Study on the mechanism of Is uptake and translocation in rice

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1)The form of Si in xylem sap was identified by ^<29>Si-nuclear magnetic resonance (NMR) spectroscopy. In rice (cv.Oochikara) cultured in a monosilicic acid solution containing 0.5mM Si, the Si concentration in the xylem reached 6mM within 30 min. In the ^<29>Si-NMR spectra of the xylem sap, only one signal was observed at a chemical shift of -72.6 ppm, which is consistent with that of monosilicic acid. An ^1H-NMR study of xylem sap did not show any significant difference between the wild-type rice and mutant rice defective in Si uptake, and the ingredient of the xylem sap was not affected by the Si supply. The Si concentration in the xylem sap in vitro decreased from an initial 18 mM to 2.6 mM with time. Addition of xylem sap to a solution containing 8 mM Si did not prevent the polymerization of silicic acid. All these results indicate that Si is translocated in the form of monosilicic acid through the xylem and that the concentration of monosilicic acid is high in the xylem only transiently.2)A gene, Low silicon rice 1 (Lsi1), controlling silicon accumulation in rice, a typical silicon-accumulating plant, was cloned. This gene belongs to the aquaporin family and is constitutively expressed in the roots. Lsi1 is localized on the plasma membrane of the distal side of both the exodermis and endodermis cells, where Casperian strips are located. Suppression of Lsi1 expression resulted in reduced silicon uptake. Furthermore, expression of Lsi1 in Xenopus oocytes showed transport activity for silicon only.3)A novel mutant defective in Si uptake was isolated. This mutant showed lower Si uptake compared to the wild type, but there is no any difference in the uptake of other minerals such as N,P, and K. Mapping result showed that the gene responsible for the mutation is located on the chromosome 3.