EVIDENCE FOR DIRECT UTILIZATION OF A SIDEROPHORE, FERRIOXAMINE-B, IN AXENICALLY GROWN CUCUMBER

To eliminate the confounding effects of microorganisms and to examine the direct utilization of microbial siderophores as iron sources by hi gher plants, a hydroponic cultural system and methodology was develope d to grow plants with axenic roots. This report presents a description of this system, and also its use to determine the efficacy of the mic robial siderophore ferrioxamine B (FOB), compared to the synthetic iro n 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 whe n chelates were supplied at 5 mmol m-3 concentrations and nutrient sol ution was buffered against pH change at 7.4 with solid phase CaCO3. In addition, autoradiograms indicated that Fe-59 from FOB was rapidly tr anslocated to shoots through vascular tissues and was specifically dis tributed to regions of rapid growth and to iron-stressed, but still ex panding young leaves. The siderophore itself could be detected within 2 h in xylem exudates, regardless of whether or not plants were expose d 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 cu cumber may transport FOB through the transpiration stream to upper par ts of plants, where the iron would be reductively released from the si derophore for shoot nutrition.