The arbuscular mycorrhizal fungal protein glomalin is a putative homolog of heat shock protein 60

Immobilization mechanisms of Sr(II), Ni(II), and Cd(II) on glomalin-related soil protein in mangrove sediments at the microscopic scale

Glomalin-related soil protein (GRSP) is a significant component in the sequestration of heavy metal in soils, but its mechanisms for metal adsorption are poorly known. This study combined spectroscopic data with molecular docking simulations to reveal metal adsorption onto GRSP's surface functional groups at the molecular level. The EXAFS combined with FTIR and XPS analyses indicated that the adsorption of Cd(II), Sr(II), and Ni(II) by GRSP occurred mainly through the coordination of -OH and -COOH groups with the metal. The -COOH and -OH groups bound to the metal as electron donors and the electron density of the oxygen atom decreased, suggesting that electrostatic attraction might be involved in the adsorption process. Two-dimensional correlation spectroscopy revealed that preferential adsorption occurred on GRSP for the metal in sequential order of -COOH groups followed by -OH groups. The presence of the Ni-C shell in the Ni EXAFS spectrum suggested that Ni formed organometallic complexes with the GRSP surface. However, Sr-C and Cd-C were absent in the second shell of the Sr and Cd spectra, which was attributed to the adsorption of Sr and Cd ions with large hydration ion radius by GRSP to form outer-sphere complexes. Through molecular docking simulations, negatively charged residues such as ASP151 and ASP472 in GRSP were found to provide electrostatic attraction and ligand combination for the metal adsorption, which was consistent with the spectroscopic analyses. Overall, these findings provided new insights into the interaction mechanisms between GRSP and metals, which will help deepen our understanding of the ecological functions of GRSP in metal sequestration.

Enhanced Cr(VI) stabilization by terrestrial-derived soil protein: Photoelectrochemical properties and reduction mechanisms

Glomalin-related soil protein (GRSP) is a stable iron-organic carbon mixture that can enhance heavy metal sequestration in soils. However, the roles of GRSP in the transformation and fate of Cr(VI) have been rarely reported. Herein, we investigated the electrochemical and photocatalytic properties of GRSP and its mechanisms in Cr(VI) adsorption and reduction. Results showed that GRSP had a stronger ability for Cr(VI) adsorption and reduction than other biomaterials, with the highest adsorption amount of up to 0.126 mmol/g. The removal efficiency of Cr(VI) by GRSP was enhanced (4-7%) by ultraviolet irradiation due to the hydrated electrons produced by GRSP. Fe(II) ions, persistent free radicals, and oxygen-containing functional groups on the GRSP surface as electron donors participated in the reduction of Cr(VI) under dark condition. Moreover, Cr(III) was mainly adsorbed on the -COOH groups of GRSP via electrostatic interactions. Based on 2D correlation spectroscopy, the preferential adsorption occurred on the GRSP surface for Cr(VI) in the sequential order of CO → COO- → O-H → C-O. This work provides new insights into the Cr(VI) adsorption and reduction mechanism by GRSP. Overall, GRSP can serve as a natural iron-organic carbon for the photo-reduction of Cr(VI) pollution in environments.

Research progress and prospect of glomalin-related soil protein in the remediation of slightly contaminated soil

Soil pollution caused by organic pollutants and potentially toxic elements poses a serious threat to sustainable agricultural development, global food security and human health. Therefore, strategies for reducing soil pollution are urgently required. Arbuscular mycorrhizal fungi (AMF)-assisted phytoremediation is widely recognized for its ability to remediate slightly-contaminated soil. Glomalin-related soil protein (GRSP) production by AMF is considered a vital mechanism of AMF-assisted phytoremediation. GRSP is widespread in soils and may contribute to the remediation of slightly contaminated soils. GRSP facilitates stabilization of pollutants in soils by interacting with pollutants owing to its abundant functional groups, recalcitrance, and long turnover time. It also enhances soil bioremediation and phytoremediation by stimulating soil microbial activity, improving soil structure, and providing nutrients for plants. However, research on GRSP is still in its early stages, and studies on contaminated soil remediation are limited. The effectiveness of GRSP in situ remediation remains to be proved. This review summarizes current knowledge regarding the GRSP distribution and its contribution to the remediation of slightly contaminated soils. Additionally, we present strategies to increase the GRSP content in contaminated soils, as well as prospects for future studies on the use of GRSP in contaminated soil remediation. This study focuses on recent developments that aim to improve awareness of the role of GRSP in soil remediation and relevant future directions.

Potential interaction mechanisms between PAHs and glomalin related-soil protein (GRSP)

Glomalin-related soil protein (GRSP) has gained widespread attention because of its benefits to carbon sequestration, improving soil quality and fixing heavy metals. However, studies on how GRSP affects the environmental fate of organic contaminants are scarce. In this study, different types of GRSPs were isolated from forest soils and characterized to study the binding of GRSPs and PAHs under different environmental conditions. The results indicated that GRSPs contain abundant functional groups (such as -NH, -COOH, and CO) and material composition, like humic acid, proteins, and lipids. For the tested GRSPs, EE-GRSP has lower DOC, SUVA₂₆₀ and SUVA₂₈₀ values, as well as higher E2/E3 values, indicating that EE-GRSP has lower hydrophobicity and molecular weight. These properties can lead to strong interactions between GRSP and PAHs, especially with benzopyrene, which has a high K_ow and K_sw and a large molecular size, with binding constants ranging from 16,119 to 163,697 L kg^-1. Furthermore, low pH (pH = 3) and temperature (15 °C) could increase GRSP's aggregation, enhance the GRSP binding ability with PAHs, whose binding constants were 11,595 and 5067.3 L kg^-1. Therefore, the binding between GRSP and PAHs may lead to changes in the fate of PAHs in the soil and affect the environmental risk of PAHs. The results presented here will deepen our understanding of the environmental function of GRSPs and provide a theoretical basis to further elucidate the mechanisms of GRSPs and organic pollutants.

