Cross-scale integration of mycorrhizal function

Dynamics and drivers of mycorrhizal fungi after glacier retreat

The development of terrestrial ecosystems depends greatly on plant mutualists such as mycorrhizal fungi. The global retreat of glaciers exposes nutrient-poor substrates in extreme environments and provides a unique opportunity to study early successions of mycorrhizal fungi by assessing their dynamics and drivers. We combined environmental DNA metabarcoding and measurements of local conditions to assess the succession of mycorrhizal communities during soil development in 46 glacier forelands around the globe, testing whether dynamics and drivers differ between mycorrhizal types. Mycorrhizal fungi colonized deglaciated areas very quickly (< 10 yr), with arbuscular mycorrhizal fungi tending to become more diverse through time compared to ectomycorrhizal fungi. Both alpha- and beta-diversity of arbuscular mycorrhizal fungi were significantly related to time since glacier retreat and plant communities, while microclimate and primary productivity were more important for ectomycorrhizal fungi. The richness and composition of mycorrhizal communities were also significantly explained by soil chemistry, highlighting the importance of microhabitat for community dynamics. The acceleration of ice melt and the modifications of microclimate forecasted by climate change scenarios are expected to impact the diversity of mycorrhizal partners. These changes could alter the interactions underlying biotic colonization and belowground-aboveground linkages, with multifaceted impacts on soil development and associated ecological processes.

Ectomycorrhizal trees rely on nitrogen resorption less than arbuscular mycorrhizal trees globally

Nitrogen (N) resorption is an important pathway of N conservation, contributing to an important proportion of plant N requirement. However, whether the ratio of N resorption to N requirement may be affected by environmental factors, mycorrhizal types or atmospheric CO2 concentration remains unclear. Here, we conducted a meta-analysis on the impacts of environmental factors and mycorrhizal types on this ratio. We found this ratio in ectomycorrhizal (EM) trees decreased with mean annual precipitation, mean annual temperature, soil total N content and atmospheric CO2 concentration and was significantly lower than that in arbuscular mycorrhizal (AM) trees. An in situ 15 N tracing experiment further confirmed that AM trees have a stronger reliance on N resorption than EM trees. Our study suggests that AM and EM trees potentially have different strategies for alleviation of progressive N limitation, highlighting the necessity of incorporating plant mycorrhizal types into Earth System Models.

Successional adaptive strategies revealed by correlating arbuscular mycorrhizal fungal abundance with host plant gene expression

The shifts in adaptive strategies revealed by ecological succession and the mechanisms that facilitate these shifts are fundamental to ecology. These adaptive strategies could be particularly important in communities of arbuscular mycorrhizal fungi (AMF) mutualistic with sorghum where strong AMF succession replaces initially ruderal species with competitive ones and where the strongest plant response to drought is to manage these AMF. Although most studies of agriculturally important fungi focus on parasites, the mutualistic symbionts, AMF, constitute a research system of human-associated fungi whose relative simplicity and synchrony are conducive to experimental ecology. First, we hypothesize that, when irrigation is stopped to mimic drought, competitive AMF species should be replaced by AMF species tolerant to drought stress. We then, for the first time, correlate AMF abundance and host plant transcription to test two novel hypotheses about the mechanisms behind the shift from ruderal to competitive AMF. Surprisingly, despite imposing drought stress, we found no stress tolerant AMF, likely due to our agricultural system having been irrigated for nearly six decades. Remarkably, we found strong and differential correlation between the successional shift from ruderal to competitive AMF and sorghum genes whose products (i) produce and release strigolactone signals, (ii) perceive mycorrhizal-lipochitinoligosaccharide (Myc-LCO) signals, (iii) provide plant lipid and sugar to AMF and, (iv) import minerals and water provided by AMF. These novel insights frame new hypotheses about AMF adaptive evolution and suggest a rationale for selecting AMF to reduce inputs and maximize yields in commercial agriculture.

Local Responses and Systemic Induced Resistance Mediated by Ectomycorrhizal Fungi

Ectomycorrhizal fungi (EMF) grow as saprotrophs in soil and interact with plants, forming mutualistic associations with roots of many economically and ecologically important forest tree genera. EMF ensheath the root tips and produce an extensive extramatrical mycelium for nutrient uptake from the soil. In contrast to other mycorrhizal fungal symbioses, EMF do not invade plant cells but form an interface for nutrient exchange adjacent to the cortex cells. The interaction of roots and EMF affects host stress resistance but uncovering the underlying molecular mechanisms is an emerging topic. Here, we focused on local and systemic effects of EMF modulating defenses against insects or pathogens in aboveground tissues in comparison with arbuscular mycorrhizal induced systemic resistance. Molecular studies indicate a role of chitin in defense activation by EMF in local tissues and an immune response that is induced by yet unknown signals in aboveground tissues. Volatile organic compounds may be involved in long-distance communication between below- and aboveground tissues, in addition to metabolite signals in the xylem or phloem. In leaves of EMF-colonized plants, jasmonate signaling is involved in transcriptional re-wiring, leading to metabolic shifts in the secondary and nitrogen-based defense metabolism but cross talk with salicylate-related signaling is likely. Ectomycorrhizal-induced plant immunity shares commonalities with systemic acquired resistance and induced systemic resistance. We highlight novel developments and provide a guide to future research directions in EMF-induced resistance.

