FIGURE 3.

Iron uptake into the infected cell and transport into the symbiosome. Ferrous iron transport into the infected cells, across the plasma membrane (PM) could me mediated by transporters belonging to the NRAMP, ZIP, or YSL family. A citrate effluxer, LjMATE1 is expressed in infected cells, it is predicted to release citrate (blue arc) into the apoplast surrounding infected cells to aid in iron uptake. It is likely to make iron in the apoplast more soluble. Both ferrous and ferric iron are transported across the symbiosome membrane (SM) and bacteroid membrane (BM). However, ferrous iron is taken up more rapidly. A ferric iron transporter, GmDMT1, is present on the SM, but it may not be the only ferrous iron transporter on the SM. Members of the VIT and ZIP transporter families are possible ferrous iron transporters on the SM. Although ferric iron is thought to cross the SM, no plant transporters are known to transport iron-citrate. Ferric chelate reductase activity (FCR) is present on the SM, however, the orientation on the SM has not been confirmed. Within the symbiosome space (SS) ferric iron could chelate siderophores produced by the bacteroid. Ferrous and ferric iron are transported across the bacteroid membrane (BM), but the transporters responsible have not been identified.