Abstract
Silicon (Si) alleviates cadmium (Cd) toxicity and accumulation in a number of plant species, but the exact molecular mechanisms responsible for this effect are still poorly understood. Here, we investigated the effect of Si on Cd toxicity and accumulation in rice (Oryza sativa) by using two mutants (lsi1 and lsi2) defective in Si uptake and their wild types (WTs). Root elongation was decreased with increasing external Cd concentrations in both WTs and mutants, but Si did not show an alleviative effect on Cd toxicity in all lines. By contrast, the Cd concentration in both the shoots and roots was decreased by Si in the WTs, but not in the mutants. Furthermore, Si supply resulted in a decreased Cd concentration in the root cell sap and xylem sap in the WTs, but not in the mutants. Pre-treatment with Si also decreased Cd accumulation in the WTs, but not in the mutants. Silicon slightly decreased Cd accumulation in the cell wall of the roots. The expression level of OsNramp5 and OsHMA2 was down-regulated by Si in the WTs, but not in the mutants. These results indicate that the Si-decreased Cd accumulation was caused by down-regulating transporter genes involved in Cd uptake and translocation in rice.
Highlights
Cadmium (Cd) is a heavy metal highly toxic for all organisms in its ionic form
Root elongation was decreased with increasing external Cd concentrations in both wild types (WTs) and mutants, but Si did not show an alleviative effect on Cd toxicity in all lines
We found that Si does not have a direct effect on alleviating Cd toxicity in rice but that Si decreases Cd accumulation by down-regulating transporter genes involved in uptake and root-to-shoot translocation of Cd
Summary
Cadmium (Cd) is a heavy metal highly toxic for all organisms in its ionic form. In plants, Cd toxicity has been associated with leaf chlorosis, growth inhibition and the disruption of key physiological processes such as photosynthesis (Atal et al, 1991; Chugh and Sawhney, 1999; Clemens and Ma, 2016). Consumption of foods containing Cd is potentially health-threatening in humans. Itai-itai disease was caused by the consumption of rice grown in cadmium-polluted soils in Japan in the mid-1950s and mid-1960s (Horiguchi et al, 1994). Limiting the entry of Cd into the food chain from the soil is important for reducing potential health risks to humans. This is especially important for rice as it is a staple food for nearly half of the world’s population and the largest source of dietary intake of Cd (Watanabe et al, 2004; Cheng et al, 2006)
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