affinity transporter may be needed when the Kconcentration in the soil solution is less than 1.0 mM
[134]. At much more negative potentials Kchannels may suffice for a high-affinity Kinflux [135].
In the marine alga Chara australisNa-Ksymport was demonstrated. Such symport employs the
naturally existing and inward-directed electrochemical Nagradient for high-affinity Ktransport
against its electrochemical potential gradient. Utilization of the natural Nagradient for symport of var-
ious solutes was suggested for other halophytes and ions, namely Na-NO 3 symport in the marine higher
plantZoospora maritima[136] and Na-urea, Na-sugar, and Na-lysine symport in charophyte algae
[137,138]. However, all investigated terrestrial plants were able to sustain growth and Kuptake in the
absence of Na[139]. The latter authors suggested that in terrestrial species Na-coupled Ktransport
has no or limited physiological significance, whereas in certain aquatic angiosperms and algae it plays a
significant role. Two complementary DNAs (cDNAs) encoding high-affinity Kantiporters, HKT1 [140]
and KUP [141], with Kmvalues of 29 and 22 M were isolated from wheat and Arabidopsis, respectively.
Both transporters were up-regulated by Kdeficiency [141,142]. The wheat HKT1 functioned as an H-
Ksymporter with an H/Kratio of 1.0. Potassium transport by the wheat HKT1 was stimulated by mi-
cromolar Naconcentrations and K-activated high-affinity Nauptake. However, Kuptake was in-
creasingly inhibited by Naconcentrations above 1.0 mM [143].
Calcium. Kasai and Muto [144] suggested that Ca^2 /Hantiport operates at the plasma mem-
brane, in addition to the Ca^2 -ATPase. Genes encoding a high-affinity Ca^2 /Hantiporter (CAX1) and a
low-affinity Ca^2 /Hantiporter (CAX2) have now been cloned from Arabidopsis[145]. When expressed
in a hypersensitive yeast strain, CAX1catalyzedpH-dependent Ca^2 transport with a Kmof 13 M.
Hence,CAX1along with Ca^2 -ATPase is proposed to keep cytosolic Ca^2 below 1 M in resting plant
cells.
A Ca^2 -ATPase and Ca^2 /Hantiport are apparently also involved in the transport of Ca^2 from the
cytosol to the vacuole. The Ca^2 concentration in plant cell vacuoles is in the millimolar range. Accord-
ingly, the apparent KMvalues for Ca^2 of the Ca^2 /Hantiporter in tonoplasts from oat roots [146,147]
andB. vulgaris[148] were 10–14 mM and 42–200 M, respectively. In the latter case, the KMvaried with
the pH in the tonoplast vesicles. The Ca^2 /Hantiport created an 800 to 2000-fold Ca^2 gradient in tono-
plast vesicles [149]; an H/Ca^2 stoichiometry of 3 was indicated [150].
MINERAL ANIONS The accumulation of anions in the negatively charged cytosol should be an ac-
tive process whether they are transported to the cytosol across the plasma membrane from the free space
or across the tonoplast from the vacuole. Anion-proton symport is expected to expedite such transport.
The early evidence for symport of protons with Cl[151], NO 3 [152,153], H 2 PO 4 [154,155], and SO 42
[156] emanates from experiments that showed a transient depolarization of the plasma membranes, or al-
kalization of the medium [155], upon addition of these anions to intact roots. In all these investigations
the degree of transient depolarization was related to the transport rate of the anions. Transient depolar-
ization indicates electrogenic transport with a stoichiometry exceeding 1 for H/anionic charge trans-
ported. Depolarization is transient because the membrane potential is restored by increased proton pump
activity. Sakano [155] measured the stoichiometry of H 2 PO 4 and Huptake (transient external alkalin-
ization) in Caranthus roseuscell cultures and calculated an H/H 2 PO 4 stoichiometry of 4.
Chloride. Proton-chloride symport was shown [157] for barley roots that were de-energized by
anaerobic pretreatment. In such roots, Clinflux could be induced by application of an artificial pH gra-
dient (acid outside). However, Clinflux was not induced by a low pH outside in the absence of a pH,
obtained with a weak acid such as acetic acid. In the latter case the high membrane permeability of acetic
acid molecules resulted in the dissipation of the initial pH gradient. Sanders [151] reviewed Cltransport
in plants and concluded that the H/Clstoichiometry of symport should be at least 2. Such stoichiom-
etry is consistent with electrogenic Cltransport and with transient plasma membrane depolarization
upon application of Cl. The symport of protons with Clinto isolated barley root plasma membrane
vesicles was also characterized [158]. The ATP-mediated acidification of the vesicles was strongly de-
pendent on the presence of permeant anions. Also,^36 Cltransport into the vesicles depended on the elec-
trochemical potential generated by the H-ATPase.
Phosphate. The symport of phosphate and protons across the plasma membrane was demon-
strated in outside-out B. vulgarisplasma membrane vesicles (SR Stutz, B Jacoby, unpublished). Phos-
phate uptake and dissipation of the pH gradient occurred concomitantly when H 2 PO 4 was added to these
vesicles in the presence of an artificial pH gradient. The initial rate of Pi transport depended on the mag-
MINERAL NUTRIENT TRANSPORT IN PLANTS 349