Mechanisms of cadmium accumulation in rice grains and molecular breeding for its reduction

Cadmium (Cd) has an important impact on agriculture, as the excessive consumption of this element from contaminated food crops leads to toxicity in humans. Rice is the greatest source of dietary Cd intake in populations consuming rice as a staple food. Therefore, reducing the Cd concentration of rice grains for diminishing the potential risk of Cd for human health is a major challenge. This review summarizes the main achievements on Cd accumulation in rice by our research group. Using a positron-emitting tracer imaging system, we succeeded to visualize differences in real-time Cd dynamics from roots to panicles in rice varieties with different Cd accumulation abilities. Several loci or genes responsible for Cd accumulation in rice were found through quantitative trait loci (QTLs) analysis. Using the QTL qCdp7 responsible for efficiently extracting Cd from paddy soils, practical rice varieties for Cd-phytoextraction were developed by DNA marker-assisted breeding. A rice variety named 'Koshihikari Kan No.1MODIFIER LETTER PRIME carrying a mutant allele of OsNramp5 for reducing Cd concentration in rice grains was produced by mutant breeding with ion-beams, and breeding programs have been implemented to transfer this allele into many Japanese rice varieties by DNA marker-assisted breeding. Growing low-Cd varieties aerobically would be a feasible way to simultaneously reduce inorganic arsenic and Cd concentrations in rice grains. These results provide basis for reducing Cd concentration in rice through breeding new varieties.