Evidence that arbuscular mycorrhizal and phosphate-solubilizing fungi alleviate NaCl stress in the halophyte Kosteletzkya virginica: nutrient uptake and ion distribution within root tissues

C-terminally encoded peptides act as signals to increase cotton root nitrate uptake under nonuniform salinity

Soil salinity is often heterogeneous in saline fields. Nonuniform root salinity increases nitrate uptake into cotton (Gossypium hirsutum) root portions exposed to low salinity, which may be regulated by root portions exposed to high salinity through a systemic long-distance signaling mechanism. However, the signals transmitted between shoots and roots and their precise molecular mechanisms for regulating nitrate uptake remain unknown. Here, we showed that nonuniform root salinity treatment using split-root systems increases the expression of C-TERMINALLY ENCODED PEPTIDE (GhCEP) genes in high-saline-treated root portions. GhCEP peptides originating in high-saline-treated root portions act as ascending long-distance mobile signals transported to the shoots to promote the expression of CEP DOWNSTREAM (GhCEPD) genes by inducing the expression of CEP receptor (GhCEPR) genes. The shoot-derived GhCEPD polypeptides act as descending mobile signals transported to the roots through the phloem, increasing the expression of nitrate transport genes NITRATE TRANSPORTER 1.1 (GhNRT1.1), GhNRT2.1, and GhNRT1.5 in nonsaline-treated root portions, thereby increasing nitrate uptake in the nonsaline-treated root portions. This study indicates that GhCEP and GhCEPD signals are transported between roots and shoots to increase nitrate uptake in cotton, and the transport from the nonsaline root side is in response to nonuniform root salinity distribution.

Label-free quantitative proteomics of arbuscular mycorrhizal Elaeagnus angustifolia seedlings provides insights into salt-stress tolerance mechanisms

INTRODUCTION

Soil salinization has become one of the most serious environmental issues globally. Excessive accumulation of soluble salts will adversely affect the survival, growth, and reproduction of plants. Elaeagnus angustifolia L., commonly known as oleaster or Russian olive, has the characteristics of tolerance to drought and salt. Arbuscular mycorrhizal (AM) fungi are considered to be bio-ameliorator of saline soils that can enhance the salt tolerance of the host plants. However, there is little information on the root proteomics of AM plants under salt stress.

METHODS

In this study, a label-free quantitative proteomics method was employed to identify the differentially abundant proteins in AM E. angustifolia seedlings under salt stress.

RESULTS

The results showed that a total of 170 proteins were significantly differentially regulated in E.angustifolia seedlings after AMF inoculation under salt stress. Mycorrhizal symbiosis helps the host plant E. angustifolia to respond positively to salt stress and enhances its salt tolerance by regulating the activities of some key proteins related to amino acid metabolism, lipid metabolism, and glutathione metabolism in root tissues.

CONCLUSION

Aspartate aminotransferase, dehydratase-enolase-phosphatase 1 (DEP1), phospholipases D, diacylglycerol kinase, glycerol-3-phosphate O-acyltransferases, and gamma-glutamyl transpeptidases may play important roles in mitigating the detrimental effect of salt stress on mycorrhizal E. angustifolia . In conclusion, these findings provide new insights into the salt-stress tolerance mechanisms of AM E. angustifolia seedlings and also clarify the role of AM fungi in the molecular regulation network of E. angustifolia under salt stress.

Assessing of growth, antioxidant enzymes, and phytohormone regulation in Cucurbita pepo under cadmium stress

