Phloem loading and transport characteristics of iron in interaction with plant‐endogenous ligands in castor bean seedlings

The concentration of iron within cells has to be precisely regulated because shortage as well as surplus may be precarious for the survival of the cell. The maintenance of iron homeostasis in shoot organs requires an efficient signalling of the leaf cells’ iron status to the uptake sites of the roots. This ‘iron signal’ may be transferred by the phloem. The handling of iron in the symplast and during phloem transport calls for mechanisms taking into account the specific physicochemical properties of this element.Seedlings of Ricinus communis were used as model plants to investigate characteristic features of phloem loading, and of speciation and valence of iron during transport in the sieve tubes. When the storage endosperm is removed from the cotyledons, phloem is loaded from the reserve pool of the mesophyll cells. In this situation, iron and the other micronutrients copper, manganese and zinc are loaded in a constant stoichiometric ratio of 1:1 to the endogenous complexor nicotianamine (NA). Application of the chelators 1,4-di(4-phenylsulphonate)-1,10-phenanthroline (BPDS) and ethylenediaminetetraacetic acid to the cotyledon apoplast did not decrease the loading rate of iron, indicating symplastic loading. Supply of ferrous ions in various concentrations to the apoplast revealed the existence of two loading systems. One of them is linearly dependent upon the concentration, and remained unsaturated up to an apoplastic concentration of 200 μM. The other one, whose activity steeply inclines already with a slight increase of the apoplastic concentration, is saturable at a supply of 100 μM. The loading of iron is slowed down with time depending on whether iron is supplied to the apoplast as a complex with NA or citrate instead of as free ferrous ions. This effect may be caused by competition of these chelators with an iron uptake receptor in the plasmalemma of the mesophyll cells. In spite of the close relationship between iron and NA during phloem loading, the Fe-NA complex seems not to be the predominant transport species in the sieve tubes. A molecule of much larger mass than NA probably serves as a transport vehicle, as concluded from microdialysis experiments. Only 4% of the total iron in the sieve tube exudate was found to exist as Fe(II) and about 45% as Fe(III). The residue of more than 50% was tightly bound and not accessible even in the presence of the reductant sodium dithionite and the chelator BPDS. The conclusions regarding the nature of the transport species and the results on the valence of iron in the sieve tubes were confirmed by calculations with the software programs GEOCHEM [Sposito G, Mattigod SV (1979) A computer program for calculating chemical equilibria in soil solutions and other natural water systems. Kearney Foundation of Soil Sci, Univ. of California, Riverside, CA] and PHREEQC [Parkhurst (1995) PHREEQC-A computer program for speciation, reaction-path, adjective-transport, and inverse geochemical calculations. US Geological Survey]. A model is outlined on the basis of the experimental findings on the fate of iron from mobilisation in the endosperm to trans-chelation in the sieve tubes.