A common garden test of host-symbiont specificity supports a dominant role for soil type in determining AMF assemblage structure in Collinsia sparsiflora

Plant hosts may influence arbuscular mycorrhizal fungal community composition in mangrove estuaries

We investigated the role of plant host and soil variables in determining arbuscular mycorrhizal fungi (AMF) community composition in plant roots of two spatially separated mangrove estuaries on the rivers Aghanashini (14° 30' 30″ N-74° 22' 44″ E) and Gangavali (14° 35' 26″ N-74° 17' 51″ E) on the west coast of India. Both mangrove estuaries had similar plant species composition but differed in soil chemistries.We amplified a 550-bp portion of 18S small subunit (SSU) rDNA from mangrove plant roots and analysed it by restriction fragment length polymorphism (RFLP). Clones representing unique RFLP patterns were sequenced. A total of 736 clones were obtained from roots of seven and five plant species sampled at Aghanashini and Gangavali, respectively. AMF phylotype numbers in plant roots at Aghanashini (12) were higher than at Gangavali (9) indicating quantitative differences in the AMF community composition in plant roots at the two mangrove estuaries. Because both estuaries had similar plant species composition, the quantitative difference in AMF communities between the estuaries could be an attribute of the differences in rhizospheric chemistry between the two sites.Non-metric multidimensional scaling (NMDS) revealed overlap in the AMF communities of the two sites. Three and two AMF phylotypes had significant indicator value indices with specific hosts at Aghanashini and Gangavali, respectively. Environmental vector fitting to NMDS ordination did not reveal a significant effect of any soil variable on AMF composition at the two sites. However, significant effects of both plant hosts and sites were observed on rhizospheric P. Our results indicate that root AMF community composition may be an outcome of plant response to rhizospheric variables. This suggests that plant identity may have a primary role in shaping AMF communities in mangroves.

Divergence in Diversity and Composition of Root-Associated Fungi Between Greenhouse and Field Studies in a Semiarid Grassland

Investigations of plant-soil feedbacks (PSF) and plant-microbe interactions often rely exclusively on greenhouse experiments, yet we have little understanding of how, and when, results can be extrapolated to explain phenomena in nature. A systematic comparison of microbial communities using the same host species across study environments can inform the generalizability of such experiments. We used Illumina MiSeq sequencing to characterize the root-associated fungi of two foundation grasses from a greenhouse PSF experiment, a field PSF experiment, field monoculture stands, and naturally occurring resident plants in the field. A core community consisting < 10% of total fungal OTU richness but > 50% of total sequence abundance occurred in plants from all study types, demonstrating the ability of field and greenhouse experiments to capture the dominant component of natural communities. Fungal communities were plant species-specific across the study types, with the core community showing stronger host specificity than peripheral taxa. Roots from the greenhouse and field PSF experiments had lower among sample variability in community composition and higher diversity than those from naturally occurring, or planted monoculture plants from the field. Core and total fungal composition differed substantially across study types, and dissimilarity between fungal communities did not predict plant-soil feedbacks measured in experiments. These results suggest that rhizobiome assembly mechanisms in nature differ from the dynamics of short-term, inoculation studies. Our results validate the efficacy of common PSF experiment designs to test soil inoculum effects, and highlight the challenges of scaling the underlying microbial mechanisms of plant responses from whole-community inoculation experiments to natural ecosystems.

Arbuscular mycorrhizal symbiosis and achene mucilage have independent functions in seedling growth of a desert shrub

Arbuscular mycorrhizal (AM) symbiosis can play a role in improving seedling establishment in deserts, and it has been suggested that achene mucilage facilitates seedling establishment in sandy deserts and that mucilage biodegradation products may improve seedling growth. We aimed to determine if AM symbiosis interacts with achene mucilage in regulating seedling growth in sand dunes. Up to 20 A M fungal taxa colonized Artemisia sphaerocephala roots in the field, and mycorrhizal frequency and colonization intensity exhibited seasonal dynamics. In the greenhouse, total biomass of AM fungal-colonized plants decreased, whereas the root/shoot ratio increased. AM symbiosis resulted in increased concentrations of nutrients and chlorophyll and decreased concentrations of salicylic acid (SA) and abscisic acid (ABA). Achene mucilage had a weaker effect on biomass and on nutrient, chlorophyll, and phytohormone concentration than did AM symbiosis. We suggest that AM symbiosis and achene mucilage act independently in enhancing seedling establishment in sandy deserts.

