Overexpression of a Defensin-Like Gene Enhances Cadmium Accumulation in Plants.

Accumulation and detoxification of cadmium in rice shoots are of great importance for adaptation to grow in cadmium contaminated soils and for limiting the transport of Cd to grains. However, the molecular mechanisms behind the processes involved in this regulation remain largely unknown. Defensin proteins play important roles in heavy metal tolerance and accumulation in plants. In rice, the cell wall-localized defensin protein (CAL1) is involved in Cd efflux and partitioning to the shoots. In the present study, we functionally characterized the CAL2 defensin protein and determined its contribution to Cd accumulation. CAL2 shared 66% similarity with CAL1, and its mRNA accumulation is mainly observed in roots and is unaffected by Cd stress, but its transcription level was lower than that of based on the relative expression of / observed in this study and that reported previously. A promoter- assay revealed that is expressed in root tips. Stable expression of the fusion protein indicated that is also localized in the cell walls. An Cd binding experiment revealed that CAL2 has Cd chelation activity. Overexpression of increased Cd accumulation in and rice shoots, but it had no effect on the accumulation of other essential elements. Heterologous expression of enhanced Cd sensitivity in , whereas overexpression of had no effect on Cd tolerance in rice. These findings indicate that CAL2 positively regulates Cd accumulation in ectopic overexpression lines of and rice. We have identified a new gene regulating Cd accumulation in rice grain, which would provide a new genetic resource for molecular breeding.