Intra- and intercellular iron trafficking and subcellular compartmentation within roots

Abstract

Iron is abundant in most soils, but ferric compounds are almost insoluble. Therefore, plant roots use as tools acidification and enzymatic reduction of iron at the outer cell surface (strategy I) or solubilization by phytosiderophores, which are specific ferric chelators (strategy II). In the first case, iron is taken up as Fe²⁺ into the root symplast, and in the latter one, iron is taken up as Fe(III) complex. The path of iron from the root surface, up to the point of the xylem vessels within the central cylinder, may be completely symplasmic. However, a part of this route also may be an apoplasmic one, through the free space of the cell walls of rhizodermis and cortex (apoplast). In the endodermis, the Casparian band forms a strict barrier for apoplasmic transport; to move past this site, all ions must enter the symplast. During symplasmic transport, the intracellular environment is protected against the reactive species of iron by handling of iron in chelated forms. A promising candidate for this purpose is the plant-endogenous chelator nicotianamine. At the apoplasmic site, iron can be oxidized followed by precipitation as hydroxide or phosphate compounds. Thus, a pool of apoplastic iron can be formed, as shown by reductive mobilization or by proton-induced X-ray emission. This pool may be remobilized when iron deficiency takes place. During radial transport to the vessels, vacuoles may compete with the transport stream forming an iron store. When there is an iron excess, as in plants growing in waterlogged soils or by experimental techniques, plants can escape the deleterious effects of free iron by depositing it in phytoferritin, a storage protein inducible under iron excess. Also, nicotianamine forms a pool of metabolically available iron. Thus, in roots cells of the nicotianamine-free tomato mutant chloronerva iron precipitations occur as evidenced by energy dispersive X-ray analysis and the electron microscopic energy loss technique of energy spectroscopic imaging. Future research concerning the plant root's iron metabolism are needed to clarify the function of nicotianamine in intra- and intercellular iron trafficking and to identify the so-called iron-sensor which mediates the regulation of iron acquisition reactions of rhizodermal cells in response to the iron nutritional status of the plant.