The visualized knowledge map and hot topic analysis of glomalin-related soil proteins in the carbon field based on Citespace

Arbuscular mycorrhizal fungi (AMF) in the soil have many positive effects on growth, nutrient acquisition, and stress tolerance of host plants, as well as soil fertility, soil structure, and soil ecology. Glomalin-related soil proteins (GRSP) are a mixture of humic substances and heat-stable glycoproteins, primarily of AMF origin. GRSP are as an important component of soil organic carbon (C) pools, which can stabilize and sequestrate C, thus reducing soil C emissions for slowing down global warming. Based on the CiteSpace software and the core collection of Web of Science as the database, this study made a visual analysis of GRSP’s literature in the C field published from 1999 to 2022, including the number of publications, countries, institutions, co-cited literature, keywords, top cited papers, etc. The study regarding the GRSP in the C field could be divided into the initial stage (1999–2009), the steady stage (2010–2018), and the explosive stage (2019–2022). The Chinese Academy of Sciences is the organization with the most publications, and the United States, China, and India are the three leading nations in the C field of GRSP. However, there was little collaboration among the participating countries and the study’s institutions. The focus of the research has shifted from the composition and content of GRSP in C to the question of whether C in GRSP affects soil properties. Future research was also prospected.

Graphical Abstract

Decoding the PLFA profiling of microbial community structure in soils contaminated with municipal solid wastes

This study aimed to assess the influence of municipal solid waste (MSW) disposal on soil microbial communities. Soil samples from 20 different locations of an MSW dumping site contaminated with toxic heavy metals (HMs) and a native forest (as control) were collected for phospholipid fatty acid (PLFA) profiling to predict microbial community responses towards unsegregated disposal of MSW. PLFA biomarkers specific to arbuscular mycorrhizal fungi (AMF), Gram-negative and Gram-positive bacteria, fungi, eukaryotes, actinomycetes, anaerobes, and microbial stress markers-fungi: bacteria (F/B) ratio, Gram-positive/Gram-negative (GP/GN) ratio, Gram-negative stress (GNStr) ratio and predator/prey ratio along with AMF spore density and the total HM content (Cu, Cr, Cd, Mn, Zn, and Ni) were assessed. The results showed that all of the PLFA microbial biomarkers and the F/B ratio were positively correlated, while HMs and microbial stress markers were negatively correlated. The significant correlation of AMF biomass with all microbial groups, the F/B ratio, and T. PLFA confirmed its significance as a key predictor of microbial biomass. With AMF and T. PLFA, Cd and Cr had a weak or negative connection. Among the toxic HMs, Zn and Cd had the greatest impact on microbial populations. Vegetation did not have any significant effect on soil microbial communities. This research will aid in the development of bioinoculants for the bioremediation of MSW-polluted sites and will improve our understanding of the soil microbial community's ability to resist, recover, and adapt to toxic waste contamination.

Evaluation of the Presence of Arbuscular Mycorrhizae and Cadmium Content in the Plants and Soils of Cocoa Plantations in San Martin, Peru

Cocoa (Theobroma cacao L.) is an important crop in Peru. International regulations require products derived from cocoa to be free of heavy metals (HMs), such as cadmium. Arbuscular mycorrhizae (AM) contribute to reduced HM content in the plant, preventing its accumulation in the fruit and facilitating the rhizodeposition of HMs through glomalin-related soil proteins (GRSP). We studied the presence of mycorrhizal symbiosis in cocoa plants and cadmium in three plantations in San Martín, Peru. The maximum Cd content detected in soils was 1.09 (mg/kg), an amount below the tolerable limit for agricultural soil (≥1.4 mg/kg). Cocoa roots showed 68–86% active mycorrhizal colonization; agronomic management did not cause differences between plantations. Levels of GRSP were between 7.67 (GRSP-EE) and 13.75 (GRSP-T) mg protein g soil−1. Morphological and molecular analysis of Glomeromycota fungi showed the presence of families Claroideoglomeraceae, Paraglomeraceae, Gigasporaceae, Glomeraceae, Acaulosporaceae, Archaeosporaceae, and Diversisporaceae. Our results show the presence of arbuscular mycorrhizal symbiosis in cocoa plantations and suggest that T. cacao may phytostabilize HM in its rhizosphere through the production of GRSP. The presence of mycorrhizal symbiosis indicates the potential for the preparation of biofertilizers for cocoa since the production of GRSP is promissory for the biostabilization of soil HMs.

Warhorses in soil bioremediation: Seed biopriming with PGPF secretome to phytostimulate crop health under heavy metal stress

The fungal symbiosis with the plant root system is importantly recognized as a plant growth promoting fungi (PGPFs), as well as elicitor of plant defence against different biotic and abiotic stress conditions. Thus PGPFs are playing as a key trouper in enhancing agricultural quality and increased crop production and paving a way towards a sustainable agriculture. Due to increased demand of food production, the over and unscientific usage of chemical fertilizers has led to the contamination of soil by organic and inorganic wastes impacting on soil quality, crops quality effecting on export business of agricultural products. The application of microbial based consortium like plant growth promoting fungi is gaining worldwide importance due to their multidimensional activity. These activities are through plant growth promotion, induction of systemic resistance, disease combating and detoxification of organic and inorganic toxic chemicals, a heavy metal tolerance ability. The master key behind these properties exhibited by PGPFs are attributed towards various secretory biomolecules (secondary metabolites or enzymes or metabolites) secreted by the fungi during interaction mechanism. The present review is focused on the multidimensional role PGPFs as elicitors of Induced systemic resistance against phytopathogens as well as heavy metal detoxifier through seed biopriming and biofortification methods. The in-sights on PGPFs and their probable mechanistic nature contributing towards plants to withstand heavy metal stress and stress alleviation by activating of various stress regulatory pathways leading to secretion of low molecular weight compounds like organic compounds, glomalin, hydrophobins, etc,. Thus projecting the importance of PGPFs and further requirement of research in developing PGPFs based molecules and combining with trending Nano technological approaches for enhanced heavy metal stress alleviations in plant and soil as well as establishing a sustainable agriculture.

