Characterization of silicon uptake by rice roots

Rice (Oryza sativa) is a typical Si-accumulating plant and it has been suggested that it has a specific uptake system for silicic acid in the roots. Here, we characterized this specific system in rice roots. The ability of rice roots to take up Si was much higher than that of other gramineous species. A kinetic study indicated that Si uptake was mediated by a type of proteinaceous transporter; the K-m value was estimated to be 0.32 mm, suggesting that the transporter had a low affinity for silicic acid. Si uptake increased linearly with time, but pretreatment with Si did not affect the uptake of Si, suggesting that the system for Si uptake was not inducible. Mercuric chloride and phloretin, significantly inhibited Si uptake, but 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) did not. Mercuric chloride and phloretin also inhibited water uptake, but to a lesser extent. Si uptake was unaffected by the presence of boric acid. Taken together, the data indicate that the uptake of Si by rice roots is a transporter-mediated process and this transporter contains Cys residues but not Lys residues.