which is preferentially absorbed by roots (Römheld, 1987). Apparently, microbial siderophores
are also capable of solubilizing ferric compounds in the rhizosphere (Jurkevitch et al., 1986), although they are the least preferred in uptake. Iron is toxic when in excess inside cells. However, unlike in microbes, its regulation in plants still remains unknown. Microbial uptake of Fe is regulated by a chromosomal repressor, which shuts down expression of all components of the siderophore biosynthetic pathway, thus restricting further uptake of Fe into the cell (Neilands, 1987). Given that mainly grasses and cereal crops exude phytosiderophores, different plant species must have different ways of regulating internal Fe concentration than that observed in microbes.