The synthetically produced fluorescent siderophore NBD-desferrioxamine B (NBD-DFO), an analog of the natural siderophore ferrioxamine B, was used to study iron uptake by plants. 5"Fe uptake rates by cotton (Gossypium spp.) and maize (Zea mays L.) plants from the modified siderophore were similar to those of the natural one. In longer-term uptake experiments (3 weeks), both siderophore treatments resulted in similar leaf chlorophyll concentration and dry matter yield. These results suggest that the synthetic derivative acts similarly to the natural siderophore. The NBD-DFO is fluorescent only when unferrated and can thus be used as a probe to follow iron removal from the siderophore. Monitoring of the fluorescence increase in a nutrient solution containing Fe3"-NBD-DFO showed that iron uptake by plants occurs at the cell membrane. The rate of iron uptake was significantly lower in both plant species in the presence of antibiotic agent, thus providing evidence for iron uptake by rhizosphere microbes that otherwise could have been attributed to plant uptake. Confocal fluorescence microscopy revealed that iron was taken up from the complex by cotton plants, and to a much lesser extent by maize plants. The active cotton root sites were located at the main and lateral root tips. Significant variations in the location and the intensity of the uptake were noticed under nonaxenic conditions, which suggested that rhizosphere microorganisms play an important role in NBD-DFO-mediated iron uptake.binding capacity, has raised the question of whether these siderophores may be used by plants as an Fe source (14).A number of researchers investigated the possibility of plant utilization of various microbial siderophores including ferrichrome A (19), agrobactin (3), pseudobactin (1, 2, 12, 13), and ferrioxamine B (9, 10). A variety of plant species were shown to acquire Fe from ferrioxamine B, such as tomato (23), oat (9, 20, 21), sunflower and sorghum (8), and peanut and cotton (1).The mechanism by which plants utilize microbial siderophores has not yet been elaborated to a satisfying extent and it is a topic of controversy. Although some investigators proposed a specific Fe uptake mechanism (9, 11), others did not support this hypothesis (1, 5,22).In this study, Fe uptake from ferrioxamine B by maize (Zea mays L.) and cotton (Gossypium spp.) plants was investigated using the synthetic fluorescent ferrioxamine B analog NBD-DFO2 as a probe. NBD-DFO has been designed to be nonfluorescent when saturated with Fe3+, but to gain fluorescence upon Fe3+ release. NBD-DFO, therefore, allows one to monitor the location and the dynamics of cellular Fe uptake. This research focused on the uptake mechanism in maize and cotton (monocot and dicot, respectively) and on the possibility of microbial involvement in the Fe uptake process.
MATERIALS AND METHODS
Studies
