Cloning and characterization of potassium channels and transporters in Mesembryanthemum crystallinum

K+ contributes most to the osmotic pressure in plant cells. Under saline conditions, accumulation of Na+ disturbs the ion homeostasis and causes toxicity to the cell. Potassium channels and transporters may mediate Na+ entry, yet K+ contents must be maintained at a certain level. Regulation of potassium transport under salt stress is important for ion homeostasis and stress tolerance in plants. This dissertation presents analyses of the potassium channels and transporters in the halophyte Mesembryanthemum crystallinum (common ice plant) emphasizing their regulation in the context of ion homeostasis under salinity conditions.; Short-term changes of ion content in leaf tissues were measured for salt stressed ice plants. Na was found to interfere with potassium uptake. Net potassium uptake was reduced at least 50% after a few hours. Potassium contents in the ice plant roots and leaves are known to be reduced after long term salt stress.; The cDNAs of three potassium channels MKT1, MKT2 and KMT1, three HAK-type transporters McHAK1-3, and a HKT-type transporter McHKT1 were isolated from the ice plant. McHAK1, McHAK2 and McHKT1 were able to complement a yeast mutant defective in potassium uptake. MKT1 transcripts were mainly detected in roots; MKT2, KMT1 and McHKT1 were expressed primarily in shoots. The McHAK transcripts were found in various tissues.; Expression patterns of the channel and transporter transcripts under salt stress were investigated by RNA blot analysis and RT-PCR. In roots, transcripts of the MKT1 channel were reduced significantly after 6 h of stress, while the transcripts of McHAK1, McHAK2 and McHAK3 increased till 24 h of stress. In leaves, McHKT1 was transiently up-regulated within hours of stress and decreased rapidly afterward, but the McHAK1–3 transcripts increased up to 48 h of treatment. The transcripts of KMT1 channel showed transient up-regulation.; The tissue-specific expression and coordinated regulation of these genes suggested that upon salt stress, the K+ uptake mechanism in the ice plant switches from the MKT1 channel to other systems such as the McHAK and McHKT1 transporters. Physiological relevance of these changes to the salinity adaptive strategy in the ice plant is discussed.