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Plant Physiol, September 2001, Vol. 127, pp. 283-294
Characterization of Two HKT1 Homologues from Eucalyptus
camaldulensis That Display Intrinsic Osmosensing
Capability1
Weihong
Liu,
David J.
Fairbairn,
Rob J.
Reid, and
Daniel P.
Schachtman2*
CSIRO Plant Industry Horticulture Unit, G.P.O. Box 350, Glen
Osmond, South Australia 5064, Australia (W.L., D.P.S.); Department of
Botany, The University of Queensland, Brisbane, Queensland 4072, Australia (D.J.F.); and University of Adelaide, Department of Plant
Science, Adelaide, South Australia 5001, Australia (R.J.R.)
Plants have multiple potassium (K+) uptake and efflux
mechanisms that are expressed throughout plant tissues to fulfill
different physiological functions. Several different classes of
K+ channels and carriers have been identified at the
molecular level in plants. K+ transporters of the HKT1
superfamily have been cloned from wheat (Triticum
aestivum), Arabidopsis, and Eucalyptus
camaldulensis. The functional characteristics as well as the
primary structure of these transporters are diverse with orthologues
found in bacterial and fungal genomes. In this report, we provide a
detailed characterization of the functional characteristics, as
expressed in Xenopus laevis oocytes, of two cDNAs
isolated from E. camaldulensis that encode proteins
belonging to the HKT1 superfamily of K+/Na+
transporters. The transport of K+ in
EcHKT-expressing oocytes is enhanced by Na+,
but K+ was also transported in the absence of
Na+. Na+ is transported in the absence of
K+ as has been demonstrated for HKT1 and AtHKT1. Overall,
the E. camaldulensis transporters show some
similarities and differences in ionic selectivity to HKT1 and AtHKT1.
One striking difference between HKT1 and EcHKT is the sensitivity to
changes in the external osmolarity of the solution. Hypotonic solutions
increased EcHKT induced currents in oocytes by 100% as compared with
no increased current in HKT1 expressing or uninjected
oocytes. These osmotically sensitive currents were not enhanced by
voltage and may mediate water flux. The physiological function of these
osmotically induced increases in currents may be related to the
ecological niches that E. camaldulensis inhabits, which are
periodically flooded. Therefore, the osmosensing function of EcHKT may
provide this species with a competitive advantage in maintaining
K+ homeostasis under certain conditions.
1
This work was funded by the Australian Research
Council (grant to D.P.S.).
2
Present address: Donald Danforth Plant Science Center,
7425 Forsyth Boulevard, Box 1098, St. Louis, MO 63105.
*
Corresponding author; e-mail dschachtman{at}danforthcenter.org;
fax 314-935-8605.
© 2001 American Society of Plant Physiologists
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