Common Mycorrhizae Network: A Review of the Theories and Mechanisms Behind Underground Interactions

Most terrestrial plants establish symbiotic associations with mycorrhizal fungi for accessing essential plant nutrients. Mycorrhizal fungi have been frequently reported to interconnect plants via a common mycelial network (CMN), in which nutrients and signaling compounds can be exchanged between the connected plants. Several studies have been performed to demonstrate the potential effects of the CMN mediating resource transfer and its importance for plant fitness. Due to several contrasting results, different theories have been developed to predict benefits or disadvantages for host plants involved in the network and how it might affect plant communities. However, the importance of the mycelium connections for resources translocation compared to other indirect pathways, such as leakage of fungi hyphae and subsequent uptake by neighboring plant roots, is hard to distinguish and quantify. If resources can be translocated via mycelial connections in significant amounts that could affect plant fitness, it would represent an important tactic for plants co-existence and it could shape community composition and dynamics. Here, we report and critically discuss the most recent findings on studies aiming to evaluate and quantify resources translocation between plants sharing a CMN and predict the pattern that drives the movement of such resources into the CMN. We aim to point gaps and define open questions to guide upcoming studies in the area for a prospect better understanding of possible plant-to-plant interactions via CMN and its effect in shaping plants communities. We also propose new experiment set-ups and technologies that could be used to improve previous experiments. For example, the use of mutant lines plants with manipulation of genes involved in the symbiotic associations, coupled with labeling techniques to track resources translocation between connected plants, could provide a more accurate idea about resource allocation and plant physiological responses that are truly accountable to CMN

Carbon investment into mobilization of mineral and organic phosphorus by arbuscular mycorrhiza

To overcome phosphorus (P) deficiency, about 80% of plant species establish symbiosis with arbuscular mycorrhizal fungi (AMF), which in return constitute a major sink of photosynthates. Information on whether plant carbon (C) allocation towards AMF increases with declining availability of the P source is limited. We offered orthophosphate (OP), apatite (AP), or phytic acid (PA) as the only P source available to arbuscular mycorrhiza (AM) (Solanum lycopersicum x Rhizophagus irregularis) in a mesocosm experiment, where the fungi had exclusive access to each P source. After exposure, we determined P contents in the plant, related these to the overall C budget of the system, including the organic C (OC) contents, the respired CO2, the phospholipid fatty acid (PLFA) 16:1ω5c (extraradical mycelium), and the neutral fatty acid (NLFA) 16:1ω5c (energy storage) at the fungal compartment. Arbuscular mycorrhizal (AM) plants incorporated P derived from the three P sources through the mycorrhizal pathway, but did this with differing C-P trading costs. The mobilization of PA and AP by the AM plant entailed larger mycelium infrastructure and significantly larger respiratory losses of CO2, in comparison with the utilization of the readily soluble OP. Our study thus suggests that AM plants invest larger C amounts into their fungal partners at lower P availability. This larger C flux to the AM fungi might also lead to larger soil organic C contents, in the course of forming larger AM biomass under P-limiting conditions. © 2020, The Author(s)

Designing a Robust and Versatile System to Investigate Nutrient Exchange in, and Partitioning by, Mycorrhiza (Populus x canesces x Paxillus involutus) Under Axenic or Greenhouse Conditions

Phosphorus (P) bioavailability affects plant nutrition. P can be present in soils in different chemical forms that are not available for direct plant uptake and have to be acquired by different mechanisms, representing different resource niches. These mechanisms, of which many seem to be attributed to mycorrhiza, likely influence the diversity and stability of plant communities in natural ecosystems, as they also might help to overcome a future shortage of P supply in agro-ecosystems. In order to understand the mechanisms of P acquisition, the associated carbon costs, and the resource partitioning by mycorrhizal fungi, the ecosystem situation has to be mimicked in smaller scaled experiments. Here, different experimental setups are evaluated using plantlets of Populus x canescens and its functional ectomycorrhizal (ECM) fungus Paxillus involututs strain MAJ. To investigate resource partitioning involving mycorrhizae, the protocols of this study describe preparation of an in vitro and a rhizotrone culture systems for studies under axenic conditions as well as a mesocosm culture system for greenhouse conditions. We also describe the construction of separate compartments containing nutrients and excluding plant roots as well as the progress that has been made in in vitro propagation of plant and ECM fungal material. The practical experience made in our study shows that the in vitro culture system is prone to desiccation and its construction and maintenance are more time consuming and complicated. In contrast, with the axenic rhizotrone culture system and the mesocosms we have created more robust and very versatile systems that are also suitable for greenhouse conditions

