Comparative transcriptomic analysis of the nodulation-competent zone and inference of transcription regulatory network in silicon applied Glycine max [L.]-Merr. Roots.

KEY MESSAGE: The study unveils Si's regulatory influence by regulating DEGs, TFs, and TRs. Further bHLH subfamily and auxin transporter pathway elucidates the mechanisms enhancing root development and nodulation. Soybean is a globally important crop serving as a primary source of vegetable protein for millions of individuals. The roots of these plants harbour essential nitrogen fixing structures called nodules. This study investigates the multifaceted impact of silicon (Si) application on soybean, with a focus on root development, and nodulation employing comprehensive transcriptomic analyses and gene regulatory network. RNA sequence analysis was utilised to examine the change in gene expression and identify the noteworthy differentially expressed genes (DEGs) linked to the enhancement of soybean root nodulation and root development. A set of 316 genes involved in diverse biological and molecular pathways are identified, with emphasis on transcription factors (TFs) and transcriptional regulators (TRs). The study uncovers TF and TR genes, categorized into 68 distinct families, highlighting the intricate regulatory landscape influenced by Si in soybeans. Upregulated most important bHLH subfamily and the involvement of the auxin transporter pathway underscore the molecular mechanisms contributing to enhanced root development and nodulation. The study bridges insights from other research, reinforcing Si's impact on stress-response pathways and phenylpropanoid biosynthesis crucial for nodulation. The study reveals significant alterations in gene expression patterns associated with cellular component functions, root development, and nodulation in response to Si.

[Effect of Silica Fertilizer(Husk Ash) to Improve Soil Quality and Reduce Cd and As Accumulation in Rice].

A rice pot experiment was conducted to identify the effect of silica fertilizer prepared from husk ash on the soil bioavailability of cadmium (Cd) and arsenic (As), enzyme activities, microbial community structure, and heavy metal content in brown rice at different growth stages. The results showed that the application of 0.1%-1.0% silica fertilizer-husk ash increased the pH value of soil by 0.04-0.24 units and the content of soil available silicon by 44.2%-97.5%. It also decreased the content of available Cd and available As by 16.2%-21.4% and 16.0%-24.9%, respectively. With the increase in application amount, the soil enzyme activities increased at all growth stages, and the sucrase activity and the dehydrogenase activity significantly increased by 6.3%-145.7% and 6.7%-224.1%, respectively. The analysis of the soil microbial community composition structure at mature stages showed that the application of silica fertilizer-husk ash had no effect on microbial α-diversity, but it had a significant effect on microbial ß-diversity and then promoted microbial growth and maintained the stability of the community structure. With the increase in application amount, the contents of Cd in brown rice decreased by 29.3%-89.7%, and the contents of total As and inorganic As in brown rice decreased by 7.8%-42.3% and 17.2%-44.5%, respectively. Under the application of 0.5% and 1.0% silica fertilizer-husk ash, the Cd contents in brown rice were lower than 0.2 mg·kg-1, and the inorganic As contents in brown rice were lower than 0.35 mg·kg-1. In conclusion, the silica fertilizer-husk ash can improve soil quality and reduce the contents of Cd and As in brown rice, and it is eco-friendly and can be used to remedy the paddy soil contaminated with Cd and As.

[Effects of Equal Amounts Silicon Fertilizer Application on Soil Bioavailability of Cadmium and Cadmium Uptake by Rice].

Cadmium (Cd) contamination of paddy fields is a global concern, as it can cause the accumulation of Cd in food. To explore the effects of equal application of silicon fertilizers on the bioavailability of cadmium and soil Cd uptake at different growth stages of rice, a field experiment was conducted with five silicon fertilizers under the same silicon dose (225 kg·hm-2). The results revealed that the Cd contents in roots, stems, and leaves increased with the extension of the rice growth stage. The application of silicon fertilizers reduced the Cd contents in roots, stems, and leaves in brown rice by 14.9%, 28.2%, and 12.2%, respectively. Compared with that in the control, the Cd content of brown rice in the SiCaMgFe and SiW treatments was decreased by 21.1% (P<0.05) and 21.2% (P<0.05), respectively. Similarly, Cd content in iron plaque (DCB-Cd) increased with the extension of the rice growth period, which accounted for 15.8%-42.8% of the total Cd content in roots, and the DCB-Cd content was different in each stage of rice. The content of exchangeable Cd (Exc-Cd) in soil at the mature stage of rice decreased by 36.4%, and the other fractions increased by 12.5%-48.2%. The results showed significant negative correlations between the Cd contents and Si in roots, DCB-Cd and soil available Cd and available Si, Exc-Cd and Car-Cd, and soil available Cd and pH value. Cd content in roots was positively correlated with DCB-Cd. With the equal dose of silicon fertilizer, the treatments of SiCaMgFe and SiW could effectively reduce the Cd content in rice. The application of silicon fertilizer promoted the transfer of Exc-Cd to Carb-Cd by increasing the soil pH value and the soil available Si content, meanwhile reducing the soil available Cd, Exc-Cd contents, the adsorption of Cd by the iron film on the root surface, and the adsorption capacity of iron plaque and root, thereby reducing the absorption of Cd by rice.

