Cell death in response to osmotic and salt stresses in two rice (Oryza sativa L.) ecotypes

Cell death in the roots of two rice ecotypes under PEG-induced osmotic and NaCl stresses were assayed by TUNEL and DAPI techniques. Although both ecotypes exhibited cell death in response to the osmotic and salt stresses, they appeared to adapt to the two types of stresses through different timing and tissue specificity of cell death. Under the stresses, the occurrence of cell death was always delayed in IRAT109, a drought tolerant upland line, than in IR20, a drought sensitive lowland line. Under the salt stress, cell death progressed successively in a well regulated manner, starting from the outer layer cells in the epidermis and exodermis of roots and subsequently to the endodermis and stele, suggesting a possible function of the dead cells in preventing the influx of excess Na+ ions into the inner parts of roots and into shoots, leading to salt exclusion. In contrast, cell death induced by PEG-induced osmotic stress occurred randomly in roots, allowing a better ability to recover after stress. Thus, the coincidence of the difference in drought tolerance between the two rice ecotypes and their differences in the osmotic/salt induced cell death indicate that cell death may have played an important role contributing to rice tolerances to different abiotic stresses.