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Summary Ectomycorrhizal fungi (EMF) play a crucial role in facilitating plant nutrient uptake from the soil although inorganic nitrogen (N) can potentially diminish this role. However, the effect of inorganic N availability and organic matter on shaping EMF‐mediated plant iron (Fe) uptake remains unclear.To explore this, we performed a microcosm study onPinus taedaroots inoculated withSuillus cothurnatustreated with +/−Fe‐coated sand, +/−organic matter, and a gradient of NH₄NO₃concentrations.Mycorrhiza formation was most favorable under conditions with organic matter, without inorganic N. Synchrotron X‐ray microfluorescence imaging on ectomycorrhizal cross‐sections suggested that the effect of inorganic N on mycorrhizal Fe acquisition largely depended on organic matter supply. With organic matter, mycorrhizal Fe concentration was significantly decreased as inorganic N levels increased. Conversely, an opposite trend was observed when organic matter was absent. Spatial distribution analysis showed that Fe, zinc, calcium, and copper predominantly accumulated in the fungal mantle across all conditions, highlighting the mantle's critical role in nutrient accumulation and regulation of nutrient transfer to internal compartments.Our work illustrated that the liberation of soil mineral Fe and the EMF‐mediated plant Fe acquisition are jointly regulated by inorganic N and organic matter in the soil. 

Each gram of rich soil can harbor 100 million to a billion microorganisms, meaning every inch of our soil is alive (Raynaud and Nunan 2014) (Fig. 1). These underground organisms keep soil healthy. Balancing the community of microbes can benefit plant yield, plant health, and soil sustainability. While it is recognized that many soil microbes perform key roles in crop productivity, the importance of these underground activities is easily overlooked because of their small size. By focusing on a specific group of microbes living on or near plant roots, this publication provides understanding for these questions: Who are these microbes and how do they improve plant and soil health? 

Abstract Zinc (Zn) is a major soil contaminant and high Zn levels can disrupt growth, survival, and reproduction of fungi. Some fungal species evolved Zn tolerance through cell processes mitigating Zn toxicity, although the genes and detailed mechanisms underlying mycorrhizal fungal Zn tolerance remain unexplored. To fill this gap in knowledge, we investigated the gene expression of Zn tolerance in the ectomycorrhizal fungus Suillus luteus. We found that Zn tolerance in this species is mainly a constitutive trait that can also be environmentally dependent. Zinc tolerance in S. luteus is associated with differences in the expression of genes involved in metal exclusion and immobilization, as well as recognition and mitigation of metal-induced oxidative stress. Differentially expressed genes were predicted to be involved in transmembrane transport, metal chelation, oxidoreductase activity, and signal transduction. Some of these genes were previously reported as candidates for S. luteus Zn tolerance, while others are reported here for the first time. Our results contribute to understanding the mechanisms of fungal metal tolerance and pave the way for further research on the role of fungal metal tolerance in mycorrhizal associations. 

Abstract Profiling the taxonomic and functional composition of microbes using metagenomic (MG) and metatranscriptomic (MT) sequencing is advancing our understanding of microbial functions. However, the sensitivity and accuracy of microbial classification using genome– or core protein-based approaches, especially the classification of eukaryotic organisms, is limited by the availability of genomes and the resolution of sequence databases. To address this, we propose the MicroFisher, a novel approach that applies multiple hypervariable marker genes to profile fungal communities from MGs and MTs. This approach utilizes the hypervariable regions of ITS and large subunit (LSU) rRNA genes for fungal identification with high sensitivity and resolution. Simultaneously, we propose a computational pipeline (MicroFisher) to optimize and integrate the results from classifications using multiple hypervariable markers. To test the performance of our method, we applied MicroFisher to the synthetic community profiling and found high performance in fungal prediction and abundance estimation. In addition, we also used MGs from forest soil and MTs of root eukaryotic microbes to test our method and the results showed that MicroFisher provided more accurate profiling of environmental microbiomes compared to other classification tools. Overall, MicroFisher serves as a novel pipeline for classification of fungal communities from MGs and MTs. 

