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 becaus e shortage as well as surplus may be precarious for the survival of the cel l. The maintenance of iron homeostasis in shoot organs requires an efficien t signalling of the leaf cells' iron status to the uptake sites of the root s. This 'iron signal' may be transferred by the phloem. The handling of iro n in the symplast and during phloem transport caps for mechanisms taking in to account the specific physicochemical properties of this element. Seedlings of Ricinus communis were used as model plants to investigate char acteristic features of phloem loading, and of speciation and valence of iro n during transport in the sieve tubes. When the storage endosperm is remove d from the cotyledons, phloem is loaded from the reserve pool of the mesoph yll cells. In this situation, iron and the other micronutrients copper, man ganese and zinc are loaded in a constant stoichiometric ratio of 1:1 to the endogenous complexor nicotianamine OVA). Application of the chelators 1,4- di(4-phenylsulphonate)-1,10-phenanthroline (BPDS) and ethylenediaminetetraa cetic acid to the cotyledon apoplast did not decrease the loading rate of i ron, indicating symplastic loading. Supply of ferrous ions in various conce ntrations to the apoplast revealed the existence of two loading systems. On e of them is linearly dependent upon the concentration, and remained unsatu rated up to an apoplastic concentration of 200 mu M. The other one, whose a ctivity steeply inclines already with a slight increase of the apoplastic c oncentration, is saturable at a supply of 100 mu M. 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 effe ct may be caused by competition of these chelators with an iron uptake rece ptor in the plasmalemma of the mesophyll cells. In spite of the close relat ionship 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 conc luded from microdialysis experiments. Only 4% of the total iron in the siev e tube exudate was found to exist as Fe(IF) and about 45% as Fe(III). The r esidue of more than 50% was tightly bound and not accessible even in the pr esence of the reductant sodium dithionite and the chelator BPDS. The conclu sions 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 wa ter systems. Kearney Foundation of Soil Sci, Univ. of California, Riverside , CAI and PHREEQC [Parkhurst (1995) PHREEQC-A computer program for speciati on, reaction-path, adjective-transport and inverse geochemical calculations . US Geological Survey]. A model is outlined on the basis of the experiment al findings on the fate of iron from mobilisation in the endosperm to trans -chelation in the sieve tubes.