Glycoproteins of arbuscular mycorrhiza for soil carbon sequestration: Review of mechanisms and controls

Glycoproteins, e.g., glomalin related soil proteins (GRSP), are sticky organic substances produced by arbuscular mycorrhizal fungi (AMF). This review summarizes the information on i) the biochemical nature, physical state and origin of GRSP, ii) GRSP decomposition and residence time in soil, iii) GRSP functions, in particular the physical, chemical, and biochemical roles for soil aggregation and carbon (C) sequestration, and finally iv) how land use and agricultural management affect GRSP production and subsequently, organic C sequestration. GRSP augment soil quality by increasing water holding capacity, nutrient storage and availability, microbial and enzymatic activities, and microbial production of extracellular polysaccharides. After release into the soil, GRSP become prone to microbial decomposition due to stabilization with organic matter and sesquioxides, and thereby increasing the residence time between 6 and 42 years. Temperate soils contain 2-15 mg GRSP g^-1, whereas arid and semiarid grasslands amount for 0.87-1.7 mg g^-1, and GRSP are lower in desert soils. GRSP content is highest in acidic soils as compared to neutral and calcareous soils. Conservation tillage, organic fertilizers and AMF inhabiting crops (e.g. maize, sorghum, soybean, and wheat) increase GRSP production and transform C into stable forms, thereby sustaining soil health and reducing CO₂ emissions. Crop rotations with non-mycorrhizal species (e.g. rapeseed) and fallow soils reduce AMF growth and consequently, the GRSP production. The GRSP production increases under nutrient and water deficiency, soil warming and elevated CO₂. In the context of global climate change, increased C sequestration through GRSP induced aggregate formation and organic matter stabilization prolong the mean residence time of soil C. Protecting soils against degradation under intensive land use, stable aggregate formation, and prolonging the residence time of C calls for strategies that maximize GRSP production and functions based on reduced tillage, AMF-relevant crop rotations and organic farming.

A targeted bioinformatics approach identifies highly variable cell surface proteins that are unique to Glomeromycotina

Diversity in arbuscular mycorrhizal fungi (AMF) contributes to biodiversity and resilience in natural environments and healthy agricultural systems. Functional complementarity exists among species of AMF in symbiosis with their plant hosts, but the molecular basis of this is not known. We hypothesise this is in part due to the difficulties that current sequence assembly methodologies have assembling sequences for intrinsically disordered proteins (IDPs) due to their low sequence complexity. IDPs are potential candidates for functional complementarity because they often exist as extended (non-globular) proteins providing additional amino acids for molecular interactions. Rhizophagus irregularis arabinogalactan-protein-like proteins (AGLs) are small secreted IDPs with no known orthologues in AMF or other fungi. We developed a targeted bioinformatics approach to identify highly variable AGLs/IDPs in RNA-sequence datasets. The approach includes a modified multiple k-mer assembly approach (Oases) to identify candidate sequences, followed by targeted sequence capture and assembly (mirabait-mira). All AMF species analysed, including the ancestral family Paraglomeraceae, have small families of proteins rich in disorder promoting amino acids such as proline and glycine, or glycine and asparagine. Glycine- and asparagine-rich proteins also were found in Geosiphon pyriformis (an obligate symbiont of a cyanobacterium), from the same subphylum (Glomeromycotina) as AMF. The sequence diversity of AGLs likely translates to functional diversity, based on predicted physical properties of tandem repeats (elastic, amyloid, or interchangeable) and their broad pI ranges. We envisage that AGLs/IDPs could contribute to functional complementarity in AMF through processes such as self-recognition, retention of nutrients, soil stability, and water movement.

Glomalin-related soil protein: The particle aggregation mechanism and its insight into coastal environment improvement

Glomalin-related soil protein (GRSP), a ubiquitous microbial product, plays a crucial role in particle aggregation and metal adsorption, but the underlying mechanisms remain unknown. Here, GRSP fraction was extracted from estuarine ecosystems and systematically characterized to elucidate the aggregation mechanisms and its impact on coastal environment improvement. We found that GRSP fraction (gravimetric mass of extracted GRSP, 5.1-24.3 mg g^-1) was a globally relevant novel bioflocculant and that protein (linked to Bradford protein assay, 1.64-4.37 mg g^-1) was the active flocculant constituent. The zeta potential, FTIR, XPS, and ¹³C NMR analyses identified its key constituents and structure, and revealed that the charge neutralization and bridging were GRSP fraction aggregation mechanisms. Thermogravimetric-infrared spectrometry analysis showed that GRSP fraction was highly thermostable, and the main volatile pyrolysis products included H₂O, CO₂, CO, and CH₄. The SEM-EDX and XPS Fe valence spectroscopy suggested that GRSP fraction contained rich Fe (11.91 ± 0.48%) and could form Fe-rich flocs with particles. We also found that GRSP fraction has a high adsorption capacity (76-95%) for Cu, Zn, Pb and Cd, and its flocculation properties provide new insights into metal adsorption. The analysis of particle aggregation mechanism and its metal adsorption capacity is of great significance to elucidate the role of GRSP fraction in coastal environment improvement.

Grey and Black Anti-Hail Nets Ameliorated Apple (Malus × domestica Borkh. cv. Golden Delicious) Physiology under Mediterranean Climate

The use of anti-hail nets on orchards changes the microclimate underneath the net. This might be of great importance in apple growing regions characterized by high radiation levels and hot and dry climates during the summer season. But, depending on the net colour and on the local climatic conditions, the shade promoted triggers different responses by the trees. Grey and black anti-hail nets were applied in an apple orchard (cv. 'Golden Delicious') located in Northeast Portugal. Under the nets a lower concentration of glomalin related-soil proteins was observed, along with an improvement on trees water status, stomatal conductance, net photosynthetic rate, total chlorophylls, N, Mg, Fe and Cu concentrations, as well as an increase in mean fruit weight. The major difference between nets was on the photosynthetic efficiency, being higher on black net in sunny days, while grey net performed better under cloudy conditions. The use of netting systems proved to be effective in improving "Golden Delicious" apple trees performance under a Mediterranean climate, mainly when the radiation reaching the plants surpass the tree saturation point for photosynthesis. Therefore, these findings anticipate solutions for current and forecasted negative effects of climate change.