Shading and litter mediate the effects of soil fertility on the performance of an understorey herb

BACKGROUND AND AIMS

Soil fertility and topographic microclimate are common determinants of plant species distributions. However, biotic conditions also vary along these abiotic gradients, and may mediate their effects on plants. In this study, we investigated whether soils and topographic microclimate acted directly on the performance of a focal understorey plant, or indirectly via changing biotic conditions.

METHODS

We examined direct and indirect relationships between abiotic variables (soil fertility and topographic microclimate) and biotic factors (overstorey and understorey cover, litter depth and mycorrhizal colonization) and the occurrence, density and flowering of a common understorey herb, Trientalis latifolia, in the Klamath-Siskiyou Mountains, Oregon, USA.

RESULTS

We found that the positive effects of soil fertility on Trientalis occurrence were mediated by greater overstorey shading and deeper litter. However, we did not find any effects of topographic microclimate on Trientalis distribution that were mediated by the biotic variables we measured. The predictive success of Trientalis species distribution models with soils and topographic microclimate increased by 12 % with the addition of the biotic variables.

CONCLUSIONS

Our results reinforce the idea that species distributions are the outcome of interrelated abiotic gradients and biotic interactions, and suggest that biotic conditions, such as overstorey density, should be included in species distribution models if data are available.

Soil Characteristics Driving Arbuscular Mycorrhizal Fungal Communities in Semiarid Mediterranean Soils

We investigated communities of arbuscular mycorrhizal fungi (AMF) in the roots and the rhizosphere soil of Brachypodium retusum in six different natural soils under field conditions. We explored phylogenetic patterns of AMF composition using indicator species analyses to find AMF associated with a given habitat (root versus rhizosphere) or soil type. We tested whether the AMF characteristics of different habitats or contrasting soils were more closely related than expected by chance. Then we used principal-component analysis and multivariate analysis of variance to test for the relative contribution of each factor in explaining the variation in fungal community composition. Finally, we used redundancy analysis to identify the soil properties that significantly explained the differences in AMF communities across soil types. The results pointed out a tendency of AMF communities in roots to be closely related and different from those in the rhizosphere soil. The indicator species analyses revealed AMF associated with rhizosphere soil and the root habitat. Soil type also determined the distribution of AMF communities in soils, and this effect could not be attributed to a single soil characteristic, as at least three soil properties related to microbial activity, i.e., pH and levels of two micronutrients (Mn and Zn), played significant roles in triggering AMF populations.

IMPORTANCE

Communities of arbuscular mycorrhizal fungi (AMF) are main components of soil biota that can determine the productivity of ecosystems. These fungal assemblages vary across host plants and ecosystems, but the main ecological processes that shape the structures of these communities are still largely unknown. A field study in six different soil types from semiarid areas revealed that AMF communities are significantly influenced by habitat (soil versus roots) and soil type. In addition, three soil properties related to microbiological activity (i.e., pH and manganese and zinc levels) were the main factors triggering the distribution of AMF. These results contribute to a better understanding of the ecological factors that can shape AMF communities, an important soil microbial group that affects multiple ecosystem functions.

