# Usage of Si, P, Se, and Ca Decrease Arsenic Concentration/Toxicity in Rice, a Review
### Silicon (Si) and As  
Si is an essential element in the soil and crust of the earth, and soil Si content makes up 1 to 45% of soil's dry weight, but only 0.1 to 0.6% is soluble [33; 34]. Si is a motionless element in plants, and rice is absorbed as ionized Si(OH)\({}_{3}\)O and silicic acid (H\({}_{4}\)SiO\({}_{4}\)). Si content of rice's dry weight can reach up to 10% [35; 36], and rice is a stacked Si plant, often used as a sample plant for Si uptake, transport, and accumulation [3]. According to research, silicon is a practical and valuable element for plant growth, especially in various stress conditions such as plant diseases, plant pests, drought, salinity [37], while one of the significant effects of Si is in reducing the toxicity of metal by decreasing absorption and transport of metals in plants. [38].  
Studies have shown that rice roots absorb arsenite via two Si carriers, _Lsil_ (the aquaporin NIP2; 1) and _Lsi_ (an efflux carrier) [39]. _Lsi_ is an invasive transporter responsible for transporting silicon and arsenic (III) from culture solutions to rice root cells [40]. As (III) is absorbed within cells in _Escherichia coli_, the yeast (_Saccharomyces cerevisiae_) via aqualyceroporins and several aquaporin channels of plants are dependent on the Nodulin 26 subfamily, the same as the intrinsic protein (NIP) is permeable to As (III) in the case of heterogeneity in yeast [41; 42]. The Si rice carrier _Lsi_ (OsNIP2; 1) is also penetrable to As (III) expressed in Xenopus laevis and yeast oocytes [39]. The second silicon transporter (_Lsi_2) in rice roots transfers silicon efflux from epidermal and endodermal cells to stele to load xylem, and this transporter also mediates the efflux of As (III) [39; 43].  
However, adding Si to soil and water significantly decreases the concentration of arsenic in rice. In addition, some studies have shown that Si can help increase methylation in rice tissue and affect the concentration of different As types in rice [44]. Therefore, adding silicon to the soil can reduce As uptake and accumulation in rice, and Si foliar application may also be an alternative route to reduce arsenic accumulation in rice [22].  
Different studies have been performed on the efficacy of Si in reducing the uptake and transport of arsenic in rice. For instance, [PERSON] and [PERSON] [44] detected that the  
Figure 1: Mechanisms of arsenic uptake, transport, accumulation, and distribution in rice. Nomenclature is as proposed by [PERSON] and [PERSON] [30] with some modifications.  
concentration of As in the grain was significantly reduced by adding Si to the pore water. Si application has also been reported to decrease concentrations of different types of arsenic in stem, leaf, husk, and brown rice (Figure 2) [45].  
It has also been observed that Si usage significantly decreases the concentration of iAs, while the DMA concentration increases in rice plant tissues [45]. It was found that silicic acid can also dislocate arsenite, arsenate, and Dimethylarsinate from the surface of soil particles, which increases the aggregate concentration of arsenic in the soil solution [47]. Hence, the effects of adding silicon to soil on arsenic bioavailability depend on the interactions between As and Si in pore water, particles of soil, and the absorption of rice roots [48]. According to research, to control the adverse efficacy of silicon usage in paddy soil, Si foliar usage can be a more impressive way to affect the arsenic accumulation rate in rice [22].