Role of arbuscular mycorrhizal fungi as an underground saviuor for protecting plants from abiotic stresses

To increase food production, prevalent agricultural malpractices such as intensive use of fertilizers and pesticides have led to degradation of the ecosystem. In this situation, there is a need to encourage eco-friendly and sustainable methods for improving crop production under ever increasing abiotic stress conditions. One such method can be through use of arbuscular mycorrhizal fungi (AMF or AM fungi). Soil microorganisms such as AMF serve as a link between plants and the soil resources. AMF represent a key functional group of soil microbiota that is fundamental for soil fertility, crop productivity, yield, quality and ecosystem resilience. AMF potentially increases bioavailability of water as well as various micro- and macro- nutrients which enhances production of plant photosynthates. In plants, inoculation with AMF led to increased photochemical efficiency ultimately resulting in enhanced plant growth. In this review we have summarized amelioration of drought or water scarcity, salt stress, increasing temperature or high temperature and heavy metal stresses etc. in crop plants by AMF through its effects on various physiological and biochemical processes including photosynthesis. The review also highlights AMF induced tolerance and adaptive mechanisms which protect crops from stresses. We conclude the review with a discussion of unseen issues and suggestions for future researches.

A critical review of 25 years of glomalin research: a better mechanical understanding and robust quantification techniques are required

Arbuscular mycorrhizal fungi (AMF) are important contributors to both plant and soil health. Twenty-five years ago, researchers discovered 'glomalin', a soil component potentially produced by AMF, which was unconventionally extracted from soil and bound by a monoclonal antibody raised against Rhizophagus irregularis spores. 'Glomalin' can resist boiling, strong acids and bases, and protease treatment. Researchers proposed that 'glomalin' is a 60 kDa heat shock protein produced by AMF, while others suggested that it is a mixture of soil organic materials that are not unique to AMF. Despite disagreements on the nature of 'glomalin', it has been consistently associated with a long list of plant and soil health benefits, including soil aggregation, soil carbon storage and enhancing growth under abiotic stress. The benefits attributed to 'glomalin' have caused much excitement in the plant and soil health community; however, the mechanism(s) for these benefits have yet to be established. This review provides insights into the current understanding of the identity of 'glomalin', 'glomalin' quantification, and the associated benefits of 'glomalin'. We invite the community to think more critically about how glomalin-associated benefits are generated. We suggest a series of experiments to test hypotheses regarding the nature of 'glomalin' and associated health benefits.

Differential Effects of Exogenous Glomalin-Related Soil Proteins on Plant Growth of Trifoliate Orange Through Regulating Auxin Changes

Multiple functions of glomalin released by arbuscular mycorrhizal fungi are well-recognized, whereas the role of exogenous glomalins including easily extractable glomalin-related soil protein (EE-GRSP) and difficultly extractable glomalin-related soil protein (DE-GRSP) is unexplored for plant responses. Our study was carried out to assess the effects of exogenous EE-GRSP and DE-GRSP at varying strengths on plant growth and chlorophyll concentration of trifoliate orange (Poncirus trifoliata) seedlings, along with changes in root nutrient acquisition, auxin content, auxin-related enzyme and transporter protein gene expression, and element contents of purified GRSP. Sixteen weeks later, exogenous GRSP displayed differential effects on plant growth (height, stem diameter, leaf number, and biomass production): the increase by EE-GRSP and the decrease by DE-GRSP. The best positive effect on plant growth occurred at exogenous EE-GRSP at ½ strength. Similarly, the GRSP application also differently affected total chlorophyll content, root morphology (total length, surface area, and volume), and root N, P, and K content: positive effect by EE-GRSP and negative effect by DE-GRSP. Exogenous EE-GRSP accumulated more indoleacetic acid (IAA) in roots, which was associated with the upregulated expression of root auxin synthetic enzyme genes (PtTAA1, PtYUC3, and PtYUC4) and auxin influx transporter protein genes (PtLAX1, PtLAX2, and PtLAX3). On the other hand, exogenous DE-GRSP inhibited root IAA and indolebutyric acid (IBA) content, associated with the downregulated expression of root PtTAA1, PtLAX1, and PtLAX3. Root IAA positively correlated with root PtTAA1, PtYUC3, PtYUC4, PtLAX1, and PtLAX3 expression. Purified EE-GRSP and DE-GRSP showed similar element composition but varied in part element (C, O, P, Ca, Cu, Mn, Zn, Fe, and Mo) concentration. It concluded that exogenous GRSP triggered differential effects on growth response, and the effect was associated with the element content of pure GRSP and the change in auxins and root morphology. EE-GRSP displays a promise as a plant growth biostimulant in citriculture.

Deciphering the dynamics of glomalin and heavy metals in soils contaminated with hazardous municipal solid wastes

Heavy metals (HMs) accumulation in the soils poses risks towards the environment and health. Glomalin related soil protein (GRSP) produced by arbuscular mycorrhizal fungi (AMF) has metal-sorption and soil aggregation properties and is critical in the survival of plants and AMF. For the first time, this study attempted to examine the GRSP mediated bio-stabilization of HMs in soils contaminated with municipal solid wastes (MSW). The content and interrelationship of GRSP and HMs, along with soil physicochemical properties were studied in 20 different soil samples from the dumping site. Higher amount of GRSP indicated potential bio-stabilization of HMs at some sites. GRSP exhibited weak positive correlation with essential (Zn, Cu) and toxic HMs (Cd, Ni). Cr and Mn were possibly sequestered in AMF structures and thus found to be negatively correlated with GRSP. The positive correlation observed between GRSP and soil nutrients like N, P and soil organic carbon (SOC) indicating potential of AMF-GRSP in sustaining soil health. Results revealed that AMF residing at contaminated sites produced higher amount of GRSP potentially to bio-stabilize the HMs, and reduce their bioavailability and also facilitate SOC sequestration.

