| Silicon(Si) is a beneficial element among plant nutrients for Poaceae. Numerous studies have shown that Si is able to protect plants from multiple abiotic and biotic stresses. Rice(Oryza sativa) is a typical Si-accumulating plant, and the Si content can be up to 10% of the shoot. Rice plants take up Si by roots involving multiple transporters. Following its uptake, Si is translocated to shoots as monomeric silicic acid or disilicic acid and deposited in specialized cells or on cell walls as amorphous silica. In addition to amorphous silica, trace Si maybe covalently crosslink cell wall components to form organosilicon. Despite reseachers attempt to look for evidence of organosilicon complexes, most investigations have been carried out at the level of whole plant or tissue, the diversity and complexity of cell types severely interfere the study. Thus, we cultivated biochemical and genetic background identical rice suspension cells as research materials, while the isolated cell walls and protoplasts as auxiliary materials to perform a comprehensive investigation of the existing form and functions of Si, especially investigated whether Si could covalently crosslink cell wall components and its effects on the properties of cell walls by multiple physical and chemical methods, including inductively coupled plasma mass spectrometry(ICP-MS), X-ray photoelectron spectroscopy(XPS), atomic force microscope(AFM) and combining the observation means of cell biology, such as confocal laser scanning microscopy(CLSM). The main results are summarized as following:1、 Organosilicon that Si covalently crosslink cell wall components exists in rice suspension cells, and it is important for maintaining the shape, viability of cells and stability of cell wall structures.We measured the content of Si in rice suspension cells by ICP-MS, and found that there has a trace amount of Si(113 μg·g-1) in cells, and most of Si exists in the cell wall. After treated with weak acid, alkali or even enzymes(cellulase, hemicellulase or pectinase), the Si could exist stably in cell walls, suggesting that Si maybe firmly crosslink cell wall components. Then, we analyzed the chemical speciation of Si by XPS, and found that there has an obvious peak at 101.3 e V in the Si 2p XPS spectra of rice suspension cells and isolated cell walls, which is neither the elemental Si(~99.2 e V), silicate/(poly)silicic acid(~102.1 e V) nor silica(103.2 e V), maybe the chemical environment of Si-O-C or O-Si-C bonds for crosslinking the cell wall. This wall-bound form of Si exhibits a significance in maintaining cellular integrity and structure of the cell walls, thereby highlighting the biochemical and structural role of Si at the trace level in the preservation of cell shape and mechanical properties to ensure subsequent wall expansion and cell division, which may be crucial for the survival of cells.2、 Hemicellulose is the major ligand binding to silicon, silicon-hemicellulose complexes could improve the mechanical properties of cell wallsWe fractionated cell walls into three main fractions including cellulose, hemicellulose and pectin, and found that 62% of Si exists in hemicellulose, following 26% in cellulose and 12% in pectin as determined by ICP-MS. Combined with the results of XPS, we confirm that Si mainly crosslink hemicellulose forming silicon-hemicellulose complexes. The results of AFM showed that the microfibrils of +Si cell walls are much more dense than those of –Si cell walls. Moreover, Si could improve the mechanical properties of cell walls.3、 Si could improve the cell wall regeneration of protoplastsWe cultured propoplasts by removing the cell walls of rice suspension cells using a method of enzyme digestion, and observed cell wall regeneration using CLSM and AFM. We found that Si could improve the wall regeneration processes and the assembly of the cellulose microfibrils, showing less defects on the surface of protoplast-derived cells(less porous). The Young’s modudus of the protoplast-derived cells in the absence and presence of Si were measured to be 11.76 ± 2.45 and 16.13 ± 2.43 GPa, respectively, suggesting that Si could significantly enhance the wall mechanical properties.4、 Si is benefit to form a hydrophobic lipid layer in the cell wallsBesides the polysaccharide of cell walls, lipid in the cell walls may also be a ligand oSi. We stained the lipid of rice suspension cells before and after plasmolysis by auraminO, and observed that there has a hydrophobic lipid layer in the cell walls of ricsuspension cells using CLSM. Moreover, the fluorescent intensity in the presence of Si istronger. The results of q RT-PCR showed that the expression of lipid transfer protein(LTP) in the presence of Si is 1.4-1.8 higher than that of absence of Si. These resultindicated that Si is benefit to form a hydrophobic lipid layer in the cell walls. |
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