A hemicellulose‐bound form of silicon inhibits cadmium ion uptake in rice (Oryza sativa) cells

Summary Silicon ( S i) alleviates cadmium ( C d) toxicity in rice ( O ryza sativa ). However, the chemical mechanisms at the single‐cell level are poorly understood. Here, a suspension of rice cells exposed to C d and/or S i treatments was investigated using a combination of plant cell nutritional, molecular biological, and physical techniques including in situ noninvasive microtest technology ( NMT ), polymerase chain reaction ( PCR ), inductively coupled plasma mass spectroscopy ( ICP ‐ MS ), and atomic force microscopy ( AFM ) in Kelvin probe mode ( KPFM ). We found that S i‐accumulating cells had a significantly reduced net Cd 2+ influx, compared with that in Si‐limited cells. PCR analyses of the expression levels of C d and S i transporters in rice cells showed that, when the S i concentration in the medium was increased, expression of the Si transporter gene Low silicon rice 1 ( L si1 ) was up‐regulated, whereas expression of the gene encoding the transporter involved in the transport of C d, Natural resistance‐associated macrophage protein 5 ( N ramp5 ), was down‐regulated. ICP ‐ MS results revealed that 64% of the total S i in the cell walls was bound to hemicellulose constituents following the fractionation of the cell walls, and consequently inhibited C d uptake. Furthermore, AFM in KPFM demonstrated that the heterogeneity of the wall surface potential was higher in cells cultured in the presence of Si than in those cultured in its absence, and was homogenized after the addition of C d. These results suggest that a hemicellulose‐bound form of S i with net negative charges is responsible for inhibition of C d uptake in rice cells by a mechanism of [ S i‐hemicellulose matrix] C d complexation and subsequent co‐deposition.