Assembly processes lead to divergent soil fungal communities within and among 12 forest ecosystems along a latitudinal gradient

Latitudinal gradients provide opportunities to better understand soil fungal community assembly and its relationship with vegetation, climate, soil and ecosystem function. Understanding the mechanisms underlying community assembly is essential for predicting compositional responses to changing environments. We quantified the relative importance of stochastic and deterministic processes in structuring soil fungal communities using patterns of community dissimilarity observed within and between 12 natural forests and related these to environmental variation within and among sites. The results revealed that whole fungal communities and communities of arbuscular and ectomycorrhizal fungi consistently exhibited divergent patterns but with less divergence for ectomycorrhizal fungi at most sites. Within those forests, no clear relationships were observed between the degree of divergence within fungal and plant communities. When comparing communities at larger spatial scales, among the 12 forests, we observed distinct separation in all three fungal groups among tropical, subtropical and temperate climatic zones. Soil fungal β-diversity patterns between forests were also greater when comparing forests exhibiting high environmental heterogeneity. Taken together, although large-scale community turnover could be attributed to specific environmental drivers, the differences among fungal communities in soils within forests was high even at local scales.

An arbuscular mycorrhizal fungus increased the macroaggregate proportion and reduced cadmium leaching from polluted soil

AMF significantly increased the GRSP content and the macroaggregate proportion in soil, which contributed to reducing the Cd concentration in pore water and its leaching loss from soil.

The extraradical proteins of Rhizophagus irregularis: A shotgun proteomics approach

Arbuscular Mycorrhizal fungi (AMF, Glomeromycota) form obligate symbiotic associations with the roots of most terrestrial plants. Our understanding of the molecular mechanisms enabling AMF propagation and AMF-host interaction is currently incomplete. Analysis of AMF proteomes could yield important insights and generate hypotheses on the nature and mechanism of AMF-plant symbiosis. Here, we examined the extraradical mycelium proteomic profile of the arbuscular mycorrhizal fungus Rhizophagus irregularis grown on Ri T-DNA transformed Chicory roots in a root organ culture setting. Our analysis detected 529 different peptides that mapped to 474 translated proteins in the R. irregularis genome. R. irregularis proteome was characterized by a high proportion of proteins (9.9 % of total, 21.4 % of proteins with functional prediction) mediating a wide range of signal transduction processes, e.g. Rho1 and Bmh2, Ca-signaling (calmodulin, and Ca channel protein), mTOR signaling (MAP3K7, and MAPKAP1), and phosphatidate signaling (phospholipase D1/2) proteins, as well as members of the Ras signaling pathway. In addition, the proteome contained an unusually large proportion (53.6 %) of hypothetical proteins, the majority of which (85.8 %) were Glomeromycota-specific. Forty-eight proteins were predicted to be surface/membrane associated, including multiple hypothetical proteins of yet-unrecognized functions. However, no evidence for the overproduction of specific proteins, previously implicated in promoting soil health and aggregation was obtained. Finally, the comparison of R. irregularis proteome to previously published AMF proteomes identified a core set of pathways and processes involved in AMF growth. We conclude that R. irregularis growth on chicory roots requires the activation of a wide range of signal transduction pathways, the secretion of multiple novel hitherto unrecognized Glomeromycota-specific proteins, and the expression of a wide array of surface-membrane associated proteins for cross kingdom cell-to-cell communications.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na+ extrusion to the soil solution, K+ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na+:K+ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

Combined application of arbuscular mycorrhizal fungi and steel slag improves plant growth and reduces Cd, Pb accumulation in Zea mays

Little attention has been paid to the combined use of arbuscular mycorrhizal fungus (AMF) and steel slag (SS) for ameliorating heavy metal polluted soils. A greenhouse pot experiment was conducted to study the effects of SS and AMF-Funneliformis mosseae (Fm), Glomus versiforme (Gv) and Rhizophagus intraradices (Ri) on plant growth and Cd, Pb uptake by maize grown in soils added with 5 mg Cd kg^-1 and 300 mg Pb kg^-1 soil. The combined usage of AMF and SS (AMF + SS) promoted maize growth, and Gv + SS had the most obvious effect. Meanwhile, single SS addition and AMF + SS decreased Cd, Pb concentrations in maize, and the greater reductions were found in combined utilization, and the lowest Cd, Pb concentrations of maize appeared in Gv + SS. Single SS amendment and AMF + SS enhanced soil pH and decreased soil diethylenetriaminepentaacetic acid (DTPA)-extractable Cd, Pb concentrations. Furthermore, alone and combined usage of AMF and SS increased contents of soil total glomalin. Our research indicated a synergistic effect between AMF and SS on enhancing plant growth and reducing Cd, Pb accumulation in maize, and Gv + SS exerted the most pronounced effect. This work suggests that AMF inoculation in combination with SS addition may be a potential method for not only phytostabilization of Pb-Cd-contaminated soil but maize safety production.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na⁺ extrusion to the soil solution, K⁺ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na⁺:K⁺ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

Glomalin-related soil protein influences the accumulation of polycyclic aromatic hydrocarbons by plant roots

Studies have demonstrated that the inoculation of soil with arbuscular mycorrhizal fungi (AMF) enhances the content of glomalin-related soil protein (GRSP), which in turn elevates the availability of polycyclic aromatic hydrocarbons (PAHs) in soil. However, few studies have examined the influence of GRSP on PAH accumulation by plants and their tissues. Understanding of this issue would provide new perspectives on the role of GRSP in PAH uptake by plants at contaminated sites. This investigation was the first observational study of the GRSP-influenced PAH accumulation in roots of ryegrass (Lolium multiflorum Lam.). GRSP (0-120 mg/L) enhanced the root PAH accumulation in a GRSP-concentration-dependent manner, based on the observed root concentrations and root concentration factors (RCFs). The greatest enhancement of ΣPAH accumulation appeared at 40 mg/L of the total GRSP (T-GRSP) and 80 mg/L of the easily extracted GRSP (EE-GRSP), respectively. The weakly and strongly adsorbed fractions accounted for 88.8-94.4%, while the absorbed fraction contributed no >11.2% of total PAH accumulation in roots. The capacity of PAH adsorption on roots was enlarged in the presence of GRSP (0-120 mg/L). As the adsorbed fraction dominated the total PAH contents in roots overwhelmingly, the GRSP-induced changes in root PAH accumulation were ascribed to GRSP-affected PAH sorption by roots.

