Silicification in Grasses: Variation between Different Cell Types

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.

Silicon affects seed development and leaf macrohair formation in Brachypodium distachyon

“…Previous reports have indicated that Si polymerizes to form a layer directly below the cuticle in the rice shoots (Yoshida et al ). Additionally, upon water loss, Si polymerizes intra‐ or extracellularly, forming characteristic structures called silica cells, silica bodies or phytoliths (Ma and Yamaji , Kumar et al ). Our element quantification showed that most of the Si in Brachypodium was deposited in leaves and bracts (Fig.…”

The controversies of silicon's role in plant biology

“…around exodermal and endodermal root cells and leaf epidermal cells; Sangster et al ., ; Gong et al ., ), and cell wall constituents, such as (hemi)cellulose, callose, pectin and lignin, have been demonstrated to interact with Si(OH) 4 as ‘templates’ or ‘scaffolding’ for silicification (Guerriero et al ., ; and references therein). Si can also polymerize in specialized cells and cellular structures of some species (particularly grasses), such as leaf silica and long cells, and spikelet hairs and papillae (Rafi et al ., ), and interesting preliminary evidence for the biological control of this process has emerged (Kumar et al ., ,b; Kumar & Elbaum, ).…”

“…If such a mechanism exists, experimental methods that elucidated the mechanism of transporters, such as NHX1 and SOS1 (Na + /H + antiporters; Apse et al, 1999;Qiu et al, 2002), should, hypothetically, shed light on Lsi2 functionality. The mechanism of Si deposition and accumulation is also unclear, but has recently garnered increased attention (Exley, 2015;Guerriero et al, 2016;Kumar et al, 2017b). Once the solubility of Si(OH) 4 is exceeded (i.e.…”

Interplay between silica deposition and viability during the life span of sorghum silica cells

“…Grasses are well known for their high silica content of up to 10% of the dry weight (DW) (Hodson et al, 2005), mostly deposited in the root endodermal cells, leaf epidermal cells and abaxial epidermal cells of the inflorescence bracts. In many cell types in grasses, silica deposition is confined to the cell wall of the silicifying cells either as a result of autocondensation of silicic acid as water is lost through evapotranspiration or as a result of the interaction between some cell wall components and silicic acid (Kumar et al, 2017b). Silica deposition also takes place in the lumen of some cells, notably epidermal silica cells where silica is deposited on a noncell wall matrix (Hodson et al, 1985;Hodson, 2016).…”