Do Antimonite and Silicon Share the Same Root Uptake Pathway by Lsi1 in Sorghum bicolor L. Moench?

A study was conducted to further develop our understanding of antimony (Sb) uptake in plants. Unlike other metal(loid)s, such as silicon (Si), the mechanisms of Sb uptake are not well understood. However, SbIII is thought to enter the cell via aquaglyceroporins. We investigated if the channel protein Lsi1, which aids in Si uptake, also plays a role in Sb uptake. Seedlings of WT sorghum, with normal silicon accumulation, and its mutant (sblsi1), with low silicon accumulation, were grown in Hoagland solution for 22 days in the growth chamber under controlled conditions. Control, Sb (10 mg Sb L-1), Si (1mM) and Sb + Si (10 mg Sb L-1 + 1 mM Si) were the treatments. After 22 days, root and shoot biomass, the concentration of elements in root and shoot tissues, lipid peroxidation and ascorbate levels, and relative expression of Lsi1 were determined. When mutant plants were exposed to Sb, they showed almost no toxicity symptoms compared to WT plants, indicating that Sb was not toxic to mutant plants. On the other hand, WT plants had decreased root and shoot biomass, increased MDA content and increased Sb uptake compared to mutant plants. In the presence of Sb, we also found that SbLsi1 was downregulated in the roots of WT plants. The results of this experiment support the role of Lsi1 in Sb uptake in sorghum plants.

[Effects of Silicon Application on Arsenic Sequestration in Iron Plaque and Arsenic Translocation in Rice].

The As sequestration by iron plaque and the As translocation in rice significantly affect the As accumulation in brown rice, and silicon (Si) application inhibits the As accumulation in rice plants. However, little information is available concerning the effect of Si application on As sequestration by iron plaque and translocation in rice. In this study, a pot experiment using As-contaminated paddy soil with different Si supply levels was conducted to investigate the effects of Si application on the As sequestration by iron plaque on the root surface and the As translocation from different tissues to brown rice. The results showed that the Si2 (0.66 g·kg-1) treatment significantly increased the activities of CAT (1.81 times), SOD (7.98 times), and POD (1.25 times) in the roots, increased the DCB-extractable Fe concentration (44.35%), and promoted the roughness of iron plaque (108.91%), resulting in a significant increase in the DCB-extractable As concentration of iron plaque (88.32%). Moreover, the Si2 treatment significantly promoted the As accumulation in the roots and inhibited the As translocation from the roots and leaves to the brown rice, leading to a significant decrease in the brown rice As concentration (53.12%). The increase in As sequestration by iron plaque with Si application was attributed to the enhancement of iron plaque formation and the promotion of surface roughness of iron plaque, whereas the inhibition of As translocation from the roots and leaves to the brown rice in the Si application treatment was closely related to the competition between Si with As for transporters and the promotion of As-thiol complex formation and As compartmentalization in vacuolar. These findings provide more insight into the mechanisms of As translocation in rice and will be helpful for exploring strategies to reduce rice grain As through Si supply in As-contaminated paddy fields in South China.

Dynamic photoelectrical regulation of ECM protein and cellular behaviors.

