Effects of Zinc Fertilization on Grain Cadmium Accumulation, Gene Expression, and Essential Mineral Partitioning in Rice

Cadmium (Cd) is a toxic heavy metal that can cause severe health issues if ingested. Certain varieties of rice can accumulate high levels of the metal in edible tissues thereby transferring the toxin into the food chain. As chemical analogs, interactions between the essential mineral zinc and the toxic heavy metal cadmium play an important role in regulating the transport of both minerals to rice grains. Understanding these interactions is crucial for limiting cadmium and increasing zinc transfer to the food chain. Previous studies have reported conflicting results suggesting synergistic and antagonistic relationships between the minerals. The goal of this work was to identify the effect of external cadmium and zinc on the uptake and translocation of both minerals from roots to grains of rice that differ in grain cadmium concentrations. The results showed that a higher input of external zinc increased cadmium translocation and accumulation to the grain in two of three varieties, while external cadmium does not influence zinc accumulation. Cadmium synergy and antagonism with other essential minerals were also examined and the effects differed between rice lines. Our results showed that the differential expression of the transport proteins OsNramp5, OsHMA2, and OsHMA3 as well as genes involved in the synthesis of glutathione and phytochelatin could have contributed to differences in grain Cd accumulation. These results add to the knowledge of cadmium and zinc partitioning in one of the most consumed plant foods in the world and can assist fortification efforts to establish rice lines that are both safe and nutritious.