Arsenic resistance in fungi conferred by extracellular bonding and vacuole-septa compartmentalization

Metal-Binding Protein TaGlo1 Improves Fungal Resistance to Arsenite (AsIII) and Methylarsenite (MAsIII) in Paddy Soil

Trivalent arsenicals such as arsenite (AsIII) and methylarsenite (MAsIII) are thought to be ubiquitous in flooded paddy soils and have higher toxicity than pentavalent forms. Fungi are widely prevalent in the rice rhizosphere, and the latter is considered a hotspot for As uptake. However, few studies have focused on alleviating As toxicity in paddy soils using fungi. In this study, we investigated the mechanism by which the protein TaGlo1, derived from the As-resistant fungal strain Trichoderma asperellum SM-12F1, mitigates AsIII and MAsIII toxicity in paddy soils. Taglo1 gene expression in Escherichia coli BL21 conferred strong resistance to AsIII and MAsIII, while purified TaGlo1 showed a high affinity for AsIII and MAsIII. Three cysteine residues (Cys13, Cys18, and Cys71) play crucial roles in binding with AsIII, while only two (Cys13 and Cys18) play crucial roles for MAsIII binding. TaGlo1 had a stronger binding strength for MAsIII than AsIII. Importantly, up to 90.2% of the homologous TaGlo1 proteins originate from fungi by GenBank searching. In the rhizospheres of 14 Chinese paddy soils, Taglo1 was widely distributed and its gene abundance increased with porewater As. This study highlights the potential of fungi to mitigate As toxicity and availability in the soil-rice continuum and suggests future microbial strategies for bioremediation.

Nutrient and mycoremediation of a global menace 'arsenic': exploring the prospects of phosphorus and Serendipita indica-based mitigation strategies in rice and other crops

KEY MESSAGE

Serendipita indica induced metabolic reprogramming in colonized plants complements phosphorus-management in improving their tolerance to arsenic stress on multifaceted biological fronts. Restoration of the anthropic damage done to our environment is inextricably linked to devising strategies that are not only economically sound but are self-renewing and ecologically conscious. The dilemma of heavy metal (HM) dietary ingestion, especially arsenic (As), faced by humans and animals alike, necessitates the exploitation of such technologies and the cultivation of healthy and abundant crops. The remarkable symbiotic alliance between plants and 'mycorrhizas' has evolved across eons, benefiting growth/yield aspects as well as imparting abiotic/biotic stress tolerance. The intricate interdependence of Serendipita indica (S. indica) and rice plant reportedly reduce As accumulation, accentuating the interest of microbiologists, agriculturists, and ecotoxicological scientists apropos of the remediation mechanisms of As in the soil-AMF-rice system. Nutrient management, particularly of phosphorus (P), is also praised for mitigating As phytotoxicity by deterring the uptake of As molecules due to the rhizospheric cationic competition. Taking into consideration the reasonable prospects of success in minimizing As acquisition by rice plants, this review focuses on the physiological, metabolic, and transcriptional alterations underlying S. indica symbiosis, recuperation of As stress together with nutritional management of P by gathering case studies and presenting successful paradigms. Weaving together a volume of literature, we assess the chemical forms of As and related transport pathways, discuss As-P-rice interaction and the significance of fungi in As toxicity mitigation, predominantly the role of mycorrhiza, as well as survey of the multifaceted impacts of S. indica on plants. A potential strategy for simultaneous S. indica + P administration in paddy fields is proposed, followed by future research orientation to expand theoretic comprehension and encourage field-based implementation.

