The role of molecular biology in enhancing the salt-tolerance of horticultural crops

It has proved difficult to develop crops that are resistant to salinities beyond those normally experienced by the species from which that crop has evolved. Since advances in molecular biology do not appear to have helped greatly in the development of salt-tolerant genotypes, it is reasonable to reflect on the lack of progress. There is good evidence that salt tolerance in truly tolerant species (halophytes) is a multifaceted trait. In halophytes, tolerance depends upon compartmentation and compatible solutes, regulation of transpiration, control of ion leakage through the apoplast, membrane characteristics and the ability to tolerate low K:Na in the cytoplasm. A range of traits has also been demonstrated to be important in crops. This suggests that the overall trait, of salt tolerance is determined by a number of genes: that the trait is quantitative. Genetic markers can be identified for such quantitative traits. Recently, markers have been reported for traits associated with salt tolerance, but this has not so far helped with the identification of specific genes - although there is the potential for marker-assisted selection. Recent work on cold tolerance suggests that it may be possible to engineer regulatory genes: such genes may control a range of processes necessary for tolerance. If this proves possible for salt tolerance, then this may be an important advance in our ability to engineer stress tolerance into plants. Successful attempts have been made to transfer genes concerned with the synthesis of compatible solutes (osmolytes) into plants and claims made for enhanced tolerance. Whether tolerance is really enhanced is questionable as are the reasons for any putative enhancement.