Cadmium (Cd) contamination in paddy soils have become a serious threat to food safety around the globe as a result of industrial emissions, mining, wastewater irrigation, as well as fertilizers and pesticides applications. Rice (Oryza sativa L.), a staple crop for the world population, is especially prone to pollution of this hazardous metal because of Cd mobility in paddy soils and accumulation in rice grain. In addition, iron (Fe) is a nutrient element required for the proper functions in organisms. Unfortunately, its uptake by rice may be inhibited in the presence of Cd, exacerbating the problem of Fe deficiency and further threatening the human health. Therefore, it is urgent to search for approaches to simultaneously enhance Fe absorption and minimize Cd accumulation by rice.

Sulfur (S) is another essential nutrient in plants and plays critical roles in a number of cellular processes, including redox cycles, detoxification of heavy metals and xenobiotics, and metabolism of secondary products. In recent years, remarkable progress has been made in understanding interactions between S and Fe, particularly the impact of S on the mobility of Fe in plants. On the other hand, it is well known that S supply affects the translocation and toxicity of Cd in rice through the formation of CdS precipitates, or complexation of Cd with thiol compounds such as glutathione (GSH), and phytochelatins (PCs). More importantly, nicotianamine (NA) is involved in the transport of Fe in rice phloem by forming Fe-NA and its biosynthesis is also affected by the S nutrition. However, the connections among S, Cd and Fe in terms of their distribution in rice remain unknown. Here, we hypothesized that S supply could mediate the competition between Cd and Fe for the entry to rice roots and translocation to stems and leaves, thus reducing the Cd accumulation while increasing Fe concentration in these tissues.

To test this hypothesis, we conducted a hydroponic experiment, in which rice seedlings (cv. N07-63) were cultivated to examine the effect of S application on Fe and Cd uptake and translocation in rice under two levels of Cd (0 and 1.0 μmol L^-1) combined with four S levels (0, 1.75, 3.5, 7.0 mmol L^-1). Our data show that rice growth was suppressed by Cd but the toxicity was alleviated with the S addition, which also led to decline of Cd concentrations in rice roots, stems and leaves. Iron was deposited on the surface of rice root as Fe plaque, whose amount decreased with the increase of S supply. The Fe concentrations in all three parts of rice were lowered by the Cd treatments, but the S addition significantly increased the Fe levels in rice roots, stems and leaves. At the same time, S supply promoted the biosynthesis of thiol compounds and NA regardless of Cd treatment, indicating that S was involved in the complexation of Cd/Fe with these molecules and their movement in rice. Taken together, this study demonstrates that sufficient S supply is beneficial to rice by improving Fe nutrition and limiting Cd accumulation. More investigations are needed to reveal the underlying mechanisms for the interactions among the three elements in rice.