Improving the metal composition of plants for reduced Cd and increased Zn content: molecular mechanisms and genetic regulations

Cereal crops account for a large proportion of human caloric demand for billions of people worldwide. However, harmful heavy metal accumulation in grain, i.e., cadmium (Cd), creates a widespread problem in threatening human health directly or indirectly through the food chain. In contrast to Cd, zinc (Zn)—an essential element chemically similar to Cd—is usually lower in concentration in consumed cereals than the amount needed to meet dietary needs, leading to malnutrition in many developing countries. As genetics and functional genomics data become increasingly available, a large body of studies has been conducted for Cd and Zn separately in different cereal crops, making it possible to perform a comparative analysis to reveal the shared or unique metal accumulation and transport mechanisms from genetic and genomics perspectives. Here, we reviewed recent efforts in dissecting the molecular mechanisms and genetic regulations regarding metal accumulation in cereal crops, mainly focusing on the two chemically similar metals—the toxic heavy metal Cd and the essential micronutrient Zn. Based on the literature study, we highlight a potential metal accumulation pathway that can be used as a benchmark to ascertain the functional roles of transporters across species to decrease Cd accumulation, increase Zn concentrations, and provide our view into the metal composition improvement in cereal crops.