IRON UPTAKE BY SYMBIOSOMES FROM SOYBEAN ROOT-NODULES

To identify possible iron sources for bacteroids in planta, soybean (G lycine max L. Merr.) symbiosomes (consisting of the bacteroid-containi ng peribacteroid space enclosed by the peribacteroid membrane [PBM]) a nd bacteroids were assayed for the ability to transport iron supplied as various ferric [Fe(III)]-chelates. Iron presented as a number of Fe (III)-chelates was transported at much higher rates across the PBM tha n across the bacteroid membranes, suggesting the presence of an iron s torage pool in the peribacteroid space. Pulse-chase experiments confir med the presence of such an iron storage pool. Because the PBM is deri ved from the plant plasma membrane, we reasoned that it may possess a ferric-chelate reductase activity similar to that present in plant pla sma membrane. We detected ferric-chelate reductase activity associated with the PBM and suggest that reduction of Fe(III) to ferrous [Fe(II) ] plays a role in the movement of iron into soybean symbiosomes.