Metal Assimilation Pathways - Biological Inorganic Chemistry - page 148

its bioavailability is further decreased at the neutral and alkaline pH values found in semi-arid, calcareous
(calcium carbonate-rich) soils.
Plant iron uptake can be divided into two distinct families, with quite distinct strategies. Strategy I plants
reduce Fe 2 þ to Fe 3 þ outside of the roots, and then take up the Fe 2 þ . In contrast, Strategy II plants solubilise Fe 3 þ
by excreting Fe 3 þ phytosiderophores, which are taken up by specific transporters and the iron is then reduced to
Fe 2 þ in the symplasm of the root cell ( Fig. 7.14 ) . In Strategy I plants, (dicotyledons such as Arabidopsis, 11 pea,

FIGURE 7.14 Mechanisms of iron uptake by higher plants. In strategy I plants (e.g., Arabidopsis, pea and tomato), Fe(III) chelates are
reduced before the Fe(II) ion is transported across the plasma membrane. Strategy II plants (e.g., barley, maize, and rice) release siderophores
capable of solubilising external Fe(III) and then transport the Fe(III) siderophore complex into the cell. AHA2 is a P-type H þ -ATPase, FRO2 is
the Fe(III) chelate reductase, IRT1 is a Fe(II) transporter, and YS1 is the transporter of the phytosiderophore (PS) e Fe complex. (Adapted from

Schmidt, 2003 . Copyright 2003, with permission from Elsevier.)

and tomato), iron mobilization is achieved by the combined action of a proton-extruding H þ -ATPase, AHA, and
a ferric chelate reductase, FRO2, both of which are induced by iron deficiency. An iron transporter of the ZIP
family, named IRT1, seems to be the principal transporter of ferrous iron in Arabidopsis, and orthologues of IRT1
have also been characterised in tomato and rice.
In Strategy II plants, monocotyledon grasses, which include barley, maize, and rice, high-affinity Fe(III)
chelators (phytosiderophores, PS) are synthesised by the plants themselves and excreted into the environment
around their roots in order to complex and solubilise the ferric iron in the soil. Transporters specific for the Fe(III)-
siderophore complex then take the complex into the cytosol, where the iron is released from the phytosiderophore
by an as-yet-undefined mechanism. The best characterised of these transporters is YS1 (yellow stripe 1) or YSL1
(yellow stripe-like), so named after the phenotypic appearence of a maize mutant deficient in phytosiderophore
uptake. However, unlike the bacterial or fungal siderophores, phytosiderophores ( Fig. 7.15 ) of the mugeneic acid
family are synthesised from L-methionine via nicotianamine. Whereas the Strategy II grasses produce and excrete
the mugeneic acid family of siderophores, nicotianamine is found in both Strategy I and Strategy II plants, where
there is much evidence that it is involved in the intercellular transport of iron as the Fe-nicotianamine chelate.
However, evidence has begun to accumulate that grasses can also take up iron as Fe 2 þ . It is established that the
Fe 2 þ transporter IRT1 is upregulated in iron-deficient rice, and that rice can still take up Fe 2 þ evenwhen the synthesis
of nicotianamine, the precursor of phytosiderophores, is compromised. Now, it has been shown that introducing

11. Thale cress, a small flowering plant, member of the brassica family which includes mustard and cabbage, and is a model organism for
studying plant biology.