3.3 ǀ Membrane selectivity
Ferrous iron breaching the oxidation zone in the rhizosphere enters the root apolplast by diffusion and mass flow in the transpiration stream. To reach the xylem for transport to the shoot, Fe2+ions in the apoplast must bypass the Casparian strip in the endodermis by crossing cell membranes into the symplasm. This provides a potential exclusion mechanism, at least in undamaged roots. Up to 87% of the Fe entering the root apoplast in mature plants is prevented from reaching the xylem at the endodermal barrier (Yamanouchi & Yoshida, 1981). Exclusion at the plasma membranes is strongly affected by respiration inhibitors, high Fe2+ concentrations, and nutrient stresses (Yoshida, 1981). Therefore, it is unlikely to be important under sustained severe toxicity (Becker & Asch, 2005) so other mechanisms of root retention must operate.
Rice is unusual in graminaceous species in possessing Fe2+ transporters, OsIRT1 and OsIRT2, in addition to the usual genes for the synthesis and secretion of Fe(III)-chelating phytosiderphores (Bughio, Yamaguchi & Nishizawa, 2002; Ishimaru et al., 2006; Quinet et al., 2012). Transport of Fe2+ in the xylem involves complexation with nicotianamine (NA) and mugineic acid (MA), and rice possesses three NA synthase genes (OsNAS1, OsNAS2 and OsNAS3) (Inoue et al., 2003), six NA amino-transferase genes (OsNAAT1-6) (Inoue et al., 2008) and one deoxymugineic acid synthase gene (OsDMAS1) (Bashir, Ishimaru & Nishizawa, 2012). The results of microarray analyses have suggested that expression of genes involved in Fe2+ uptake (OsIRT1 and OsIRT2) and xylem transport are suppressed under different levels of Fe excess (Aung, Masuda, Kobayashi & Nishizawa, 2018a; Finatto et al., 2015; Quintet et al., 2012). The Fe-binding ubiquitin ligase HRZ is involved in the regulation of this process (Aung et al., 2018a).