Silicon amelioration of aluminium toxicity and cell death in suspension cultures of Norway spruce (Picea abies (L.) Karst.)

A role for silicon (Si) in the amelioration of aluminium (Al) toxicity in gymnosperms is suggested by their codeposition in planta, including within needles. This study was designed to investigate Al/Si interactions at the cellular level using suspension cultures of Norway spruce. Toxic effects of Al were dependent on duration of Al exposure, concentration of Al, and pH. Toxicity was reduced when Si was present, and the effect was enhanced at pH 5.0 compared to pH 4.2. Study of the ultrastructure of Al-treated cells indicated that changes in cell wall thickening, degree of vacuolation, and the degeneration of mitochondria, Golgi bodies, ER and nucleus preceded cell death, and significant amelioration was noted when Si was also present. When the fluorescent dye Morin was employed to localise free Al, cells treated with Al and Si in combination showed less fluorescence than the cells treated with Al alone. Intensity of fluorescence depended on the concentration of Al, duration of treatment and pH. Notably, presence of Si reduced the concentration of free Al in the cell wall in parallel with amelioration of Al toxicity. We therefore propose that formation of aluminosilicate complexes in the wall and apoplasm provide a significant barrier to Al penetration and cell damage in Norway spruce.     

Aluminium/silicon interactions in barley (Hordeum vulgare L.) seedlings

The response of seedlings of the monocot Hordeum vulgare L. cv. Bronze to 0,25 and 50 M aluminium in factorial combination with 0, 1.4, 2.0 and 2.8 mM Si was tested in hydroponic culture at pH 4.5. Nutrient solution (500 M calcium nitrate) and Al/Si treatments were designed to avoid the precipitation of Al from solution. Silicon treatments gave significant amelioration of the toxic effects of Al on root and shoot growth and restored calcium levels in roots and shoots at harvest to levels approaching those of control plants. Aluminium uptake by roots was also significantly diminished in the presence of Si. Silicon alone gave a slight stimulation of growth, insufficient to explain its ameliorative effect on Al toxicity. The mechanism of the Si effect on Al toxicity in monocotyledons awaits further investigation.Abbreviations ICP inductively coupled plasma     

Aluminium/silicon interactions in higher plants

Aluminium and silicon are usually abundant in soil mineral matter,but their availability for plant uptake is limited by low solubilityand, in the case of Al, high soil pH causes precipitation ofthe element in insoluble forms. Al toxicity is a major problemin naturally occurring acid soils and in soils affected by acidicprecipitation. Al has no known role in higher plants, and isgenerally known as a toxic element, whereas Si is generallyregarded as a beneficial element. Recently, it has been suggestedthat Al toxicity can be ameliorated by Si in a variety of animalsystems. In this review the evidence that amelioration of Altoxicity by Si can also occur in plants is assessed. At presentsuch amelioration has been shown in sorghum, barley, teosinte,and soybean, but not in rice, wheat, cotton, and pea. Plantspecies vary considerably in the amounts of Al and Si that theytransport into their tissues, and it seems that very high Siaccumulation and very high Al accumulation are mutually exclusive.The mechanisms considered for amelioration are: solution effects;codeposition of Al and Si within the plant; effects in the cytoplasmand on enzyme activity; and indirect effects. Key words: Aluminium, silicon, biomineralization, codeposition, toxicity, tolerance     

Release of Si from Silicon,a Ferrosilicon (FeSi) Alloy and a Synthetic Silicate Mineral in Simulated Biological Media

