Arctic arbuscular mycorrhizal spore community and viability after storage in cold conditions

A New Technique for the Extraction of Arbuscular Mycorrhizae Fungal Spores from Rhizosphere

Monitoring the dynamics of the spore bank of arbuscular mycorrhizal fungi (AMF) is essential for the sustainable management and protection of agroecosystems. The most common method for extracting AMF spores from soil is the wet-sieving technique (WST). However, this method has many disadvantages. In this study, we modified the WST using new approaches: the ultrasound wet-sieving technique (UWST) and the ultrasound centrifuge technique (UCT). We enumerated and compared the numbers and quality of spores obtained from WST, UWST, and UCT to validate the new modified techniques. We extracted AMF spores from the rhizospheres of different plants, including wheat (Triticum aestivum L.), bean (Phaseolus vulgaris L.), tomato (Solanum lycopersicum L.), pepper (Piper nigrum L.), parsley (Petroselinum crispum Mill.), and turfgrass (Lolium perenne L.) collected from the Van Lake basin, Turkey. The highest and lowest AMF spore numbers were observed in wheat and turfgrass rhizospheres. The UCT allowed for the extraction of the highest number of spores from all rhizospheres, followed by the UWST and WST. The UWST and WST allowed for the extraction of similar spore numbers from wheat, pepper, parsley, and turfgrass rhizospheres. Beyond the high extracted spore number, UCT was shown to be a fast and low-material-consuming approach. These findings demonstrate that the UCT can be used to efficiently extract AMF spores in future research.

Factors in the Distribution of Mycorrhizal and Soil Fungi

Soil fungi are crucial microorganisms in the functioning of ecosystems. They shape the soil properties, facilitate nutrient circulation, and assist with plant growth. However, their biogeography and distribution studies are limited compared to other groups of organisms. This review aims to provide an overview of the main factors shaping the spatial distribution of soil fungi (with a special focus on mycorrhizal fungi). The review also tries to identify the field frontier where further studies are needed. The main drivers of soil fungal distribution were classified and reviewed into three groups: soil properties, plant interactions, and dispersal vectors. It was apparent that ectomycorrhizal and arbuscular fungi are relatively overrepresented in the body of research, while the other mycorrhiza types and endophytes were grossly omitted. Notwithstanding, soil pH and the share of ectomycorrhizal plants in the plant coverage were repeatedly reported as strong predictors of mycorrhizal fungal distribution. Dispersal potential and vector preferences show more variation among fungi, especially when considering long-distance dispersal. Additionally, special attention was given to the applications of the island biogeography theory to soil fungal assemblages. This theory proves to be a very efficient framework for analyzing and understanding not only the soil fungal communities of real islands but even more effective islands, i.e., isolated habitats, such as patches of trees discontinuous from more enormous forests.

Models of mycorrhizal colonization patterns and strategies induced by biostimulator treatments in Zea mays roots

Agronomic inputs and technologies, especially fertilizers, act on the evolution of the symbiotic partnership between arbuscular mycorrhizal fungi and cultivated plants. The use of the MycoPatt method for the assessment of mycorrhizas in maize roots leads to the extraction of large parameter databases with an increased resolution over the colonization mechanism. The application of a biostimulator treatment on plants acted toward a reduction of root permissiveness for mycorrhizas. The phenomenon was noticeable through an increased colonization variability that overlapped with plant nutritional needs. The annual characteristic of the plant was highlighted by the simultaneous presence of arbuscules and vesicles, with a high share of arbuscules in the advanced phenophases. Colonized root parts presented numerous arbuscule-dominated areas in all phenophases, which indicated a continuous formation of these structures and an intense nutrient transfer between partners. Mycorrhizal maps showed the slowing effect of the biostimulators on colonization, with one phenophase delay in the case of biostimulated plants compared to the ones without biostimulators. The forecast models presented gradual colonization in plants without biostimulators, with the expansion of new hyphal networks. The use of biostimulators on plants exhibited a lower permissiveness for new colonization areas, and the mechanism relies on hyphae developed in the former phenophases.

