## 4 Si Transporters
### Influx and Efflux of Si Transporters in Rice
plasma membrane. Lsi2 also localizes on exodermal and endodermal cells; however, unlike Lsi1, Lsi2 is located proximally. This gene's activity is opposite to the uptake activity of Lsi1 in _X. laevis_ oocytes, as shown by heterologous expression experiments. This indicates that OsLsi2 cannot transport Si from an external solution into cells, and the transporter OsLsi2 only has the function of Si export, with the Si exporting activity being silicon is inversely proportional to the acidity of the medium. The addition of three protonophores, 2,4-dinitrophenol, carbonylcyanide 3-chlorophenylhydrazone and carbonylcyanide \(p\)-(trifluoromethyloromethoxy) phenylhydra-zone and a low temperature treatment significantly reduced the Si export activity, which differed from OsLsi1 (Figure 1). Therefore, it has been concluded that OsLsi2-mediated Si transport is an active transport process driven by a proton gradient [46]. Due to the different localization and transport properties of OsLsi1 and OsLsi2 in rice root cells, deletion of any of these genes results in a dramatic decrease in the rice uptake of Si [50]. It has been speculated that the two proteins have a synergistic effect on Si absorption in rice. The identification of this Si transporter revealed a unique mechanism of nutrient transport in plants: an influx transporter on one side of the cell and an efflux transporter on the other to ensure efficient nutrient transport across cells. In recent years, homology-based studies of the rice genome found that _Lsi6_ and _Lsi1_ have a close homologous relationship. However, after verifying the expression of _Lsi6_ in different tissues, in contrast with _Lsi1_ and _Lsi2_, _Lsi6_ is expressed in leaf sheaths, leaves and root tips. A knockout of _Lsi6_ affected the distribution of Si in the shoots. Moreover, a knockout of _Lsi6_ did not affect the uptake of Si by the roots, but resulted in the accelerated excretion of Si in the spit fluid. These results suggest that OsLsi6 is a transporter involved in the transport of Si out of the xylem and in the distribution of Si in leaves. [PERSON] et al. [47] showed that OsLsi3 also has a Si export transport activity. In addition, we expressed _Lsi3_ in the _lsi2_ rice mutant under the control of the _Lsi2_ promoter. In the above two independent transgenic lines, the introduction of _Lsi3_ significantly increased the Si uptake. Unlike Lsi2 and Lsi6, Lsi3 shows no polar distribution, and OsLsi3 localizes apolarly in the parenchyma between enlarged and diffuse vascular bundles. Similar to that of _Lsi6_, knockouts of _Lsi2_ and _Lsi3_ reduced Si distributions in panicles but increased Si in flag leaves. Among them, the Si content in the panicle decreased the most after knocking out _Lsi6_. The high Si accumulation in rice is mainly related to the Si transport gene expression pattern and its root structure. The polar localizations of Lsi1 and Lsi2 on the same cell and the double-layer structure of the Casparian are important factors in the active Si uptake  
Figure 1: Schematic model of the Si transport system in rice. system [58]. In fact, the absence of Casparian strips in the endodermis significantly reduces Si uptake [59]. The expressions of _OsLsi1_ and _OsLsi6_ in rice roots were down-regulated after a Si application in the external environment. In diatoms (high-Si accumulating model plants), the gene _SIT_, encoding the Si transporter, is also down-regulated after exogenous Si application. This is an important signaling process for diatoms to perceive the Si concentration in the external environment. Studies have shown that another necessary condition for high-Si accumulation in rice is the down-regulation of Si transport genes in response to exogenous Si applications [60]. Therefore, it is very likely that a diatom-like mechanism exists in rice.