with Kþ for uptake into roots. The transcript levels of
several Kþ transporter genes are either down- or upregulated
by salt stress, probably reflecting the different
capacities of plants to maintain Kþ uptake under salt
stress. Salt stress increases the transcript level of the
Arabidopsis root Kþ-transporter gene AtKC1 [38]. In the
common ice plant, salt stress upregulates the expression
of KMT1 (a AKT/KAT family member) and various HAK/
KUP (high affinity Kþ transporter/Kþ uptake transporter)-type
genes, whereas it downregulates the expression
of MKT1 (another AKT/KAT family member) [39,40].
The significance of this transcript-level regulation is
difficult to determine because the transport characteristics
and in-vivo roles of the transporters are unclear. At the
activity level, Kþ channels are regulated by protein
kinases [41] and phosphatases [42]. Whether salt stress
regulates the activities of Kþ-uptake transporters through
these or other protein kinases or phosphatases remains to
be determined. A particularly novel mode of activity
regulation has been found for two HKT1 homologs from
Eucalyptus camaldulensis [43]. These Naþ–Kþ co-transporters
display intrinsic osmosensing capabilities when
expressed in Xenopus oocytes. Their Naþ- and Kþ-transport
activities are enhanced by a downshift in extracellular
osmolarity.