The biogeochemistry of phytosiderophores in rhizosphere soil

Phytosiderophores (PS) are extremely efficient Fe-mobilising root exudate compounds which are released by grass species only (Strategy II). Surprisingly, detailed information on their concentration, activity and fate in the rhizosphere is still largely incomplete, since the vast majority of PS studies were conducted in hydroponic conditions. Studies under in situ conditions have previously been hampered by the challenges involved in measuring phytosiderophores in the rhizosphere. Recent advances in analytical techniques have made it possible to measure trace concentrations of phytosiderophores in the rhizosphere and to observe the isotopic composition of iron in plants as a tracer for uptake mechanisms and possibly iron status. Using these techniques in combination with advanced spacial sampling techniques will allow understanding the rhizosphere geochemistry involved in iron acquisition. In particular, we will be able to elucidate and quantify the processes that may limit iron uptake under in situ conditions including, e.g., PS exudation, adsorption, degradation as well as the rates of iron mobilization and the mobilization of competing trace metals. The parameterization of these processes in a numerical geochemical reactive-transport model will allow us to understand which of these processes are limiting iron uptake. Clearly, a detailed understanding of such limits is necessary in order to remediate iron limitation of agricultural production or to enhance plant iron uptake in order to promote human iron nutrition. The research work will proceed in the following phases: (i) determine actual concentrations of main PS compounds in the rhizosphere, (ii) elucidate the fate of PS in rhizosphere soil (adsorption, degradation, mobilisation of Fe and other elements), (iii) develop new analytical techniques for the measurement of free and complexed PS species, (iv) assess the shift from Strategy II to Strategy I-like mobilisation mechanisms under changing redox conditions, and (v) model the mobilisation and uptake of Fe. This project will deliver new insights into the efficiency Fe mobilisation in rhizosphere soil and quantitatively elucidate the key thermodynamic and kinetic factors limiting strategy II iron acquisition under in-situ conditions. We believe that the results of these studies will constitute an important advance in the field of plant iron nutrition serving to remediate iron limitation in a rational manner.

Phytosiderophores (PS) are very specific root exudates, which are only released by members of the grass family. In soil solution this compounds form stable complexes with iron and thus contribute to the iron nutrition of plants. In addition to iron, also other micronutrients, such as copper or zinc, are mobilized. Almost all previous research work on PS has been carried out in nutrient solution culture. Consequently, hardly anything was known about exudation rates and soil solution concentrations. Thus, the aim of the project was to determine the PS release rates of soil-grown plants and to measure the PS concentrations around roots. To achieve this aim, it was necessary to develop specific methods in analytical chemistry, but also to synthesize PS compounds, which were partly labelled with a stable C isotope. Based on the experimental results, the mobilisation efficiency of PS for iron and other micronutrients was validated using mathematical modelling.Using a novel tool for collecting exudates of soil-grown we found out that the PS exudation rates of soil-grown plants were about 50 times lower than previously determined. This implies that the carbon and energy investment in Fe mobilisation is much less than previously assumed. Moreover, part of the released PS is degraded by microorganisms around roots. Here, we observed a faster degradation of PS by microorganisms living very close to roots compared to those occurring at higher distance.Extraction experiments and mathematical modelling revealed that the iron solubilisation efficiency is reduced by competing ions, e.g. copper. Also, we could show that at higher PS concentrations in soil solutions other elements than iron are increasingly mobilized.All experimental approaches to determine PS release rates as well as PS concentrations in soil solution would have been unsuccessful without the concurrent development of methods to measure PS in different sample matrices. In this context, a sensitive, accurate and robust analytical mass spectrometric method for investigation of metal complexes in soil and plant related samples in rhizosphere research was developed. Moreover, for the first time 2-deoxymugineic acid was quantified in in-situ samples from controlled plants. Generally, the analytical-chemical developments in the project allow the performance of experiments, which are significantly contributing to the better understanding of the iron metabolism of plants acquiring iron in the form of a complex.