## 4 Si Transporters
### Influx and Efflux of Si Transporters in Rice  
Rice requires a large amount of Si to improve its growth, development and yield; consequently, the first Si-influx transporter was identified from rice. [PERSON] et al. [35] found that a mutant Si transporter (Lsi1) was defective in Si uptake in rice, and further isolated mutant seeds from a Germanium (Ge)-containing environment. [PERSON] et al. [19] used map-based cloning to clone _OsLsi1_ on the second chromosome of rice. It consists of four introns and five exons, and the cDNA content is 1409 bp, containing 298 aa. On the basis of a BLAST search and CLUSTALW analysis, it is speculated that _Lsi1_ belongs to the NIP subfamily. It has been predicted that the _Lsi1_ amino acid sequence contained six transmembrane regions conserved in typical aquaporins and two Asn-Pro-Ala motifs. [PERSON] et al. [19] conducted a series of experiments on transgenic rice and concluded that the Lsi1 promoter region tagged with GFP is located in the lateral and main roots of plants, but they do not exist in root hairs. This result indicates that only the roots are involved in the uptake of Si. A subcellular localization of Lsi1 revealed that cells expressing _Lsi1_-GFP fusion only displayed GFP signals on the plasma membrane. Therefore, Lsi1 is localized in the distal regions of the endodermis and exodermis of the root [56]. In addition, abscisic acid and dehydration stresses inhibit and down-regulate the expression of Lsi1. Injections of _Xenopus laevis_ oocytes showed that Lsi1 is a bidirectional transporter of silicic acid [21]; however, it only exhibits the function of Si uptake into rice roots [56]. OsLsi1 is an import transporter of Si in rice roots, primarily responsible for Si transport from external solutions into root cells. The expression of the Si transporter was found to follow a diurnal pattern in the maturation zones of root tips. This typically results in the fluctuating expression of the Lsi1 transporter of Si in many monocots, with Lsi1 expression being highest from 12 PM to 12 AM and then declining from 4 AM to 8 AM. Moreover, Lsi1 shows differential expression at each growth stage. Studies have shown that approximately 60% of Si is taken up by transporters at the beginning of the rice reproductive period. The depletion of Si at this time leads to a reduction in the number of grains; conversely, high yields require more Si. By comparing the transgenic material with the wild type, it has also been confirmed that the OsLsi1 transporter plays a key role in the uptake of Si in rice, increasing the uptake and accumulation of Si, thereby enhancing the activity of antioxidant enzymes and improving the morphological traits of rice [57]. After cloning the _Lsi1_ transporter gene, an export transporter gene _Lsi2_, located on chromosome 3, was cloned. The gene contains one intron and two exons, and its full-length cDNA is 1416 bp, encoding 472 aa [46]. Based on a BLAST search and CLUSTALW analysis, OsLsi2 is a putative anion transporter, and it is not similar to the Si-influx transporter OsLsi1. The subcellular localization of Lsi2 confirmed that it is also expressed on the \begin{table}
\begin{tabular}{c c c c c} \hline
**Si Accumulation** & **Species** & **Si Content (\%d.w.)** & **Si Transporter** & **Reference** \\ \hline \multirow{4}{*}{High} & Horsetail & 10\% [44] & EaLsi2-1; EaLsi2-2 & [23; 45] \\ \cline{2-5}  & Rice & 10\% [35] & OsLsi1; OsLsi2; OsLsi3; OsLsi6 & [19; 46; 47; 48] \\ \cline{2-5}  & Maize & 4–7.5\% [49] & ZmLsi1; ZmLsi2; ZmLsi6 & [50] \\ \cline{2-5}  & Barely & 2–4\% [49] & HvLsi1; HvLsi2; HvLsi6 & [20; 51] \\ \hline Intermediate & Cucumber & 2–4\% [49] & CmLsi1; CmLsi2 & [26; 52] \\ \hline \multirow{4}{*}{Low} & Tomato & \textless{}0.2\% [49] & SlLsi1; SILi2 & [27] \\ \cline{2-5}  & Potato & \textless{}0.5\% [53] & StLsi1; StLsi2 & [54] \\ \cline{1-1} \cline{2-5}  & Soybean & \textless{}0.5\% [55] & GmNIP2-1; GmNIP2-2 & [25] \\ \hline \end{tabular}
\end{table} 
Table 1: Si content and Si transporters in various plants with different Si uptake types.