Elevation and plant species identity jointly shape a diverse arbuscular mycorrhizal fungal community in the High Arctic

Knowledge about the distribution and local diversity patterns of arbuscular mycorrhizal (AM) fungi are limited for extreme environments such as the Arctic, where most studies have focused on spore morphology or root colonization. We here studied the joint effects of plant species identity and elevation on AM fungal distribution and diversity. We sampled roots of 19 plant species in 18 locations in Northeast Greenland, using next generation sequencing to identify AM fungi. We studied the joint effect of plant species, elevation and selected abiotic conditions on AM fungal presence, richness and composition. We identified 29 AM fungal virtual taxa (VT), of which six represent putatively new VT. Arbuscular mycorrhizal fungal presence increased with elevation, and as vegetation cover and the active soil layer decreased. Arbuscular mycorrhizal fungal composition was shaped jointly by elevation and plant species identity. We demonstrate that the Arctic harbours a relatively species-rich and nonrandomly distributed diversity of AM fungi. Given the high diversity and general lack of knowledge exposed herein, we encourage further research into the diversity, drivers and functional role of AM fungi in the Arctic. Such insight is urgently needed for an area with some of the globally highest rates of climate change.

Mechanisms of stress tolerance and their effects on the ecology and evolution of mycorrhizal fungi

Stress is ubiquitous and disrupts homeostasis, leading to damage, decreased fitness, and even death. Like other organisms, mycorrhizal fungi evolved mechanisms for stress tolerance that allow them to persist or even thrive under environmental stress. Such mechanisms can also protect their obligate plant partners, contributing to their health and survival under hostile conditions. Here we review the effects of stress and mechanisms of stress response in mycorrhizal fungi. We cover molecular and cellular aspects of stress and how stress impacts individual fitness, physiology, growth, reproduction, and interactions with plant partners, along with how some fungi evolved to tolerate hostile environmental conditions. We also address how stress and stress tolerance can lead to adaptation and have cascading effects on population- and community-level diversity. We argue that mycorrhizal fungal stress tolerance can strongly shape not only fungal and plant physiology, but also their ecology and evolution. We conclude by pointing out knowledge gaps and important future research directions required for both fully understanding stress tolerance in the mycorrhizal context and addressing ongoing environmental change.

Legume germination is delayed in dry soils and in sterile soils devoid of microbial mutualists: Species-specific implications for upward range expansions

Climate change is affecting species and their mutualists and can lead to the weakening or loss of important interspecific interactions. Through independent shifts in partner phenology and distribution, climatic stress can separate mutualists temporally or spatially, leading to alterations in partner functional traits and fitness. Here, we explored the effects of the loss of microbial mutualists on legume germination success and phenology. In particular, we assessed the effects of mutualism loss via soil sterilization, increased drought, and introduction to novel soils found beyond the current distributions of two focal legume species in subalpine environments. Through common garden experiments in controlled environments, we found evidence that soil sterilization (and consequent microbial absence) and dry soils caused species‐specific phenological delays of 2–5 weeks in germination, likely as a result of interaction loss between legumes and specialized germination‐promoting soil microbes, such as mutualistic rhizobia. Delays in germination caused by a mismatch between legumes and beneficial microbes could negatively affect legume fitness through increased plant–plant competition later in the season. Additionally, we found evidence of the presence of beneficial microbes beyond the current elevational range of one of our focal legumes, which may allow for expansion of the leading edge, although harsh abiotic factors in the alpine may hinder this. Alterations in the strength of soil microbe‐legume mutualisms may lead to reduced fitness and altered demography for both soil microbes and legumes.

Mycorrhizae Helper Bacteria: Unlocking Their Potential as Bioenhancers of Plant-Arbuscular Mycorrhizal Fungal Associations

The dynamic interactions of plants and arbuscular mycorrhizal fungi (AMF) that facilitate the efficient uptake of minerals from soil and provide protection from various environmental stresses (biotic and abiotic) are now also attributed to a third component of the symbiosis. These are the less investigated mycorrhizae helper bacteria (MHB), which constitute a dense, active bacterial community, tightly associated with AMF, and involved in the development and functioning of AMF. Although AMF spores are known to host several bacteria in their spore walls and cytoplasm, their role in promoting the ecological fitness and establishment of AMF symbiosis by influencing spore germination, mycelial growth, root colonization, metabolic diversity, and biocontrol of soil borne diseases is now being deciphered. MHB also promote the functioning of arbuscular mycorrhizal symbiosis by triggering various plant growth factors, leading to better availability of nutrients in the soil and uptake by plants. In order to develop strategies to promote mycorrhization by AMF, and particularly to stimulate the ability to utilize phosphorus from the soil, there is a need to decipher crucial metabolic signalling pathways of MHB and elucidate their functional significance as mycorrhiza helper bacteria. MHB, also referred to as AMF bioenhancers, also improve agronomic efficiency and formulations using AMF along with enriched population of MHB are a promising option. This review covers the aspects related to the specificity and mechanisms of action of MHB, which positively impact the formation and functioning of AMF in mycorrhizal symbiosis, and the need to advocate MHB as AMF bioenhancers towards their inclusion in integrated nutrient management practices in sustainable agriculture.

