Impact of nanoparticles and their ionic counterparts derived from heavy metals on the physiology of food crops

Cu-II-directed self-assembly of fullerenols to ameliorate copper stress in maize seedlings

Widespread use of copper-based agrochemical may cause copper excessive accumulation in agricultural soil to seriously threaten crop production. Recently, fullerenols are playing important roles in helping crops build resistance to abiotic stresses by giving ingenious and successful resolutions. However, there is a lack of knowledge on their beneficial effects in crops under stresses induced by heavy metals. Herein, the visual observation of Cu2+-mediated assembly of fullerenols via electrostatic and coordination actions was carried out in vitro, showing that water-soluble nanocomplexes and water-insoluble cross-linking nanohybrids were selectively fabricated by precisely adjusting feeding ratios of fullerenols and CuSO4. Furthermore, maize simultaneous exposure of fullerenols and CuSO4 solutions was tested to investigate the comparative effects of seed germination and seedling growth relative to exposure of CuSO4 alone. Under moderate Cu2+ stresses (40 and 80 μM), fullerenols significantly mitigated the detrimental effects of seedlings, including phenotype, root and shoot elongation, biomass accumulation, antioxidant capacity, and Cu2+ uptake and copper transporter-related gene expressions in roots. Under 160 μM of Cu2+ as a stressor, fullerenols also accelerated germination of Cu2+-stressed seeds eventually up to the level of the control. Summarily, fullerenols can enhance tolerance of Cu2+-stressed maize mainly due to direct detoxification through fullerenol-Cu2+ interactions restraining the Cu2+ intake into roots and reducing free Cu2+ content in vivo, as well as fullerenol-maize interactions to enhance resistance by maintaining balance of reactive oxygen species and optimizing the excretion and transport of Cu2+. This will unveil valuable insights into the beneficial roles of fullerenols and its mechanism mode in alleviating heavy metal stress on crop plants.

Manganese Nanoparticles: Synthesis, Mechanisms of Influence on Plant Resistance to Stress, and Prospects for Application in Agricultural Chemistry

Manganese (Mn) is an important microelement for the mineral nutrition of plants, but it is not effectively absorbed from the soil and mineral salts added thereto and can also be toxic in high concentrations. Mn nanoparticles (NPs) are less toxic, more effective, and economical than Mn salts due to their nanosize. This article critically reviews the current publications on Mn NPs, focusing on their effects on plant health, growth, and stress tolerance, and explaining possible mechanisms of their effects. This review also provides basic information and examples of chemical, physical, and ecological ("green") methods for the synthesis of Mn NPs. It has been shown that the protective effect of Mn NPs is associated with their antioxidant activity, activation of systemic acquired resistance (SAR), and pronounced antimicrobial activity against phytopathogens. In conclusion, Mn NPs are promising agents for agriculture, but their effects on gene expression and plant microbiome require further research.

Exogenous titanium dioxide nanoparticles alleviate cadmium toxicity by enhancing the antioxidative capacity of Tetrastigma hemsleyanum

Nanotechnology is one of the most recent approaches employed to defend plants against both biotic and abiotic stress including heavy metals such as Cadmium (Cd). In this study, we evaluated the effects of titanium dioxide (TiO2) nanoparticles (TiO2 NPs) in alleviating Cd stress in Tetrastigma hemsleyanum Diels et Gilg. Compared with Cd treatment, TiO2 NPs decreased leaf Cd concentration, restored Cd exposure-related reduction in the biomass to about 69% of control and decreased activities of antioxidative enzymes. Integrative analysis of transcriptome and metabolome revealed 325 differentially expressed genes associated with TiO2 NP treatment, most of which were enriched in biosynthesis of secondary metabolites. Among them, the flavonoid and phenylpropanoid biosynthetic pathways were significantly regulated to improve the growth of T. hemsleyanum when treated with Cd. In the KEGG Markup Language (KGML) network analysis, we found some commonly regulated pathways between Cd and Cd+TiO2 NP treatment, including phenylpropanoid biosynthesis, ABC transporters, and isoflavonoid biosynthesis, indicating their potential core network positions in controlling T. hemsleyanum response to Cd stress. Overall, our findings revealed a complex response system for tolerating Cd, encompassing the transportation, reactive oxygen species scavenging, regulation of gene expression, and metabolite accumulation in T. hemsleyanum. Our results indicate that TiO2 NP can be used to reduce Cd toxicity in T. hemsleyanum.

