Role of polar localization of the silicon transporter OsLsi1 in metalloid uptake by rice roots

Low silicon (Si) rice 1 (OsLsi1) is a key transporter mediating Si uptake in rice (Oryza sativa). It is polarly localized at the distal side of the root exodermis and endodermis. Although OsLsi1 is also permeable to other metalloids, such as boron (B), germanium (Ge), arsenic (As), antimony (Sb), and selenium (Se), the role of its polar localization in the uptake of these metalloids remains unclear. In this study, we investigated the role of OsLsi1 polar localization in metalloid uptake by examining transgenic rice plants expressing polarly or nonpolarly localized OsLsi1 variants. Loss of OsLsi1 polar localization resulted in decreased accumulation of Ge, B, and As in shoots but increased Sb accumulation, while Se accumulation remained unaffected under normal conditions. Experiments with varying B concentrations revealed that B uptake is significantly lower at low B concentrations (0.3 to 3 μm) but higher at high B concentrations (300 μm) in plants expressing nonpolarly localized OsLsi1, despite the similar B permeability of both OsLsi1 variants in Xenopus oocytes and their comparable protein abundance in roots. Additionally, the loss of OsLsi1 polarity did not affect the abundance, localization, or high B-induced degradation of the borate transporter 1 (OsBOR1), an efflux transporter that cooperates with OsLsi1 for B uptake. Taken together, our findings demonstrate that the polar localization of OsLsi1 plays a critical role in regulating metalloid uptake, depending on the presence or absence of efflux transporters cooperating with OsLsi1.