Abstract
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.
Highlights
Silicon is a ubiquitous soil element that along with oxygen forms 50–70% of soil mass (Ma and Yamaji, 2006)
Based on the matrix that templates the silicification and the participation of transpiration in this process, we identified three types of silica deposition in grasses that describe silicification in most of the cell types
(1) Passive cell wall silicification: This type is distinctive to mature and/or intensely transpiring organs, where the condensation of Si is driven by dehydration
Summary
Silicon is a ubiquitous soil element that along with oxygen forms 50–70% of soil mass (Ma and Yamaji, 2006). The first is based on a passive mode of silicification, relying on the spatial correlation between silica deposition and organ transpiration (Yoshida et al, 1962; Sangster and Parry, 1971; Rosen and Weiner, 1994; Euliss et al, 2005) In this case, specific cell wall components and cuticular structures may affect the location of bio-silicification (reviewed by Exley, 2015; Guerriero et al, 2016). Specific cell wall components and cuticular structures may affect the location of bio-silicification (reviewed by Exley, 2015; Guerriero et al, 2016) This hypothesis infers that silica deposition in plants is a spontaneous process resulting from auto-condensation of Si molecules as the sap undergoes dehydration (Yoshida et al, 1962). A review of literature is pertinent to better understand this biomineralization process
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