Dynamic regulation of cell-extracellular matrix (ECM)-material interactions is crucial for various biomedical applications. In this study, a light-activated molecular switch for the modulation of cell attachment/detachment behaviors was established on monolayer graphene (Gr)/n-type Silicon substrates (Gr/Si). Initiated by light illumination at the Gr/Si interface, pre-adsorbed proteins (bovine serum albumin, ECM proteins collagen-1, and fibronectin) underwent protonation to achieve negative charge transfer to Gr films (n-doping) through π-π interactions. This n-doping process stimulated the conformational switches of ECM proteins. The structural alterations in these ECM interactors significantly reduced the specificity of the cell surface receptor-ligand interaction (e.g., integrin recognition), leading to dynamic regulation of cell adhesion and eventual cell detachment. RNA-sequencing results revealed that the detached bone marrow mesenchymal stromal cell sheets from the Gr/Si system manifested regulated immunoregulatory properties and enhanced osteogenic differentiation, implying their potential application in bone tissue regeneration. This work not only provides a fast and feasible method for controllable cells/cell sheets harvesting but also gives new insights into the understanding of cell-ECM-material communications.

Organosilicon and inorganic silica inhibit polystyrene nanoparticles uptake in rice.

Nanoplastics (NPs) have become an emerging global environmental problem, and the toxicity of polystyrene nanoplastics (PS-NPs) in rice plants has received widespread attention. However, few studies have focused on silicon (Si)-mediated interactions between PS-NPs and rice. Thus, two forms of Si (organosilicon/inorganic silica) treated rice cells were exposure of positively or negatively charged NPs, PS-NH2 and PS-COOH, to evaluate the effects of Si for defense against PS-NPs toxicity in rice. The result showed PS-NH2 nanoparticles were accumulated at relatively low levels in cells compared with that of PS-COOH, but induced a higher accumulation of hydrogen peroxide (H2O2) and superoxide radicals (O2•-). However, both organosilicon and inorganic silica can generate more negative potential on the surfaces of cell wall to absorb large numbers of positively charged PS-NH2. In addition, they can prevent the uptake of both PS-NH2 and PS-COOH through reducing the porosity on the surface of the cell walls. These finally alleviated the toxicity of oxidative stress caused by PS-NPs and improved the viability of rice cells. Our findings demonstrated the significant contribution of Si in combating PS-NPs in rice.

Effect of silicon on root growth, ionomics and antioxidant performance of maize roots exposed to As toxicity.

Nowadays, one of the biggest challenges of plant physiology is to find out the ways how to mitigate negative impacts of abiotic stress on plants. It is the pollution of groundwater or soil by various metals and metalloids that significantly affects the quality of life. Both arsenic (As) and silicon (Si) are metalloids - while the first one is toxic in general, the latter one is considered as beneficial for plants suffering from various kinds of stresses. The aim of our work was to elucidate the growth and development of young maize (Zea mays L.) plants exposed to both of these metalloids simultaneously. Experiments were focused on the comparison of root growth and biomass allocation, changes in uptake of macro- and micronutrients, visualisation of free radicals along with monitoring of the dynamics of main antioxidant enzymes activity in roots. The results showed that increasing concentration of As (75 and 150 µM As) severely inhibited root length and the amount of biomass, and addition of Si (2.5 mM) to the medium containing As did not have a significant effect on root growth. Similarly, the application of Si did not influence the uptake of macro- and microelements into the roots (mainly Ca, P, K, Mo, Cu, Zn and Ni) which was mostly decreased due to As. On the other hand, Si significantly decreased the presence of both superoxide and hydrogen peroxide in roots that suffered from As toxicity. Although the overall growth of maize plants was not improved by Si amendment, we assume that Si might affect the functionality of key antioxidant enzymes in time, and in this way at least partially help to overcome negative effects of As on maize roots.

Multiple effects of silicon on alleviation of nickel toxicity in young maize roots.

Although beneficial metalloid silicon (Si) has been shown to alleviate the toxicity of various heavy metals, there is a lack of knowledge about the role of Si in possible alleviation of phytotoxicity caused by excess of essential nickel (Ni). In the present study we investigated the growth and biomass production, reactive oxygen species (ROS) formation and activities of selected antioxidants, as well as combined effect of Ni and Si on the integrity of cell membranes and electrolyte leakage in young maize roots treated for 24, 48 and 72 h with excess of Ni and/or Si. By histochemical methods we also visualized Ni distribution in root tissues and compared the uptake of Ni and Si with the development of root apoplasmic barriers. Ni enhanced the root lignification and suberization and shifted the development of apoplasmic barriers towards the root tip. Similarly, localization of Ni correlated with lignin and suberin deposition in root endodermis, further supporting the barrier role of this tissue in Ni uptake. Si reversed the negative impact of Ni on root anatomy. Additionally, improved cell membrane integrity, and enhanced ascorbate-based antioxidant system might be the mechanisms how Si partially mitigates the deleterious effects of Ni excess in maize plants.

