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The Regulation of Iron Uptake and Possible Functions of Nicotianamine in Higher Plants

1988; Elsevier BV; Volume: 183; Issue: 4 Linguagem: Inglês

10.1016/s0015-3796(88)80015-5

0015-3796

Günter Scholz, Roswitha Becker, Udo W. Stephan, Armin Rudolph, Axel Pich,

Cassava research and cyanide

At present two different strategies of iron uptake under conditions of iron shortage are known for higher plants. In most cases, with the exception of grasses, an increased release of protons and reductants by root tips is accompanied by an increase of the reduction potential at the surface of root cells, leading to a higher availability of iron present in the rhizosphere. Anatomically, the most striking response is the formation of numerous transfer cells within the root epidermis which very likely are the sites of increased metabolic activity (Strategy I). Roots of graminaceous plants respond to iron limitation in the environment by the release of phytosiderophores of the mugineic acid type, potent chelators of ferric iron. Ferri-phytosiderophores are re-absorbed by the roots, thus improving the iron nutrition of the plant (Strategy II). Both strategies imply the presence of a trigger mechanism, perhaps an iron-sensitive sensor, that responds to the iron status of the cell and delivers a signal for switching on and off specific inductive iron uptake mechanisms. Nicotianamine, a compound structurally related to mugineic acid, is of general occurrence among plants. Its biological activity is very likely linked with its ability to form stable complexes with iron (II) and other divalent heavy metal ions. Experiments with the nicotianamine-auxotroph tomato mutant chloronerva revealed its regulatory function in the uptake of Fe, Cu, Mn, and Zn. It is assumed that nicotianamine could act within both strategies as an iron (II) carrier between the site(s) of iron (III) reduction and the ‘sensor’ which perhaps is located in the mitochondria. The regulation of other divalent cations can be explained by the antagonism between iron and heavy metals.