INCREASING SALT TOLERANCE OF PLANTS THROUGH CELL-CULTURE REQUIRES GREATER UNDERSTANDING OF TOLERANCE MECHANISMS

Despite numerous attempts, increasing the salt tolerance of cultured cells has rarely led to increased salt tolerance in regenerated plants. Clearly the role of cultured cells in selecting for salt tolerance needs reappraisal. Although the relationship between salt tolerance of cultured cells and whole plants is crucial to the use of cultured cells in selecting for increased salt tolerance, it is not understood, and neither are the mechanisms of salt tolerance in cultured cells understood. Our understanding of salt tolerance in cultured cells has been limited mainly by poor methodology, particularly failure to consider the large free space volume and the effect of high NaCl on the various phases of culture growth. Further, there has been a reluctance to search for mechanisms and test hypotheses. The relationship of salt tolerance in cultured cells and whole plants needs to be studied since much of the tolerance of whole plants is associated with their integrated functioning. Furthermore, cultured cells grow slowly and have different hormonal, osmotic and nutritional environments from cells in whole plants. Selection of cultured cells may be more productive if focused on specific cell-based physiological traits (such as Na+ accumulation, turgor regulation or tolerance to high Na+:Ca2+) rather than on tolerance to high NaCl only. Expression in whole plants of traits in cultured cells also needs to be studied.