Unique quantitative bioaccessibility data has been generated, and the influence of surface/material and test media characteristics on the elemental release process were assessed for silicon containing materials in specific synthetic body fluids at certain time periods at a fixed loading. The metal release test protocol, elaborated by the KTH team, has previously been used for classification, ranking, and screening of different alloys and metals. Time resolved elemental release of Si, Fe and Al from particles, sized less than 50 µm, of two grades of metallurgical silicon (high purity silicon, SiHG, low purity silicon, SiLG), an alloy (ferrosilicon, FeSi) and a mineral (aluminium silicate, AlSi) has been investigated in synthetic body fluids of varying pH, composition and complexation capacity, simple models of for example dermal contact and digestion scenarios. Individual methods for analysis of released Si (as silicic acid, Si(OH)₄) in synthetic body fluids using GF-AAS were developed for each fluid including optimisation of solution pH and graphite furnace parameters. The release of Si from the two metallurgical silicon grades was strongly dependent on both pH and media composition with the highest release in pH neutral media. No similar effect was observed for the FeSi alloy or the aluminium silicate mineral. Surface adsorption of phosphate and lactic acid were believed to hinder the release of Si whereas the presence of citric acid enhanced the release as a result of surface complexation. An increased presence of Al and Fe in the material (low purity metalloid, alloy or mineral) resulted in a reduced release of Si in pH neutral media. The release of Si was enhanced for all materials with Al at their outermost surface in acetic media.     

Silicon has opposite effects on the accumulation of inorganic and methylated arsenic species in rice

Background and aims

Rice (Oryza sativa) is a main source of human exposure to inorganic arsenic and mitigation measures are needed to decrease As accumulation in this staple crop. It has been shown that silicon decreases the accumulation of arsenite but, unexpectedly, increases the accumulation of dimethylarsinic acid (DMA) in rice grain. The aim of this study was to investigate why Si increases DMA accumulation.

Methods

Pot and incubation experiments were conducted to investigate how the addition of sparingly soluble silicate gel affected As speciation in the soil solution and the accumulation of different As species in rice tissues.

Results

Silicon addition significantly decreased the concentration of inorganic As (mainly arsenite) but increased the concentration of DMA in both the vegetative and reproductive tissues of rice. Silicon increased the concentration of DMA in the soil solution, whereas autoclaving soil decreased DMA concentration. Less DMA was adsorbed by the soil than arsenate and Si addition significantly inhibited DMA adsorption.

Conclusions

Silicon increased DMA accumulation and decreased arsenite accumulation in rice through different mechanisms. Silicic acid released from the silicate gel increased the availability of DMA for rice uptake by inhibiting DMA adsorption on the soil solid phase or by displacing adsorbed DMA. Although silicic acid also increased the concentration of inorganic As in the soil solution, this effect was much smaller than the inhibitory effect of Si on arsenite uptake by rice roots.     

The effect of silicic acid on rice in a P-deficient soil

A pot experiment was conducted to analyze the effect of silicon on the growth of rice grown in a P-deficient soil and on P availability in the soil. Silicic acid was used, rather than a silicate salt, to avoid the complication of changes in soil pH.Shoot dry weight on silicic acid treated soil (0.47 mg Si g^–1 soil) increased significantly under both nonflooded and flooded conditions. Shoot Si concentration also increased although P concentration did not. Mn concentration decreased with silicic acid, resulting in a higher P/Mn ratio in shoots.An adsorption and desorption experiment showed that silicic acid did not displace P nor decrease the ability of the soil to adsorb P. In contrast, Si desorption increased with increasing P concentration in the solution, and Si adsorption was reduced when P was applied first.These results suggest that silicic acid does not increase P availability in soil. Increased dry weight may be attributed to a higher P/Mn ratio in the shoot, which may improve P utilization in the plant.     

Effect of silicate on phosphate availability for rice in a P-deficient soil

In a pot experiment the effect of silicate on P availability for rice grown in a P-deficient soil with and without flooding was analyzed. Treatments were designed as follows: C (control: Yakuno soil), SS (sodium silicate application, at 0.47 mg Si g^-1 soil) and SC (sodium carbonate application). In order to separate pH effect from Si effect, SC was adjusted to the same pH as SS.Soil pH of SS and SC increased by 1.0 unit. Shoot dry weight of SC plants, and more so of SS plants, increased under both nonflooded and flooded conditions. P concentrations in the shoots were not increased under either condition of SS and SC. With SS, Si concentration in the shoots significantly increased, Mn concentration significantly decreased, resulting in a higher P/Mn ratio in the shoot, but not with SC. Both SS and SC increased N concentration in the shoots nearly two times compared with control under both conditions.Adsorption experiments showed that neither SS nor SC decreased P adsorption by soil. SS also could not displace the adsorbed P in soil samples which had previously either received P or not.These results suggest that the beneficial effects of silicate on rice growth do not result from increasing P availability in soil. The Si effect may be attributed to decreasing Mn uptake, thus indirectly improving P utilization in the plant.     

