Abstract: Cadmium (Cd) is a highly toxic heavy metal that threatens human health. Rice is one of food crops that can accumulate Cd in the grain to levels that are unsafe for human consumption. With increasing contamination of heavy metals in paddy soils in China, considerable proportions of rice grain produced in some areas of southern China exceed the 0.2 mg·kg^-1 Cd limit of the Chinese food standard, which causes widespread public concern. Molecular breeding of rice varieties that accumulate Cd in straw for removing Cd from paddy soil while producing safe grain is one of the strategies for phytoremediation of contaminated soils. Recently, Luo et al. identified a quantitative trait locus CAL1 in rice that specifically regulates the accumulation of Cd in leaves. CAL1 encodes a defensin-like protein that can chelate Cd in the cytosol and facilitates Cd secretion from xylem parenchyma cells into xylem vessels for long-distance transport. The chelation of Cd to CAL1 appears to prevent Cd from being loaded into the phloem for transport to rice grain. Thus, CAL1 does not affect the accumulation of Cd in rice grain. These findings shed light on understanding the molecular mechanism of Cd translocation and allocation in rice and provide a molecular tool to breed rice varieties that may be used to remove Cd from the soil without affecting grain Cd concentration.

Key words: rice, cadmium, phytoremediation, defensin protein