Gene transformation
Significant advances in the field of molecular biology technology have been made during the past decade. The use of molecular techniques to selectively introduce desired genes may provide alternative ways to classical plant breeding to achieve salinity tolerance. These techniques will benefit the development of salinity-tolerant cultivars based on specific traits that are controlled by one gene, eg a transcription factor or an important ion channel. The work of Blumwald and colleagues (e.g. Zhang and Blumwald, 2001) shows the progress made by using molecular technology. The authors reported the development of a salinity tolerant transgenic tomato plant in which over-expression of the vacuolar Na+/H+ antiporter shows dramatic improvement of vegetative growth and of fruit yield. This antiporter is the only known transporter that would compartmentalise Na+ in the vacuole, where Na+ has little chance of toxic effect on metabolism, or to be transported to younger leaves and fruits. These studies indicate great potential for transgenic methodology, but so far the evidence of the mechanism is not proven.
Significant advances in the field of molecular biology technology have been made during the past decade. The use of molecular techniques to selectively introduce desired genes may provide alternative ways to classical plant breeding to achieve salinity tolerance. These techniques will benefit the development of salinity-tolerant cultivars based on specific traits that are controlled by one gene, eg a transcription factor or an important ion channel. The work of Blumwald and colleagues (e.g. Zhang and Blumwald, 2001) shows the progress made by using molecular technology. The authors reported the development of a salinity tolerant transgenic tomato plant in which over-expression of the vacuolar Na+/H+ antiporter shows dramatic improvement of vegetative growth and of fruit yield. This antiporter is the only known transporter that would compartmentalise Na+ in the vacuole, where Na+ has little chance of toxic effect on metabolism, or to be transported to younger leaves and fruits. These studies indicate great potential for transgenic methodology, but so far the evidence of the mechanism is not proven.
Flowers (2004) has questioned the current 'hype' (hyperbole) involving transgenics and concluded that so far no useful increase in salinity tolerance has been achieved. A number of genes, encoding proteins with known functions in ion transport or in synthesis of compatible organic solutes, as well as genes whose functions are not fully understood, have been used to transform a number of species in efforts to improve salinity tolerance. Despite numerous claims of improved salinity tolerance, poor experimental designs and choices of parameters measured to evaluate tolerance mean that much doubt remains. None of these transgenics has been proven in the field. Since salinity tolerance is a multigenic trait, large improvements based on modification of only one gene could only occur if the gene is a transcription factor and regulates a number of genes that control ion transport or some other process involved in salinity tolerance.
Given the natural diversity that exists, and given the current social antipathy to genetically-engineered crops, it might be more realistic to consider using the genes identified as perfect markers for naturally-occurring diversity.
Molecular markers
The development of molecular markers for physiological traits has made significant headway in recent years with the advancement of new technologies. Consequently, the use of molecular markers in breeding programs is increasing rapidly as they have been shown to greatly improve the efficiency of the breeding programs. Marker-assisted selection is non-destructive and can provide information on the genotype of a single plant without exposing the plant to the stress. The technology is capable of handling large numbers of samples. PCR-based molecular markers have the potential to reduce the time, effort and expense often associated with physiological screening. In order to use marker-assisted selection in breeding programs, the markers must be closely linked to the trait, and work across different genetic backgrounds.
The development of robust markers that are reliable across a wide range of backgrounds can be quite difficult, and is entirely dependent on an accurate phenotype screen. Understanding the physiology of sodium uptake is critical to the development of a reliable and accurate phenotype test, and thereby to the identification of a QTL (Quantitative Trait Locus) and a molecular marker linked to the locus.
QTL mapping and marker-assisted selection is a technique that has many advantages over phenotypic screening as a selection tool. The efficiency of genetic mapping has improved greatly in recent years, with the advent of high-density maps incorporating microsatellite markers, RFLP markers, and population-specific polymorphic fragments identified by the AFLP technique. The approach has been widely used to successfully map agronomic traits in a variety of cereal species. Although developing a suitable population for QTL analysis is laborious, and identifying a QTL is expensive, the markers that are linked to the QTL may be sufficient to use as the sole selection tool for a specific trait in a breeding program. QTLs for salinity tolerance have been described in several cereal species, including rice, barley and wheat. However, these studies have not yet yielded robust markers that can be used across a range of germplasm, significant associations between the trait and the marker being confined to the populations in which they were derived. The success of these studies could be limited by the small amount of genetic diversity present within modern cultivars, and the use of parental lines with small differences in the traits.
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