The cadmium tolerance enhancement through regulating glutathione conferred by vacuolar compartmentalization in Aspergillus sydowii

Aspergillus was found to be a vital hyperaccumulation species for heavy metal removal with admirable tolerance capacity. But the potential tolerance mechanism has not been completely studied. This study quantified the amounts of total cadmium (Cd), Cd2+, glutathione (GSH), and reactive oxygen species (ROS) in the protoplasts and vacuoles of mycelium. We modulated GSH synthesis using buthionine sulfoximine (BSO) and 2-oxothiazolidine-4-carboxylic acid (OTC) to investigate the subcellular regulatory mechanisms of GSH in the accumulation of Cd. The results confirmed that GSH plays a crucial role in vacuolar compartmentalization under Cd stress. GSH and GSSG as a redox buffer to keep the cellular redox state in balance and GSH as a metal chelating agent to reduce toxicity. When regulating the decreased GSH content with BSO, and increased GSH content with OTC, the system of Cd-GSH-ROS can change accordingly, this also supported that vacuolar compartmentalization is a detoxification strategy that can modulate the transport and storage of substances inside and outside the vacuole reasonably. Interestingly, GSH tended to be distributed in the cytoplasm, the battleground of redox takes place in the cytoplasm but not in the vacuole. These finding potentially has implications for the understanding of tolerance behavior and detoxification mechanisms of cells. In the future bioremediation of Cd in soil, the efficiency of soil remediation can be improved by developing organisms with high GSH production capacity.

Insights into the influence of Fe(III) on the interaction between roxarsone and humic acid using multi-spectroscopic techniques

The influence of Fe(III) on the interaction between roxarsone (ROX) and humic acid (HA) was investigated by multi-spectroscopic techniques. The fluorescence quenching experiment indicated that the fluorescence intensity of HA-ROX was quenched by Fe(III) through a static quenching process. Synchronous fluorescence spectra provided further information concerning the competitive combination between ROX and Fe(III) for HA. The results of the dialysis equilibrium experiment confirmed the existence of Fe(III) (0.05-0.1 mmol/L) promotes the combination of HA and ROX. Binding mechanisms were further characterized by FTIR spectroscopy, and the carboxyl functional group is involved in the binding process of HA/Fe/ROX. In addition, acidic and neutral conditions are more conducive to the combination of ROX and HA/Fe than alkaline conditions. The above discussion is of great significance in understanding the environmental fate of ROX under the coexistence of Fe(III) and HA.

Consequences of Arsenic Contamination on Plants and Mycoremediation-Mediated Arsenic Stress Tolerance for Sustainable Agriculture

Arsenic contamination in water and soil is becoming a severe problem. It is toxic to the environment and human health. It is usually found in small quantities in rock, soil, air, and water which increase due to natural and anthropogenic activities. Arsenic exposure leads to several diseases such as vascular disease, including stroke, ischemic heart disease, and peripheral vascular disease, and also increases the risk of liver, lungs, kidneys, and bladder tumors. Arsenic leads to oxidative stress that causes an imbalance in the redox system. Mycoremediation approaches can potentially reduce the As level near the contaminated sites and are procuring popularity as being eco-friendly and cost-effective. Many fungi have specific metal-binding metallothionein proteins, which are used for immobilizing the As concentration from the soil, thereby removing the accumulated As in crops. Some fungi also have other mechanisms to reduce the As contamination, such as biosynthesis of glutathione, cell surface precipitation, bioaugmentation, biostimulation, biosorption, bioaccumulation, biovolatilization, methylation, and chelation of As. Arsenic-resistant fungi and recombinant yeast have a significant potential for better elimination of As from contaminated areas. This review discusses the relationship between As exposure, oxidative stress, and signaling pathways. We also explain how to overcome the detrimental effects of As contamination through mycoremediation, unraveling the mechanism of As-induced toxicity.

Endofungal Rhizobium species enhance arsenic tolerance in colonized host plant under arsenic stress