Changes in Bulk and Rhizosphere Soil Microbial Diversity and Composition Along an Age Gradient of Chinese Fir (Cunninghamia lanceolate) Plantations in Subtropical China

Soil microorganisms play key roles in biogeochemical cycling in forest ecosystems. However, whether the responses of microbial community with stand development differed in rhizosphere and bulk soils remains unknown. We collected rhizosphere and bulk soil in Chinese fir plantations with different stand ages (7a, 15a, 24a, and 34a) in subtropical China, and determined bacterial and fungal community variation via high-throughput sequencing. The results showed that soil bacterial, but not fungal, community diversity significantly differed among stand ages and between rhizosphere and bulk soils (p < 0.05). The differences in Shannon-Wiener and Simpson's indices between rhizosphere and bulk soil varied with stand age, with significant higher soil bacterial diversity in rhizosphere than bulk soils in 7a and 34a plantations (p < 0.05), but there were no significant difference in soil bacterial diversity between rhizosphere and bulk soils in 15a and 24a plantations (p > 0.05). Soil microbial community composition varied significantly with stand age but not between the rhizosphere and bulk soil. The dominant bacterial phyla at all ages were Acidobacteria and Proteobacteria, while the dominant fungal phyla were Ascomycota and Basidiomycota in both rhizosphere and bulk soil. They showed inconsistent distribution patterns along stand age gradient (7-34a) in the rhizosphere and bulk soil, suggesting distinct ecological strategy (r-strategist vs. k-strategist) of different microbial taxa, as well as changes in the microenvironment (i.e., nutrient stoichiometry and root exudates). Moreover, bacterial and fungal community composition in rhizosphere and bulk soil were governed by distinct driving factors. TP and NH4 +-N are the two most important factors regulating bacterial and fungal community structure in rhizosphere soil, while pH and NO3 --N, DON, and TN were driving factors for bacterial and fungal community structure in bulk soil, respectively. Collectively, our results demonstrated that the changes in microbial diversity and composition were more obvious along stand age gradients than between sampling locations (rhizosphere vs. bulk soil) in Chinese fir plantations.

New method for arbuscular mycorrhizal fungus spore separation using a microfluidic device based on manual temporary flow diversion

Arbuscular mycorrhizal fungi are beneficial components often included in biofertilizers. Studies of the biology and utilization of these fungi are key to their successful use in the biofertilizer industry. The acquisition of isolated spores is a required step in these studies; however, spore quality control and spore separation are bottlenecks. Filtered and centrifuged spores have to be hand-picked under a microscope. The conventional procedure is skill-demanding, labor-intensive, and time-consuming. Here, we developed a microfluidic device to aid manual separation of spores from a filtered and centrifuged suspension. The device is a single spore streamer equipped with a manual temporary flow diversion (MTFD) mechanism to select single spores. Users can press a switch to generate MTFD when the spore arrives at the selection site. The targeted spore flows in a stream to the collection chamber via temporary cross flow. Using the device, spore purity, the percentage of spore numbers against the total number of particles counted in the collecting chamber reached 96.62% (median, n = 10) which is greater than the spore purity obtained from the conventional method (88.89% (median, n = 10)).

Soil legacy determines arbuscular mycorrhizal spore bank and plant performance in the low Arctic

Human impact is rapidly changing vegetation globally. The effect of plant cover that no longer exists in a site may still affect the development of future vegetation. We focused on a little studied factor-arbuscular mycorrhizal (AM) fungus spore bank-and its effect on three test plant species. In a low Arctic field site, plots were maintained for 6 years, devoid of any vegetation or with a Solidago virgaurea monoculture cover. We analysed the AM fungal morphospecies composition and identified 21 morphospecies in the field plots. The AM morphospecies community was dominated by members of Acaulosporaceae. Monoculturing under low Arctic field conditions changed the soil AM spore community, which became dominated by Glomus hoi. We tested the soil feedback in the greenhouse and grew Solidago virgaurea, Potentilla crantzii and Anthoxanthum odoratum in the field soils from the plots without plant cover, covered with Solidago virgaurea or with intact vegetation. Our results suggest that monoculturing resulted in improved N acquisition by the monocultured plant species Solidago virgaurea which may be related to the AM fungus community. Our results show that a rich community of AM fungus spores may remain viable under field conditions for 6 years in the low Arctic. Spore longevity in field soil in the absence of any host plants differed among AM fungus species. We suggest that AM fungus spore longevity be considered an AM fungal life-history trait.