Alleviation mechanisms of metal(loid) stress in plants by silicon: a review.

Silicon (Si), although not considered as an essential element for plants in general, can ameliorate the phytotoxicity induced by excess metal(loid)s whether non-essential (e.g. Cd, Pb, Cr, Al, As, and Sb) or essential (e.g. Cu, Ni, and Zn). The Si-enhanced resistance allowing plants to cope with this type of abiotic stress has been developed at multiple levels in plants. Restriction of root uptake and immobilization of metal(loid)s in the rhizosphere by Si is probably one of the first defence mechanism. Further, retention of elements in the root apoplasm might enhance the resistance and vigour of plants. At the cellular level, the formation of insoluble complexes between Si and metal(loid)s and their storage within cell walls help plants to decrease available element concentration and restrict symplasmic uptake. Moreover, Si influences the oxidative status of plants by modifying the activity of various antioxidants, improves membrane stability, and acts on gene expression, although its exact role in these processes is still not well understood. This review focuses on all currently known plant-based mechanisms related to Si supply and involved in amelioration of stress caused by excess metal(loid)s.

Silicon Uptake and Localisation in Date Palm (Phoenix dactylifera) - A Unique Association With Sclerenchyma.

Date palm (Phoenix dactylifera) can accumulate as much as 1% silicon (Si), but not much is known about the mechanisms inherent to this process. Here, we investigated in detail the uptake, accumulation and distribution of Si in date palms, and the phylogeny of Si transporter genes in plants. We characterized the PdNIP2 transporter following heterologous expression in Xenopus oocytes and used qPCR to determine the relative expression of Si transporter genes. Silicon accumulation and distribution was investigated by light microscopy, scanning electron microscopy coupled with X-ray microanalysis and Raman microspectroscopy. We proved that PdNIP2-1 codes for a functional Si-permeable protein and demonstrated that PdNIP2 transporter genes were constitutively expressed in date palm. Silicon aggregates/phytoliths were found in specific stegmata cells present in roots, stems and leaves and their surfaces were composed of pure silica. Stegmata were organized on the outer surface of the sclerenchyma bundles or associated with the sclerenchyma of the vascular bundles. Phylogenetic analysis clustered NIP2 transporters of the Arecaceae in a sister position to those of the Poaceae. It is suggested, that Si uptake in date palm is mediated by a constitutively expressed Si influx transporter and accumulated as Si aggregates in stegmata cells abundant in the outer surface of the sclerenchyma bundles (fibers).

Si permeability of a deficient Lsi1 aquaporin in tobacco can be enhanced through a conserved residue substitution.

Silicon (Si) is a beneficial substrate for many plants, conferring heightened resilience to environmental stress. A plant's ability to absorb Si is primarily dependent on the presence of a Si-permeable Lsi1 (NIP2-1) aquaporin in its roots. Structure-function analyses of Lsi1 channels from higher plants have thus far revealed two key molecular determinants of Si permeability: (a) the amino acid motif GSGR in the aromatic/arginine selectivity filter and (b) 108 amino acids between two highly conserved NPA domains. Curiously, tobacco (Nicotiana sylvestris) stands as a rare exception as it possesses an Lsi1 (NsLsi1) with these molecular signatures but is reported as a low Si accumulator. The aim of this study was therefore to identify whether additional determinants influence Si permeability via Lsi1 channels, focusing on the role of residues that differ uniquely in NsLsi1 relative to functional Lsi1 homologs. We observed tobacco indeed absorbed Si poorly (0.1% dw), despite NsLsi1 being expressed constitutively in planta. Si influx measured in NsLsi1-expressing Xenopus oocytes was very low (<13% that of OsLsi1 from rice (Oryza sativa) over a 3-hr time course), which likely explains why tobacco is a low Si accumulator. Interestingly, NsLsi1P125F displayed a significant gain of function (threefold increase in Si influx relative to NsLsi1WT), which coincided with a threefold increase in plasma membrane localization in planta, as measured by transient expression of GFP constructs in Nicotiana benthamiana leaves. These findings thus reveal a novel molecular determinant of Si transport in plants and inform breeding, biotechnological, and agricultural practices to effectively utilize Si in the context of plant resilience to environmental stress.