The amelioration of aluminium toxicity by silicon in wheat (Triticum aestivum L.): malate exudation as evidence for an in planta mechanism

Two wheat (Triticum aestivum L.) cultivars, one aluminium tolerant (Atlas 66) and one sensitive (Scout 66), were grown in a continuous-flow culture system (≤pH 5.0) containing aluminium (0–100 μM) and silicon (0–2000 μM) in factorial combination. Treatment with silicon resulted in a highly significant amelioration of aluminium toxicity as assessed by root growth in both cultivars. Amelioration was influenced by wheat cultivar and silicon concentration, as 2000 μM silicon significantly ameliorated the toxic effects of 100 μM aluminium in Atlas 66, and only 5 μM silicon alleviated the effect of 1.5 μM aluminium on Scout 66. Nutrient medium pH was critical, as an amelioration by silicon was apparent only at pH > 4.2 for Atlas 66, and at pH > 4.6 for Scout 66. Silicon neither reduced levels of toxic aluminium species in the growth solutions, nor the amount of aluminium taken up by roots. In experiments to assess exudation of malate by Atlas 66 roots treated with 100 μM aluminium, the presence of 2000 μM silicon (pH 4.6) was found to have a negligible effect on exudation. In contrast, citrate, a known aluminium chelator, reduced aluminium-induced exudation of malate at 5–40 μM and completely inhibited it at 100 μM citrate. The results indicate that silicon does not reduce aluminium phytotoxicity as a result of aluminium/silicon interactions in the external media, and that the mechanism of amelioration has an in planta component. Received: 22 April 1997 / Accepted: 16 August 1997     

Silicate-Mediated Alleviation of Pb Toxicity in Banana Grown in Pb-Contaminated Soil

Silicate (Si) can enhance plant resistance or tolerance to the toxicity of heavy metals. However, it remains unclear whether Si can ameliorate lead (Pb) toxicity in banana (Musa xparadisiaca) roots. In this study, treatment with 800 mg kg⁻¹ Pb decreased both the shoot and root weight of banana seedlings. The amendment of 800 mg kg⁻¹ Si (sodium metasilicate, Na₂SiO₃·9H₂O) to the Pb-contaminated soil enhanced banana biomass at two growth stages significantly. The amendment of 800 mg kg⁻¹ Si significantly increased soil pH and decreased exchangeable Pb, thus reducing soil Pb availability, while Si addition of 100 mg kg⁻¹ did not influence soil pH. Results from Pb fractionation analysis indicated that more Pb were in the form of carbonate and residual-bound fractions in the Si-amended Pb-contaminated soils. The ratio of Pb-bound carbonate to the total Pb tended to increase with increasing growth stages. Treatment with 100 mg kg⁻¹ Si had smaller effects on Pb forms in the Si-amended soils than that of 800 mg kg⁻¹ Si. Pb treatment decreased the xylem sap greatly, but the addition of Si at both levels increased xylem sap and reduced Pb concentration in xylem sap significantly in the Si-amended Pb treatments. The addition of Si increased the activities of POD, SOD, and CAT in banana roots by 14.2% to 72.1% in the Si-amended Pb treatments. The results suggested that Si-enhanced tolerance to Pb toxicity in banana seedlings was associated with Pb immobilization in the soils, the decrease of Pb transport from roots to shoots, and Si-mediated detoxification of Pb in the plants.     

Analysis of Antioxidant and Anti-inflammatory Activity of Silicon in Murine Macrophages

The purpose of this study is to investigate the antioxidant and anti-inflammatory properties of silicon (Si) in the RAW 264.7 murine macrophage cell line. Lipopolysaccharide (LPS) was used to induce inflammatory conditions, and cells were treated with 0, 1, 5, 10, 25, 50, and 100 μM Si in the form of sodium metasilicate. Tert-butylhydroquinone (TBHQ), a well-known antioxidative substance, was used as a positive control to assess the degree of antioxidative and anti-inflammatory properties of Si. Sodium metasilicate at 100 μM suppressed LPS-induced nitric oxide generation from macrophages 36 h after treatment. In addition, 50 μM sodium metasilicate decreased interleukin-6 production, and the degree of suppression was comparable to that of 10 μM TBHQ treatment. LPS-induced messenger RNA (mRNA) expression of tumor necrosis factor-α and inducible nitric oxide synthase was significantly decreased by 1, 5, 10, and 50 μM sodium metasilicate. Cyclooxygenase-2 mRNA expression was also suppressed by 1, 5, 25, and 50 μM sodium metasilicate. Based on these data, Si has the ability to suppress the production of inflammatory cytokines and mediators, possibly through the suppression of radical scavenger activity and down-regulation of gene expression of inflammatory mediators.     

