Stable NaCl Tolerance of Tobacco Cells Is Associated with Enhanced Accumulation of Osmotin

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Abstract
Osmotin is a major protein which accumulates in tobacco cells (Nicotiana tabacum L. var Wisconsin 38) adapted to low water potentials. Quantitation of osmotion levels by immunoblots indicated that cells adapted to 428 millimolar NaCl contained 4 to 30 times the level of osmotin found in unadapted cells, depending on the stage of growth. Unadapted cells accumulated low levels of osmotion with apparent isoelectric points, (pI) of 7.8 and > 8.2. Upon transfer of NaCl-adapted cells to medium without NaCl and subsequent growth for many cell generations, the amount of osmotin declined gradually to a level intermediate between that found in adapted and unadapted cells. NaCl-adapted cells grown in the absence of NaCl accumulated both pI forms; however, the form accumulated by cells adapted to NaCl (pI > 8.2) was most abundant. Adapted cells grown in the absence of NaCl exhibited absolute growth rates and NaCl tolerance levels which were intermediate to those of NaCl-adapted and unadapted cells. The association between osmotin accumulation and stable NaCl tolerance indicates that cells with a stable genetic change affecting the accumulation of osmotin are selected during prolonged exposure to high levels of NaCl. This stable alteration in gene expression probably affects salt tolerance.