nitude of the pH. Phosphate transport into the vesicles was also mediated by a negative (inside) electri-
cal gradient (negative EM) in the absence of a pH. This indicated electrogenic H-phosphate symport
and an H/H 2 PO 4 stoichiometry1.0.
Two cDNAs (AtPT1andAtPT2) encoding plant phosphate transporters have been isolated from
phosphate-starvedArabidopsis thalianaroots [159]. The corresponding protein belongs to a family of
transporter proteins with 12 putative transmembrane proteins. It is highly homologous to Pi transporters
isolated from yeasts and fungi. When expressed in a Pi uptake–deficient yeast mutant, it exhibited high-
affinity phosphate transport activity. The transcripts of both genes were expressed in roots but were not
detectable in leaves. Similar genes, LePT1andLePT2, were highly expressed in tomato roots [160]; only
LePT1was expressed in the leaves as well. The expression was induced by phosphate starvation but not
by nitrate, potassium, and iron starvation. The transcripts were localized to the root epidermis cells and
LePT1also to leaf palisade cells. When LePT1was expressed in a Pi uptake–deficient yeast mutant [161]
it exhibited a Kmof 31 mM, but the expressed enzyme was still active at submicromolar Pi concentration
and mediated highest uptake at pH 5.0. The transport activity of LePT1depended on an electrochemical
proton gradient.
Nitrate. Nitrate-Hsymport was characterized in plasma membrane vesicles isolated from maize
[162] and cucumber [163] roots (nitrate was labeled with^36 ClO 3 , an NO 3 analogue). The transport was
driven by an artificially imposed pH. The initial rate of nitrate transport depended on the magnitude of
thepH. The imposed pH affected the Kmbut not the Vmax. Nitrate transport was higher into vesicles
isolated from both nitrate- and starvation-induced cucumber plants than that observed in vesicles obtained
from uninduced plants [163].
An additional kind of nitrate transport, namely Na-NO 3 symport, was demonstrated in cells of the
marine plant Zostera marinaL. [136]. In this marine environment the innate Nagradient is utilized for
the active transport of NO 3 . The plasma membranes of Z. marinacells were depolarized in the presence
of both Naand NO 3 ions but not in the presence of one of these ions alone. The depolarization was in-
hibited by monensin, a sodium ionophore. The depolarization indicated a transport stoichiometry of at
least 2Na/NO 3 .
TheCHL1(NRT1) gene of Arabidpsisencodes a nitrate-inducible nitrate transporter. This trans-
porter is thought to be a component of the low-affinity nitrate uptake system in plants [164]. It functions
in the low-affinity (10 mM) as well as in the high-affinity (0.1 M) concentration range. It may be a dual-
affinity nitrate transporter. An additional gene encoding a putative high-affinity nitrate transporter (Gm-
NRT2) was isolated from a soybean root cDNA library and sequenced [165]. It is related to high-affinity
nitrate transporters in Chlamydomonas reinhardtiiandAspergillus nidulansand putative high-affinity ni-
trate transporters in barley and tobacco. Expression of the gene was selectively regulated by different N
sources. The expression was barely detected in NH 4 grown plants; it was higher in N-deprived plants and
highest in NO 3 grown plants. Induction resulted in a fourfold increase of NO 3 uptake from 0.1 M ex-
ternal NO 3 and occurred within 1 hr.
Sulfate. Sulfate uptake by Brassica napusroot plasma membrane vesicles was characterized
[166]. It was driven by an artificially imposed pH. The Kmfor sulfate was strongly pH dependent (at
constantpH). It decreased from 1.0 M at pH 5.0 outside to 64 M at pH 6.4. The initial rate of sulfate
uptake and the equilibrium concentration in vesicles isolated from sulfate-starved roots were approxi-
mately twofold greater than observed in those isolated from sulfate-fed plants.
A cDNA encoding a high-affinity sulfate transporter was isolated from barley [167]. This cDNA,
designatedHVST1, encodes a polypeptide that has a high sequence homology with other identified eu-
karyotic sulfate transporters. The Kmfor SO 42 was 6.9 nM when the HVST1cDNA was expressed in a
yeast mutant.
Mineral Anion Transport to Vacuoles. The electrochemical potential gradients of the major
anions and of Hacross the tonoplast suggest anion-proton symport from the vacuole to the cytosol.
However, evidence for such symport is limited. Blumwald and Poole [168] formed an ATPase-dependent
proton motive force in B. vulgaristonoplast vesicles and subsequently added Clor NO 3 . Both ions dis-
sipated the electrical component of the proton motive force (the EM), but only NO 3 dissipated the pH
as well. They concluded that both anions entered the vesicles by uniport, dissipating the EM, and that only
NO 3 was excreted again by symport with protons, thus dissipating the pH. In similar experiments of
Schumaker and Sze [169] with oat root tonoplast vesicles, both Cland NO 3 dissipated the EMas well
as the pH.
350 JACOBY AND MORAN