The controversies of silicon's role in plant biology.

Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

Expression of Genes for Si Uptake, Accumulation, and Correlation of Si with Other Elements in Ionome of Maize Kernel.

The mineral composition of cells, tissues, and organs is decisive for the functioning of the organisms, and is at the same time an indicator for understanding of physiological processes. We measured the composition of the ionome in the different tissues of maize kernels by element microanalysis, with special emphasis on silicon (Si). We therefore also measured the expression levels of the Si transporter genes ZmLsi1, ZmLsi2 and ZmLsi6, responsible for Si uptake and accumulation. Two weeks after pollination ZmLsi1 and ZmLsi6 genes were expressed, and expression continued until the final developmental stage of the kernels, while ZmLsi2 was not expressed. These results suggest that exclusively ZmLsi1 and ZmLsi6 are responsible for Si transport in various stages of kernel development. Expression level of ZmLsi genes was consistent with Si accumulation within kernel tissues. Silicon was mainly accumulated in pericarp and embryo proper and the lowest Si content was detected in soft endosperm and the scutellum. Correlation linkages between the distribution of Si and some other elements (macroelements Mg, P, S, N, P, and Ca and microelements Cl, Zn, and Fe) were found. The relation of Si with Mg was detected in all kernel tissues. The Si linkage with other elements was rather specific and found only in certain kernel tissues of maize. These relations may have effect on nutrient uptake and accumulation.

Synergistic effects between [Si-hemicellulose matrix] ligands and Zn ions in inhibiting Cd ion uptake in rice (Oryza sativa) cells.

MAIN CONCLUSION: Our study demonstrated that Zn alleviated Cd toxicity in the presence of Si in the cell walls by Zn 2+ binding to ligands through the formation of the [Si-hemicellulose matrix]Zn complexes that restrict the uptake of Cd. The plant cell wall exhibits preferential sites for the accumulation of metals at toxic concentrations. Through modification of wall polysaccharide components, elements, such as silicon (Si) and zinc (Zn), may play active roles in alleviating the toxicity of heavy metals, including cadmium (Cd). However, enhanced tolerance for Cd stress may rely on synergistic effects between nutrient elements. Here, we cultured Si-accumulating suspension cells of rice (Oryza sativa) exposed to Cd and Zn treatments, either separately or in combination, and investigated cells using noninvasive microtest technology (NMT), inductively coupled plasma mass spectroscopy (ICP-MS) and atomic force microscopy (AFM). We found that Zn alleviated Cd toxicity in the presence of Si in the cell walls by binding of Zn2+ to ligands through the formation of the [Si-hemicellulose matrix]Zn complexes and co-precipitates to greatly inhibit Cd2+ uptake into cells. This, in turn, induced the lower expression of Cd-related transporters. This synergistic effect could be decisive for the survival of cells under conditions of high Cd concentrations.

iTRAQ-based proteomic analysis reveals the mechanisms of silicon-mediated cadmium tolerance in rice (Oryza sativa) cells.

Silicon (Si) can alleviate cadmium (Cd) stress in rice (Oryza sativa) plants, however, the understanding of the molecular mechanisms at the single-cell level remains limited. To address these questions, we investigated suspension cells of rice cultured in the dark environment in the absence and presence of Si with either short- (12 h) or long-term (5 d) Cd treatments using a combination of isobaric tags for relative and absolute quantitation (iTRAQ), fluorescent staining, and inductively coupled plasma mass spectroscopy (ICP-MS). We identified 100 proteins differentially regulated by Si under the short- or long-term Cd stress. 70% of these proteins were down-regulated, suggesting that Si may improve protein use efficiency by maintaining cells in the normal physiological status. Furthermore, we showed two different mechanisms for Si-mediated Cd tolerance. Under the short-term Cd stress, the Si-modified cell walls inhibited the uptake of Cd ions into cells and consequently reduced the expressions of glycosidase, cell surface non-specific lipid-transfer proteins (nsLTPs), and several stress-related proteins. Under the long-term Cd stress, the amount of Cd in the cytoplasm in Si-accumulating (+Si) cells was decreased by compartmentation of Cd into vacuoles, thus leading to a lower expression of glutathione S-transferases (GST). These results provide protein-level insights into the Si-mediated Cd detoxification in rice single cells.

Silicon Enhances Water Stress Tolerance by Improving Root Hydraulic Conductance in Solanum lycopersicum L.