One of the major problems worldwide is soil pollution by trace metal elements, which limits plant productivity and threatens human health. In this work, we have studied the effect of different concentrations of cadmium on Cucurbita pepo plants, evaluating different physiological and biochemical parameters: hormone signaling, metabolite concentration (malondialdehyde and hydrogen peroxide) and, in addition, the antioxidant enzyme activities of catalase and superoxide dismutase were evaluated. The production of biomass decreased under the Cd-stress. The results showed that C. pepo accumulates higher amounts of Cd2+ in roots than in shoots and fruits. Cd2+ differently affected the content of endogenous phytohormones. Furthermore, data suggest an essential involvement of roots in the regulation of tolerance to trace elements. As a result, indole acetic acid content increased in roots of treated plants, indicating that this phytohormone can stimulate root promotion and growth under Cd-stress. Similarly, salicylic acid content in roots and shoots increased in response to Cd2+, as well as abscisic acid levels in roots and fruits. In roots, the rambling accumulation pattern observed for jasmonic acid and salicylic acid suggests the lack of a specific regulation role against trace element toxicity. The activity of catalase and superoxide dismutase decreased, disrupted by the metal stress. However, the proline, malondialdehyde and hydrogen peroxide content significantly increased in Cd2+in all the analyzed tissues of the stressed plants. All these data suggest that C. pepo plants are equipped with an effective antioxidant mechanism against oxidative stress induced by cadmium up to a concentration of 500 μM.

Evaluation of Microbial Assemblages in Various Saline-Alkaline Soils Driven by Soluble Salt Ion Components

Land degraded by salinization and alkalization is widely distributed globally and involves a wide range of ecosystem types. However, the knowledge of the indigenous microbial assemblages and their roles in various saline-alkaline soils is limited. This study demonstrated microbial assemblages in various saline-alkaline soils from different regions of Inner Mongolia and revealed the key driving factors to influence microbiome. The correlation network analysis indicates the difference in adaptability of bacterial and fungal communities under stimulation by saline-alkaline stress: fungal community shows higher tolerance, stability, and resilience to various saline-alkaline soils than a bacterial community. The keystone bacteria and fungi that have potential adaptability to various saline-alkaline environments are further identified, and they may confer benefits in restoring saline-alkaline soils by their own effects or assisting plants. For salt-rich soils in different regions, the soluble salt ion components are the major determinant to drive microbial assemblages of different saline-alkaline soils, rather than salinity. Thus, these saline-alkaline soils are clustered into sulfated, chlorinated, and soda-type saline-alkaline soils. Multivariate analysis reveals unique, dominant, and common microbial taxa in three saline-alkaline soils. This result of the conceptual mode indicates that potential roles of unique and dominant microbial taxa on regulating saline-alkaline functions are more vital.

Biochar applications combined with paddy-upland rotation cropping systems benefit the safe use of PAH-contaminated soils: From risk assessment to microbial ecology

This study aimed to establish a method allowing the safe use of polycyclic aromatic hydrocarbon (PAH)-contaminated soils through the combination of biochar applications and different cropping systems. The impact of biochar applications under different cropping systems on the human health risks of PAHs and soil microbiology was elucidated. The residual PAHs were the lowest in rhizosphere soils amended with 2% corn straw-derived biochar pyrolyzed at 300 °C (CB300) under the paddy-upland rotation cropping (PURC) system. Human health risks resulting from the ingestion of PAH-contaminated carrot roots / rice grains under the PURC system were significantly lower than those under continuous upland cropping systems. The greatest diversity, richness and network complexity of soil microbial communities occurred under the PURC system combined with the 2% CB300 treatment. Soil microbial functions associated with soil health and PAH biodegradation were enhanced under this strategy, while the pathogen group was inhibited. Primarily owing to its high sorption capacity, bamboo-derived biochar pyrolyzed at 700 °C realized in the reduction of PAHs, but weakly influenced shifts in soil microbial communities. Overall, the combination of PURC systems and low-temperature-pyrolyzed nutrient-rich biochar could efficiently reduce the human health risks of PAHs and improve soil microbial ecology in agricultural fields.

Rhizoplane and Rhizosphere Fungal Communities of Geographically Isolated Korean Bellflower (Campanula takesimana Nakai)

Simple Summary

The current study reports fungal diversities in the rhizoplane (RP) and rhizosphere (RS) samples of the geographically isolated Korean bellflower (Campanulatakesimana) obtained from its original habitats of the eastern coast of the Korean Peninsula for the first time. The identification of specific taxa in each site may provide a better understanding of the interaction between the soil fungi and Korean bellflower.

