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. 2013 Sep 13;4:359. doi: 10.3389/fpls.2013.00359

FIGURE 2.

FIGURE 2

A summary of possible transport pathways for iron from the xylem to the infected cell (A) and a more detailed depiction of the iron species and transporters involved (B). (A) The majority of iron entering the nodule is derived from iron citrate, translocated within the xylem. However, the presence of a ferric chelate reductase and a ZIP family, transporter related to A. thaliana IRT1, on the surface of the nodule, suggests a mechanism for direct iron uptake from the soil. The nodule is divided into three main sections the outer cortex (OC), inner cortex (IC), and infected region (IR). The xylem and infected cells are separated by a number of cell layers. Movement of iron to the infected cell could be either via an apoplastic (1) or symplastic (2) route. (B) Apoplastic transport involves the unloading of ferric citrate from the xylem into the apoplast. Ferric-citrate diffuses through the apoplast toward the infected cell, stabilized by the low pH. Uptake by the infected cells involve the reduction of ferric iron to ferrous iron. The resulting ferrous iron could then be imported into the infected cell by a member of the NRAMP or ZIP transporter family. Alternatively ferrous iron could chelate to nicotianamine (NA) and be imported into the infected cell via a member of the YSL transporter family. A symplastic path to the infected cell would require the initial reduction of ferric citrate by a ferric chelate reductase, localized on cells surrounding the xylem, possibly a pericycle cell (PC). Ferrous iron could then be imported into the cell via similar transport mechanisms utilized for uptake into the infected cell, highlighted above. Iron could move symplastically through the pericycle cells, inner cortex cells (ICS) and uninfected cells (UC) as a ferrous iron-NA chelate. Uninfected cells neighboring the infected cells possibly efflux ferrous iron-Na or dissociated ferrous iron. Now in the apoplast the ferrous iron could be oxidized and chelated to citrate to increase solubility. However, for import into the infected cell ferric iron must be reduced to ferrous iron.