High Concentration of Sodium Metasilicate Impairs Autophagic Flux and Induces Apoptosis in Human Umbilical Vein Endothelial Cells

Silicon-doped materials have been widely used in bone regeneration research; however, a consensus on the safety range of silicon ions has not been reached and its toxicity mechanism remains to be further elucidated. This study aims to explore whether high level of sodium metasilicate can induce toxicity effect in human umbilical vein endothelial cells (HUVEC) and the role of autophagy and apoptosis in its toxic mechanism. HUVEC was treated with different level of high silicon and then investigated with respect to morphologic change, cell viability, immunofluorescence, the level of autophagy, and apoptosis-related protein. Moreover, bafilomycin A1 (Baf A1) was applied to detect whether autophagic flux is disrupted, and 3-methyladenine (3-MA, an autophagy inhibitor) was used to determine the relationship between autophagy and apoptosis. Results demonstrated that high-level silicon induced cell viability to decrease; LC3-II, p62, and apoptosis-related proteins were up-regulated after exposure to high-dose silicon (sodium metasilicate concentration more than 1 mM). There is no significant difference in LC3-II and p62 between Baf A1 and sodium metasilicate-exposed group. Besides, 3-MA further increased the apoptotic rate by inhibiting autophagy after high silicon exposure. Collectively, high concentration of silicon can impair autophagy and induce apoptosis in human umbilical vein endothelial cells, and autophagy may play a protective role in HUVEC apoptosis. Furthermore, silicon concentration used in HUVEC should not be more than 1 mM.

     

Silicon reduces sodium uptake in rice (Oryza sativa L.) in saline conditions and this is accounted for by a reduction in the transpirational bypass flow

Rice is relatively sensitive to salinity and is classified as a silicon accumulator. There have been reports that silicon can reduce sodium uptake in crop grasses in saline conditions, but the mechanism by which silicon might alleviate salinity damage is unclear. We report on the effects of silicon on growth, gas exchange and sodium uptake in rice genotypes differing in salt tolerance. In non-saline media there were no effects of supplementary silicate upon shoot fresh or dry weight or upon root dry weight, indicating that the standard culture solution was not formally deficient with respect to silicon. Plants grown with supplementary silicate had slightly, but significantly, shorter leaves than plants grown in a standard culture solution. Salinity reduced growth and photosynthetic gas exchange. Silicate supplementation partly overcame the reduction in growth and net photosynthesis caused by salt. This amelioration was correlated with a reduction in sodium uptake. Silicate supplementation increased the stomatal conductance of salt-treated plants, showing that silicate was not acting to reduce sodium uptake via a reduction in the transpiration rate. Silicate reduced both sodium transport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). This implies that the mode of action of silicate was by partial blockage of the transpirational bypass flow, the pathway by which a large proportion of the uptake of sodium in rice occurs. Mechanisms by which silicate might reduce the transpirational bypass flow directly are discussed.     

Effects of Silicon on Osteoblast Activity and Bone Mineralization of MC3T3-E1 Cells

Previous studies have reported that dietary silicon (Si) intake is positively associated with bone health including bone mineral density. Although the amount of Si intake is high among trace elements in humans, how dietary Si affects bone formation at the cellular level is not well addressed. The purpose of this study was to investigate the role of Si in osteoblast activity and bone mineralization. MC3T3-E1 was cultured as mature osteoblasts and treated with sodium metasilicate (0, 1, 5, 10, 25, 50, and 100 μM) as a source of Si. After 7 days of treatment, 5 and 10 μM of sodium metasilicate significantly increased intracellular alkaline phosphatase activity (p?相似文献   

