots respectively after analyses of the compatibility of primers. The result indicated that the sensitivity was in the same range as that of the corresponding single PCR reaction. Overall accuracy was 95%. The efficiency and sensitivity of this multiplex PCR procedure provided a rapid and easy way to simultaneously detect several of arbuscular mycorrhizal fungal species in a same plant root system.