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).