Successional patterns along soil development gradients formed by glacier retreat in the Maritime Antarctic, King George Island

Background: Maritime Antarctica is severely affected by climate change and accelerating glacier retreat forming temporal gradients of soil development. Successional patterns of soil development and plant succession in the region are largely unknown, as are the feedback mechanisms between both processes. Here we identify three temporal gradients representing horizontal and vertical glacier retreat, as well as formation of raised beaches due to isostatic uplift, and describe soil formation and plant succession along them. Our hypotheses are (i) plants in Antarctica are able to modulate the two base parameters in soil development, organic C content and pH, along the temporal gradients, leading to an increase in organic carbon and soil acidity at relatively short time scales, (ii) the soil development induces succession along these gradients, and (iii) with increasing soil development, bryophytes and Deschampsia antarctica develop mycorrhiza in maritime Antarctica in order to foster interaction with soil.  Results: All temporal gradients showed soil development leading to differentiation of soil horizons, carbon accumulation and increasing pH with age. Photoautptroph succession occurred rapidly after glacier retreat, but occurrences of mosses and lichens interacting with soils by rhizoids or rhizines were only observed in the later stages. The community of ground dwelling mosses and lichens is the climax community of soil succession, as the Antarctic hairgrass D. antarctica was restricted to ornithic soils. Neither D. antarctica nor mosses at the best developed soils showed any sign of mycorrhization.  Conclusion: Temporal gradients formed by glacier retreat can be identified in maritime Antarctic, where soil development and plant succession of a remarkable pace can be observed, although pseudo-succession occurs by fertilization gradients caused by bird feces. Thus, the majority of ice-free surface in Antarctica is colonized by plant communities which interact with soil by litter input rather than by direct transfer of photoassimilates to soil

Arbuscular mycorrhizal assemblages along contrasting andean forests of Southern Chile

Southern Chilean pristine temperate rainforests have been floristically stable during the Holocene, thus representing a pre-industrial baseline of forest ecology. Given this and its edaphic limitations, it is imperative to better understand these forests ecological patterns of mycorrhizal symbiosis. Therefore, here we compare the arbuscular mycorrhizal (AM) communities in three treeline Nothofagus pumilio contrasting plots of Chilean Andes (a volcano crater, pristine forest, and disturbed forest). The AM community assemblages were determined by morphological identification and spore counting, in three A horizon soil samples by plot. In the same nine soil samples, standard chemical analysis was performed. Eighteen AM species were described; Acaulospora was the most abundant genus. The forest plot had the highest AM species richness compared to the disturbed and crater plots. Interestingly, soils Olsen P (plant available phosphorus), pH, and Al+++ saturation similarly affected the AM assemblages. We suggest that some AM species could be specially adapted to extremely high Al saturation and extremely low plant available P conditions, as those experienced on Andean Nothofagus forests. These species may help initiate biological succession on highly disturbed ecosystems. We suggest that mycorrhizal fungi play a key role in seedling colonization of extreme environments such as the Andean treeline.CONICYT/21150047FONDECYT/1141060FONDECYT/315017

Mycorrhizal Mediated Partitioning of Phosphorus: Ectomycorrhizal (Populus x canescens x Paxillus involutus) Potential to Exploit Simultaneously Organic and Mineral Phosphorus Sources

