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Glutamine and glutamate metabolism in ectomycorrhizal fungus Paxillus involutus involves the function of glutamate synthase and a transamination pathway. Glutamine and glutamate pass through the tricarboxylic acid (TCA) cycle and form CO2 to function as respiratory substrates. Amino acid precursors, synthesized during the TCA cycle, regulate aspartate and alanine synthesis in P. involutus.

Abstract Background Symbiotic ectomycorrhizal associations of fungi with forest trees play important and economically significant roles in the nutrition, growth and health of boreal forest trees, as well as in nutrient cycling. The ecology and physiology of ectomycorrhizal associations with Pinus sp are very well documented but very little is known about the molecular mechanisms behind these mutualistic interactions with gymnosperms as compared to angiosperms. Results Using a micro-array approach, the relative abundance of 2109 EST transcripts during interaction of Pinus sylvestris roots with the ectomycorrhizal fungus was profiled. The results reveal significant differential expression of a total of 236 ESTs, 96 transcripts differentially abundant after 1 day of physical contact with the fungus, 134 transcripts after 5 days and only 6 after 15 days at early stages of mantle formation on emerging lateral roots. A subset of cell wall modification and stress related genes was further assessed by quantitative reverse transcription PCR at late stages of mycorrhizal development coinciding with Hartig net formation. The results reveal down regulation of gene transcripts involved in general defence mechanism (e.g. antimicrobial peptide) as well as those involved in cell wall modification (e.g. glycine rich protein, xyloglucan endo transglycosylase). Conclusion This study constitutes the first attempt to characterize the transcriptome of the plant partner in the Pinus sylvestris – Laccaria bicolor model system. We identified 236 ESTs which are potentially important for molecular regulation of a functional symbiotic association in conifer host. The results highlight similarities with other studies based on angiosperm model systems, nevertheless some differences were found in the timing and spatial scale of gene regulation during ectomycorrhiza development in gymnosperms. The present study has identified a number of potentially important molecular events responsible for the initiation and regulation of biochemical, physiological and morphological changes during development of a fully functional symbiosis that are relevant for gymnosperm hosts.

Better understanding and quantifying the relative influence of plants, associated mycorrhizal fungi, and abiotic factors such as elevated CO2 on biotic weathering is essential to constraining weathering estimates. We employed a column microcosm system to examine the effects of elevated CO2 and Pinus sylvestris seedlings, with or without the ectomycorrhizal fungi Piloderma fallax and Suillus variegatus , on rhizosphere soil solution concentrations of low-molecular-weight organic acids (LMWOAs) and on the weathering of primary minerals. Seedlings significantly increased mineral weathering, as estimated from elemental budgets of Ca, K, Mg, and Si. Elevated CO2 increased plant growth and LMWOA concentrations but had no effect on weathering. Colonization by ectomycorrhizal fungi, particularly P. fallax , showed some tendency to increase weathering. LMWOA concentrations correlated with seedling biomass across both CO2 and mycorrhizal treatments but not with total weathering. We conclude that nutrient uptake, which reduces transport limitation to weathering, is the primary mechanism by which plants enhanced weathering in this system. While the experimental system used departs from conditions in forest soils in a number of ways, these results are in line with weathering studies performed at the ecosystem, macrocosm, and microcosm scale, indicating that nutrient uptake by plants and microbes is an important biological mechanism by which mineral weathering is enhanced. [ABSTRACT FROM AUTHOR]

Better understanding and quantifying the relative influence of plants, associated mycorrhizal fungi, and abiotic factors such as elevated CO2 on biotic weathering is essential to constraining weathering estimates. We employed a column microcosm system to examine the effects of elevated CO2 and Pinus sylvestris seedlings, with or without the ectomycorrhizal fungi Piloderma fallax and Suillus variegatus, on rhizosphere soil solution concentrations of low molecular weight organic acids (LMWOA) and weathering of primary minerals. Seedlings significantly increased mineral weathering, as estimated from elemental budgets of Ca, K, Mg, and Si. Elevated CO2 increased plant growth and LMWOA concentrations, but had no effect on weathering. Colonization by ectomycorrhizal fungi, particularly P. fallax, showed some tendency to increase weathering. LMWOA concentrations correlated with seedling biomass across both CO2 and mycorrhizal treatments, but not with total weathering. We conclude that nutrient uptake, which reduces transport limitation to weathering, is the primary mechanism by which plants enhanced weathering in this system. While the experimental system used departs from conditions in forest soils in a number of ways, these results are in line with weathering studies performed at the ecosystem, macrocosm, and microcosm scale, indicating that nutrient uptake by plants and microbes is an important biological mechanism by which mineral weathering is enhanced. [ABSTRACT FROM AUTHOR]

