Cadmium Distribution and Characteristics of Cadmium-binding Proteins in Rice (<i>Oryza sativa L.</i>) Kernel

Abstract

Studies were performed to investigate the Cadmium (Cd) distribution and Cd-binding proteins in rice kernel. Rice seeds [Oryza sativa L. (Akitakaomachi)] were cultured under Cd stress and after harvesting, the Cd distribution was determined using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Cd concentrations and amino acid compositions in different storage protein fractions were analyzed using ICP-MS and an amino acid analyzer, respectively. The molecular weights of the Cd-binding proteins were determined by size exclusion chromatography-inductively coupled plasma mass spectrometry (SEC-ICP-MS). The Cd concentrations in rice kernel gradually decreased from the outer bran layers to the endosperm and Cd was particularly concentrated in the embryo, the outer pericarp and the aleurone layer. The Cd concentrations in different proteins fractions decreased in the following sequence: globulin > albumin > glutelin > prolamin. The presence of cysteine and methionine were closely related to the Cd concentration. The molecular weights of three prominent Cd-binding protein fractions were > 2000 k Da, 570 k Da and 131.9 k Da, respectively.

Changes of Cadmium Storage Forms and Isotope Ratios in Rice During Grain Filling

“…In the grains, almost all Cd was bound to S ( Figure 4b ). This observation coincides with previously published size exclusion ICP-MS data that identified S-bearing proteins as the major storage form of Cd in rice ( Wei et al, 2017 ). In addition, μXANES studies revealed that large fractions of up to 92% of Cd can be bound to S in rice and wheat grains ( Gu et al, 2020 ; Yan et al, 2020 ).…”

“…Little is known about Cd speciation in rice grains. Size exclusion chromatography–inductively coupled plasma mass spectrometry (ICP-MS) techniques ( Wei et al, 2017 ) indicated that the major fraction of Cd was stored with S-rich protein fractions in rice. Recent X-ray absorption spectroscopy (XAS) studies for rice and wheat showed that Cd–S can be an important storage form in grains (27–92%, Gu et al, 2020 ; Yan et al, 2020 ).…”

Cadmium distribution in mature durum wheat grains using dissection, laser ablation-ICP-MS and synchrotron techniques

“…3. In rice grains, Cd is associated with S-containing amino acids including cysteine and methionine (Wei et al 2017). The thiol groups of these amino acids are involved in the disulfide bonds that control protein folding and may be preferential sites of Cd binding and retention thereby explaining the relatively high concentration of Cd in the endosperm.…”

Chemical Speciation and Distribution of Cadmium in Rice Grain and Implications for Bioavailability to Humans

“…Furthermore, these few studies appear to be contradictory, reporting that Cd was uniformly distributed throughout the grain, was present at higher in the outer layers of grain, or was present at higher concentrations in the endosperm. [19][20][21] Generally, Cd concentrations in rice grain, even for plants growing in highly Cd-contaminated soils, are only in the order of a few mg/kg or lower. 22 These low Cd concentrations present substantial analytical challenges for synchrotron-based X-ray absorption spectroscopy (XAS).…”