Soil acidity factors and nodulation ofTrifolium repens

Summary Effects of factors associated with soil acidity (low pH, low calcium, high aluminium and high manganese) on theTrifolium repens-Rhizobium trifolii symbiosis were investigted under laboratory conditions using an axenic solution-culture technique. 200 μM manganese increased root elongation in the range pH 4.3–5.5, but had no effect on root hair formation, the number of Rhizobium in the rhizosphere, or nodule formation. Root elongation and root hair formation were unaffected at pH 4.3 when 500 or 1000μM calcium was supplied, whereas multiplication of Rhizobium in the rhizosphere and nodulation were inhibited at pH 4.3 and 4.7.50–1000μM calcium had no effect either on the multiplication of Rhizobium in the range pH 4.3–5.5, or on nodule formation in the absence of aluminium. 50 μM aluminium inhibited, root elongation and root hair formation at pH 4.3 and 4.7; the effect on root elongation was reduced by increasing the calcium concentration from 50 to 1000μM. 50μM aluminium also inhibited Rhizobium multiplication in the rhizosphere and reduced nodule formation at pH 5.5 (at which aluminium precipitated out of solution), but root elongation and root hair formation were unaffected. These, effects of aluminium at pH 5.5 may explain the poor response to inoculation by white clover in acid mineral soils after liming.     

Leaf silicon content in banana (Musa spp.) reveals the weathering stage of volcanic ash soils in Guadeloupe

Several plant species accumulate silicon, which is taken up by roots in soil solution. The Si concentration in soil solution can be governed by silicate dissolution and formation, and thus soil constitution. Here, we study the Si leaf content of mature banana plants (Musa acuminata cv Grande Naine) cropped on soils derived from andesitic ash in Guadeloupe through standard foliar analysis. The soils strongly differ in weathering stage and total Si content. The most desilicated soils (Andosol–Nitisol–Ferralsol) occur in the wettest areas, on the Eastern slopes (Es) of the volcano exposed to rain bearing winds. Least weathered soils (Andosol–Cambisol) occur on Western slopes (Ws). The average leaf Si concentration ranges from 2.7 to 3.9 g kg^?1 for bananas cropped in Es soils, and from 7.7 to 9.6 g kg^?1 in Ws soils. The leaf Si concentrations are lowest for the Es gibbsite-rich Andosols and Ferralsols. The leaf Si concentration is positively correlated with soil CaCl₂-extractable Si content, soil Si content and total reserve in weatherable minerals. The silicon content of banana leaves thus reveals the weathering stage of volcanic ash soils in Guadeloupe.     

Model studies of the precipitation of silica in the presence of aluminium; implications for biology and industry

The unique chemical affinity between the oxides of silicon and aluminium has been cited as a potential route for the amelioration of the detrimental effects of aluminium in the environment and in biological systems. A greater understanding of silicon-aluminium interactions may assist in this endeavour and also provide a means of overcoming silica fouling problems encountered by industry which are exacerbated by the presence of aluminium. It is also conceivable that this increased knowledge may demonstrate a positive use for aluminium in the processing of the silicon dioxide phase. In this study we report the effect of aluminium ions, derived from aluminium chloride, on silicic acid species obtained from potassium catecholato complexes of silicon at circumneutral pH at the molar ratios 1000Si:Al, 100Si:Al and 50Si:Al. Silica and low levels of aluminium-rich silica materials were formed with Si:Al ratios of about 3.5:1 comparable with the element ratios detected in senile plaques and aluminium-rich scale. A kinetic study showed that aluminium in the reaction medium slowed down the rate of formation of one of the silica species formed early in the condensation process, e.g. trimers, but increased the rate at which silicic acid was removed from sub 1 nm diameter particles. The materials precipitated in the presence of aluminium were composed of smaller particles and aggregates with smaller pores (Si100:Al and Si50:Al systems) or larger pores (Si1000:Al) compared to the control. The nature of the interactions responsible for these differences is discussed. The effects described here demonstrate the ability of silica and aluminium to interact under conditions such as those found in biological systems. That silica reacts with aluminium in the presence of catechol supports the protective role assigned to silicon.     

The amelioration of aluminium toxicity by silicon in higher plants: Solution chemistry or an in planta mechanism?