Arsenic (As) is a toxic metalloid that is present in natural surroundings in many forms with severe consequences to sustainable agriculture and human health. Plant growth-promoting Rhizobia have been found involved in the induction of plant tolerance under various biotic and abiotic stresses. An endofungal Rhizobium species associated with arbuscular mycorrhizal fungi (AMF) Serendipita indica deploy beneficial role in the promotion of plant growth and tolerance against various biotic and abiotic stresses. In the current study, we have determined the role of endofungal Rhizobium species in protection of host plant growth under As stress. We observed that endofungal Rhizobium species strain Si001 tolerate AsV up to 25 mM and its inoculation enhances tomato seed germination and seedling growth. A hyper-colonization of Rhizobium species Si001 in tomato roots was observed under As stress and results in modulation of GSH and proline content with reduced ROS. Rhizobium species Si001 colonization in host plant recovered pigment contents (chlorophyll-a and chlorophyll-b up to 189.5% and 192%, respectively), photosynthesis (157%), and water use efficiency (166%) compared to As-treated plants. Interestingly, bacterial colonization results in 40% increased As accumulation in the root, while a reduction in As translocation from root to shoot up to 89% was observed as compared to As treated plants. In conclusion, endofungal Rhizobium species Si001 association with the host plant may improve plant health and tolerance against As stress with reduced As accumulation in the crop produce.

Cadmium in Cereal Crops: Uptake and Transport Mechanisms and Minimizing Strategies

Cadmium (Cd) contamination in soils and accumulation in cereal grains have posed food security risks and serious health concerns worldwide. Understanding the Cd transport process and its management for minimizing Cd accumulation in cereals may help to improve crop growth and grain quality. In this review, we summarize Cd uptake, translocation, and accumulation mechanisms in cereal crops and discuss efficient measures to reduce Cd uptake as well as potential remediation strategies, including the applications of plant growth regulators, microbes, nanoparticles, and cropping systems and developing low-Cd grain cultivars by CRISPR/Cas9. In addition, miRNAs modulate Cd translocation, and accumulation in crops through the regulation of their target genes was revealed. Combined use of multiple remediation methods may successfully decrease Cd concentrations in cereals. The findings in this review provide some insights into innovative and applicable approaches for reducing Cd accumulation in cereal grains and sustainable management of Cd-contaminated paddy fields.

Endophytic fungus Serendipita indica reduces arsenic mobilization from root to fruit in colonized tomato plant

The accumulation of arsenic in crop plants has become a worldwide concern that affects millions of people. The major source of arsenic in crop plants is irrigation water and soil. In this study, Serendipita indica, an endophytic fungus, was used to investigate the protection against arsenic and its accumulation in the tomato plant. We found that inoculation of S. indica recovers seed germination, plant growth and improves overall plant health under arsenic stress. A hyper-colonization of fungus in the plant root was observed under arsenic stress, which results in reduced oxidative stress via modulation of antioxidative enzymes, glutathione, and proline levels. Furthermore, fungal colonization restricts arsenic mobilization from root to shoot and fruit by accumulating it exclusively in the root. We observed that fungal colonization enhances the arsenic bioaccumulation factor 1.48 times in root and reduces the arsenic translocation factor by 2.96 times from root to shoot and 13.6 times from root to fruit compared to non colonized plants. Further, investigation suggests that S. indica can tolerate arsenic by immobilizing it on the cell wall and accumulating it in the vacuole. This study shows that S. indica may be helpful for the reduction of arsenic accumulation in crops grown in arsenic-contaminated agriculture fields.

Impact of arsenic on phosphate solubilization, acquisition and poly-phosphate accumulation in endophytic fungus Serendipita indica

Symbiotic interactions play a crucial role in the phosphate (Pi) nutrient status of the host plant and offer resilience during biotic and abiotic stresses. Despite a competitive behavior of arsenic (AsV) with Pi, Serendipita indica association promotes plant growth by reducing arsenic bioavailability in the rhizosphere. Reduced arsenic availability is due to the adsorption, accumulation, and precipitation of arsenic in the fungus. The present investigation focused on the fitness and performance of Pi acquisition and utilization in S. indica for growth and metabolism under arsenic stress. The fungus accumulates a massive amount of arsenic up to 2459.3 ppm at a tolerable limit of arsenic supply (1 mM) with a bioaccumulation factor (BAF) 32. Arsenic induces Pi transporter expression to stimulate the arsenic acquisition in the fungus. At the same time, Pi accumulation was also enhanced by 112.2 times higher than the control with an increase in poly-P (polyphosphate) content (6.69 times) of the cell. This result suggests arsenic does not hamper poly-P storage in the cell but shows a marked delocalization of stored poly-P from the vacuoles. Furthermore, an enhanced exopolyphosphatase activity and poly-P storage during arsenic stress suggest induction of cellular machinery for the utilization of Pi is required to deal with arsenic toxicity and competition. However, at high arsenic supply (2.5 and 5 mM), 14.55 and 22.07 times reduced Pi utilization, respectively, was observed during the Pi uptake by the fungus. The reduction of Pi uptake reduces the cell growth and biomass due to competition between arsenic and phosphate. The study suggests no negative impact of arsenic on the Pi acquisition, storage, and metabolism in symbiotic fungus, S. indica, under environmental arsenic contamination.