Differences in Arbuscular Mycorrhizal Fungal Community Composition in Soils of Three Land Use Types in Subtropical Hilly Area of Southern China

Land use type is key factor in restoring the degraded soils due to its impact on soil chemical properties and microbial community. In this study, the influences of land use type on arbuscular mycorrhizal fungal (AMF) community and soil chemical properties were assessed in a long-run experimental station in subtropical hilly area of southern China. Soil samples were collected from forest land, orchard and vegetable field. Soil chemical properties were analyzed, and PCR-DGGE was performed to explore the AMF community structure. Cloning and sequencing of DGGE bands were conducted to monitor AMF community composition. Results indicate that the contents of total P, available P and available K were the highest while the contents of soil organic matter, total N, total K and available N were the lowest in vegetable field soils, with forest land soils vice versa. According to DGGE profiling, AMF community in forest soils was more closely related to that in orchard soils than that in vegetable field soils. Sequencing indicated that 45 out of 53 excised bands were AMF and 64.4% of AMF belonged to Glomeraceae, including some “generalists” present in all soils and some “specialists” present only in soils of particular land use. Category principle component analysis demonstrated that total N, soil organic matter and available P were the most important factors affecting AMF community, and some AMF phylotypes were closely associated with particular soil chemical properties. Our data suggest that AMF communities are different with different land use types.

Nitrogen and phosphorus additions impact arbuscular mycorrhizal abundance and molecular diversity in a tropical montane forest

Increased nitrogen (N) depositions expected in the future endanger the diversity and stability of ecosystems primarily limited by N, but also often co-limited by other nutrients like phosphorus (P). In this context a nutrient manipulation experiment (NUMEX) was set up in a tropical montane rainforest in southern Ecuador, an area identified as biodiversity hotspot. We examined impacts of elevated N and P availability on arbuscular mycorrhizal fungi (AMF), a group of obligate biotrophic plant symbionts with an important role in soil nutrient cycles. We tested the hypothesis that increased nutrient availability will reduce AMF abundance, reduce species richness and shift the AMF community toward lineages previously shown to be favored by fertilized conditions. NUMEX was designed as a full factorial randomized block design. Soil cores were taken after 2 years of nutrient additions in plots located at 2000 m above sea level. Roots were extracted and intraradical AMF abundance determined microscopically; the AMF community was analyzed by 454-pyrosequencing targeting the large subunit rDNA. We identified 74 operational taxonomic units (OTUs) with a large proportion of Diversisporales. N additions provoked a significant decrease in intraradical abundance, whereas AMF richness was reduced significantly by N and P additions, with the strongest effect in the combined treatment (39% fewer OTUs), mainly influencing rare species. We identified a differential effect on phylogenetic groups, with Diversisporales richness mainly reduced by N additions in contrast to Glomerales highly significantly affected solely by P. Regarding AMF community structure, we observed a compositional shift when analyzing presence/absence data following P additions. In conclusion, N and P additions in this ecosystem affect AMF abundance, but especially AMF species richness; these changes might influence plant community composition and productivity and by that various ecosystem processes.

Modularity reveals the tendency of arbuscular mycorrhizal fungi to interact differently with generalist and specialist plant species in gypsum soils

Patterns in plant-soil biota interactions could be influenced by the spatial distribution of species due to soil conditions or by the functional traits of species. Gypsum environments usually constitute a mosaic of heterogeneous soils where gypsum and nongypsum soils are imbricated at a local scale. A case study of the interactions of plants with arbuscular mycorrhizal fungi (AMF) in gypsum environments can be illustrative of patterns in biotic interactions. We hypothesized that (i) soil characteristics might affect the AMF community and (ii) there are differences between the AMF communities (modules) associated with plants exclusive to gypsum soils (gypsophytes) and those associated with plants that show facultative behavior on gypsum and/or marly-limestone soils (gypsovags). We used indicator species and network analyses to test for differences between the AMF communities harbored in gypsophyte and gypsovag plants. We recorded 46 operational taxonomic units (OTUs) belonging to nine genera of Glomeromycota. The indicator species analysis showed two OTUs preferentially associating with gypsum soils and three OTUs preferentially associating with marly-limestone soils. Modularity analysis revealed that soil type can be a major factor shaping AMF communities, and some AMF groups showed a tendency to interact differently with plants that had distinct ecological strategies (gypsophytes and gypsovags). Characterization of ecological networks can be a valuable tool for ascertaining the potential influence of above- and below-ground biotic interactions (plant-AMF) on plant community composition.

