Fe homeostasis in plant cells: Does nicotianamine play multiple roles in the regulation of cytoplasmic Fe concentration?

The cellular and intracellular localization of the non-proteogenic amino ac id nicotianamine (NA) in leaves and root elongation zones was immunochemica lly investigated in pea (Pisum sativum L.) and tomato (Lycopersicon esculen tum Mill.) plants grown under various iron regimes and in three mutants def ective in the regulation of iron uptake. Strongest immunostaining was obser ved in the over-accumulating pea mutants brz and dgl, and in iron-loaded wi ld-type plants. Fe concentration and NA level paralleled staining intensity , indicating that NA synthesis is induced by high iron availability. While label was mainly present in the cytoplasm under normal (10 muM) Fe supply a nd under Fe deprivation, most of the labeling was present in the vacuole in iron-loaded plants. This pattern resembled the distribution of NA in Fe ov er-accumulating mutants, indicating the possible importance of vacuolar seq uestration in the detoxification of excess Fe. Based on the dependence of t he cellular distribution of NA on the iron nutritional status of the plant, a possible role of NA in buffering free Fe in root and leaf cells was infe rred. We show here for the first time that the NA concentration is increase d in response to iron overload, indicating that, besides other classes of i ntracellular metal-binding ligands, NA may play an essential role in iron t olerance.