Interactions with Arsenic: Mechanisms of Toxicity and Cellular Resistance in Eukaryotic Microorganisms

Arsenic (As) is quite an abundant metalloid, with ancient origin and ubiquitous distribution, which represents a severe environmental risk and a global problem for public health. Microbial exposure to As compounds in the environment has happened since the beginning of time. Selective pressure has induced the evolution of various genetic systems conferring useful capacities in many microorganisms to detoxify and even use arsenic, as an energy source. This review summarizes the microbial impact of the As biogeochemical cycle. Moreover, the poorly known adverse effects of this element on eukaryotic microbes, as well as the As uptake and detoxification mechanisms developed by yeast and protists, are discussed. Finally, an outlook of As microbial remediation makes evident the knowledge gaps and the necessity of new approaches to mitigate this environmental challenge.

Dark septate endophyte Falciphora oryzae-assisted alleviation of cadmium in rice

Dark septate endophytes (DSEs) are the typical representatives of root endophytic fungi in heavy metal (HM)-contaminated environments. However, little is known about their roles in the HMs tolerance of hosts and the underlying mechanism. Here, we investigated the biological roles and molecular mechanisms of a DSE strain Falciphora oryzae in alleviating cadmium (Cd) toxicities in rice. It was found that F. oryzae possessed a capacity of accumulating Cd in its vacuoles and chlamydospores. During symbiosis, F. oryzae conferred improved Cd tolerance to rice, decreasing Cd accumulation in roots and translocation to shoots. F. oryzae alleviated Cd toxicity to rice by sequestering Cd in its vacuoles. Further application of F. oryzae as fertilizer in the field could reduce Cd content in rice grains. We identified a SNARE Syntaxin 1 gene through proteomics, which participated in Cd tolerance of F. oryzae by regulating chlamydospore formation and vacuole enlargement. This study provided novel insights into how the DSEs and their host plants combat Cd stress.

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

Mine wastes pollute the environment with metals and metalloids in toxic concentrations, causing problems for humans and wildlife. Microorganisms colonize and inhabit mine wastes, and can influence the environmental mobility of metals through metabolic activity, biogeochemical cycling and detoxification mechanisms. In this article we review the microbiology of the metals and metalloids most commonly associated with mine wastes: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. We discuss the molecular mechanisms by which bacteria, archaea, and fungi interact with contaminant metals and the consequences for metal fate in the environment, focusing on long-term field studies of metal-impacted mine wastes where possible. Metal contamination can decrease the efficiency of soil functioning and essential element cycling due to the need for microbes to expend energy to maintain and repair cells. However, microbial communities are able to tolerate and adapt to metal contamination, particularly when the contaminant metals are essential elements that are subject to homeostasis or have a close biochemical analog. Stimulating the development of microbially reducing conditions, for example in constructed wetlands, is beneficial for remediating many metals associated with mine wastes. It has been shown to be effective at low pH, circumneutral and high pH conditions in the laboratory and at pilot field-scale. Further demonstration of this technology at full field-scale is required, as is more research to optimize bioremediation and to investigate combined remediation strategies. Microbial activity has the potential to mitigate the impacts of metal mine wastes, and therefore lessen the impact of this pollution on planetary health.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.