Abstract Studying the signatures of evolution can help to understand genetic processes. Here, we demonstrate how the existence of balancing selection can be used to identify the breeding systems of fungi from genomic data. The breeding systems of fungi are controlled by self-incompatibility loci that determine mating types between potential mating partners, resulting in strong balancing selection at the loci. Within the fungal phylum Basidiomycota, two such self-incompatibility loci, namely HD MAT locus and P/R MAT locus, control mating types of gametes. Loss of function at one or both MAT loci results in different breeding systems and relaxes the MAT locus from balancing selection. By investigating the signatures of balancing selection at MAT loci, one can infer a species’ breeding system without culture-based studies. Nevertheless, the extreme sequence divergence among MAT alleles imposes challenges for retrieving full variants from both alleles when using the conventional read-mapping method. Therefore, we employed a combination of read-mapping and local de novo assembly to construct haplotypes of HD MAT alleles from genomes in suilloid fungi (genera Suillus and Rhizopogon). Genealogy and pairwise divergence of HD MAT alleles showed that the origins of mating types predate the split between these two closely related genera. High sequence divergence, trans-specific polymorphism, and the deeply diverging genealogy confirm the long-term functionality and multiallelic status of HD MAT locus in suilloid fungi. This work highlights a genomics approach to studying breeding systems regardless of the culturability of organisms based on the interplay between evolution and genetics. 

Zinc (Zn) is a plant essential micronutrient involved in a wide range of cellular processes. Ectomycorrhizal fungi (EMF) are known to play a critical role in regulating plant Zn status. However, how EMF control uptake and translocation of Zn and other nutrients in plant roots under different Zn conditions is not well known. Using X-ray fluorescence imaging, we found the EMF species Suillus luteus increased pine root Zn acquisition under low Zn concentrations and reduced its accumulation under higher Zn levels. By contrast, non-mycorrhizal pine roots exposed to high Zn indiscriminately take up and translocate Zn to root tissues, leading to Zn stress. Regardless of S. luteus inoculation, the absorption pattern of Ca and Cu was similar to Zn. Compared to Ca and Cu, effects of S. luteus on Fe acquisition were more marked, leading to a negative association between Zn addition and Fe concentration within EMF roots. Besides, higher nutrient accumulation in the fungal sheath, compared to hyphae inhabiting between intercellular space of cortex cells, implies the fungal sheath serves as a barrier to regulate nutrient transportation into fungal Hartig net. Our results demonstrate the crucial roles EMF play in plant nutrient uptake and how fungal partners ameliorate soil chemical conditions either by increasing or decreasing element uptake. 

High-throughput amplicon sequencing that primarily targets the 16S ribosomal DNA (rDNA) (for bacteria and archaea) and the Internal Transcribed Spacer rDNA (for fungi) have facilitated microbial community discovery across diverse environments. A three-step PCR that utilizes flexible primer choices to construct the library for Illumina amplicon sequencing has been applied to several studies in forest and agricultural systems. The three-step PCR protocol, while producing high-quality reads, often yields a large number (up to 46%) of reads that are unable to be assigned to a specific sample according to its barcode. Here, we improve this technique through an optimized two-step PCR protocol. We tested and compared the improved two-step PCR meta-barcoding protocol against the three-step PCR protocol using four different primer pairs (fungal ITS: ITS1F-ITS2 and ITS1F-ITS4, and bacterial 16S: 515F-806R and 341F-806R). We demonstrate that the sequence quantity and recovery rate were significantly improved with the two-step PCR approach (fourfold more read counts per sample; determined reads ≈90% per run) while retaining high read quality (Q30 > 80%). Given that synthetic barcodes are incorporated independently from any specific primers, this two-step PCR protocol can be broadly adapted to different genomic regions and organisms of scientific interest. 

Recent studies have shown that M. elongata (M. elongata) isolated from Populus field sites has a dual endophyte–saprotroph lifestyle and is able to promote the growth of Populus. However, little is known about the host fidelity of M. elongata and whether M. elongata strains differ from one another in their ability to promote plant growth. Here, we compared the impacts of three Populus-associated M. elongata isolates (PMI 77, PMI 93, and PMI 624) on the growth of seven different crop species by measuring plant height, plant dry biomass, and leaf area. M. elongata isolates PMI 624 and PMI 93 increased the plant height, leaf area, and plant dry weight of Citrullus lanatus, Zea mays, Solanum lycopersicum, and Cucurbita to a much greater degree than PMI 77 (33.9% to 14.1%). No significant impacts were observed for any isolate on the growth of Abelmoschus esculentus or Glycine max. On the contrary, Glycine max significantly decreased in height by 30.6% after the inoculation of M. elongata PMI 77. In conclusion, this study demonstrates that M. elongata generally promoted metrics of the plant performance among a diverse set of importantly non-leguminous crop species. Future research on understanding the molecular mechanisms that underlie strain and host variability is warranted. 

Summary 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.