Contribution of glomalin to dissolve organic carbon under different land uses and seasonality in dry tropics

Glomalin related soil protein (GRSP) is a hydrophobic glycoprotein that is significant for soil organic carbon (SOC) persistence and sequestration, owing to its large contribution to SOC pool and long turnover time. However, the contribution of GRSP to dissolve OC (DOC) leach from soil is not yet comprehensively explored, though it could have implication in understanding SOC dynamics. We, therefore, aim to measure the contribution of GRSP to DOC, in a range of land uses and climatic seasons in the dry tropical ecosystem. Our results demonstrated that a significant proportion of GRSP (water soluble GRSP; WS-GRSP) leached with DOC (7.9-21.9 mg kg^-1), which accounts for 0.2-0.23% of soils total GRSP (T-GRSP). Forest exhibited significantly higher WS-GRSP and DOC leaching than fallow and agriculture. WS-GRSP and DOC accumulations were higher in the dry season (summer and winter) than in rainy. The extent of seasonal variations was higher in forest than in other two land uses, indicating the role of vegetation and biological activity in soil dissolve organic matter (DOM) dynamics. The regression analysis among WS-GRSP, T-GRSP, DOC and SOC prove that the accumulations and leaching of GRSP and other soil OM (SOM) depend on similar factors. The ratio of WS-GRSP-C to DOC was higher in agriculture soil than in forest and fallow, likely a consequence of altered soil chemistry, and organic matter quantity and quality due to soil management practices. Multivariate analysis reflects a strong linkage among GRSP and SOC storage and leaching, soil nutrients (nitrogen and phosphorus) and other important soil properties (pH and bulk density), suggesting that improving GRSP and other SOM status is an urgent need for the both SOC sequestration and soil health in dry tropical agro-ecosystems.

Effects of soil salinity on the content, composition, and ion binding capacity of glomalin-related soil protein (GRSP)

Soil aggregation, an ecosystem function correlated with the concentration of glomalin-related soil protein (GRSP), is highly disturbed in saline soil. However, few studies have focused on differences in amount, composition, and ion binding capacity of GRSP in typical sodic-saline soils. In this study, a field study was performed in Songnen Plain. Combined indicators of soil salinity (Q value) were significant negatively correlated with GRSP concentration by Principal Component Analysis. Multiple linear regression models showed that soil salinity might account for 46%, 25% and 44% variation in total GRSP (T-GRSP), easily-extractable GRSP (EE-GRSP) and difficultly-extractable GRSP (DE-GRSP), respectively. Soil bulk density had most important impact on GRSP concentration, followed by the pH, soil EC had the weak influence. Comparative analysis was carried out between low-salinity and high-salinity soil. Purified T-GRSP of high-saline soil contained higher N content (13.13%), lower C content (43.41%) and lower functional groups relative content (e.g. CO and SiOSi). Purified T-GRSP of high-salinity soil had a greater binding capacity with calcium and phosphorus, the binding capacity could compensate the GRSP loss about 29.8% and 14.1%, respectively. Our findings suggested that sodic salinization of the soil led to a decrease in GRSP concentration and a change in the component percentages. This change in composition might be related to adaptation of fungi-plant systems to varied environments. The calcium and phosphorus binding capacity had a positive dependent of soil salinization, which was possible to develop ecological management or recovery technology in the future.

Sensitivity of glomalin-related soil protein to wildfires: Immediate and medium-term changes

Forest fires are part of many ecosystems, especially in the Mediterranean Basin. Depending on the fire severity, they can be a great disturbance, so it is of special importance to know their impact on the ecosystem elements. In this study, we measured the sensitivity of glomalin related soil protein (GRSP), a glycoprotein produced by arbuscular mycorrhizal fungi (AMF), to fire perturbation. Two wildfire-affected areas in the SE Spain (Gata and Gorga) were studied. Soil organic carbon (SOC) was also measured. Effects on GRSP immediately after fire were analyzed in both areas, while in Gorga a monitoring of GRSP stocks over a year period after the fire was also carried out. Soil samplings were carried out every 4months. Plots (1×2m²) were installed beneath pines and shrubs in burned and an adjacent control area. Results of GRSP content immediately after a fire only showed significant differences for shrub plots (burned vs control) (p<0.01) in the Gorga site. However, a year of monitoring showed significant fire effect on GRSP content in both plot types (pines and shrubs). Control plots varied considerably over time, while in burned plots GRSP content remained constant during the whole studied period. This research provides evidence of the sensitivity of GRSP to a wildfire perturbation.

Prospects for arbuscular mycorrhizal fungi (AMF) to assist in phytoremediation of soil hydrocarbon contaminants

Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with the roots of 80-90% of vascular plant species and may constitute up to 50% of the total soil microbial biomass. AMF have been considered to be a tool to enhance phytoremediation, as their mycelium create a widespread underground network that acts as a bridge between plant roots, soil and rhizosphere microorganisms. Abundant extramatrical hyphae extend the rhizosphere thus creating the hyphosphere, which significantly increases the area of a plant's access to nutrients and contaminants. The paper presents and evaluates the role and significance of AMF in phytoremediation of hydrocarbon contaminated sites. We focused on (1) an impact of hydrocarbons on arbuscular mycorrhizal symbiosis, (2) a potential of AMF to enhance phytoremediation, (3) determinants that influence effectiveness of hydrocarbon removal from contaminated soils. This knowledge may be useful for selection of proper plant and fungal symbionts and crucial to optimize environmental conditions for effective AMF-mediated phytoremediation. It has been concluded that three-component phytoremediation systems based on synergistic interactions between plant roots, AMF and hydrocarbon-degrading microorganisms demonstrated high effectiveness in dissipation of organic pollutants in soil.

