The Expression of HAK-Type K+ Transporters Is Regulated in Response to Salinity Stress in Common Ice Plant

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Abstract
Four transcripts homologous to K+ transporters of the HAK/KT/KUP family have been characterized from the common ice plant (Mesembryanthemum crystallinum). We report tissue-specific expression of McHAK1 andMcHAK4 transcripts abundant in roots, leaves, and stems.McHAK2 was predominantly present in stems andMcHAK3 in root tissues. By in situ hybridizations, the McHAKs showed signals in the leaf vascular bundles, mesophyll, and epidermal cells as well as in epidermal bladder cells. In mature roots, transcripts were mainly localized to the vasculature, and in differentiated root tips, the strongest signals were obtained from the epidermis. Expression of McHAK1, McHAK2, and McHAK4 complemented a yeast mutant defective in low- and high-affinity K+ uptake. Growth of the yeast mutant was restored at low-millimolar K+ concentrations and was inhibited by Rb+ and Cs+ but was not affected by Na+. Transcript levels of McHAK1 andMcHAK4 increased by K+ starvation and by salt stress of 400 mm NaCl in leaves and roots. Expression of McHAK2 and McHAK3 was stimulated in leaves and was transiently induced in roots in response to high salinity with prestress transcript levels restored in salt-adapted plants. We discuss possible roles for such transporters in ion homeostasis at high salinity.