Silicon oxide in the form of hydrated silica (SiO2·nH2O) is a constituent of plant tissues that in certain taxa can make several percent per dry weight. Nonetheless, the formation mechanism of plant silica is mostly unknown. Si is taken up from the soil by roots in the form of mono-silicic acid molecules (H4SiO4). The silicic acid is carried in the xylem and subsequently polymerizes in target sites to silica. In roots of sorghum (Sorghum bicolor), silica aggregates form in an orderly pattern along the inner tangential cell walls of endodermis cells. Using Raman micro-spectroscopy, auto-fluorescence and scanning electron microscopy we investigated the structure and composition of developing aggregates in roots of sorghum seedling. Putative silica aggregation loci were identified in roots grown under Si starvation. These micrometer-scale spots were constructed of tightly packed modified lignin, and nucleated trace concentrations of silicic acid. Substantial variation in cell wall auto-fluorescence between Si+ and Si- roots demonstrated the impact of Si on cell wall chemistry. We propose that in Si- roots, the modified lignin crosslinked into the cell wall and lost its ability to nucleate silica. In Si+ roots, silica polymerized on the modified lignin and altered its structure. Our work demonstrates a profound control over lignin and silica deposition in cell walls.
Background Silicon and aluminum oxides make the bulk of agricultural soils. Plants absorb dissolved silicon as silicic acid into their bodies through their root. The silicic acid moves with transpiration to target tissues in the plant body where it polymerizes into biogenic silica. Mostly, the mineral forms on a matrix of cell wall polymers that create a composite material. Historically, silica deposition (silicification) was supposed to occur once water evaporated from the plant surface leaving behind increased concentration of silicic acid within plant tissues. However, recent publications indicate that certain cell wall polymers and proteins initiate and control the extent of plant silicification. Scope Here we review recent publications on the polymers that scaffold the formation of biogenic plant silica, and propose a paradigm shift from spontaneous polymerization of silicic acid to dedicated active metabolic processes that control both location and extent of the mineralization. Conclusion Protein activity concentrate silicic acid beyond its saturation and polymeric structures at the cell wall stabilize it and allow its flow with the transpiration stream, or bind it and allow its initial condensation. Nucleation and further polymerization is enabled on a polymeric scaffold, which is embedded within the mineral. Deposition is terminated once free silicic acid is consumed or the chemical moieties for its binding are saturated.
Hydrated silica (SiO2·nH2O) aggregates in the root endodermis of grasses. Application of soluble silicates (Si) to roots is associated with variations in the balance of reactive oxygen species (ROS), increased tolerance to a broad range of stresses affecting ROS levels, and early lignin deposition. In sorghum (Sorghum bicolor L.), silica aggregation is patterned in an active silicification zone (ASZ) by a special type of aromatic material with a signature typical to lignin. Since lignin polymerization is mediated by ROS, we studied the formation of root lignin and silica under varied conditions of ROS by modulating hydrogen peroxide (H2O2) concentration in the growth medium. Sorghum seedlings were grown hydroponically and supplemented with Si, H2O2, and KI, a salt that catalyzes H2O2 decomposition. Lignin and silica deposits in the endodermis were studied by histology, scanning electron and Raman microscopies. Cell wall composition was quantified by thermal gravimetric analysis. The endodermal H2O2 concentration correlated to the extent of the aromatic polymer deposition along the root, but did not affect its patterning in spots. Our results show that the ASZ spots were necessary for root silica aggregation, and suggest that silicification is intensified under oxidative stress as a result of increased ASZ lignin-like deposition.
<p>Silicon oxides are the most abundant mineral group in soils. Therefore, plant roots are always exposed to some silicic acid (Si(OH)<sub>4</sub>), which is the soluble form of silicates. Monosilicic acid molecules are taken up by roots, carried in the xylem, and subsequently polymerize to silica in varied silicifying target sites. This biogenic silica (SiO<sub>2</sub>&#183;<em>n</em>H<sub>2</sub>O) can constitute several percent by dry weight in certain plant taxa. However, the mechanisms of its formation remain mostly unknown. In the roots of sorghum (<em>Sorghum bicolor</em>), silica aggregates form in an orderly pattern along the cell walls of endodermis cells. To investigate the structure and composition of root silica aggregates, we studied their development along roots of hydroponically grown sorghum seedlings. By using Raman micro-spectroscopy, auto-fluorescence, and scanning electron microscopy, we found that putative silica aggregation loci could be identified in roots grown under Si starvation. These micrometer-scale spots were constructed of tightly packed modified lignin and were capable of nucleating trace concentrations of silicic acid. Substantial variation in cell wall auto-fluorescence between roots grown with and without silicic acid demonstrated the impact of silicon on cell wall chemistry. Taken together, this work demonstrates a high degree of control over lignin and silica deposition in cell walls. Such regulation implies an important, yet unknown, function for silicon in plant biology.</p>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.