Abstract

Fungal communities in the rhizoplane (RP) and rhizosphere (RS) of geographically isolated C. takesimana habitats in different environments such as oceanic (Seodo, the Dokdo Islands), coastline (Sadong, Ulleungdo Island), and inland (Taeha, Ulleungdo Island) regions were analyzed by MiSeq sequencing. In total, 1279 operational taxonomic units (OTUs) were obtained and they were further classified into 185 genera belonging to five phyla. The total number of fungal taxa in the RP samples was lower than those in the RS samples in all the sampled locations, providing an indication of the existence of a certain level of the selective pressures from the host plant. The richness of the RP in the Dokdo Islands was higher than that of Ulleungdo Island, but the richness of the RS in the Dokdo Islands was lower than that of Ulleungdo Island. These results suggest evidence for strong effects of a harsh geo-climate on the RP and RS fungal diversities in the Dokdo Islands. Additionally, a total of 82 fungal genera were identified in all three RP samples and 63 genera (77%) were uniquely found in each of the geographical regions and 43 genera (52.4%) showed high dependency on the C. takesimana vegetation. It was found that the genus Mortierella was the most dominant taxon in all the samples. The geo-ecological isolation of the Korean bellflower may have caused unique formation of the RP and RS fungal communities in the natural habitats.

Effect of arbuscular mycorrhizal symbiosis on ion homeostasis and salt tolerance-related gene expression in halophyte Suaeda salsa under salt treatments

Halophytes can remove large quantities of salts from saline soils, so their importance in ecology has received increasing attention. Preliminary studies have shown that arbuscular mycorrhizal (AM) fungi can improve the salt tolerance of halophytes. However, few studies have focused on the molecular mechanisms and effects of AM fungi in halophytes under different salt conditions. A pot experiment was carried out to investigate the effects of Funneliformis mosseae inoculation on growth, nutrient uptake, ion homeostasis and the expression of salt tolerance-related genes in Suaeda salsa under 0, 100, 200 and 400 mM NaCl. The results showed that F. mosseae promoted the growth of S. salsa and increased the shoot Ca²⁺ and Mg²⁺ concentrations under no-salt condition and high-salt condition. In addition, AM fungi increased the K⁺ concentration and maintained a high K⁺/Na⁺ ratio at 400 mM NaCl, while AM fungi decreased the K⁺ concentration and reduced the K⁺/Na⁺ ratio at 0 mM NaCl. AM fungi downregulated the expression of SsNHX1 in shoots and the expression of SsSOS1 in roots at 400 mM NaCl. These effects may decrease the compartmentation of Na⁺ into leaf vacuoles and restrict Na⁺ transport from roots to shoots, leading to an increase in root Na⁺ concentration. AM symbiosis upregulated the expression of SsSOS1 in shoots and downregulated the expression of SsSOS1 and SsNHX1 in roots at 100 mM NaCl. However, regulation of the genes (SsNHX1, SsSOS, SsVHA-B and SsPIP) was not significantly different with AM symbiosis at 0 mM or 200 mM NaCl. The results revealed that AM symbiosis might induce diverse modulation strategies in S. salsa, depending on external Na⁺ concentrations. These findings suggest that AM fungi may play significant ecological roles in the phytoremediation of salinized ecosystems.

Apophysomyces jiangsuensis sp. nov., a Salt Tolerant and Phosphate-Solubilizing Fungus from the Tidelands of Jiangsu Province of China

