Plants take up silicon as mono-silicic acid, which is released to soil by the weathering of silicate minerals. Silicic acid can be taken up by plant roots passively or actively, and later it is deposited in its polymerized form as amorphous hydrated silica. Major silica depositions in grasses occur in root endodermis, leaf epidermal cells, and outer epidermal cells of inflorescence bracts. Debates are rife about the mechanism of silica deposition, and two contrasting scenarios are often proposed to explain it. According to the passive mode of silicification, silica deposition is a result of silicic acid condensation due to dehydration, such as during transpirational loss of water from the aboveground organs. In general, silicification and transpiration are positively correlated, and continued silicification is sometimes observed after cell and tissue maturity. The other mode of silicification proposes the involvement of some biological factors, and is based on observations that silicification is not necessarily coupled with transpiration. Here, we review evidence for both mechanisms of silicification, and propose that the deposition mechanism is specific to the cell type. Considering all the cell types together, our conclusion is that grass silica deposition can be divided into three modes: spontaneous cell wall silicification, directed cell wall silicification, and directed paramural silicification in silica cells.

A model is proposed in which the formation of silica aggregates in sorghum roots is predetermined by a modified cell wall architecture and takes place as governed by endodermal development. The interaction with silica is provided by arabinoxylan-ferulic acid complexes and interferes with further deposition of lignin. Due to contrasting hydrophobicity, silicification and lignification do not represent functionally equivalent modifications of plant cell walls.

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).

Silicon (Si) is not considered an essential element, however, its tissue concentration can exceed that of many essential elements in several evolutionary distant plant species. Roots take up Si using Si transporters and then translocate it to aboveground organs. In some plant species, root tissues are also places where a high accumulation of Si can be found. Three basic modes of Si deposition in roots have been identified so far: (1) impregnation of endodermal cell walls (e.g., in cereals, such as Triticum (wheat)); (2) formation of Si-aggregates associated with endodermal cell walls (in the Andropogoneae family, which includes Sorghum and Saccharum (sugarcane)); (3) formation of Si aggregates in “stegmata” cells, which form a sheath around sclerenchyma fibers e.g., in some palm species (Phoenix (date palm)). In addition to these three major and most studied modes of Si deposition in roots, there are also less-known locations, such as deposits in xylem cells and intercellular deposits. In our research, the ontogenesis of individual root cells that accumulate Si is discussed. The documented and expected roles of Si deposition in the root is outlined mostly as a reaction of plants to abiotic and biotic stresses.

Silica phytoliths are microscopic structures of amorphous hydrated silica (SiO2 · nH2O) formed by specialized plant cells. Besides their biological roles, physical, chemical, and structural properties of biogenic silica offer a wide spectrum of applications in many fields of industry and technology. Therefore, processes involved in their formation recently become a very interesting topic to study. However, optical transparency and microscopic sizes of silica phytoliths do not allow their visualization and localization by classical light microscopy methods. Their observation thus requires phytolith isolation, technically difficult or lengthy sample preparation procedures, or a work with toxic chemicals. In this paper we are proposing a novel method for visualization of silica phytoliths in Sorghum bicolor root endodermal cells by fluorescence microscopy using alkali mounting solution (pH 12). This method offers an easy and quick preparation of the samples and high contrast imaging. Based on our results we can assume that the proposed fluorescent method for silica phytolith investigation allows observation of multiple samples in relatively short time period and thus might be applicable also for high-throughput screenings. Using this method we found out that after a 3-day cultivation of sorghum plants the minimal needed concentration of sodium silicate, limiting the formation of silica phytoliths in the root endodermis, was 25 µmol dm(-3). The positive correlation of sodium silicate concentration in the substrate with the phytolith diameter was also observed.

Silica, a mineral reducing the digestibility of grass tissues, is assumed to form spontaneously in planta. We overturn this model, showing that silica deposition occurs during lignification and demands metabolic energy.

Background and Aims Cell walls of the peri-endodermis, a layer adjacent to the endodermis in alpine pennycress (Noccaea caerulescens) roots, form C-shaped peri-endodermal thickenings (PETs). Despite its specific position close to the endodermis, the assumed similarity of PETs to phi thickenings in many other species, and the fact that N. caerulescens is a well-studied heavy-metal-hyperaccumulating plant, the PET as a root trait is still not understood. Methods Here, we characterized PET cell walls by histochemical techniques, Raman spectroscopy, immunolabelling and electron microscopy. Moreover, a role of PETs in solute transport was tested and compared with Arabidopsis thaliana plants, which do not form PETs in roots. Key Results Cell walls with PETs have a structured relief mainly composed of cellulose and lignin. Suberin, typical of endodermal cells, is missing but pectins are present on the inner surface of the PET. Penetrating dyes are not able to cross PETs either by the apoplasmic or the symplasmic pathway, and a significantly higher content of metals is found in root tissues outside of PETs than in innermost tissues. Conclusions Based on their development and chemical composition, PETs are different from the endodermis and closely resemble phi thickenings. Contrarily, the different structure and dye impermeability of PETs, not known in the case of phi thickenings, point to an additional barrier function which makes the peri-endodermis with PETs a unique and rare layer.

Silicon (Si) is an element frequently associated with remedial properties in plant defence against various biotic stressors. Application of Si in plant–fungus interactions offers an intriguing research subject for its potential to reduce fungal disease‐related losses in agriculture. However, Si‐mediated changes in the plant metabolism must be first elucidated. The present microscopic study explores the effect of Si amendment on ultrastructural, cytological and anatomical aspects of the interaction between Sorghum bicolor ‘Gadambalia’ primary roots and the necrotrophic fungus Alternaria alternata. Bright‐field microscopy observations revealed that Si supplementation helped sorghum seedlings to restrict the growth of A. alternata, possibly by cell wall modifications of the root exodermis, limiting further fungal invasion. Ultrastructural analyses by transmission electron microscopy showed intense vesiculation in exodermal cells of inoculated roots. Fluorescence microscopy with suberin‐ and phenolic‐specific staining indicated a significant enhancement in the deposition of these substances in exodermal cell walls upon Si supplementation. The increased accumulation of phenolics in Si‐supplied plants was also confirmed by Raman spectroscopy. In conclusion, Si amendment seemed to alleviate the negative effect of A. alternata infection in sorghum roots, which is shown by restricted distribution of fungal hyphae in root tissues probably due to Si‐mediated deposition of suberin and phenolics in the exodermis.