Transport of silicon from roots to panicles in plants

Abstract

Silicon (Si) is the most abundant minerals in soil and exerts beneficial effects on plant growth by alleviating various stresses. The transport of Si from soil to the panicles is mediated by different transporters. Lsi1, belonging to a NIP group of the aquaporin family, is responsible for the uptake of Si from soil into the root cells in both dicots and monocots although its expression patterns and cellular localization differ with plant species. The subsequent transport of Si out of the root cells towards the stele is medicated by an active efflux transporter, Lsi2. Lsi1 and Lsi2 are polarly localized at the distal and proximal sides, respectively, of both exodermis and endodermis in rice root. Silicon in the xylem sap is presented in the form of monosilicic acid and is unloaded by Lsi6, a homolog of Lsi1 in rice. Lsi6 is also involved in the inter-vascular transfer of Si at the node, which is necessary for preferential Si distribution to the panicles.

Figure 1.

Examples showing beneficial effects of Si in plants. A, Effect of Si on pest damage. B, Protective role of Si in excess transpiration of panicles in rice. C, Alleviative effect of Si on Mn toxicity in pumpkin. The plants were exposed to high Mn in the presence (+Si) or absence (−Si) of Si. D–E, Decreased fertility due to low Si in rice. A–B, D–E, wild type rice (High Si) and a mutant defective in Si uptake (Low Si) were grown in the field. From Tamai and Ma (2008).¹⁵⁾

Figure 2.

Deposition of Si in rice leaf blade. A–B, soft X-ray image of rice leave with (A) or without (B) Si. Black dot shows silica bodies. C, SEM-EDX image of a cross section of rice leaf blade. Yellow color shows Si deposition under cuticle and bulliform motor cells known as plant opal.

Figure 3.

Phenotype of lsi1 mutant. A, Symptoms of Ge toxicity in the leaf of wild-type rice (WT) and mutant lsi1. B–C, Growth of WT and lsi1 in the absence (B) and presence (C) of Ge. D, Si uptake in the mutant and WT. From Ma et al. (2002).¹⁶⁾

Figure 4.

Localization of Si transporters in rice. A, Polar localization of influx Si transporter Lsi1 (green) and efflux Si transporter Lsi2 (red) in lateral roots. Ex, exodermis; En, endodermis; nuclei stained by DAPI (cyan). From Yamaji and Ma (2011).¹⁸⁾ B, Schematic presentation of Si uptake system in rice root exodermis/endodermis cells.

Figure 5.

Schematic presentation of different Si uptake system in barley/maize (A) and rice (B) roots. From Mitani et al. (2009).²¹⁾

Figure 6.

Localization of Lsi6 in rice leaf blade. A, Immunostaining of Lsi6 (red) showing localization at xylem parenchyma cells. B, Pathway of Si from leaf xylem via Lsi6. From Yamaji et al. (2008).²⁷⁾

Figure 7.

Localization of Lsi6 at the node I in rice. A, Cross section showing localization of Lsi6 (red) at the transfer cells surrounding enlarged vascular bundles. Enlarged vascular bundles (orange) and diffuse vascular bundles (green) are indicated. B, Longitudinal section of rice node I. Modified from Yamaji and Ma (2009).³³⁾