Many natural and anthropogenic soils are phosphorus (P) limited often due to larger P stocks sequestered in forms of low bioavailability. One of the strategies to overcome this shortage lies in the symbiosis of plants with mycorrhizal fungi, increasing the plant P uptake of these hardly accessible sources. However, little is known about mycorrhizal fungal mediated partitioning of differently available P forms, which could contribute to more efficient use of P by plants and, thereby, reduce competition for soil P. This study aimed to investigate the uptake of P from differently bioavailable P sources by ectomycorrhiza. For that, we conducted a rhizotrone study using Populus x canescens and its compatible ectomycorrhizal fungus Paxillus involutus. Four different P sources [ortho-phosphate (oP), adenosine monophosphate (AMP), hydroxyapatite (HAP), and oP bound to goethite (gP)] or only HAP as 1P control were supplied in separate compartments, where only the fungal partner had access to the P sources. The amount of the specific P sources was increased according to their decreasing bioavailability. In order to distinguish between the P sources, we applied 33P to track its incorporation in plants by a non-destructive analysis via digital autoradiography. Our results show that an ectomycorrhizal plant is able to utilize all provided P sources via its mycorrhizal fungal associate. The acquisition timing was determined by the most bioavailable P sources, with oP and AMP over HAP and gP, and a mixed P pool over a single P source. In contrast, the magnitude was defined by the by the amount of supplied P source provision of additional nitrogen, hence AMP over oP and gP, as well as by P source complexity, with gP as the least favorable P form. Nevertheless, the results of the present study provide evidence that an ectomycorrhiza has the potential to occupy fundamental niches of various P sources differing in their bioavailability, indicating that being a generalist in P nutrition can facilitate adaptation to various nutritional settings in soil

Mobilisierbarkeit von Glucose-6-Phosphat und Orthophosphat aus Adsorptionskomplexen mit Ferrihydrit

Die Mobilisierung von organischem Phosphor aus schwerverfügbaren Quellen wie mineral-organischen Assoziationen gewinnt im Zuge der Bodenentwicklung an Relevanz. Im Rahmen dieser Studie wurde die Mobilisierbarkeit von organischem Phosphor (in Form von Glucose-6-Phosphat) aus Adsorptionskomplexen mit Ferrihydrit exemplarisch untersucht und mit der Mobilisierbarkeit von Orthophosphat verglichen. Dabei zeigte sich, dass Glucose-6-Phosphat durch Auflösung der Adsorptionskomplexe mittels organischer Liganden (Oxalat, Ascorbat und Desferrioxamin B) in stärkerem Maße mobilisiert werden kann als Orthophosphat. Dies lässt sich vermutlich auf die bevorzugte Adsorption von Glucose-6-Phosphat an den äußeren Sorptionsplätzen des Ferrihydrits und eine damit einhergehende leichtere Zugänglichkeit für die organischen Liganden zurückführen

Influence of small-scale spatial variability of soil properties on yield formation of winter wheat

Background: With the increasing development of sophisticated precision farming techniques, high-resolution application maps are frequently discussed as a key factor in increasing yield potential. However, yield potential maps based on multiple soil properties measurements are rarely part of current farming practices. Furthermore, small-scale differences in soil properties have not been taken into account. Methods: To investigate the impact of soil property changes at high resolution on yield, a field trial has been divided into a sampling grid of 42 plots. The soil properties in each plot were determined at three soil depths. Grain yield and yield formation of winter wheat were analyzed at two sites. Results: Multiple regression analyses of soil properties with yield measures showed that the soil contents of organic carbon, silt, and clay in the top and subsoil explained 45–46% of the variability in grain yield. However, an increasing clay content in the topsoil correlated positively with grain yield and tiller density. In contrast, a higher clay content in the subsoil led to a decrease in grain yield. A cluster analysis of soil texture was deployed to evaluate whether the soil´s small-scale differences caused crucial differences in yield formation. Significant differences in soil organic carbon, yield, and yield formation were observed among clusters in each soil depth. Conclusion: These results show that small-scale lateral and vertical differences in soil properties can strongly impact crop yields and should be considered to improve site-specific cropping techniques further

