Inoculation with mycorrhizal fungi modifies proline metabolism and increases chromium tolerance in pepper plants (Capsicum annuum L.)

Deciphering the alleviation potential of nitric oxide, for low temperature and chromium stress via maintaining photosynthetic capacity, antioxidant defence, and redox homeostasis in rice (Oryza sativa)

Sodium nitroprusside (SNP) is a potent nitric oxide (NO) donor that enhances plant tolerance to various abiotic stresses. This research aims to assess the effect of SNP application on rice seedlings subjected to individual and combined exposure to two abiotic stresses viz., low-temperature (LT) and chromium (Cr). Exposure to LT, Cr, and LT+Cr caused severe oxidative damage by stimulating greater production and accumulation of reactive oxygen species (ROS) leading to lipid peroxidation and cell membrane instability. The combined LT+CR stress more intensly increased the cellular oxidative stress and excessive Cr uptake that in turn deteriorated the chlorophyll pigments and photosynthesis, as well as effected the level of tetrapyrrole biosynthesis in rice plants. The reduction in rice seedling growth was more obvious under LT+Cr treatment than their individual effects. The exogenous application of SNP diminished the toxic impact of LT and Cr stress. This was attributed to the positive role of SNP in regulating the endogenous NO levels, free amino acids (FAAs) contents, tetrapyrrole biosynthesis and antioxidants. Consequently, SNP-induced NO decreased photorespiration, ROS generation, lipid peroxidation, and electrolyte leakage. Moreover, exogenous SNP diminished the Cr uptake and accumulation by modulating the ionic homeostasis and strengthening the heavy metals detoxification mechanism, thus improving plant height, biomass and photosynthetic indexes. Essentially, SNP boosts plant tolerance to LT and Cr stress by regulating antioxidants, detoxification mechanism, and the plant's physio-biochemical. Hence, applying SNP is an effective method for boosting rice plant resilience and productivity in the face of escalating environmental stresses and pollutants.

Arbuscular mycorrhizae reduced arsenic induced oxidative stress by coordinating nutrient uptake and proline-glutathione levels in Cicer arietinum L. (chickpea)

Accumulation of Arsenic (As) generates oxidative stress by reducing nutrients availability in plants. Arbuscular mycorrhizal (AM) symbiosis can impart metalloid tolerance in plants by enhancing the synthesis of sulfur (S)-rich peptides (glutathione- GSH) and low-molecular-weight nitrogenous (N) osmolytes (proline- Pro). The present study, therefore investigated the efficiency of 3 AM fungal species (Rhizoglomus intraradices-Ri, Funneliformis mosseae -Fm and Claroideoglomus claroideum- Cc) in imparting As (arsenate-AsV -40 at 60 mg kg-1 and arsenite- AsIII at 5 and 10 mg kg-1) tolerance in two Cicer arietinum (chickpea) genotypes (HC 3 and C 235). As induced significantly higher negative impacts in roots than shoots, which was in accordance with proportionately higher reactive oxygen species (ROS) in the former, with AsIII more toxic than AsV. Mycorrhizal symbiosis overcame oxidative stress by providing the host plants with necessary nutrients (P, N, and S) through enhanced microbial enzyme activities (MEAs) in soil, which increased the synthesis of Pro and GSH and established a redox balance in the two genotypes. This coordination between nutrient status, Pro-GSH levels, and antioxidant defense was stronger in HC 3 than C 235 due to its higher responsiveness to the three AM species. However, Ri was most beneficial in inducing redox homeostasis, followed by Fm and Cc, since the Cicer arietinum-Ri combination displayed the maximum ability to boost antioxidant defense mechanisms and establish a coordination with Pro synthesis. Thus, the results highlighted the importance of selecting specific chickpea genotypes having an ability to establish effective mycorrhizal symbiosis for imparting As stress tolerance.

Trichoderma Inoculation Alleviates Cd and Pb-Induced Toxicity and Improves Growth and Physiology of Vigna radiata (L.)