Elevated CO2 increases glomalin-related soil protein (GRSP) in the rhizosphere of Robinia pseudoacacia L. seedlings in Pb- and Cd-contaminated soils

Glomalin-related soil protein (GRSP), which contains glycoproteins produced by arbuscular mycorrhizal fungi (AMF), as well as non-mycorrhizal-related heat-stable proteins, lipids, and humic materials, is generally categorized into two fractions: easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP). GRSP plays an important role in soil carbon (C) sequestration and can stabilize heavy metals such as lead (Pb), cadmium (Cd), and manganese (Mn). Soil contamination by heavy metals is occurring in conjunction with rising atmospheric CO₂ in natural ecosystems due to human activities. However, the response of GRSP to elevated CO₂ combined with heavy metal contamination has not been widely reported. Here, we investigated the response of GRSP to elevated CO₂ in the rhizosphere of Robinia pseudoacacia L. seedlings in Pb- and Cd-contaminated soils. Elevated CO₂ (700 μmol mol^-1) significantly increased T- and EE- GRSP concentrations in soils contaminated with Cd, Pb or Cd + Pb. GRSP contributed more carbon to the rhizosphere soil organic carbon pool under elevated CO₂ + heavy metals than under ambient CO₂. The amount of Cd and Pb bound to GRSP was significantly higher under elevated (compared to ambient) CO₂; and elevated CO₂ increased the ratio of GRSP-bound Cd and Pb to total Cd and Pb. However, available Cd and Pb in rhizosphere soil under increased elevated CO₂ compared to ambient CO₂. The combination of both metals and elevated CO₂ led to a significant increase in available Pb in rhizosphere soil compared to the Pb treatment alone. In conclusion, increased GRSP produced under elevated CO₂ could contribute to sequestration of soil pollutants by adsorption of Cd and Pb.

Cd-induced production of glomalin by arbuscular mycorrhizal fungus (Rhizophagus irregularis) as estimated by monoclonal antibody assay

Glomalin is a specific fungal glycoprotein produced by arbuscular mycorrhizal (AM) fungi belonging to the Glomerales which could efficiently sequestrate heavy metals. The glomalin has been introduced as a heat shock protein and there are evidences that increasing levels of heavy metals could enhance its production. We examined the influence of Cd concentrations on glomalin production by AM fungus, as well as its contribution to the sequestration of Cd in both pot and in vitro culture conditions. Pot experiment was carried out using pure sand with Trifolium repens L. as host plant, mycorrhized by Rhizophagus irregularis and treated with Cd levels of 0, 15, 30, and 45 μM. In vitro experiment was performed in two-compartment plates containing the transformed carrot roots mycorrhized with the same fungus and treated with Cd levels of 0, 0.001, 0.01, and 0.1 mM. The immunoreactive and Bradford reactive glomalin contents in both experiments increased as so raising Cd concentration. Total Cd sequestrated by hyphal glomalin in both cultures was significantly increased as the levels of Cd increased. The highest contents of Cd sequestration in pot (75.78 μg Cd/mg glomalin) and in vitro (11.44 μg Cd/mg glomalin) cultures were recorded at the uppermost levels of Cd, which significantly differed with other levels. Our results suggested that under Cd-induced stress, stimulated production of glomalin by AM fungus may be a protective mechanism against the toxic effect of Cd.

Proteomic Investigation of Rhizoctonia solani AG 4 Identifies Secretome and Mycelial Proteins with Roles in Plant Cell Wall Degradation and Virulence

Rhizoctonia solani AG 4 is a soilborne necrotrophic fungal plant pathogen that causes economically important diseases on agronomic crops worldwide. This study used a proteomics approach to characterize both intracellular proteins and the secretome of R. solani AG 4 isolate Rs23A under several growth conditions, the secretome being highly important in pathogenesis. From over 500 total secretome and soluble intracellular protein spots from 2-D gels, 457 protein spots were analyzed and 318 proteins positively matched with fungal proteins of known function by comparison with available R. solani genome databases specific for anastomosis groups 1-IA, 1-IB, and 3. These proteins were categorized to possible cellular locations and functional groups and for some proteins their putative roles in plant cell wall degradation and virulence. The majority of the secreted proteins were grouped to extracellular regions and contain hydrolase activity.

Arbuscular mycorrhizal fungal responses to abiotic stresses: A review

The majority of plants live in close collaboration with a diversity of soil organisms among which arbuscular mycorrhizal fungi (AMF) play an essential role. Mycorrhizal symbioses contribute to plant growth and plant protection against various environmental stresses. Whereas the resistance mechanisms induced in mycorrhizal plants after exposure to abiotic stresses, such as drought, salinity and pollution, are well documented, the knowledge about the stress tolerance mechanisms implemented by the AMF themselves is limited. This review provides an overview of the impacts of various abiotic stresses (pollution, salinity, drought, extreme temperatures, CO2, calcareous, acidity) on biodiversity, abundance and development of AMF and examines the morphological, biochemical and molecular mechanisms implemented by AMF to survive in the presence of these stresses.