Soil nitrogen and water management by winter-killed catch crops

Improving N cycling in agroecosystems is one of the key challenges in reducing the environmental footprint of agriculture. Further, uncertainty in precipitation makes crop water management relevant in regions where it has not been necessary thus far. Here, we focus on the potential of winter-killed catch crops (CCs) to reduce N leaching losses from N mineralization over the winter and from soil water management. We compared four single CCs (white mustard, phacelia, Egyptian clover and bristle oat) and two CC mixtures with 4 and 12 plant species (Mix4 and Mix12) with a fallow treatment. High-resolution soil mineral N (Nmin) monitoring in combination with the modelling of spatiotemporal dynamics served to assess N cycling under winter-killed CCs, while soil water was continuously monitored in the rooting zone. Catch crops depleted the residual Nmin pools by between 40 % and 72 % compared to the fallow. The amount of residual N uptake was lowest for clover and not significantly different among the other CCs. Catch crops that produce high N litter materials, such as clover and mustard leaves, showed an early N mineralization flush immediately after their termination and the highest leaching losses from litter mineralization over the winter. Except for clover, all CCs showed Nmin values between 18 % and 92 % higher on the sowing date of the following maize crop. However, only Mix12 was statistically significant. Catch crops depleted the soil water storage in the rooting zone during their growth in autumn and early winter, but preserved water later on when their residues covered the ground. The shallow incorporation of CC residues increased water storage capacity during the cropping season of the main crop even under reduced soil water availability. Hence, catch cropping is not just a simple plant cover for the winter but improves the growth conditions for the following crop with decreased N losses. Mixtures have been shown to compensate for the weaknesses of individual CC species in terms of nutrient capture, mineralization and transfer to the following main crop as well as for soil water management. Detailed knowledge about plant performance during growth and litter mineralization patterns is necessary to make optimal use of their potential

Unrecognized diversity and distribution of soil algae from Maritime Antarctica (Fildes Peninsula, King George Island)

IntroductionEukaryotic algae in the top few centimeters of fellfield soils of ice-free Maritime Antarctica have many important effects on their habitat, such as being significant drivers of organic matter input into the soils and reducing the impact of wind erosion by soil aggregate formation. To better understand the diversity and distribution of Antarctic terrestrial algae, we performed a pilot study on the surface soils of Meseta, an ice-free plateau mountain crest of Fildes Peninsula, King George Island, being hardly influenced by the marine realm and anthropogenic disturbances. It is openly exposed to microbial colonization from outside Antarctica and connected to the much harsher and dryer ice-free zones of the continental Antarctic. A temperate reference site under mild land use, SchF, was included to further test for the Meseta algae distribution in a contrasting environment.MethodsWe employed a paired-end metabarcoding analysis based on amplicons of the highly variable nuclear-encoded ITS2 rDNA region, complemented by a clone library approach. It targeted the four algal classes, Chlorophyceae, Trebouxiophyceae, Ulvophyceae, and Xanthophyceae, representing key groups of cold-adapted soil algae.ResultsA surprisingly high diversity of 830 algal OTUs was revealed, assigned to 58 genera in the four targeted algal classes. Members of the green algal class Trebouxiophyceae predominated in the soil algae communities. The major part of the algal biodiversity, 86.1% of all algal OTUs, could not be identified at the species level due to insufficient representation in reference sequence databases. The classes Ulvophyceae and Xanthophyceae exhibited the most unknown species diversity. About 9% of the Meseta algae species diversity was shared with that of the temperate reference site in Germany.DiscussionIn the small portion of algal OTUs for which their distribution could be assessed, the entire ITS2 sequence identity with references shows that the soil algae likely have a wide distribution beyond the Polar regions. They probably originated from soil algae propagule banks in far southern regions, transported by aeolian transport over long distances. The dynamics and severity of environmental conditions at the soil surface, determined by high wind currents, and the soil algae’s high adaptability to harsh environmental conditions may account for the high similarity of soil algal communities between the northern and southern parts of the Meseta