Heavy metals (HMs) pose a serious threat to agricultural productivity. Therefore, there is a need to find sustainable approaches to combat HM stressors in agriculture. In this study, we isolated Trichoderma sp. TF-13 from metal-polluted rhizospheric soil, which has the ability to resist 1600 and 1200 μg mL-1 cadmium (Cd) and lead (Pb), respectively. Owing to its remarkable metal tolerance, this fungal strain was applied for bioremediation of HMs in Vigna radiata (L.). Strain TF-13 produced siderophore, salicylic acid (SA; 43.4 μg mL-1) and 2,3-DHBA (21.0 μg mL-1), indole-3-acetic acid, ammonia, and ACC deaminase under HM stressed conditions. Increasing concentrations of tested HM ions caused severe reduction in overall growth of plants; however, Trichoderma sp. TF-13 inoculation significantly (p ≤ 0.05) increased the growth and physiological traits of HM-treated V. radiata. Interestingly, Trichoderma sp. TF-13 improved germination rate (10%), root length (26%), root biomass (32%), and vigor index (12%) of V. radiata grown under 25 μg Cd kg-1 soil. Additionally, Trichoderma inoculation showed a significant (p ≤ 0.05) increase in total chlorophyll, chl a, chl b, carotenoid content, root nitrogen (N), and root phosphorus (P) of 100 μg Cd kg-1 soil-treated plants over uninoculated treatment. Furthermore, enzymatic and nonenzymatic antioxidant activities of Trichoderma inoculated in metal-treated plants were improved. For instance, strain TF-13 increased proline (37%), lipid peroxidation (56%), catalase (35%), peroxidase (42%), superoxide dismutase (27%), and glutathione reductase (39%) activities in 100 μg Pb kg-1 soil-treated plants. The uptake of Pb and Cd in root/shoot tissues was decreased by 34/39 and 47/38% in fungal-inoculated and 25 μg kg-1 soil-treated plants. Thus, this study demonstrates that stabilizing metal mobility in the rhizosphere through Trichoderma inoculation significantly reduced the detrimental effects of Cd and Pb toxicity in V. radiata and also enhanced development under HM stress conditions.

Arbuscular mycorrhizal fungi-induced tolerance to chromium stress in plants

Chromium (Cr) is one of the toxic elements that harms all forms of life, including plants. Industrial discharges and mining largely contribute to Cr release into the soil environment. Excessive Cr pollution in arable land significantly reduces the yield and quality of important agricultural crops. Therefore, remediation of polluted soil is imperative not only for agricultural sustainability but also for food safety. Arbuscular mycorrhizal fungi (AMF) are widespread soil-borne endophytic fungi that form mutualistic relationships with the vast majority of land plants. In mycorrhizal symbiosis, AMF are largely dependent on the host plant-supplied carbohydrates and lipids, in return, AMF aid the host plants in acquiring water and mineral nutrients, especially phosphorus, nitrogen and sulfur from distant soils, and this distinguishing feature of the two-way exchange of resources is a functional requirement for such mutualism and ecosystem services. In addition to supplying nutrients and water to plants, the AMF symbiosis enhances plant resilience to biotic and abiotic stresses including Cr stress. Studies have revealed vital physiological and molecular mechanisms by which AMF alleviate Cr phytotoxicity and aid plants in nutrient acquisition under Cr stress. Notably, plant Cr tolerance is enhanced by both the direct effects of AMF on Cr stabilization and transformation, and the indirect effects of AMF symbiosis on plant nutrient uptake and physiological regulation. In this article, we summarized the research progress on AMF and associated mechanisms of Cr tolerance in plants. In addition, we reviewed the present understanding of AMF-assisted Cr remediation. Since AMF symbiosis can enhance plant resilience to Cr pollution, AMF may have promising prospects in agricultural production, bioremediation, and ecological restoration in Cr-polluted soils.

The participation of nitric oxide in hydrogen sulphide-mediated chromium tolerance in pepper (Capsicum annuum L) plants by modulating subcellular distribution of chromium and the ascorbate-glutathione cycle