A newly isolated phosphate-solubilizing fungus from the topsoil of Spartina alterniflora habitats in Yancheng coastal salt marsh was cultivated. Scanning electron microscopy observation revealed that the sporangia are nearly spherical, peach-shaped, and the spores formed on the top of sporangia. The spores are ellipsoidal with raised white nubbins on the surface. Based on a polyphasic study and the genetic distance analysis referring to the sequence analysis of ITS (ITS1 + 5.8S + ITS2) and 28S rDNA (D1/D2 domains) genes, the novel species belongs to the genus Apophysomyces and is named as A. jiangsuensis. The optimum growth temperature and salinity of the new species were 28 °C and 1.15% NaCl, respectively. A study of its phosphate-solubilizing ability revealed that the fungus had an obvious decomposition effect on lecithin, Ca₃(PO₄)₂, and AlPO₃, respectively. The pH of the fermented liquid progressively decreased from 6.85 to 2.27 after 7 days of incubation, indicating that the low molecular weight organic acids excreted into the culture liquor were oxalic, succinic, and malic acids and a trace amount of citric acid. Among these, oxalic acid was the major organic acid, and its amount reached 652.5 mg/L. These results indicated that the main mechanism underlying the dissolved phosphorus was related to the secretion of large amounts of organic acids.

Structural and functional microbial diversity of sandy soil under cropland and grassland

Background

Land use change significantly alters soil organic carbon content and the microbial community. Therefore, in the present study, the effect of changing cropland to grassland on structural and functional soil microbial diversity was evaluated. The specific aims were (i) to identify the most prominent members of the fungal communities and their relevant ecological guild groups; (ii) to assess changes in the diversity of ammonia-oxidizing archaea; (iii) to determine the relationships between microbial diversity and selected physical and chemical properties.

Methods

We investigated microbial diversity and activity indicators, bulk density and the water-holding capacity of sandy soil under both cropland and 25-year-old grassland (formerly cropland) in Trzebieszów, in the Podlasie Region, Poland. Microbial diversity was assessed by: the relative abundance of ammonia-oxidizing archaea, fungal community composition and functional diversity. Microbial activity was assessed by soil enzyme (dehydrogenase, β-glucosidase) and respiration tests.

Results

It was shown that compared to cropland, grassland has a higher soil organic carbon content, microbial biomass, basal respiration, rate of enzyme activity, richness and diversity of the microbial community, water holding capacity and the structure of the fungal and ammonia-oxidizing archaea communities was also altered. The implications of these results for soil quality and soil health are also discussed. The results suggest that grassland can have a significant phytosanitary capacity with regard to ecosystem services, due to the prominent presence of beneficial and antagonistic microbes. Moreover, the results also suggest that grassland use may improve the status of soil organic carbon and nitrogen dynamics, thereby increasing the relative abundance of fungi and ammonia-oxidizing archaea.

Spatio-temporal dynamics of arbuscular mycorrhizal fungi and soil organic carbon in coastal saline soil of China

A comprehensive understanding of the relationship between arbuscular mycorrhizal (AM) fungi and coastal saline soil organic carbon (SOC) is crucial for analysis of the function of coastal wetlands in soil carbon sequestration. In a field experiment, the temporal and spatial dynamics of AM fungi, glomalin-related soil protein (GRSP) – which is described as a N-linked glycoprotein and the putative gene product of AM fungi, SOC, and soil aggregates were investigated in halophyte Kosteletzkya virginica rhizosphere soil of coastal saline areas of North Jiangsu, China. Soil samples were collected from a depth of up to 30 cm in two plantation regions from August 2012 to May 2013. Results showed K. virginica formed a strong symbiotic relationship to AM fungi. AM colonization and spore density were the highest in the 10–20 cm soil layer of Jinhai farm in August 2012, because of the presence of numerous fibrous roots in this soil layer. The total GRSP and SOC were the highest in the 0–10 cm soil layer in May 2013 and November 2012, respectively. Correlation coefficient analysis revealed that AM colonization and spore density were positively correlated with total GRSP. Meanwhile, total GRSP was significantly positively correlated with large macroaggregates (>3 mm), SOC, total P, Olsen P, and soil microbial biomass carbon (SMBC), but negatively correlated with microaggregates (<0.25 mm), soil EC, total N, and pH. SOC was positively correlated with spore density, large macroaggregates, small macroaggregates (2–0.25 mm), alkaline N, and SMBC and negatively correlated with microaggregates, EC, pH, and total K. Although it may be a statistical artifact, we found an interesting phenomenon that there was no significant correlation between soil aggregates and AM colonization or spore density. Hence, total GRSP is a vital source of saline soil C pool and an important biological indicator for evaluating coastal saline SOC pool and soil fertility, while AM colonization or spore density may not be.

