Evidence for direct utilization of a siderophore, ferrioxamine B, in axenically grown cucumber

ABSTRACT To eliminate the confounding effects of microorganisms and to examine the direct utilization of microbial siderophores as iron sources by higher plants, a hydroponic cultural system and methodology was developed to grow plants with axenic roots. This report presents a description of this system, and also its use to determine the efficacy of the microbial siderophore ferrioxamine B (FOB), compared to the synthetic iron chelate FeEDTA, and the phytosiderophores (PS) of barley as an iron source for alleviating iron stress in the model dicot cucumber. It was observed that FOB gave superior plant biomass and was preferentially utilized to restore chlorophyll synthesis in long‐term experiments when chelates were supplied at 5mmol m−3 concentrations and nutrient solution was buffered against pH change at 7.4 with solid phase CaCO3. In addition, autoradiograms indicated that 59Fe from FOB was rapidly translocated to shoots through vascular tissues and was specifically distributed to regions of rapid growth and to iron‐stressed, but still expanding young leaves. The siderophore itself could be detected within 2h in xylem exudates, regardless of whether or not plants were exposed to metabolic inhibitors. It was concluded that the FOB and iron were taken up by the axenic roots of cucumber in a highly efficient manner, most likely as the iron‐siderophore complex, and at rates that could be significant to dicot nutrition. The results also suggested that cucumber may transport FOB through the transpiration stream to upper parts of plants, where the iron would be reductively released from the siderophore for shoot nutrition.