Silicon (Si) can improve drought tolerance in plants, but the mechanism is still not fully understood. Previous research has been concentrating on Si's role in leaf water maintenance in Si accumulators, while little information is available on its role in water uptake and in less Si-accumulating plants. Here, we investigated the effects of Si on root water uptake and its role in decreasing oxidative damage in relation to root hydraulic conductance in tomato (Solanum lycopersicum 'Zhongza No.9') under water stress. Tomato seedlings were subjected to water stress induced by 10% (w/v) polyethylene glycol-6000 in the absence or presence of 2.5 mM added silicate. The results showed that Si addition ameliorated the inhibition in tomato growth and photosynthesis, and improved water status under water stress. The root hydraulic conductance of tomato plants was decreased under water stress, and it was significantly increased by added Si. There was no significant contribution of osmotic adjustment in Si-enhanced root water uptake under water stress. The transcriptions of plasma membrane aquaporin genes were not obviously changed by Si under water stress. Water stress increased the production of reactive oxygen species and induced oxidative damage, while added Si reversed these. In addition, Si addition increased the activities of superoxide dismutase and catalase and the levels of ascorbic acid and glutathione in the roots under stress. It is concluded that Si enhances the water stress tolerance via enhancing root hydraulic conductance and water uptake in tomato plants. Si-mediated decrease in membrane oxidative damage may have contributed to the enhanced root hydraulic conductance.

Silicon alleviates cadmium toxicity by enhanced photosynthetic rate and modified bundle sheath's cell chloroplasts ultrastructure in maize.

Silicon was shown to alleviate the negative effects of various biotic and abiotic stresses on plant growth. Although the positive role of Si on toxic and heavy metal Cd has been already described, the mechanisms have been explained only partially and still remain unclear. In the present study we investigated the effect of Si on photosynthetic-related processes in maize exposed to two different levels of Cd via measurements of net photosynthetic rate (AN), chlorophyll a fluorescence and pigment analysis, as well as studies of leaf tissue anatomy and cell ultrastructure using bright-field and transmission electron microscopy. We found that Si actively alleviated the toxic syndromes of Cd by increasing AN, effective photochemical quantum yield of photosystem II (ϕPSII) and content of assimilation pigments, although did not decrease the concentration of Cd in leaf tissues. Cadmium did not affect the leaf anatomy and ultrastructure of leaf mesophyll's cell chloroplasts; however, Cd negatively affected thylakoid formation in chloroplasts of bundle sheath cells, and this was alleviated by Si. Improved thylakoid formation in bundle sheath's cell chloroplasts may contribute to Si-induced enhancement of photosynthesis and related increase in biomass production in C4 plant maize.

A hemicellulose-bound form of silicon inhibits cadmium ion uptake in rice (Oryza sativa) cells.

Silicon (Si) alleviates cadmium (Cd) toxicity in rice (Oryza sativa). However, the chemical mechanisms at the single-cell level are poorly understood. Here, a suspension of rice cells exposed to Cd and/or Si treatments was investigated using a combination of plant cell nutritional, molecular biological, and physical techniques including in situ noninvasive microtest technology (NMT), polymerase chain reaction (PCR), inductively coupled plasma mass spectroscopy (ICP-MS), and atomic force microscopy (AFM) in Kelvin probe mode (KPFM). We found that Si-accumulating cells had a significantly reduced net Cd(2+) influx, compared with that in Si-limited cells. PCR analyses of the expression levels of Cd and Si transporters in rice cells showed that, when the Si concentration in the medium was increased, expression of the Si transporter gene Low silicon rice 1 (Lsi1) was up-regulated, whereas expression of the gene encoding the transporter involved in the transport of Cd, Natural resistance-associated macrophage protein 5 (Nramp5), was down-regulated. ICP-MS results revealed that 64% of the total Si in the cell walls was bound to hemicellulose constituents following the fractionation of the cell walls, and consequently inhibited Cd uptake. Furthermore, AFM in KPFM demonstrated that the heterogeneity of the wall surface potential was higher in cells cultured in the presence of Si than in those cultured in its absence, and was homogenized after the addition of Cd. These results suggest that a hemicellulose-bound form of Si with net negative charges is responsible for inhibition of Cd uptake in rice cells by a mechanism of [Si-hemicellulose matrix]Cd complexation and subsequent co-deposition.