Effect of decreasing levels of Fe availability on S assimilation pathway in durum wheat (Triticum durum L.) seedlings

Iron is an essential element required for many crucial cellular functions. Studies in many different organisms have shown that Fe deficiency induces several responses including increased uptake of Fe from the environment. In plants, it is achieved through the activation of several components of the Strategy I and II Fe uptake system. Wheat belongs to Strategy II plants and therefore cope with Fe deficiency by increasing both the synthesis and secretion of phytosiderophores (PS), and the uptake of Fe-PS complexes. Besides the processes described for Strategy II, sulfate assimilation pathway is known to be induced upon Fe deprivation in various plant species (maize, barley and wheat), most likely because PS are derived from nicotianamine, whose precursor is methionine. Aim of this study was to investigate if sulfate assimilation rate could be modulated in an Fe concentration-dependent manner. To this purpose, durum wheat seedlings were grown hydroponically for eight days with Fe(III)-EDTA concentrations ranging from 0 to 75 µM. The amount of PS released by roots gradually increased with decreasing external Fe concentrations, this result being correlated to the increase in root thiol concentration. This was supported by roots showing higher activity of both ATP sulfurylase and Oacetylserine(thiol)lyase, the first and the last enzyme of S metabolism, respectively. These preliminary results suggest that the capability to induce sulfate assimilation pathway when plants sense the onset of Fe limitation seems to correlate with the general S use efficiency of a plant species as an ecological adaptation to Fe deficiency.