Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 258

In Strategy I, the goal of the plant is to solubilize the Fe
(III). The Fe(III) is solubilized by either reduction with
enzymatic reductases or the release of protons to reduce
the soil pH and increase Fe(II) solubility; the Fe(II) produced
is then free to enter the cell. Iron uptake typically follows
this pathway; the Fe(III) is reduced to Fe(II) by a plasma
membrane redox reaction (ferrireductase), and the Fe(II) is
taken up by a transporter (Guerinot and Ying 1994). The
reduction of ferric iron is done by dicotyledonous plants. In
fact, low iron availability induces the production of ferric
reductase. The reduction occurs in the plasma membrane of
the epidermal cells of roots. By comparison, soil bacteria
that encounter other metals, such as mercury (Hg), quickly
route the toxic element through the cell by conjugation with
organic functional groups, such as -CH 4 , to increase Hg
solubility and decrease uptake by increased elimination.
The Strategy I system for iron works in reverse; organic
compounds are released to increase solubility to increase
cell uptake.
In some cases, plants can release enough organic matter
to decrease the content of dissolved oxygen near the
oxidized iron and render ferric iron to ferrous iron by
microbially mediated iron-reduction reactions. This process
also can occur in the uptake of ferrous iron by plants that
grow in periodically flooded soils or high water tables where
concentrations of dissolved oxygen are depressed. If the
organic matter released is acidic, the pH is lowered and,
with the presence of chelators, enhances Fe(II) uptake. Iron-
ically, it is plants that ultimately are to be blamed the
inaccessibility of iron, because iron was initially oxidized
by early photosynthesizers that released oxygen.
In Strategy II, plants release organic acids into the root zone
that then complex with the Fe(III) into a form, called a chelate,
that can be taken up. Strategy II is more common for mono-
cotyledonous plants. These plants synthesize, secrete, and then
take up phytosiderophores that are used to chelate the Fe(III)
(Romheld and Marschner 1986). Siderophores are nonprotein
amino acids (Graham and Stangoulis 2003), such as mugineic
acid (Kawai and Alam 2006) and rhizoferrin. These phyto-
siderophores are released primarily in the root tips and by
newer roots. The chelation of iron then permits uptake to
occur by the apoplastic pathway rather than the symplastic
pathway. The Fe(III) complexes then can enter cells.
Strategy II is similar to methods used by bacteria that also
release siderophores, such as ferrichrome. However, plant
siderophores, or phytosiderophores, result in much faster
iron uptake than bacterial siderophores (R

facilitate uptake (Robinson et al. 1999). The enzyme is the
bridge needed to connect the roots with the ferric iron in the
soil. This bridge is used to funnel electrons from reduced
organic matter in the plant roots to ferric iron to complete the
reduction and mobilization and subsequent uptake by spe-
cific transport systems. In some cases, these same plants can
release excess protons into the pore water to decrease the pH
and increase iron solubility.
The uptake of bioavailable forms of iron by plant roots
is by diffusion along concentration gradients and advec-
tion within the transpiration stream. For diffusion, natural
chelators produced by plants increase the extent of diffusion
by increasing the iron concentration outside of the root zone.
This increased iron solubility is achieved even in the pres-
ence of dissolved oxygen (Oborn 1962). The production of
siderophores by rhizospheric fungi and bacteria may help
explain their presence on most plant roots—each is compet-
ing for iron. Once inside the plants, iron is less mobile, as
evidenced by the production of new growth of yellow or
chloritic leaves after iron supplies have been depleted,
whereas older leaves remain green. This yellowing can be
observed in both old and new leaves, however, because
chlorophyll is constantly breaking down and needs to be
synthesized continually.
Additional insight into the interaction among plant roots,
iron, and iron uptake has been provided by studies using the
stable isotopes of iron, which exist as 54 Fe and 56 Fe. Guelke
and Von Blanckenburg (2007) investigated the isotopic frac-
tionation of iron stable isotopes during the uptake of iron by
the two pathways described above. During Strategy I iron
uptake, the reduction of Fe(III) results in the uptake of Fe
(II), which is isotopically lighter, or depleted, in percent
heavy isotope by 1.6 per mil relative to that remaining in
the soil iron pool. During Strategy II iron that is associated
with siderophores results in the uptake of iron that is isoto-
pically heavier, or enriched, in percent heavy isotope by 0.2
per mil, than the soils.
Entry of iron into the roots and transport within the plant
occurs by two methods. Passive uptake of iron occurs by
diffusion along concentration gradients through the apoplast,
or the cell walls and spaces between cells. The endodermis
interrupts this transport to the inner part of the root (stele),
however, by way of the Casparian strip, which is made of
hydrophobic suberin. Still, solute does enter through this
barrier. Alternatively, active uptake of iron occurs from
cell-to-cell in the symplast, which requires selective trans-
port from cell membrane to cell membrane. With metals,
such as iron, the free ions can be taken up, and the metal ion-
chelate complexes can be taken up by the apoplastic path-
way along concentration gradients. The chelate EDTA and
its metal complexes have been detected in plant xylem sap
(Nowack et al. 2006). Once in the plant, chelates are then
absorbed by shoots.

omheld and
Marschner 1986). One molecule of chelate will chelate one
molecule of a particular metal, such as Fe(II) or Fe(III). The
Fe(III)-chelate complex is extremely stable under environ-
mental conditions. Researchers also have found that the
weed Arabidopsis thaliana contains the enzyme ferric reduc-
tase, which directly can reduce ferric iron to ferrous iron to

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