- Uptake of Heavy Metals
Genes encoding proteins involved in the transport of copper, manganese, zinc, and iron have been iden-
tified [109]. In most cases it is not known whether these proteins have a direct role in the transport across
the plasma membrane or whether they function as carriers or channels. More detailed information is avail-
able for iron uptake.
IRON The low solubility of Fe-bearing minerals restricts the available Fe pools in most soils. The free
Fe concentration in soil solutions is usually less than 10^15 M [170]. Plants employ two distinct and in all
known cases mutually exclusive strategies for solubilization and absorption of Fe [171].
All plants except grasses employ a procedure termed strategy I to acquire Fe. These plants reduce
and solubilize Fe(III) prior to transport of Fe(II) across the plasma membrane of root cells. The initial re-
duction and solubilization are carried out by two plasma membrane–bound enzymes, an H-ATPase and
an Fe(III) chelate reductase. Proton release, Fe(III) chelate reductase, and Fe(II) transport activities are
all enhanced under Fe deficiency [109]. Proton release lowers the rhizosphere pH and increases Fe(III)
solubility. In Arabidopsisone of the genes encoding H-ATPase,AHA2, is up-regulated in response to
Fe deficiency and may be involved in this acidification. An FRO2-encoding gene was isolated from Ara-
bidopsisroots. The FRO2 protein belongs to a superfamily of flavocytochromes that transport electrons
across membranes [172]. The authors showed that FRO2 is allelic to the frd1mutations that impair Fe(III)
chelate reductase activity. Introduction of functional FRO2 complemented the frd1-1 phenotype in trans-
genic plants. Iron deficiency induces the expression of a further Arabidopsisgene; it encodes IRT1 (iron-
regulated transporter). Expression of IRT1 in yeast restored iron-limited growth to a yeast mutantant de-
fective in Fe uptake [173]. Yeasts expressing IRT1 possess an iron uptake system that is specific for Fe(II)
over Fe(III) and other potential substrates such as Cu(I), Cu(II), Mn(II), and Zn(II). However, Cd^2 in-
hibited Fe^2 uptake by IRT1. It is proposed that IRT1 is an Fe^2 transporter and that it may transport
Cd^2 as well.
Grasses employ another procedure, termed strategy II, for Fe acquisition [171]. These plants syn-
thesize and secrete phytosiderophores. These are low-molecular-weight, Fe(III)-specific ligands. The
phytosiderophores involved in strategy II are mucigenic acids, namely nonproteinous amino acids syn-
thesized from methionine [174]. They posses a high chelation affinity for Fe(III) but not for other poly-
valent cations. Iron transport is regulated by a specific uptake system that transports the phy-
tosiderophore-Fe(III) complex across the plasma membrane [175]. Nicotine amine (NA) and the
enzyme nicotine amine aminotransferase (NAAT) are implicated in the synthesis of mucigenic acids.
NAAT-encoding cDNAs were identified in barley and NAAT was strongly induced under Fe defi-
ciency [109]. In order to identify the Fe(III)–mucigenic acid transporter, a yeast mutant ctr1that is un-
able to grow on Fe-deficient media was transformed with a barley cDNA expression library. A clone
designated SFD1 (suppressor of ferrous uptake defect) that could use Fe(III)–mucigenic acid as an Fe
source was isolated.
VI. CHARGE BALANCE
Uniport, cotransport, and primary active transport may all result in the transport of unbalanced charges;
that is, they may all perform electrogenic transport. Primary active transport generates a membrane po-
tential, and electrogenic cotransport and uniport dissipate it. Continued electrogenic contransport and uni-
port are sustained by persistent proton motive force turnover (dissipation, followed by regeneration by
primary active proton transport). An extremely small imbalance of cation and anion fluxes results in a
considerableEM. The ion concentration difference that sustains the membrane potential of plant cells (up
to about 240 mV at the plasma membrane) can be calculated from its relation to the capacitance Cand
the electrical charge QMof the membrane: QMCEM[2]. For a spherical cell with a radius of 50 m,
such a calculation shows that an uncompensated ion concentration difference of about 1.5 M is needed
to sustain a membrane potential of 240 mV. The total cytosolic salt concentration may be assumed to
be about 50 mM. An anion concentration difference of about 1.5 M then constitutes an anion excess of
only 0.003% and is tantamount to equivalent anion and cation concentrations.
Considerably different amounts of the anion and cation of a salt are absorbed by plant cells, this in
spite of near anion and cation balance in the cells. The difference is compensated by proton fluxes, re-
sulting in transient pH shifts in the cells. Plant cells regulate the cytosolic pH at about 7.0 [176]. Synthe-
MINERAL NUTRIENT TRANSPORT IN PLANTS 351