New Aspects of the Interaction between Polysaccharides and Metal Ions in Relation to the Mineral Nutrition of Plant Roots

Recent discoveries emphasize the newly emerging concept that an extracellular apparatus, rich in polyuronates, allows plant roots to remove nutrient cations from clay particles [1, 2]. The study of metal binding to polygalacturonic acid (Fig. 1) has drawn attention to the possible role of root polysaccharides in the mineral nutrition of plants. Evidence was found to show that specific interactions, which could have relevant functional and physiological implications, are established by the polysaccharide with metal ions [3-5]. Metal-polyion interactions of electrostatic nature are possible mechanisms accounting for the uptake and buffering of faster-moving nutrient ions, In this case, metals bound to the polysaccharide moiety retain enough mobility to reach the cells and translocate into the plant. On the other hand, innersphere binding to carboxylate groups could be necessary for the uptake of certain micronutrients which, being in low concentration, are less competitive in the exchange process. With regard to the iron uptake, it has been shown that Fe(III) gives rise to polynuclear structures on the surfaces of the polysaccharide, whereas Fe(II) is more weakly retained as hydrated ion [4]. Recent developments have been concerned with redox processes occurring upon interaction of polygalacturonic acid with soil mineral species. Reduction of Fe(III) to Fe(II), V(V) to V(IV) and Mo(VI) to Mo(V), followed by complexation of reduced ions, has been observed. The reaction is due to the reducing properties of the polysaccharide end-units. For example, the following mechanism has been assessed for the reduction of VO - 3 to VO(IV): Such processes appear to be highly significant, particularly in relation to iron uptake by plant roots. In fact, ferric species are not available for plants unless reduction to the divalent state occurs [6]. Based on our results, it is suggested that polysaccharides are active also in redox interactions and provide suitable pathways for the adsorption an transport of iron, which is essential for the survival of plants.