Hydroxylated phytosiderophore species possess an enhanced chelate stability and affinity for iron(III)

Graminaceous plant species acquire soil iron by the release of phytosiderop hores and subsequent uptake of iron(III)-phytosideroyhore complexes. As pla nt species differ in their ability for phytosiderophore hydroxylation prior to release, an electrophoretic method was set up to determine whether hydr oxylation affects the net charge of iron (III)-phytosiderophere complexes, and thus chelate stability. At pH 7.0, non-hydroxylated (deoxymugineic acid ) and hydroxylated (mugineic acid; epi-hydroxymugineic acid) phytosideropho res form single negatively charged iron(III) complexes, in contrast to iron (III)nicotianamine. As the degree of phytosiderophore hydroxylation increas es, the corresponding iron(III) complex was found to be less readily proton ated. Measured pKa values of the amino groups and calculated free iron(III) concentrations in presence of a 10-fold chelator excess were also found to decrease with increasing degree of hydroxylation, confirming that phytosid erophore hydroxylation protects against acid-induced protonation of the iro n(III)-phytosideruphore complex. These effects are almost certainly associa ted with intramolecular hydrogen bonding between the hydroxyl and amino fun ctions. We conclude that introduction of hydroxyl groups into the phytoside rophore skeleton increases iron(III)-chelate stability in acid environments such as those found in the rhizosphere or the root apoplasm and may contri bute to an enhanced iron acquisition.