Fungal community shifts in soils with varied cover crop treatments and edaphic properties

Cover cropping is proposed to enhance soil microbial diversity and activity, with cover crop type affecting microbial groups in different ways. We compared fungal community compositions of bulk soils differing by cover crop treatment, season, and edaphic properties in the third year of an organic, conventionally tilled rotation of corn-soybean-wheat planted with winter cover crops. We used Illumina amplicon sequencing fungal assemblages to evaluate effects of nine treatments, each replicated four times, consisting of six single winter cover crop species, a three-species mixture, a six-species mixture, and fallow. Alpha-diversity of fungal communities was not affected by cover crop species identity, function, or diversity. Sampling season influenced community composition as well as genus-level abundances of arbuscular mycorrhizal (AM) fungi. Cover crop mixtures, specifically the three-species mixture, had distinct AM fungal community compositions, while cereal rye and forage radish monocultures had unique Core OTU compositions. Soil texture, pH, permanganate oxidizable carbon, and chemical properties including Cu, and P were important variables in models of fungal OTU distributions across groupings. These results showed how fungal composition and potential functions were shaped by cover crop treatment as well as soil heterogeneity.

Variation in Soil Fungal Composition Associated with the Invasion of Stellera chamaejasme L. in Qinghai-Tibet Plateau Grassland

Stellera chamaejasme L. is the most problematic weed in China’s grasslands. Its root exudates affect co-occurring plants and thus may also affect soil fungi. Soils (0–20 cm depth) on two adjacent sites, one invaded the other uninvaded, were compared for a range of physiochemical parameters and by DNA sequencing of fungal communities. At the invaded site, relationships between S. chamaejasme abundance, soil physiochemical factors, and fungal communities were further investigated to determine whether these relationships corroborated conclusions on the basis of site differences that could be translated into functional variation. Results showed that the invaded soils had lower N, P, organic matter, fungal alpha diversity, and relative abundance of arbuscular mycorrhizal fungi (AMF), but greater abundance of pathogenic fungi. Organic matter and P were the edaphic factors most strongly linked to site differences in total fungal communities. Within the invaded site, organic matter rather than S. chamaejasme cover was closely linked to total fungal composition. However, on this site, a number of fungal species that had various ecological functions and that differentiated the two sites were related to S. chamaejasme cover. This study indicates that lower fertility soils may be more susceptible to invasion by S. chamaejasme. Although the influence of S. chamaejasme on total fungal community composition was limited, there was evidence of effects on particular fungal species. Further research is needed to determine whether these effects influence S. chamaejasme invasiveness.

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.

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.

Non-uniform salinity in the root zone alleviates salt damage by increasing sodium, water and nutrient transport genes expression in cotton

Non-uniform salinity alleviates salt damage through sets of physiological adjustments in Na+ transport in leaf and water and nutrient uptake in the non-saline root side. However, little is known of how non-uniform salinity induces these adjustments. In this study, RNA sequencing (RNA-Seq) analysis shown that the expression of sodium transport and photosynthesis related genes in the non-uniform treatment were higher than that in the uniform treatment, which may be the reason for the increased photosynthetic (Pn) rate and decreased Na+ content in leaves of the non-uniform salinity treatment. Most of the water and nutrient transport related genes were up-regulated in the non-saline root side but down-regulated in roots of the high-saline side, which might be the key reason for the increased water and nutrient uptake in the non-saline root side. Furthermore, the expression pattern of most differentially expressed transcription factor and hormone related genes in the non-saline root side was similar to that in the high-saline side. The alleviated salt damage by non-uniform salinity was probably attributed to the increased expression of salt tolerance related genes in the leaf and that of water and nutrient uptake genes in the non-saline root side.