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Mycorrhizal symbioses the union of roots and soil fungi are universal in terrestrial ecosystems and may have been fundamental to land colonization by plants 1, 2. Boreal, temperate and montane forests all depend on ectomycorrhizae1. Identification of the primary factors that regulate symbiotic development and metabolic activity will therefore open the door to understanding the role of ectomycorrhizae in plant development and physiology, allowing the full ecological significance of this symbiosis to be explored. Here we report the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor (Fig. 1) and highlight gene sets involved in rhizosphere colonization and symbiosis. This 65-megabase genome assembly contains 20,000 predicted protein-encoding genes and a very large number of transposons and repeated sequences. We detected unexpected genomic features, most notably a battery of effector-type small secreted proteins (SSPs) with unknown function, several of which are only expressed in symbiotic tissues. The most highly expressed SSP accumulates in the proliferating hyphae colonizing the host root. The ectomycorrhizae-specific SSPs probably have a decisive role in the establishment of the symbiosis. The unexpected observation that the genome of L. bicolor lacks carbohydrate-active enzymes involved in degradation of plant cell walls, but maintains the ability to degrade non-plant cell wall polysaccharides, reveals the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots. The predicted gene inventory of the L. bicolor genome, therefore, points to previously unknown mechanisms of symbiosis operating in biotrophic mycorrhizal fungi. The availability of this genome provides an unparalleled opportunity to develop a deeper understanding of the processes by which symbionts interact with plants within their ecosystem to perform vital functions in the carbon and nitrogen cycles that are fundamental to sustainable plant productivity.

Discusses the impact of inoculation with Paxillus involutus on the use of organic carbon compounds by birch roots in Midlothian, Scotland. Stages of the mycorrhiza formation; Synthesis of amino acids in mycorrhizal birch roots and in free living fungus; Disappearance of radioactivity on the incubation medium.

Seedlings of Betula pendula Roth. were grown in the presence of Paxillus involutus (Batsch) Fr., and metabolic changes during mycorrhiza formation were examined by measuring organic acid and amino acid pools and related enzyme activities, following sequential harvests. Glutamine, aspartate and asparagine pools were always lower in infected roots than in non-infected roots, especially during Hartig net initiation and formation. Glutamate concentration was similar in both tissues. Citrate and malate were the two major organic acids detected and their concentrations were equal in infected and non-infected roots. Aspartate aminotransferase, glutamine synthetase, NAD-dependent malate dehydrogenase and glucose-6-phosphate dehydrogenase activities were higher in infected roots than non-infected roots. For all enzymes revealed on polyacrylamide gels, both root and fungal isoforms were present in infected roots. Quantitative changes in enzyme capacities and metabolite pools indicated that mycorrhiza formation caused a re-arrangement of the main metabolic pathways during the very early stages following contact, which might be related to the structural changes. [ABSTRACT FROM AUTHOR]

Detailed examination of the structural and functional integration taking place during mycorrhiza formation necessitates rapid and aseptic synthesis of uniform mycorrhizal structures at well-defined stages of development. A system is described for formation of ectomycorrhizas between Betula pendula Roth. and Paxillus involutus (Batsch) Fr. Small sterile seedlings were placed directly on fungal colonies growing on cellophane covered agar plates. Microscopic examination during the course of development indicated an initial rapid colonization of the roots and formation of a fully developed mantle within 2-4 d. Harting net formation was evident after 8 d of contact and involved a transition from a paraepidermal organization to a mature periepidermal Harting net after 15 d of contact. Using a standard fungal growth medium, formation and development of mycorrhizas were not greatly affected by changes in the overall concentrations of nitrogen and phosphorus but changes in the ratio of nitrogen to phosphorus had strong effects on development and mycorrhiza formation was completely suppressed when the ratio of nitrogen to phosphorus was increased. These differences were not related to the effects of nitrogen or phosphorus on linear growth of the fungus. [ABSTRACT FROM AUTHOR]

Marginal lands have been proposed to produce non-food crop biomass for energy or green materials. For this purpose, the selection, implementation, and growth optimization of plant species on such lands are key elements to investigate to achieve relevant plant yields. Stinging nettle ( Urtica dioica ) is a herbaceous perennial that grows spontaneously on contaminated lands and was described as suitable to produce fibers for material applications. Two mercury-contaminated soils from industrial wastelands with different properties (grassland soil and sediment landfill) were used in this study to assess the potential growth of stinging nettle in a greenhouse mesocosm experiment. Two organic amendments were studied for their impact on nettle growth. The solid digestate from organic food wastes significantly doubled plant biomass whereas the compost from green wastes had a lower impact. The highest doses of organic amendments significantly increased the number of fibers, which doubled following digestate application, while reducing leaf Hg concentration. Both amendments significantly improved soil respiration and enzymatic activities linked to the microbial biomass in the soil from the sediment landfill by the end of the experiment. In the context of a phytomanagement scenario, solid digestate would be a preferred amendment resource to improve nettle production on industrial wastelands.

