Background/Question/Methods   Traits that serve more than one function may enhance the competitiveness of species. The replacement of an energy-demanding process by a more adaptive and energy-efficient mechanism would further enhance the overall species fitness. In a previous study, we showed that the invasive, allelopathic plant, Centaurea diffusa, utilizes a phytotoxin, 8-hydroxyquinoline (8HQ), for the acquisition of iron, a nutrient likely to be deficient in most of its alkaline invaded habitats. This novel Fe uptake mechanism is thought to be energy-efficient as the 8HQ-Fe complex (FeQ), due to its lipophilicity, is passively diffusing across the plasma membrane. We utilized synchrotron X-ray fluorescence (XFS) and X-ray absorption spectroscopy (XAS) to further examine the mechanism of this allelochemical mediated Fe uptake in wild and mutant (YS1) corn lines. Since YS1 lines lack the carrier proteins to transport Fe-phytosiderophore complex across the plasma membrane, we predicted that, presence of undissociated FeQ inside the plant tissues will prove conclusively a more energy efficient uptake of Fe.
X-ray absorption measurement utilizes intense and energy-tunable source of x-rays. The fluorescence energies emitted is unique for every atom and can be used to identify and quantify atoms in a system. XAS can be utilized to characterize all elements and crystallinity of samples is not a prerequisite. X-ray absorption near-edge spectroscopy (XANES) can be utilized to understand the oxidation state and coordination chemistry of an element and are used to characterize the metal-ligand complexes.

Results/Conclusions

The YS1 mutant lines of corn were able to utilize FeQ as iron source as evident from tissue Fe content and chlorophyll contents. Due to the hydrophobicity the FeQ complex underwent rapid adsorption to the root surfaces. This could enhance the Fe uptake efficiency by increasing the proximity of Fe to the uptake-enzyme complexes on root surfaces. XAS spectrum reveled the distribution of Fe to be more on the mature root regions than on root tips. XANES indicated Fe to be present in Fe(III) oxidation state inside the root tissue. Our results indicate that following a rapid adsorption reaction on to root surface, FeQ is partly being transported into roots in an undissociated form.