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

Abstract

Summary Silicon (Si) alleviates cadmium (Cd) toxicity in rice (Oryza sativa). However, the chemical mechanisms at the single‐cell level are poorly understood. Here, a suspension of rice cells exposed to Cd and/or Si 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 Si‐accumulating cells had a significantly reduced net Cd2+ influx, compared with that in Si‐limited cells. PCR analyses of the expression levels of Cd and Si transporters in rice cells showed that, when the Si concentration in the medium was increased, expression of the Si transporter gene Low silicon rice 1 (Lsi1) was up‐regulated, whereas expression of the gene encoding the transporter involved in the transport of Cd, Natural resistance‐associated macrophage protein 5 (Nramp5), was down‐regulated. ICP‐MS results revealed that 64% of the total Si in the cell walls was bound to hemicellulose constituents following the fractionation of the cell walls, and consequently inhibited Cd 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 Cd. These results suggest that a hemicellulose‐bound form of Si with net negative charges is responsible for inhibition of Cd uptake in rice cells by a mechanism of [Si‐hemicellulose matrix]Cd complexation and subsequent co‐deposition.

Silicon ameliorates manganese toxicity by regulating both physiological processes and expression of genes associated with photosynthesis in rice (Oryza sativa L.)

“…Nwugo and Huerta (2011) identified 60 protein spots that were differentially regulated due to Cd and/or Si treatments. The major studies that have been conducted so far focus on the Si-regulated expression level of genes responsible for metal transporters under heavy-metal stress (Kim et al 2014;Bokor et al 2014;Ma et al 2015). Further studies to determine the effect of Si-induced Mn tolerance in rice, the sequential influence of Si on gene expression, and the possible Si-gene interactions are currently being explored.…”

Mechanisms of silicon-mediated alleviation of heavy metal toxicity in plants: A review

“…For example, da Cunha and do Nascimento (2009) showed simultaneous presence of silica, Cd, and Zn deposits in root mesophyll cells of maize plants and suggested Si-metal complexes in plants. Liu et al (2013a) and Ma et al, (2015) reported the co-deposition of Si and Cd in the root cell walls via a [Si-wall matrix]Cd complex formation might reduce the Cd uptake by the plants suggesting in vivo detoxification of Cd in rice. Shi et al (2005b) reported that in rice roots Si and Cd was mainly deposited in endodermis and reduced the cell wall porosity of inner root tissues and they hypothesized the co-precipitation of Si with cadmium.…”

“…Nwugo and Huerta (2010) investigated the effect of Si on the leaf proteome of rice plants under Cd stress and identified 60 protein spots that were differentially regulated due to Cd and/or Si treatments. More recently, Ma et al (2015) reported that in rice plants expression of Lsi1 gene was up regulated in the presence of Si in the medium, whereas expression of natural resistance-associated macrophage protein 5 (Nramp5) gene, involved in the transport of Cd, was down-regulated. In addition, Song et al (2014) reported that Si application enhanced the expression of genes (Os08g02630 (PsbY), Os05g48630 (PsaH), Os07g37030 (PetC), Os03g57120 (PetH), Os09g26810 and Os04g38410) associated with photosynthesis in rice under Zn toxicity.…”

Silicon-enhanced oxalate exudation contributes to alleviation of cadmium toxicity in wheat

“…Taken together, previous studies showed that Si mainly decreased root Cd uptake and/or restricted root-to-shoot Cd translocation. Ma et al (2015) have already demonstrated, by using suspension cells of rice, that negatively charged Si-hemicellulose is formed that is able to bind Cd cations which ultimately prevents the cellular uptake of Cd. For protoplasts, Greger et al (2016) …”