Myo-inositol-dependent sodium uptake in ice plant

In salt-stressed ice plants (Mesembryanthemum crystallinum), sodium accumul ates to high concentrations in vacuoles, and polyols (myo-inositol, D-ononi tol, and D-pinitol) accumulate in the cytosol. Polyol synthesis is regulate d by NaCl and involves induction and repression of gene expression (D.E. Ne lson, B. Shen, and H.J. Bohnert [1998] Plant Cell 10: 753-764). In the stud y reported here we found increased phloem transport of myo-inositol and rec iprocal increased transport of sodium and inositol to leaves under stress. To determine the relationship between increased translocation and sodium up take, we analyzed the effects of exogenous application of myo-inositol: The NaCl-inducible ice plant myo-inositol l-phosphate synthase is repressed in roots, and sodium uptake from root to shoot increases without stimulating growth. Sodium uptake and transport through the xylem was coupled to a 10-f old increase of myo-inositol and ononitol in the xylem. Seedlings of the ic e plant are not salt-tolerant, and yet the addition of exogenous myo-inosit ol conferred upon them patterns of gene expression and polyol accumulation observed in mature, salt-tolerant plants. Sodium uptake and transport throu gh the xylem was enhanced in the presence of myo-inositol. The results indi cate an interdependence of sodium uptake and alterations in the distributio n of myo-inositol. We hypothesize that myo-inositol could serve not only as a substrate for the production of compatible solutes but also as a leaf-to -root signal that promotes sodium uptake.