Graminaceous YSL15 and exhibits a similar amino acid

Graminaceous plants however do not rely on the uptake
of reduced Fe. The plasma membrane-located TRANSPORTER OF MUGINEIC ACID1 (TOM1)
(Nozoye et al., 2011) releases
phytosiderophores (PS) that efficiently chelate ferrous Fe over a wide pH range,
from pH 4 to pH 9 under Fe deficiency (Takagi, 1976; Takagi et al., 1984; Marschner et al.,
1986). These
non-proteinogenic amino acids, such as mugineic acid (MA) or avenic acid (AA) are
produced from the initial precursor methionine (Marschner et al., 1986; Mori and Nishizawa, 1987). In a first
step, the enzyme S-adenosylmethionine
(SAM) synthethase uses methionine
to catalyze the synthesis of SAM. Three SAM molecules are subsequently
condensed by the enzyme nicotianamine
synthase (NAS) to produce the PS precursor nicotianamine (NA). The metal
chelator NA facilitates intracellular and also short- and long-distance metal
transport (Higuchi et al., 1999; Ling et al., 1999; Schuler and
Bauer, 2011). Secreted
PS solubilize and complex Fe3+ in the rhizosphere, form a stable
complex which is subsequently taken up into epidermis cells e.g. by the maize proton-coupled
transporter YELLOW STRIPE 1 (ZmYS1) (Marschner et al., 1986; Romheld and Marschner, 1986;
Curie et al., 2001; Schaaf et al., 2004). For
barley and rice, homologous transporters, YS-likes (YSL), were reported (Murata et al., 2006; Inoue et al., 2009; Araki et
al., 2011).

Several bHLH transcription factors were reported to be
Fe regulated and involved in the Fe uptake control in graminaceous plants. In
rice, OsIRO2 is a positive regulator of e.g.
OsNAS1, OsNAS2 or YSL15 and
exhibits a similar amino acid sequence as subgroup Ib bHLH transcription
factors in Arabidopsis. OsIRO2 has homologues
in other graminaceous
plants, such as in barley and is not only involved in Fe uptake, but also in Fe
transport and translocation (Ogo et al.,
2006; Ogo et al., 2007; Ogo et al., 2011). OsIRO3, however, has a negative effect on Fe
uptake and is suggested to be an orthologue of Arabidopsis POPEYE (PYE) (Zheng et al.,
2010). No orthologue of AtFIT has yet been presented.

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Recent findings that Zea mays, with ZmIRT1 (Li et al., 2013; Li et al., 2015), Oryza sativa, with OsIRT1 and OsIRT2 (Eide et al., 1996; Bughio et al., 2002; Ishimaru et
al., 2006; Lee and An, 2009), and Hordeum vulgare, with HvIRT1 (Pedas et al., 2008),
express functional homologues of genes employed by Strategy I plants raise
the question however, whether this strict separation into Strategy I and Strategy
II can be applied.