Siliplant1 (Slp1) protein precipitates silica in sorghum silica cells

Abstract

28• Silicon is absorbed by plant roots as silicic acid. The acid moves with the transpiration 29 stream to the shoot, and mineralizes as silica. In grasses, leaf epidermal cells called 30 silica cells deposit silica in most of their volume by unknown mechanism. 31• Using bioinformatics tools, we identified a previously uncharacterized protein in 32 sorghum (Sorghum bicolor), which we named Siliplant1 (Slp1). Silica precipitation 33 activity in vitro, expression profile, and activity in precipitating biosilica in vivo were 34 characterized. 35• Slp1 is a basic protein with seven repeat units rich in proline, lysine, and glutamic acid. 36 A short peptide, repeating five times in the protein precipitated silica in vitro at a 37 biologically relevant silicic acid concentration. Raman and NMR spectroscopies 38 showed that the peptide attached the silica through lysine amine groups, forming a 39 mineral-peptide open structure. We found Slp1 expression in immature leaf and 40 inflorescence tissues. In the immature leaf active silicification zone, Slp1 was localized 41 to the cytoplasm or near cell boundaries of silica cells. It was packed in vesicles and 42 secreted to the paramural space. Transient overexpression of Slp1 in sorghum resulted 43 in ectopic silica deposition in all leaf epidermal cell types. 44 • Our results show that Slp1 precipitates silica in sorghum silica cells. 45 46 47

Spectroscopic Discrimination of Sorghum Silica Phytoliths

“…Specifically in bilobate, protein residues were identified embedded in their silica (Alexandre et al, 2015). A protein (Siliplant1) was identified inside sorghum bilobate cells that is active in in planta silica deposition (Kumar et al, 2019). Nonetheless, our results did not provide direct evidence of amino acids in bilobate cells, possibly because they degraded during the phytolith extraction.…”

Phytolith Formation in Plants: From Soil to Cell

“…This process continues as long as the cells are viable [68]. This silica biogenesis in silica cell was recently elucidated by Kumar, et al [69] in sorghum to be under the molecular control of a siliplant1 protein (SIp1). The previous report by the same group on sorghum confirmed this, where they discovered that the silica cells are still viable in non-silicified conditions and may die in such conditions strongly suggesting that “silica cells lose their nucleus and cytoplasm, become empty and subsequently get silicified in a passive way” [62].…”

“…This spatiotemporal release of SIp1 needs to be further explored to determine what signals the vesicles containing SIp1 to fuse with the membrane and release it. It is evident from the work on sorghum that silicification in silica cells is dependent on the SIp1 protein, while silicification in the cell wall and other cells do not involve this protein [69]. This raises a question, whether there are possibilities that this protein is not functional in other cell types because it might not receive the signals prior to or during the programmed silica cell death?…”

“…It is interesting to note that the SIp1 protein has limited sequence homology with other known biomineralization-related proteins. The SIp1 protein has seven repeat units that are rich in lysine, histidine-aspartic acid rich regions, and several proline residues [69]. Four rice transporters ( OsLsi1, OsLsi2, OsLsi3 , and OsLsi6 ), a sorghum SIp1 gene, and the respective proteins are shown in Figure 2.…”

Accumulation of germanium (Ge) in plant tissues of grasses is not solely driven by its incorporation in phytoliths

“…In addition, a thin silica layer formed of thin silica plates is located just below the cuticle of grasses (Yoshida et al 1962;Sato et al 2017). Although still not clear, the formation of species-specific phytoliths indicates that the process of biomineralization is not a simple precipitation as a result of oversaturation but a physiologically regulated process (Kumar et al 2017a(Kumar et al , 2019Soukup et al 2017). While there is a large body of literature supporting the importance of phytoliths in biogeochemical cycling of Si, the mechanisms of biomineralization and their relevance for the accumulation of Ge are still not well understood.…”