Stress responses of tobacco cells to high temperature and salinity. Proline accumulation and phosphorylation of polypeptides

Stress responses to high temperatures (35–48°C) and salinity (170–340 mM NaCl) and thermotolerance were investigated for the salt‐sensitive wild type and the salt‐tolerant N r E s ‐1 strain of Nicotiana sylvestris L. using suspension‐cultured cells. Under saline conditions, N r E s ‐1 strain cells accumulated proline, polyamine (putrescine and spermidine) and betaine in contrast to wild‐type cells. The simultaneous treatment of salt‐tolerant cells with high temperature (40°C) and NaCl (170 or 340 mM ) resulted in a transient overproduction of proline accompanied by an increase in their thermotolerance. At the high temperature, the synthesis of polypeptides and the accumulation of heat shock protein HSP70 mRNA were not affected by salinity. The higher thermotolerance of the NaCl‐tolerant cells could not be related to osmoprotecting sugar‐starch interconversions but could rather be associated with selective phosphorylation of several polypeptides (23–24, 27, 31–32, 47 kDa) prior to the accumulation of proline. The possible role of polyamines and polypeptide phosphorylation in this respect is discussed.