Rare earth elements (REEs) are crucial elements for current high-technology and renewable energy advances. In addition to their increasing usage and their low recyclability leading to their release into the environment, REEs are also used as crop fertilizers. However, little is known regarding the cellular and molecular effects of REEs in plants, which is crucial for better risk assessment, crop safety and phytoremediation. Here, we analysed the ionome and transcriptomic response of Arabidopsis thaliana exposed to a light (lanthanum, La) and a heavy (ytterbium, Yb) REE. At the transcriptome level, we observed the contribution of ROS and auxin redistribution to the modified root architecture following REE exposure. We found indications for the perturbation of Fe homeostasis by REEs in both roots and leaves of Arabidopsis suggesting competition between REEs and Fe. Furthermore, we propose putative ways of entry of REEs inside cells through transporters of microelements. Finally, similar to REE accumulating species, organic acid homeostasis (e.g. malate and citrate) appears critical as a tolerance mechanism in response to REEs. By combining ionomics and transcriptomics, we elucidated essential patterns of REE uptake and toxicity response of Arabidopsis and provide new hypotheses for a better evaluation of the impact of REEs on plant homeostasis., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Soil microbial communities play a key role in plant nutrition and stress tolerance. This is particularly true in sites contaminated by trace metals, which often have low fertility and stressful conditions for woody plants in particular. However, we have limited knowledge of the abiotic and biotic factors affecting the richness and composition of microbial communities inhabiting the rhizosphere of plants in contaminated sites. Using high-throughput amplicon sequencing, we studied the rhizospheric bacterial and fungal community structures of 14 woody plant families planted in three contrasting sites contaminated by metals (Pb, Cd, Zn, Mn, Fe, S). The rhizospheric bacterial communities in the given sites showed no significant difference between the various woody species but did differ significantly between sites. The Proteobacteria phylum was dominant, accounting for over 25 % of the overall relative abundance, followed by Actinobacteria, Bacteroidetes and Gemmatimonadetes. Site was also the main driver of fungal community composition, yet unlike bacteria, tree species identity significantly affected fungal communities. The Betulaceae, Salicaceae and Fagaceae families had a high proportion of Basidiomycota, particularly ectomycorrhizal fungi, and the lowest diversity and richness. The other tree families and the unplanted soil harboured a greater abundance of Ascomycota and Mucoromycota. Consequently, for both bacteria and fungi, the site effect significantly impacted their community richness and composition, while the influence of plants on the richness and composition of rhizospheric microbial communities stayed consistent across sites and was dependent on the microbial kingdom. Finally, we highlighted the importance of considering this contrasting response of plant rhizospheric microbial communities in relation to their host identity, particularly to improve assisted revegetation efforts at contaminated sites., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: CHALOT Michel reports financial support was provided by French National Research Agency. CHALOT Michel reports financial support was provided by Région Franche-Comté. CHALOT Michel reports financial support was provided by Pays de Montbéliard Agglomération. ZAPPELINI Cyril reports financial support was provided by French Ministry of Higher Education and Research., (Copyright © 2023 Elsevier B.V. All rights reserved.)

High-throughput sequencing has become a prominent tool to assess plant-associated microbial diversity. Still, some technical challenges remain in characterising these communities, notably due to plant and fungal DNA co-amplification. Fungal-specific primers, Peptide Nucleic Acid (PNA) clamps, or adjusting PCR conditions are approaches to limit plant DNA contamination. However, a systematic comparison of these factors and their interactions, which could limit plant DNA contamination in the study of plant mycobiota, is still lacking. Here, three primers targeting the ITS2 region were evaluated alone or in combination with PNA clamps both on nettle ( Urtica dioica ) root DNA and a mock community. PNA clamps did not improve the richness or diversity of the fungal communities but increased the number of fungal reads. Among the tested factors, the most significant was the primer pair. Specifically, the 5.8S-Fun/ITS4-Fun pair exhibited a higher OTU richness but fewer fungal reads. Our study demonstrates that the choice of primers is critical for limiting plant and fungal DNA co-amplification. PNA clamps increase the number of fungal reads when ITS2 is targeted but do not result in higher fungal diversity recovery at high sequencing depth. At lower read depths, PNA clamps might enhance microbial diversity quantification for primer pairs lacking fungal specificity., Competing Interests: No potential conflict of interest was reported by the author(s)., (© 2024 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.)