Mimicking climate warming effects on Alaskan soil microbial communities via gradual temperature increase

Climate change can trigger shifts in community structure and may therefore pose a severe threat to soil microbial communities, especially in high northern latitudes such as the Arctic. Arctic soils are covered by snow and ice throughout most of the year. This insulation shields them from high temperature variability and low surface temperatures. If this protective layer thaws, these soils are predicted to warm up at 1.5x to 4x the rate of other terrestrial biomes. In this study, we sampled arctic soils from sites with different elevations in Alaska, incubated them for 5 months with a simulated, gradual or abrupt temperature increase of +5 °C, and compared bacterial and fungal community compositions after the incubation. We hypothesized that the microbial communities would not significantly change with a gradual temperature treatment, whereas an abrupt temperature increase would decrease microbial diversity and shift community composition. The only differences in community composition that we observed were, however, related to the two elevations. The abrupt and gradual temperature increase treatments did not change the microbial community composition as compared to the control indicating resistance of the microbial community to changes in temperature. This points to the potential importance of microbial dormancy and resting stages in the formation of a “buffer” against elevated temperatures. Microbial resting stages might heavily contribute to microbial biomass and thus drive the responsiveness of arctic ecosystems to climate change.

Beneficial Services of Arbuscular Mycorrhizal Fungi - From Ecology to Application

Arbuscular mycorrhiza (AM) is the most common symbiotic association of plants with microbes. AM fungi occur in the majority of natural habitats and they provide a range of important ecological services, in particular by improving plant nutrition, stress resistance and tolerance, soil structure and fertility. AM fungi also interact with most crop plants including cereals, vegetables, and fruit trees, therefore, they receive increasing attention for their potential use in sustainable agriculture. Basic research of the past decade has revealed the existence of a dedicated recognition and signaling pathway that is required for AM. Furthermore, recent evidence provided new insight into the exchange of nutritional benefits between the symbiotic partners. The great potential for application of AM has given rise to a thriving industry for AM-related products for agriculture, horticulture, and landscaping. Here, we discuss new developments in these fields, and we highlight future potential and limits toward the use of AM fungi for plant production.

Microbial island biogeography: isolation shapes the life history characteristics but not diversity of root-symbiotic fungal communities

Island biogeography theory is one of the most influential paradigms in ecology. That island characteristics, including remoteness, can profoundly modulate biological diversity has been borne out by studies of animals and plants. By contrast, the processes influencing microbial diversity in island systems remain largely undetermined. We sequenced arbuscular mycorrhizal (AM) fungal DNA from plant roots collected on 13 islands worldwide and compared AM fungal diversity on islands with existing data from mainland sites. AM fungal communities on islands (even those >6000 km from the closest mainland) comprised few endemic taxa and were as diverse as mainland communities. Thus, in contrast to patterns recorded among macro-organisms, efficient dispersal appears to outweigh the effects of taxogenesis and extinction in regulating AM fungal diversity on islands. Nonetheless, AM fungal communities on more distant islands comprised a higher proportion of previously cultured and large-spored taxa, indicating that dispersal may be human-mediated or require tolerance of significant environmental stress, such as exposure to sunlight or high salinity. The processes driving large-scale patterns of microbial diversity are a key consideration for attempts to conserve and restore functioning ecosystems in this era of rapid global change.

