Na+/myo-inositol symporters and Na+/H+-antiport in Mesembryanthemum crystallinum

Mitr1 and Mitr2 from Mesembryanthemum crystallinum (common ice plant) are m embers of a family of genes homologous to H+[or Na+]/myo-inositol symporter s (ITRs), not previously studied in plants. MITR1 complemented an Itr1-defi cient yeast strain. Mitr1 is strongly expressed in roots, moderately in ste ms, and weakly in leaves. Its transcripts increased in all organs, most dra matically in roots, under salinity stress. Mitr2 constitutes a rare transcr ipt, slightly upregulated by salt stress in leaves only. Mitr1 transcripts are present in all cells in the root tip, but become restricted to phloem-a ssociated cells in mature roots. Peptide antibodies against the two protein s indicated the presence of MITR1 in all organs and of MITR2 in leaves. Bot h are located in the tonoplast. MITR1 acts in removing sodium from root vac uoles, correlated with findings of low root sodium, while leaf vacuoles acc umulate sodium in the ice plant. Up-regulation in leaves and stems is also found for Na+/H+-antiporter (Nhx-type) transcripts. Under comparable stress conditions, Nhx-and Itr-like transcripts in Arabidopsis were regulated dif ferently. In the ice plant, co-ordinate induction of Na+/H+-antiporters and Na+/myo-inositol symporters transfers sodium from vacuoles in root cells i nto the leaf mesophyll as a halophytic strategy that lowers the osmotic pot ential. The tissue-specific differential expression of Itr- and Nhx-type tr anscripts suggests that the vacuolar sodium/inositol symporters function to reduce sodium amounts in cells of the root and vascular tissue, while sodi um/proton antiporters in leaf tissues function to partition sodium into vac uoles for storage.