Elucidation of heavy-metal tolerance mechanism in (Cav.) Trin. ex Steud. without iron plaque formation

Okuma, Miyu*; Yamaji, Keiko*; Nakamoto, Yukihiro*; Fukuyama, Kenjin*; Tsunashima, Yasumichi

At our study site, mill tailing pond, which is used as a temporary storage site to treat mine water containing iron, manganese, etc., (Cav.) Trin. ex Steud. has been found to grow naturally there; therefore, this plant was considered to have some metal tolerance mechanisms. Actually, was previously reported to enhance heavy metal tolerance through the formation of iron plaques; however, few iron plaques were observed on roots of growing Fe deposition site close to the entrance of mine water at our study site, suggesting that they may have another heavy metal tolerance mechanisms. The objective of this study was to elucidate the heavy metal tolerance mechanisms of , without iron plaque formation. Elemental analysis clarified that high concentrations of Fe, Al, and Zn were accumulated in healthy nodal roots; especially, excessive Fe accumulation was confirmed compared with normal plants. Detoxicant analysis in the roots indicated that phenolics and organic acids were not produced well to contribute to the tolerance. Due to the observation of sections of nodal roots stained with potassium ferrocyanide solution, Fe localization was observed in the epidermis and the cell walls of the outermost layers of the cortex cells. Since immobilization of heavy metals in the cell walls is known as one of the heavy metal tolerance mechanisms in plants, our results suggest that would show the tolerance to immobilize Fe in the cell walls to prevent from migrating into the interior cells.