Nanoparticles modulate heavy-metal and arsenic stress in food crops: Hormesis for food security/safety and public health

Heavy metal and arsenic (HM-As) contamination at the soil-food crop interface is a threat to food security/safety and public health worldwide. The potential ecotoxicological effects of HM-As on food crops can perturb normal physiological, biochemical, and molecular processes. To protect food safety and human health, nanoparticles (NPs) can be applied to seed priming and soil amendment, as 'manifestation of hormesis' to modulate HM-As-induced oxidative stress in edible crops. This review provides a comprehensive overview of NPs-mediated alleviation of HM-As stress in food crops and resulting hormetic effects. The underlying biochemical and molecular mechanisms in the amelioration of HM-As-induced oxidative stress is delineated by covering the various aspects of the interaction of NPs (e.g., magnetic particles, silicon, metal oxides, selenium, and carbon nanotubes) with plant microbes, phytohormone, signaling molecules, and plant-growth bioregulators (e.g., salicylic acid and melatonin). With biotechnical advances (such as clustered regularly interspaced short palindromic repeats (CRISPR) gene editing and omics), the efficacy of NPs and associated hormesis has been augmented to produce "pollution-safe designer cultivars" in HM-As-stressed agriculture systems. Future research into nanoscale technological innovations should thus be directed toward achieving food security, sustainable development goals, and human well-being, with the aid of HM-As stress resilient food crops.

Integrating Chlorophyll a Fluorescence and Enzymatic Profiling to Reveal the Wheat Responses to Nano-ZnO Stress

It has been shown that increased concentrations of zinc oxide nanoparticles (nano-ZnO) in the soil are harmful to plant growth. However, the sensitivity of different wheat cultivars to nano-ZnO stress is still unclear. To detect the physiological response process of wheat varieties with different tolerance to nano-ZnO stress, four wheat cultivars (viz., cv. TS1, ZM18, JM22, and LM6) with different responses to nano-ZnO stress were selected, depending on previous nano-ZnO stress trials with 120 wheat cultivars in China. The results found that nano-ZnO exposure reduced chlorophyll concentrations and photosynthetic electron transport efficiency, along with the depressed carbohydrate metabolism enzyme activities, and limited plant growth. Meanwhile, the genotypic variation in photosynthetic carbon assimilation under nano-ZnO stress was found in wheat plants. Wheat cv. JM22 and LM6 possessed relatively lower Zn concentrations and higher leaf nitrogen per area, less reductions in their net photosynthetic rate, a maximum quantum yield of the PS II (Fv/Fm), electron transport flux per cross-section (ETo/CSm), trapped energy flux per cross-section (TRo/CSm), and total soluble sugar and sucrose concentrations under nano-ZnO stress, showing a better tolerance to nano-ZnO stress than wheat cv. TS1 and ZM18. In addition, the chlorophyll a fluorescence parameters Fv/Fm, ETo/CSm, and TRo/CSm could be used to rapidly screen wheat varieties resistant to nano-ZnO stress. The results here provide a new approach for solving the issues of crop yield decline in regions polluted by heavy metal nanoparticles and promoting the sustainable utilization of farmland with heavy metal pollution.

MicroRNA: A Dynamic Player from Signalling to Abiotic Tolerance in Plants

MicroRNAs (miRNAs) are a class of non-coding single-stranded RNA molecules composed of approximately 20-24 nucleotides in plants. They play an important regulatory role in plant growth and development and as a signal in abiotic tolerance. Some abiotic stresses include drought, salt, cold, high temperature, heavy metals and nutritional elements. miRNAs affect gene expression by manipulating the cleavage, translational expression or DNA methylation of target messenger RNAs (mRNAs). This review describes the current progress in the field considering two aspects: (i) the way miRNAs are produced and regulated and (ii) the way miRNA/target genes are used in plant responses to various abiotic stresses. Studying the molecular mechanism of action of miRNAs' downstream target genes could optimize the genetic manipulation of crop growth and development conditions to provide a more theoretically optimized basis for improving crop production. MicroRNA is a novel signalling mechanism in interplant communication relating to abiotic tolerance.

Metal-tolerant and siderophore producing Pseudomonas fluorescence and Trichoderma spp. improved the growth, biochemical features and yield attributes of chickpea by lowering Cd uptake