Plant nutrients can be recycled through microbial decomposition of organic matter but replacement of base cations and phosphorus, lost through harvesting of biomass/biofuels or leaching, requires de novo supply of fresh nutrients released through weathering of soil parent material (minerals and rocks). Weathering involves physical and chemical processes that are modified by biological activity of plants, microorganisms and animals. This article reviews recent progress made in understanding biological processes contributing to weathering. A perspective of increasing spatial scale is adopted, examining the consequences of biological activity for weathering from nanoscale interactions, through in vitro and in planta microcosm and mesocosm studies, to field experiments and finally, ecosystem and global level effects. The topics discussed include: the physical alteration of minerals and mineral surfaces, the composition, amounts, chemical properties and effects of plant and microbial secretions, and the role of carbon flow (including stabilization/sequestration of C in organic and inorganic forms). Although the predominant focus is on the effects of fungi in forest ecosystems, the properties of biofilms, including bacterial interactions, are discussed. The implications of these biological processes for modelling are discussed and finally, we attempt to identify some key questions and knowledge gaps, as well as experimental approaches and areas of research in which future studies are likely to yield useful results. A particular focus of this article is to improve the representation of the ways in which biological processes complement physical and chemical processes that mobilize mineral elements, making them available for plant uptake. This is necessary to produce better estimates of weathering that are necessary for sustainable management of forests in a post-fossil fuel economy. While there are abundant examples of nm- and µm-scale physical interactions between microorganisms and different minerals, opinion appears to be divided with respect to the quantitative significance of these observations to overall weathering. Numerous in vitro experiments and microcosm studies involving plants and their associated microorganisms suggest that the allocation of plant-derived carbon, mineral dissolution and plant nutrient status are tightly coupled but there is still disagreement about the extent to which these processes contribute to field-scale observations. Apart from providing dynamically responsive pathways for the allocation of plant-derived carbon to power dissolution of minerals, mycorrhizal mycelia provide conduits for the long-distance transportation of weathering products back to plants that are also quantitatively significant sinks for released nutrients. These mycelial pathways bridge heterogeneous substrates, reducing the influence of local variation in C:N ratios. The production of polysaccharide matrices by biofilms of interacting bacteria and/or fungi at interfaces with mineral surfaces and roots, influences patterns of production of antibiotics and quorum sensing molecules, with concomitant effects on microbial community structure, and the qualitative and quantitative composition of mineral solubilizing compounds and weathering products. Patterns of carbon allocation and nutrient mobilization from both organic and inorganic substrates have been studied at larger spatial and temporal scales, including both ecosystem and global levels and there is a generally wider degree of acceptance of the "systemic" effects of microorganisms on patterns of nutrient mobilization. Theories about the evolutionary development of weathering processes have been advanced but there is still a lack of information connecting processes at different spatial scales and detailed studies of the liquid chemistry of local weathering sites at the µm-scale, together with up-scaling to soil-scale dissolution rates, are advocated, as well as new approaches involving stable isotopes. [ABSTRACT FROM AUTHOR]

Symbiotic ectomycorrhizal tree roots represent an important niche for interaction with bacteria since the fungi colonizing them have a large surface area and receive a direct supply of photosynthetically derived carbon. We examined individual root tips of P inus sylvestris at defined time points between 5 days and 24 weeks, identified the dominant fungi colonizing each root tip using Sanger sequencing and the bacterial communities colonizing individual root tips by 454 pyrosequencing. Bacterial colonization was extremely dynamic with statistically significant variation in time and increasing species richness until week 16 (3477 operational taxonomic units). Bacterial community structure of roots colonized by R ussula sp. 6 GJ-2013b, P iloderma spp., M eliniomyces variabilis and P axillus involutus differed significantly at weeks 8 and 16 but diversity declined and significant differences were no longer apparent at week 24. The most common genera were B urkholderia, S phingopyxsis, D yella, P seudomonas, A cinetobacter, A ctinospica, A quaspirillum, A cidobacter Gp1 , S phingomonas, T erriglobus, E nhydrobacter, H erbaspirillum and B radyrhizobium. Many genera had high initial abundance at week 8, declining with time but D yella and T erriglobus increased in abundance at later time points. In roots colonized by P iloderma spp. several other bacterial genera, such as A ctinospica, B radyrhizobium, A cidobacter Gp1 and R hizomicrobium appeared to increase in abundance at later sampling points. [ABSTRACT FROM AUTHOR]