Symplastic and apoplastic pathways for local distribution of silicon in rice leaves

Silicon (Si) is highly accumulated in both the leaf blade and sheath of rice, but the transporter mediating the local distribution of Si between these two tissues remains unidentified. We investigated the role of an aquaporin, OsLsi6, in the local distribution of Si in rice leaves. We also examined the interrelations between vascular structure and OsLsi6 function in xylem unloading of Si for its local distribution. OsLsi6 is polarly localized at the xylem parenchyma cells of both the large and small vascular bundles of the leaf blade and sheath. OsLsi6 was downregulated by Si supply at the leaf sheath but not in the leaf blade. The knockout of OsLsi6 increased the distribution of Si and germanium (Ge) to the leaf blade while reducing their distribution to the leaf sheath. The mestome sheath surrounding the vascular bundle was suberized in leaf sheaths and in large vascular bundles of leaf blades, but not in small vascular bundles of leaf blades. Our results indicate that there are two pathways for xylem unloading of Si for its local distribution: the OsLsi6-dependent symplastic pathway in the leaf sheath and large vascular bundles of the leaf blade, and the apoplastic pathway in the small vascular bundle of the leaf blade.