Industrialization and human urbanization have led to an increase in heavy metal (HM) pollution which often cause negative/toxic effect on agricultural crops. The soil-HMs cannot be degraded biologically however, microbe-mediated detoxification of toxic HMs into lesser toxic forms are reported. Considering the potentiality of HMs-tolerant soil microbes in metal detoxification, Pseudomonas fluorescence PGPR-7 and Trichoderma sp. T-4 were recovered from HM-affected areas. Under both normal and cadmium stress, the ability of both microorganisms to produce different plant hormones and biologically active enzymes was examined. Strains PGPR-7 and T-4 tolerated cadmium (Cd) an up-to 1800 and 2000 µg mL^-1, respectively, and produced various plant growth regulating substances (IAA, siderophore, ACC deaminase ammonia and HCN) in Cd-stressed condition. The growth promoting and metal detoxifying ability of both strains were evaluated (either singly/combined) by applying them in chickpea (Cicer arietinum L.) plants endogenously contaminated with different Cd levels (0-400 µg kg^-1 soils). The higher Cd concentration (400 µg kg^-1 soils) negatively influenced the plant parameters which, however, improved following single/combined inoculation of P. fluorescence PGPR-7 and Trichoderma sp. T-4. Both microbial strains increased the growth of Cd-treated chickpeas however, their combined inoculation (PGPR-7 + T-4) caused the most positive effect. For instance, 25 µg Cd Kg^-1 + PGPR-7 + T4 treatment caused maximum increase in germination percentage (10%), root dry biomass (71.4%) and vigour index (33%), chl-a (38%), chl-b (41%) and carotenoid content (52%). Furthermore, combined inoculation of P. fluorescence PGPR-7 and Trichoderma sp. T-4 maximally decreased the proline, MDA content, POD and CAT activities by 50%, 43% and 62%, respectively following their application in 25 µg Cd kg^-1 soils-treated chickpea. Additionally, microbial strains lowered the plant uptake of Cd. For example, Cd-uptake in root tissues was decreased by 42 and 34% when 25 µg Cd Kg^-1- treated chickpea plants were inoculated with P. fluorescence PGPR-7, Trichoderma sp. T-4 and co-inoculation (PGPR-7 + T4) of both strains, respectively. Therefore, from the current observation, it is suggested that dual inoculation of metal tolerant P. fluorescence and Trichoderma sp. may potentially be used in detoxification and reclamation of metal-contaminated soils.

Biogenic CuO and ZnO Nanoparticles as Nanofertilizers for Sustainable Growth of Amaranthus hybridus

The biogenic synthesis of CuO and ZnO nanoparticles (NPs) was carried out by Stenotrophomonas maltophilia. The shape, size, and chemical identity of the CuO and ZnO NPs were determined using FTIR, XRD, SEM, EDX, and TEM analysis. The study aimed to investigate the effects of the CuO and ZnO NPs on Amaranthus hybridus seed germination and plant growth. Two different fertilizer application modes (hydroponics and foliar) were studied with varying concentrations of CuO (0.06 µM, 0.12 µM) and ZnO (0.12 µM, 0.24 µM) nanoparticles with water control and Hoagland's media control. The hydroponic system of fertilizer application demonstrated better efficiency in terms of plant growth as compared to the foliar application. The agronomic traits, SPAD value, total reducing sugars, antioxidant activity, amount of copper, and zinc ions in root and shoot were analyzed for all experimental plants and found better with the nanoparticle application. The highlight of the study is the application of extremely low concentrations of CuO and ZnO nanoparticles, almost 70% lower than the copper and zinc salts in the Hoagland's medium for improved plant growth. The use of lower concentrations of nanoparticles can prevent their accumulation in the environment and also lower the production cost. The high antioxidant concentration exhibited by the plants treated with CuO and ZnO nanoparticles ensures the enhanced plant's resistance to infections and pests while promoting plant growth.

Heavy metal (loid)s phytotoxicity in crops and its mitigation through seed priming technology

Unexpected bioaccumulation and biomagnification of heavy metal(loid)s (HMs) in the environment have become a predicament for all living organisms, including plants. The presence of these HMs in the plant system raised the level of reactive oxygen species (ROS) and remodeled several vital cellular biomolecules. These lead to several morphological, physiological, metabolic, and molecular aberrations in plants ranging from chlorosis of leaves to the lipid peroxidation of membranes, and degradation of proteins and nucleic acid including the modulation of the enzymatic system, which ultimately affects the plant growth and productivity. Plants are equipped with several mechanisms to counteract the HMs toxicity. Among them, seed priming (SP) technology has been widely tested with the use of several inorganic chemicals, plant growth regulators (PGRs), gasotransmitters, nanoparticles, living organisms, and plant leaf extracts. The use of these compounds has the potential to alleviate the HMs toxicity through the strengthening of the antioxidant defense system, generation of low molecular weight metallothionein's (MTs), and phytochelatins (PCs), and improving seedling vigor during early growth stages. This review presents an account of the sources, uptake and transport, and phytotoxic effects of HMs with special attention to different mechanism/s, occurring to mitigate the HMs toxicity in plants employing SP technology.Novelty statement: To the best of our knowledge, this review has delineated the consequences of HMs on the crucial plant processes, which ultimately affect plant growth and development. This review also compiled the up to dated information on phytotoxicity of HMs through the use of SP technology, this review discussed how different types of SP approaches help in diminishing the concentration HMs in plant systems. Also, we depicted mechanisms, represent how HMs transport and their actions on cellular levels, and emphasized, how diverse SP technology effectiveness in the mitigation of plants' phytotoxicity in unique ways.