Mechanism for the decrease of Cd uptake and transport within wheat plants in the presence of dissolved Fe(II)

Dissolved Fe(II) influences Cd uptake and transport within wheat plants, but the specific interaction process remains elusive. This study used synchrotron radiation technology, Fourier transform infrared spectroscopy, high-performance liquid chromatography, ion chromatography, and RT-PCR to analyze the interaction processes of Fe(II) and Cd(II) ions in the roots of wheat grown in hydroponic systems containing Cd(II). The results indicated that dissolved Fe(II) decreases Cd uptake due to rhizosphere passivation, cell wall binding or blocking, and competitive absorption and regulation. Poorly crystalline Fe(III) oxides were newly formed, which facilitated the immobilization of Cd(II) on the rhizoplane (over 78%) in a hydroponic system with 5 μmol L^-1 Cd(II). Binding of Cd(II) by cell wall depended on pectin and phosphate contents in the presence of Fe(II). Higher accumulation of lignin in the root endodermis blocked Cd(II) transport at 1000 μmol L^-1 Fe(II). The Fe(II) level influenced Cd uptake by regulating the content of rhizosphere Cd(II) ions and the expression of Cd(II) uptake genes in a hydroponic system with 5 μmol L^-1 Cd(II). The content of CaCl₂-extractable Cd(II) was decreased by 45.4% and the expression of Cd(II) uptake genes was significantly down-regulated with Fe(II) concentration increasing from 15 to 1000 μmol L^-1. In the high-concentration Cd(II) (15 μmol L^-1) system, the inhibitory effect of dissolved Fe(II) on Cd uptake decreased due to increases in rhizosphere Cd content and expression of Cd(II) uptake genes. This work suggests that dissolved Fe(II) can reduce Cd uptake of wheat plants and provides fundamental data for food security.