Assessing the diversity of arbuscular mycorrhizal fungi in semiarid shrublands dominated by Artemisia tridentata ssp. wyomingensis

Variation in the abiotic environment and host plant preferences can affect the composition of arbuscular mycorrhizal (AMF) assemblages. This study analyzed the AMF taxa present in soil and seedlings of Artemisia tridentata ssp. wyomingensis collected from sagebrush steppe communities in southwestern Idaho, USA. Our aims were to determine the AMF diversity within and among these communities and the extent to which preferential AMF-plant associations develop during seedling establishment. Mycorrhizae were identified using molecular methods following DNA extraction from field and pot culture samples. The extracted DNA was amplified using Glomeromycota specific primers, and identification of AMF was based on phylogenetic analysis of sequences from the large subunit-D2 rDNA region. The phylogenetic analyses revealed seven phylotypes, two within the Claroideoglomeraceae and five within the Glomeraceae. Four phylotypes clustered with known species including Claroideoglomus claroideum, Rhizophagus irregularis, Glomus microaggregatum, and Funneliformis mosseae. The other three phylotypes were similar to several published sequences not included in the phylogenetic analysis, but all of these were from uncultured and unnamed glomeromycetes. Pairwise distance analysis revealed some phylotypes with high genetic variation. The most diverse was the phylotype that included R. irregularis, which contained sequences showing pairwise differences up to 12 %. Most of the diversity in AMF sequences occurred within sites. The smaller genetic differentiation detected among sites was correlated with differences in soil texture. In addition, multiplication in pot cultures led to differentiation of AMF communities. Comparison of sequences obtained from the soil with those from A. tridentata roots revealed no significant differences between the AMF present in these samples. Overall, the sites sampled were dominated by cosmopolitan AMF taxa, and young seedlings of A. tridentata ssp. wyomingensis were colonized in relation to the abundance of these taxa in the soil.

Mycorrhizal fungal growth responds to soil characteristics, but not host plant identity, during a primary lacustrine dune succession

Soil factors and host plant identity can both affect the growth and functioning of mycorrhizal fungi. Both components change during primary succession, but it is unknown if their relative importance to mycorrhizas also changes. This research tested how soil type and host plant differences among primary successional stages determine the growth and plant effects of arbuscular mycorrhizal (AM) fungal communities. Mycorrhizal fungal community, plant identity, and soil conditions were manipulated among three stages of a lacustrine sand dune successional series in a fully factorial greenhouse experiment. Late succession AM fungi produced more arbuscules and soil hyphae when grown in late succession soils, although the community was from the same narrow phylogenetic group as those in intermediate succession. AM fungal growth did not differ between host species, and plant growth was similarly unaffected by different AM fungal communities. These results indicate that though ecological filtering and/or adaptation of AM fungi occurs during this primary dune succession, it more strongly reflects matching between fungi and soils, rather than interactions between fungi and host plants. Thus, AM fungal performance during this succession may not depend directly on the sequence of plant community succession.

The nature of serpentine endemism

Serpentine soils are a model system for the study of plant adaptation, speciation, and species interactions. Serpentine soil is an edaphically stressful, low productivity soil type that hosts stunted vegetation and a spectacular level of plant endemism. One of the first papers on serpentine plant endemism was by Arthur Kruckeberg, titled "Intraspecific variability in the response of certain native plant species to serpentine soil." Published in the American Journal of Botany in 1951, it has been cited over 100 times. Here, I review the context and content of the paper, as well as its impact. On the basis of the results of reciprocal transplant experiments in the greenhouse, Kruckeberg made three important conclusions on the nature of serpentine plant endemism: (1) Plants are locally adapted to serpentine soils, forming distinct soil ecotypes; (2) soil ecotypes are the first stage in the evolutionary progression toward serpentine endemism; and (3) serpentine endemics are restricted from more fertile nonserpentine soils by competition. Kruckeberg's paper inspired a substantial amount of research, especially in the three areas reviewed here: local adaptation and plant traits, speciation, and the interaction of climate and soil in plant endemism. In documenting soil ecotypes, Kruckeberg identified serpentine soils as a potent selective factor in plant evolution and helped establish serpentine soils as a model system in evolution and ecology.