Silicon (Si) is the second most abundant element in soil, where its availability to plants can exhilarate to 10% of total dry weight of the plant. Si accumulation/transport occurs in the upward direction, and has been identified in several crop plants. Si application has been known to ameliorate plant growth and development during normal and stressful conditions over past two-decades. During abiotic (salinity, drought, thermal, and heavy metal etc) stress, one of the immediate responses by plant is the generation of reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide (O2−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH), which cause severe damage to the cell structure, organelles, and functions. To alleviate and repair this damage, plants have developed a complex antioxidant system to maintain homeostasis through non-enzymatic (carotenoids, tocopherols, ascorbate, and glutathione) and enzymatic antioxidants [superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)]. To this end, the exogenous application of Si has been found to induce stress tolerance by regulating the generation of ROS, reducing electrolytic leakage and malondialdehyde (MDA) contents, and immobilizing and reducing the uptake of toxic ions like Na, under stressful conditions. However, the interaction of Si and plant antioxidant enzyme system remains poorly understood, and further in-depth analyses at the transcriptomic level are needed to understand the mechanisms responsible for the Si-mediated regulation of stress responses.

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Silicon Regulates Antioxidant Activities of Crop Plants under Abiotic-Induced Oxidative Stress: A Review

The beneficial effects of silicon and its role for plants are well established; however, the advantages of silicon nanoparticles over its bulk material are an area that is less explored. Silicon nanoparticles have distinctive physiological characteristics that allow them to enter plants and influence plant metabolic activities. The mesoporous nature of silicon nanoparticles also makes them good candidates as suitable nanocarriers for different molecules that may help in agriculture. Several studies have shown the importance of silicon nanoparticles in agriculture, but an overview of the related aspects was missing. Therefore, this review brings together the literature on silicon nanoparticles and discusses the impact of silicon nanoparticles on several aspects of agricultural sciences. The review also discusses the future application of silicon nanoparticles in plant growth, plant development, and improvement of plant productivity.

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Application of silicon nanoparticles in agriculture

Microplastic particles increase arsenic toxicity to rice seedlings.

In plants, pathogen attack can induce an immune response known as systemic acquired resistance that protects against a broad spectrum of pathogens. In the search for safer agrochemicals, silica nanoparticles (SiO2 NPs; food additive E551) have recently been proposed as a new tool. However, initial results are controversial, and the molecular mechanisms of SiO2 NP-induced disease resistance are unknown. Here we show that SiO2 NPs, as well as soluble Si(OH)4, can induce systemic acquired resistance in a dose-dependent manner, which involves the defence hormone salicylic acid. Nanoparticle uptake and action occurred exclusively through the stomata (leaf pores facilitating gas exchange) and involved extracellular adsorption in the air spaces in the spongy mesophyll of the leaf. In contrast to the treatment with SiO2 NPs, the induction of systemic acquired resistance by Si(OH)4 was problematic since high Si(OH)4 concentrations caused stress. We conclude that SiO2 NPs have the potential to serve as an inexpensive, highly efficient, safe and sustainable alternative for plant disease protection. New mechanistic insights into nanoparticle–plant interactions show that specifically designed silica nanoparticles have the potential to serve as an inexpensive, highly efficient, safe and tracelessly degradable alternative for pesticides.

Silica nanoparticles enhance disease resistance in Arabidopsis plants

Silica nanoparticles were obtained from rice husk through an alternative microwave process and applied to remove Methylene Blue dye (MB) from aqueous media by adsorption. The better characteristics of microwave modified nano–silica (MW–nSiO2), regarding to the standard nano–silica (nSiO2), were demonstrated by X–ray diffraction, infrared spectroscopy, N2 adsorption isotherms, gas picnometry, differential scanning calorimetry and scanning electron microscopy. nSiO2 and MW–nSiO2 were used as adsorbents to remove MB dye from aqueous media. Adsorption kinetics, isotherms and thermodynamics were evaluated. MW–nSiO2 presented higher values of surface area, pore volume, pore diameter and porosity than nSiO2. Pseudo–n order and HSDM models were adequate to represent the kinetic profile. To represent the equilibrium curves, Sips model was the most adequate. MW–nSiO2 attained adsorption capacity of 679.9 mg g−1 coupled with removal percentage of 80%, being and excellent adsorbent for MB dye. For both adsorbents, the thermodynamic results revealed a favorable, spontaneous and exothermic process. These findings show that microwave is a powerful technique to intensify the silica production process, generating a material with better characteristics for dyes adsorption.

Microwave synthesis of silica nanoparticles and its application for methylene blue adsorption

Effect of silica ions and nano silica on rice plants under salinity stress

Background: Salinity is one of the main threats that can cause crop yield losses by limiting plant growth. Several important cultivated crops including wheat exhibit sensitivity to salt stress by severe yield reductions. Approaches to improve salt resistance are critical for improved food production in the future. Aims: Silicon (Si) is well known for its role as quasi‐essential element to enhance growth and development of monocot food crops, especially under a variety of abiotic and biotic stress conditions. However, the mechanism of Si‐induced stress resistance is still not fully understood. Here, we investigate the biochemical basis of Si‐induced salt resistance of wheat. Methods: Effect of Si treatment on wheat plants under salt stress was measured by biochemical studies of photosynthetic pigments, ions, oxidative stress indicators, enzymatic and non‐enzymatic antioxidant systems, phytohormones along with expression of two Si‐transport genes. Results: Our results suggest that Si‐induced salt resistance of wheat involves a signaling cascade that triggers the biochemical defense responses. Salinity induces production of the stress hormone abscisic acid that upregulates the expression of Si‐influx transporter gene TaLsi1 and Si‐efflux transporter gene TaLsi2, in the presence of soluble Si in cytosol, and this triggers jasmonate‐mediated activation of both enzymatic and non‐enzymatic antioxidant systems along with osmolyte production. Moreover, peculiar stoichiometric changes in photosynthetic pigments after Si application, as signatures of shaded condition, also lead us to the hypothesis that insoluble Si phytoliths in plants may aid a physical defense response by creating a “mechanically shielded leaf microenvironment” against a variety of abiotic and biotic stress conditions. Conclusion: This study demonstrates that Si activates biochemical defense responses against salt stress in wheat plants.

Noaman S. Hassan

Papers

Effect of silica ions and nano silica on rice plants under salinity stress

Biochemical responses of wheat to silicon application under salinity