Effects of Long-Term Subcultured Arbuscular Mycorrhizal Fungi on Red Pepper Plant Growth and Soil Glomalin Content

Arbuscular mycorrhizal fungi (AMF) are well-known for their ability to improve plant growth and help plants withstand abiotic stress conditions. Unlike other fungi and bacteria, AMF cannot be stored, as they are obligate biotrophs. Long-term preservation of AMF spores is challenging and may lead to the loss of viability and efficiency. This study aimed to understand the effect of prolonged subculture of AMF species on the growth and glomalin-related soil protein (GRSP) from red pepper (Capsicum annuum L.). AMF spores were mass-produced using different techniques and subcultured in pots with sorghum sudangrass as the host plant for 3 years. Experimental soil samples were collected from natural grassland. Five different AMF inocula were used in triplicate as treatments. After 70 days of growth, red pepper plants were harvested and plant dry weight, plant nutrient content, mycorrhizal colonization, AMF spore count, and soil glomalin content were determined. AMF-treated plants displayed higher dry weight than controls, with only fruit dry weight being significantly different. Similarly, significant differences in phosphorous and potassium contents of the above-ground plant parts were observed between mycorrhizal and control treatments. In addition, soil GRSP content was significantly higher in plants inoculated with Rhizophagus sp. and Gigaspora margarita. The increased plant growth and GRSP content suggest that AMF can be maintained for 3 years without losing their efficiency if subcultured regularly with different symbiotic host plants.

The paleosymbiosis hypothesis: host plants can be colonised by root symbionts that have been inactive for centuries to millenia

Paleoecologists have speculated that post-glacial migration of tree species could have been facilitated by mycorrhizal symbionts surviving glaciation as resistant propagules belowground. The general premise of this idea, which we call the 'paleosymbiosis hypothesis', is that host plants can access and be colonised by fungal root symbionts that have been inactive for millennia. Here, we explore the plausibility of this hypothesis by synthesising relevant findings from a diverse literature. For example, the paleoecology literature provided evidence of modern roots penetrating paleosols containing ancient (>6000 years) fungal propagules, though these were of unknown condition. With respect to propagule longevity, the available evidence is of mixed quality, but includes convincing examples consistent with the paleosymbiosis hypothesis (i.e. >1000 years viable propagules). We describe symbiont traits and environmental conditions that should favour the development and preservation of an ancient propagule bank, and discuss the implications for our understanding of soil symbiont diversity and ecosystem functioning. We conclude that the paleosymbiosis hypothesis is plausible in locations where propagule deposition and preservation conditions are favourable (e.g. permafrost regions). We encourage future belowground research to consider and explore the potential temporal origins of root symbioses.

Symbiont dynamics during ecosystem succession: co-occurring plant and arbuscular mycorrhizal fungal communities

Although mycorrhizas are expected to play a key role in community assembly during ecological succession, little is known about the dynamics of the symbiotic partners in natural systems. For instance, it is unclear how efficiently plants and arbuscular mycorrhizal (AM) fungi disperse into early successional ecosystems, and which, if either, symbiotic partner drives successional dynamics. This study describes the dynamics of plant and AM fungal communities, assesses correlation in the composition of plant and AM fungal communities and compares dispersal limitation of plants and AM fungi during succession. We studied gravel pits 20 and 50 years post abandonment and undisturbed grasslands in Western Estonia. The composition of plant and AM fungal communities was strongly correlated, and the strength of the correlation remained unchanged as succession progressed, indicating a stable dependence among mycorrhizal plants and AM fungi. A relatively high proportion of the AM fungal taxon pool was present in early successional sites, in comparison with the respective fraction of plants. These results suggest that AM fungi arrived faster than plants and may thus drive vegetation dynamics along secondary vegetation succession.

Impact of alien pines on local arbuscular mycorrhizal fungal communities-evidence from two continents

The introduction of alien plants can influence biodiversity and ecosystems. However, its consequences for soil microbial communities remain poorly understood. We addressed the impact of alien ectomycorrhizal (EcM) pines on local arbuscular mycorrhizal (AM) fungal communities in two regions with contrasting biogeographic histories: in South Africa, where no native EcM plant species are present; and in Argentina, where EcM trees occur naturally. The effect of alien pines on AM fungal communities differed between these regions. In South Africa, plantations of alien EcM pines exhibited lower AM fungal richness and significantly altered community composition, compared with native fynbos. In Argentina, the richness and composition of local AM fungal communities were similar in plantations of alien EcM pines and native forest. However, the presence of alien pines resulted in slight changes to the phylogenetic structure of root AM fungal communities in both regions. In pine clearcut areas in South Africa, the richness and composition of AM fungal communities were intermediate between the native fynbos and the alien pine plantation, which is consistent with natural regeneration of former AM fungal communities following pine removal. We conclude that the response of local AM fungal communities to alien EcM pines differs between biogeographic regions with different histories of species coexistence.