Iron deficiency induces sulfate uptake and modulates redistribution of reduced sulfur pool in barley plants

We studied the possibility that the sulfur (S) assimilatory pathway might be modulated by iron (Fe) starvation in barley, as a consequence of plant requirement for an adequate amount of reduced S to maintain methionine and, in turn, phytosiderophore biosynthesis. Barley seedlings were grown with or without 100 mu M Fe-III-EDTA, at three S levels in the nutrient solution (S-2 = 1200, S-1 = 60, and S-0 = 0 mu M sulfate) in order to reproduce conditions of optimal supply, latent and severe defi ciency, respectively. Fe deprivation increased root cysteine content irrespective of the S supply. However, this increase was not associated with either higher rates of (SO42-)-S-35 uptake or increased expression of the gene for the high-affinity sulfate transporter, HvST1, and these roots failed to increase their activities of ATP sulfurylase (ATPS) and O-acetylserine(thiol) lyase (OASTL). We observed a significant increase in (SO42-)-S-35 uptake rate (+ 76%) only in Fe-deficient S-1 plants and we found an increase in root ATPS activity only in S-0 plants. We observed an increase of ATPS enzyme activity in leaves of S-1 and S-2 plants, most likely suggesting increased S assimilation followed by translocation of thiols (Cys) to the root. Taken together, our results suggest that Fe deficiency affects the partitioning from the shoot to the root of the reduced S pool within the plant and can affect (SO42-)-S-35 uptake under limited S supply.