Arbuscular mycorrhizal fungi in phytoremediation of contaminated areas by trace elements: mechanisms and major benefits of their applications

In recent decades, the concentration of trace elements has increased in soil and water, mainly by industrialization and urbanization. Recovery of contaminated areas is generally complex. In that respect, microorganisms can be of vital importance by making significant contributions towards the establishment of plants and the stabilization of impacted areas. Among the available strategies for environmental recovery, bioremediation and phytoremediation outstand. Arbuscular mycorrhizal fungi (AMF) are considered the most important type of mycorrhizae for phytoremediation. AMF have broad occurrence in contaminated soils, and evidences suggest they improve plant tolerance to excess of certain trace elements. In this review, the use of AMF in phytoremediation and mechanisms involved in their trace element tolerance are discussed. Additionally, we present some techniques used to study the retention of trace elements by AMF, as well as a summary of studies showing major benefits of AMF for phytoremediation.

Spatio-temporal dynamics of arbuscular mycorrhizal fungi associated with glomalin-related soil protein and soil enzymes in different managed semiarid steppes

Temporal and spatial patterns of arbuscular mycorrhizal fungi (AMF) and glomalin and soil enzyme activities were investigated in different managed semiarid steppes located in Inner Mongolia, North China. Soils were sampled in a depth up to 30 cm from non-grazed, overgrazed, and naturally restored steppes from June to September. Roots of Leymus chinense (Trin.) Tzvel. and Stipagrandis P. Smirn. were also collected over the same period. Results showed that overgrazing significantly decreased the total mycorrhizal colonization of S. grandis; total colonization of L. chinensis roots was not significantly different in the three managed steppes. Nineteen AMF species belonging to six genera were isolated. Funneliformis and Glomus were dominant genera in all three steppes. Spore density and species richness were mainly influenced by an interaction between plant growth stage and management system (P < 0.001). Spore densities were higher in 0-10-cm soil depth. AMF species richness was significantly positively correlated with soil acid phosphatase activity, alkaline phosphatase activity, and two Bradford-reactive soil protein (BRSP) fractions (P < 0.01). It is concluded that the dynamics of AMF have highly temporal and spatial patterns that are related to soil glomalin and phosphatase activity in different managed semiarid steppes. Based on these observations, AMF communities could be useful indicators for evaluating soil quality and function of semiarid grassland ecosystems.

Glomalin: an arbuscular mycorrhizal fungal soil protein

Glomalin is abundant in soils and is closely correlated with aggregate water stability. Glomalin contains carbon and, hence, constitutes a non-trivial portion of the terrestrial carbon pool. Possibly far more importantly, however, stabilization of aggregates amplifies the role of glomalin in soils because carbonaceous compounds are protected from degradation inside of aggregates. Increased atmospheric CO2 can lead to increased production of glomalin because of the symbiotic association that exists between plants and producers of glomalin, arbuscular mycorrhizal fungi (AMF). Glomalin concentrations in soils are influenced by management practices, for example, in agroecosystems, further highlighting the role of this protein in carbon storage. Glomalin is an unusual molecule that has proven difficult to analyze biochemically due to its recalcitrance and complexity. Future research will be directed towards the elucidation of its structure and controls on its production.

The influence of different stresses on glomalin levels in an arbuscular mycorrhizal fungus--salinity increases glomalin content

Glomalin is a glycoprotein produced by arbuscular mycorrhizal (AM) fungi, and the soil fraction containing glomalin is correlated with soil aggregation. Thus, factors potentially influencing glomalin production could be of relevance for this ecosystem process and for understanding AM fungal physiology. Previous work indicated that glomalin production in AM fungi may be a stress response, or related to suboptimal mycelium growth. We show here that environmental stress can enhance glomalin production in the mycelium of the AM fungus Glomus intraradices. We applied NaCl and glycerol in different intensities to the medium in which the fungus was grown in vitro, causing salinity stress and osmotic stress, respectively. As a third stress type, we simulated grazing on the extraradical hyphae of the fungus by mechanically injuring the mycelium by clipping. NaCl caused a strong increase, while the clipping treatment led to a marginally significant increase in glomalin production. Even though salinity stress includes osmotic stress, we found substantially different responses in glomalin production due to the NaCl and the glycerol treatment, as glycerol addition did not cause any response. Thus, our results indicate that glomalin is involved in inducible stress responses in AM fungi for salinity, and possibly grazing stress.

Breeding crop plants with deep roots: their role in sustainable carbon, nutrient and water sequestration

BACKGROUND

The soil represents a reservoir that contains at least twice as much carbon as does the atmosphere, yet (apart from 'root crops') mainly just the above-ground plant biomass is harvested in agriculture, and plant photosynthesis represents the effective origin of the overwhelming bulk of soil carbon. However, present estimates of the carbon sequestration potential of soils are based more on what is happening now than what might be changed by active agricultural intervention, and tend to concentrate only on the first metre of soil depth.

SCOPE

Breeding crop plants with deeper and bushy root ecosystems could simultaneously improve both the soil structure and its steady-state carbon, water and nutrient retention, as well as sustainable plant yields. The carbon that can be sequestered in the steady state by increasing the rooting depths of crop plants and grasses from, say, 1 m to 2 m depends significantly on its lifetime(s) in different molecular forms in the soil, but calculations (http://dbkgroup.org/carbonsequestration/rootsystem.html) suggest that this breeding strategy could have a hugely beneficial effect in stabilizing atmospheric CO(2). This sets an important research agenda, and the breeding of plants with improved and deep rooting habits and architectures is a goal well worth pursuing.

Evolution of tree nutrition

Using a broad definition of trees, the evolutionary origins of trees in a nutritional context is considered using data from the fossil record and molecular phylogeny. Trees are first known from the Late Devonian about 380 million years ago, originated polyphyletically at the pteridophyte grade of organization; the earliest gymnosperms were trees, and trees are polyphyletic in the angiosperms. Nutrient transporters, assimilatory pathways, homoiohydry (cuticle, intercellular gas spaces, stomata, endohydric water transport systems including xylem and phloem-like tissue) and arbuscular mycorrhizas preceded the origin of trees. Nutritional innovations that began uniquely in trees were the seed habit and, certainly (but not necessarily uniquely) in trees, ectomycorrhizas, cyanobacterial, actinorhizal and rhizobial (Parasponia, some legumes) diazotrophic symbioses and cluster roots.