Aluminium (Al) toxicity is a very important factor limiting the growth of plants on acidic soils. Recently, a number of workers have shown that, under certain conditions, silicon (Si) can ameliorate the toxic effects of A1 in hydroponic culture. The mechanism of the amelioration is unclear, but three suggestions have been put forward: Si‐induced increase in solution pH during the preparation of hydroponic solutions; reduced availability of Al due to the formation of hydroxyaluminosilicate (HAS) species in those solutions during plant growth; or in planta detoxification. It is now known that it is possible to make up Al and Si solutions in an order in which pH is lowered prior to Al addition; in these cases amelioration has still been observed. Amelioration has also been noted in experiments where HAS formation is minimal. These observations would suggest that, at least under some circumstances, there is an in planta component to the amelioration phenomenon. Several microanalytical investigations have noted codeposition of Al and Si in root cell walls. We propose a model in which root cell walls are the main internal sites of aluminosilicate (AS) and/or HAS formation and of Al detoxification. Factors promoting AS/HAS formation in this compartment include: high apoplastic pH; the presence of organic substances (e.g. malate); and the presence of suitable local concentrations of reactive forms of Al and Si, on or within the surfaces of the wall matrix. All these are likely to be important in the amelioration of Al toxicity.     

Role of leaf apoplast in silicon-mediated manganese tolerance of Cucumis sativus L.

Silicon (Si) supplied as sodium silicate (1·8 mm ) clearly decreased symptoms of manganese (Mn) toxicity in Cucumis sativus L. (cv. Chinesische Schlange) grown in nutrient solution with low to elevated Mn concentrations (0·5–1000 µm ). Despite approximately the same total Mn content in the leaves, plants not treated with Si had higher Mn concentrations in the intercellular washing fluid (IWF) compared with plants treated with Si, especially in the BaCl₂‐ and DTPA‐exchangeable fraction of the leaf apoplast. The Mn concentration of the IWF correlated positively with the severity of Mn‐toxicity symptoms and negatively with the Si supply. Furthermore, in Si‐treated plants less Mn was located in the symplast (< 10%) and more Mn was bound to the cell wall (> 90%) compared with non‐Si‐treated plants (about 50% in each compartment). Manganese present in Si‐treated plants is therefore less available and for this reason less toxic than in plants not treated with Si. It is concluded that Si‐mediated tolerance of Mn in C. sativus is a consequence of stronger binding of Mn to cell walls and a lowering of Mn concentration within the symplast. These results support the role of Si as an important beneficial element in plant nutrition.     

The Effects of a Developing Biofilm on Chemical Changes across the Sediment-Water Interface in a Freshwater Environment

The development of a diatom biofilm on a river sediment was studied using a fluvarium channel with intensive investigations over a total duration of 16 days. The overlying solution was monitored for dissolved calcium, silicon and soluble reactive phosphorus, pH, dissolved oxygen and temperature. The surface of the sediment was sampled for chlorophyll a, algal cell density and porewater profile measurements of calcium concentration, pH and dissolved oxygen were made using microelectrodes. At the end of the experiment the sediment was longitudinally sectioned and porewaters isolated and analysed. A diatom biofilm developed within approximately 5 days leading to a decrease in the concentrations of dissolved silicon and phosphorus in the overlying solution. After approximately 270 hours, the dissolved silicon concentration remained low (average of 6.7 μM). As the diatom numbers increased, photosynthetic activity was evident from increases in dissolved oxygen and pH at the interface. By the end of the experiment diurnal changes in the overlying solution of dissolved calcium, alkalinity and soluble reactive phosphorus were evident. Vertical concentration gradients in dissolved calcium, phosphorus and silicon in the sediment porewater were found at the end of the experiment. The results are consistent with the development of a photosynthetically active diatom biofilm that acted as a barrier to the diffusion of silicon from the porewater. It also induced precipitation and dissolution of calcite and co-precipitation of phosphate with calcite. Chemical fluxes of silicon, calcium and phosphorus were estimated from concentration gradients in the sediment and found to be much smaller than fluxes measured from changes in the bulk solution indicating that processes at the sediment-water interface and biofilm mainly control the flux to the overlying solution.