Interactions of Arbuscular Mycorrhizal Fungi with Hyphosphere Microbial Communities in a Saline Soil: Impacts on Phosphorus Availability and Alkaline Phosphatase Gene Abundance

The limited availability of soil phosphorus to plants under salinity stress is a major constraint for crop production in saline soils, which could be alleviated by improving mycorrhizal and soil microbial interactions. This study investigated the effects of Funneliformis mosseae (Fm) inoculation on phosphorus (P) availability to Sorghum bicolor, and alkaline phosphatase (ALP) activity and gene abundance (phoD) in a P-deficient naturally saline soil. A greenhouse study was conducted in order to compare the experimental treatments of Fm inoculated vs. control plants grown in saline soil with and without (sterilized soil) native microbial community. A separate hyphosphere (root-free) compartment was constructed within the mycorrhizosphere and amended with phosphate. After four weeks of transplanting, shoot, roots, mycorrhizosphere, and hyphosphere samples were collected and analyzed for soil and plant P concentrations, root colonization, and abundance of ALP and phoD. The results showed significantly higher colonization in Fm-inoculated treatments compared to uninoculated. Plant available P concentrations, phoD gene abundance and ALP activity were significantly reduced (p < 0.05) in sterilized-hyphosphere as compared to unsterilized in both Fm-inoculated and uninoculated treatments. Inoculation with Fm significantly increased the plant P uptake (p < 0.05) when compared to uninoculated treatments, but only in the plants gown in unsterile mycorrhizosphere. It can be concluded that inoculation of Fm increased root colonization and the uptake of P by sorghum plant in saline soil and native microbial community interactions were critical for increasing bioavailable P concentrations. These beneficial interactions between plants, mycorrhizae, and native microbes should be considered for soil fertility management in saline soils.

Mildew Locus O facilitates colonization by arbuscular mycorrhizal fungi in angiosperms

Loss of barley Mildew Resistance Locus O (MLO) is known to confer durable and robust resistance to powdery mildew (Blumeria graminis), a biotrophic fungal leaf pathogen. Based on the increased expression of MLO in mycorrhizal roots and its presence in a clade of the MLO family that is specific to mycorrhizal-host species, we investigated the potential role of MLO in arbuscular mycorrhizal interactions. Using mutants from barley (Hordeum vulgare), wheat (Triticum aestivum), and Medicago truncatula, we demonstrate a role for MLO in colonization by the arbuscular mycorrhizal fungus Rhizophagus irregularis. Early mycorrhizal colonization was reduced in mlo mutants of barley, wheat, and M. truncatula, and this was accompanied by a pronounced decrease in the expression of many of the key genes required for intracellular accommodation of arbuscular mycorrhizal fungi. These findings show that clade IV MLOs are involved in the establishment of symbiotic associations with beneficial fungi, a role that has been appropriated by powdery mildew.

Plants make galls to accommodate foreigners: some are friends, most are foes

At the colonization site of a foreign entity, plant cells alter their trajectory of growth and development. The resulting structure - a plant gall - accommodates various needs of the foreigner, which are phylogenetically diverse: viruses, bacteria, protozoa, oomycetes, true fungi, parasitic plants, and many types of animals, including rotifers, nematodes, insects, and mites. The plant species that make galls also are diverse. We assume gall production costs the plant. All is well if the foreigner provides a gift that makes up for the cost. Nitrogen-fixing nodule-inducing bacteria provide nutritional services. Gall wasps pollinate fig trees. Unfortunately for plants, most galls are made for foes, some of which are deeply studied pathogens and pests: Agrobacterium tumefaciens, Rhodococcus fascians, Xanthomonas citri, Pseudomonas savastanoi, Pantoea agglomerans, 'Candidatus' phytoplasma, rust fungi, Ustilago smuts, root knot and cyst nematodes, and gall midges. Galls are an understudied phenomenon in plant developmental biology. We propose gall inception for discovering unifying features of the galls that plants make for friends and foes, talk about molecules that plants and gall-inducers use to get what they want from each other, raise the question of whether plants colonized by arbuscular mycorrhizal fungi respond in a gall-like manner, and present a research agenda.

Tree mycorrhizal associations mediate soil fertility effects on forest community structure in a temperate forest

Soil fertility influences plant community structure, yet few studies have focused on how this influence is affected by the type of mycorrhizal association formed by tree species within local communities. We examined the relationship of aboveground biomass (AGB) and diversity of adult trees with soil fertility (nitrogen, phosphorus, organic matter, etc.) in the context of different spatial distributions of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) trees in a temperate forest in Northeast China. Diversity showed a positive trend along the soil fertility gradient driven mostly by a positive relationship between AM tree abundance and soil fertility. By contrast, the AGB showed a negative trend along the soil fertility gradient driven mostly by a negative relationship between EM tree AGB and soil fertility. Furthermore, the opposite trend in the AGB and tree species diversity along the soil fertility gradient led to an overall negative diversity-biomass relationship at the 50-m scale but not the 20-m scale. These results suggest that tree mycorrhizal associations play a critical role in driving forest community structure along soil fertility gradients and highlight the importance of tree mycorrhizal associations in influencing how the diversity-ecosystem function (e.g. biomass) relationships change with soil fertility.