Evidence for reutilization of nodule iron in soybean seed development

Iron (Fe) is required in plants for the function of the important processes of photosynthesis, respiration, DNA synthesis, and nitrogen (N) fixation. Concentrations of Fe show tissue specific changes during development. In soybean seeds, Fe accumulates through the linear phase of seed development, but the source of seed Fe, whether remobilized from other tissues or taken from the root environment, is not known. Root nodules of legumes have higher concentrations of Fe than other vegetative organs. To examine whether nodules could provide Fe to the seeds, two cultivars (Tokyo and Arksoy), differing in seed ferritin and Fe content were grown in a phytotron and given a single dose of Fe-59-EDTA early in development [15 days after inoculation with Bradyrhizobium (DAI)]. The Fe-59 distribution as well as immunoreactive ferritin were examined throughout development in nodule, leaf, and seed tissue. Leaves, nodules, and seeds accounted for 75 to 87% of the total plant Fe-59 throughout the reproductive period with seeds increasing from 0 to 35-46% at maturity. The largest decrease in Fe-59 occurred in nodules. If all Fe-59 lost from nodules were translocated to seeds, then 40-59% of Fe-59 in seeds could have come from nodules for Tokyo or Arksoy, respectively. The remaining seed Fe came from vegetative tissue and from the rhizosphere. Seed Fe-59 in Tokyo was 2.5 times that of Arksoy. In both cultivars, Fe-59, soluble Fe, and ferritin concentrations in seed decreased from 39 DAI until maturity, suggesting that dry weight accumulation in seeds proceeds at a faster rate than Fe accumulation. Nodule ferritin remained constant suggesting a role in concentrating Fe for reutilization as nodules begin to senesce and decline in function.