Siliplant1 protein precipitates silica in sorghum silica cells

Abstract

<jats:title>Abstract</jats:title> <jats:p>Silicon is absorbed by plant roots as silicic acid. The acid moves with the transpiration stream to the shoot, and mineralizes as silica. In grasses, leaf epidermal cells called silica cells deposit silica in most of their volume using an unknown biological factor. Using bioinformatics tools, we identified a previously uncharacterized protein in Sorghum bicolor, which we named Siliplant1 (Slp1). Slp1 is a basic protein with seven repeat units rich in proline, lysine, and glutamic acid. We found Slp1 RNA in sorghum immature leaf and immature inflorescence. In leaves, transcription was highest just before the active silicification zone (ASZ). There, Slp1 was localized specifically to developing silica cells, packed inside vesicles and scattered throughout the cytoplasm or near the cell boundary. These vesicles fused with the membrane, releasing their content in the apoplastic space. A short peptide that is repeated five times in Slp1 precipitated silica in vitro at a biologically relevant silicic acid concentration. Transient overexpression of Slp1 in sorghum resulted in ectopic silica deposition in all leaf epidermal cell types. Our results show that Slp1 precipitates silica in sorghum silica cells.</jats:p>

Cellulose Intrafibrillar Mineralization of Biological Silica in a Rice Plant

“…Some proteins promoting silica polymerization were detected in the silica accumulation area in higher plants. 28 We confirmed the presence of proteins in the silica layer of a mature rice husk using the SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) ( Figure S4 in the SI). Thus, the interfibrillar mineralization would be associated with specific proteins.…”

“…38 Moreover, silica polymerization-promoting factors, such as specific proteins and long-chain polyamines synthesized in the cell, are secreted as molecular templates in the apoplast, as described in a previous report on silicification in sorghum. 28 Actually, we found several proteins in the silica layer of rice husks ( Figure S4 in the SI). Thus, silicic acid in the CNF matrix is polymerized and forms silica particles with the proteins as a molecular template.…”

“…26,27 A protein that promotes silica polymerization in the apoplast was discovered in the cytoplasm of a higher plant, sorghum. 28 The amino acid sequence of the protein in sorghum contains a lysine-rich domain similar to silaffin and a histidine-and aspartic acid-rich domain similar to glassine. These proteins and polyamines contained in diatoms, sponges, and higher plants are regarded as a molecular-level template for the promotion of silica polymerization.…”

Hydrogen peroxide modulates lignin and silica deposits in sorghum roots

“…It is exported to the cell wall, where it interacts with supersaturated silicic acid solution. Slp1 catalyzes the formation of a thick silica wall within hours (Kumar et al, 2020). In parallel, the cell cytoplasm shrinks into threads, and is eventually evacuated during a programmed cell death (Kumar and Elbaum, 2018).…”

“…Recently we reported the first protein that causes silica formation in plants (Kumar et al , 2020). Siliplant1 (Slp1) is expressed in sorghum ( Sorghum bicolor L.) epidermis cells called silica cells.…”