The promising response of chromium-stressed (Cr(VI)-S) plants to hydrogen sulphide (H₂S) has been observed, but the participation of nitric oxide (NO) synthesis in H₂S-induced Cr(VI)-S tolerance in plants remains to be elucidated. It was aimed to assess the participation of NO in H₂S-mediated Cr(VI)-S tolerance by modulating subcellular distribution of Cr and the ascorbate-glutathione (AsA-GSH) cycle in the pepper seedlings. Two weeks following germination, plants were exposed to control (no Cr) or Cr(VI)-S (50 μM K₂Cr₂O₇) for further two weeks. The Cr(VI)-S-plants grown in nutrient solution were supplied with 200 μM sodium hydrosulphide (NaHS, donor of H₂S), or NaHS plus 100 μM sodium nitroprusside (SNP, a donor of NO). Chromium stress suppressed plant growth and leaf water status, while elevated proline content, oxidative stress, and the activities of AsA-GSH related enzymes, as well as endogenous H₂S and NO contents. The supplementation of NaHS increased Cr accumulation at root cell walls and vacuoles of leaves as soluble fraction to reduce its toxicity. Furthermore it limited oxidative stress, improved plant growth, modulated leaf water status, and the AsA-GSH cycle-associated enzymes' activities, as well as it further improved H₂S and NO contents. The positive effect of NaHS was found to be augmented on those parameters in the CrS-plants by the SNP supplementation. However, 0.1 mM cPTIO, the scavenger of NO, inverted the prominent effect of NaHS by decreasing NO content. The supplementation of SNP along with NaHS + cPTIO reinstalled the positive effect of NaHS by restoring NO content, which suggested that NO might have a potential role in H₂S-induced tolerance to Cr(VI)-S in pepper plants by stepping up the AsA-GSH cycle.

Arbuscular mycorrhizal fungi-induced mitigation of heavy metal phytotoxicity in metal contaminated soils: A critical review

The heavy metal pollution is a worldwide problem and has received a serious concern for the ecosystem and human health. In the last decade, remediation of the agricultural polluted soil has attracted great attention. Phytoremediation is one of the technologies that effectively alleviate heavy metal toxicity, however, this technique is limited to many factors contributing to low plant growth rate and nature of metal toxicities. Arbuscular mycorrhizal fungi (AMF) assisted alleviation of heavy metal phytotoxicity is a cost-effective and environment-friendly strategy. AMF have a symbiotic relationship with the host plant. The bidirectional exchange of resources is a hallmark and also a functional necessity in mycorrhizal symbiosis. During the last few years, a significant progress in both physiological and molecular mechanisms regarding roles of AMF in the alleviation of heavy metals (HMs) toxicities in plants, acquisition of nutrients, and improving plant performance under toxic conditions of HMs has been well studied. This review summarized the current knowledge regarding AMF assisted remediation of heavy metals and some of the strategies used by mycorrhizal fungi to cope with stressful environments. Moreover, this review provides the information of both molecular and physiological responses of mycorrhizal plants as well as AMF to heavy metal stress which could be helpful for exploring new insight into the mechanisms of HMs remediation by utilizing AMF.

Arbuscular mycorrhiza and Aspergillus terreus inoculation along with compost amendment enhance the phytoremediation of Cr-rich technosol by Solanum lycopersicum under field conditions

Chromium (Cr) contamination is a potential threat to the agricultural soil. Arbuscular mycorrhizal (AM) fungi have potential to remediate the heavy metal polluted soils. It was hypothesized that Cr phytoremediation potentiality of AM fungi could be enhanced in combination with saprophytic filamentous fungi and soil amendment. Tomato plants were raised in Cr polluted technosol amended with compost, inoculated with mixed-culture of AM fungi and Aspergillus terreus. It was found that, triple treatment (soil amendment with compost along with AM fungi and A. terreus inoculation) enhanced biomass production (up to 315%), fruit setting (up to 49%), photosynthetic pigments (up to 214%) and carbohydrate content (up to 400%) whereas reduced the proline (up to 76.5%), catalase (up to 34.2%), peroxidase (up to 58.9%) and root membrane permeability (up to 74.2%). The effect of AM fungi with compost amendment was additive, while it was synergistic with A. terreus. AM fungi enhanced the extraction of Cr from the substrate, but retained in the mycorrhizal root, thereby reduced the translocation into shoot and in fruit, Cr translocation was undetectable. At the end of experiment Cr content in the substrate was significantly decreased (up to 37.9%). Soil amendment with compost along with AM fungi and A. terreus inoculation can be used for reclamation of Cr polluted soils at field scale.

Tomato responses to Funneliformis mosseae during the early stages of arbuscular mycorrhizal symbiosis

The concept of symbiosis can be described as a continuum of interactions between organisms ranging from mutualism to parasitism that can also change over time. Arbuscular mycorrhizal fungi (AMF) are among the most important obligate plant symbionts. Once the symbiosis is well established, mycorrhizal plants are more tolerant to biotic or abiotic stresses, so the AMF relationship with the host plant is generally described as mutualistic. However, little is known about AMF effects on the plant during the early stages of root colonization. The aim of this work was to assess the type of interaction (mutualistic or parasitic) between the arbuscular mycorrhizal (AM) fungus Funelliformis mosseae and Solanum lycopersicum cv. Rio Grande plants, at 7, 14, 21, and 28 days after inoculation (DAI), considering that in the adopted experimental design (one plant per pot), the seedling was the only carbon source for fungus development in the absence of common mycorrhizal networks with other plants. At each harvest, mycorrhizal colonization, shoot and root weights, morphometric parameters, and photosynthetic efficiency were evaluated. The presence of the AM fungus in the tomato root system was observed starting from the 14th DAI, and its level increased over time. Few effects of the fungus presence on the considered parameters were observed, and no stress symptoms ever appeared; so, we can state that the fungus behaved as a mutualistic symbiont during the early stages of plant growth. Moreover, a trend towards a positive effect on plant growth was observed at 28 DAI in mycorrhizal plants.

Engineering Mycorrhizal Symbioses to Alter Plant Metabolism and Improve Crop Health

Creating sustainable bioeconomies for the 21st century relies on optimizing the use of biological resources to improve agricultural productivity and create new products. Arbuscular mycorrhizae (phylum Glomeromycota) form symbiotic relationships with over 80% of vascular plants. In return for carbon, these fungi improve plant health and tolerance to environmental stress. This symbiosis is over 400 million years old and there are currently over 200 known arbuscular mycorrhizae, with dozens of new species described annually. Metagenomic sequencing of native soil communities, from species-rich meadows to mangroves, suggests biologically diverse habitats support a variety of mycorrhizal species with potential agricultural, medical, and biotechnological applications. This review looks at the effect of mycorrhizae on plant metabolism and how we can harness this symbiosis to improve crop health. I will first describe the mechanisms that underlie this symbiosis and what physiological, metabolic, and environmental factors trigger these plant-fungal relationships. These include mycorrhizal manipulation of host genetic expression, host mitochondrial and plastid proliferation, and increased production of terpenoids and jasmonic acid by the host plant. I will then discuss the effects of mycorrhizae on plant root and foliar secondary metabolism. I subsequently outline how mycorrhizae induce three key benefits in crops: defense against pathogen and herbivore attack, drought resistance, and heavy metal tolerance. I conclude with an overview of current efforts to harness mycorrhizal diversity to improve crop health through customized inoculum. I argue future research should embrace synthetic biology to create mycorrhizal chasses with improved symbiotic abilities and potentially novel functions to improve plant health. As the effects of climate change and anthropogenic disturbance increase, the global diversity of arbuscular mycorrhizal fungi should be monitored and protected to ensure this important agricultural and biotechnological resource for the future.

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.

Heavy metal stress and some mechanisms of plant defense response

Unprecedented bioaccumulation and biomagnification of heavy metals (HMs) in the environment have become a dilemma for all living organisms including plants. HMs at toxic levels have the capability to interact with several vital cellular biomolecules such as nuclear proteins and DNA, leading to excessive augmentation of reactive oxygen species (ROS). This would inflict serious morphological, metabolic, and physiological anomalies in plants ranging from chlorosis of shoot to lipid peroxidation and protein degradation. In response, plants are equipped with a repertoire of mechanisms to counteract heavy metal (HM) toxicity. The key elements of these are chelating metals by forming phytochelatins (PCs) or metallothioneins (MTs) metal complex at the intra- and intercellular level, which is followed by the removal of HM ions from sensitive sites or vacuolar sequestration of ligand-metal complex. Nonenzymatically synthesized compounds such as proline (Pro) are able to strengthen metal-detoxification capacity of intracellular antioxidant enzymes. Another important additive component of plant defense system is symbiotic association with arbuscular mycorrhizal (AM) fungi. AM can effectively immobilize HMs and reduce their uptake by host plants via binding metal ions to hyphal cell wall and excreting several extracellular biomolecules. Additionally, AM fungi can enhance activities of antioxidant defense machinery of plants.