In summary, the improvements in plant salt tolerance can be identified by: a) screening
for appropriate diversity in responses to salinity among cultivars or related wild species
(Flowers and Yeo, 1995); b) treatment with mutagens in order to produce mutants which
show hypersensitive or reduced responses to salinity as compared with the wild type
(Zhang et al., 1995, Wu et al, 1996, Maggio et al., 2001), and c) engineering transgenic
plants expressing one or more foreign genes which are expected to increase cellular
resistance to salinity (Bohnert and Jensen, 1996, Neumann, 1997).
Conventional methodologies of genetic manipulation in breeding for salt tolerance have been discussed (e.g. Yeo, 1998, Munns et al., 2002). These include quantitative trait locus (QTL) analysis, introgression of whole or parts of chromosomes,
Table 5. Continued…
Z Dajic .
monitored with fluorescence in situ hybridization, as well as the pedigree selection
breeding, mass selection, backcross breeding, recurrent selection and wide hybridization. The improvement of crops by investigation of favorable alleles existing in wild
relatives of crops, provides a potential opportunity for achieving advances in crop
performance, as well as screening for a range of traits for salt stress tolerance, within
large populations of mutant plants (Miflin, 2000).
New molecular marker technologies can be used for marker-assisted selection
(MAS) to improve the salinity tolerance of crops (Tanksley et al., 1989). Markers tightly
linked to salt tolerance can be used in MAS after evaluation of their reliability. The use
of molecular markers is needed for clarifying the number, chromosomal locations and
genetic contributions of genes controlling both the quantitative or complex traits and
simply-inherited traits under stress conditions (Lilley et al., 1996).
Since the various mechanisms and adaptive responses of plants to salt stress
are multigenic traits, further efforts are necessary to comprehend the gene expression
for groups of functionally related genes. In order to extend the application of gene
transformation to abiotic stresses it is important to gather information on what are the
“useful genes”, responsible for better stress tolerance (Grover et al., 1998). Three approaches are commonly used in identification of genes responsible for salt tolerance: a)
analysis of genes involved in processes associated with salt tolerance, b) identification
of genes whose expression is dependent on salt stress, and c) survey based on salt
tolerance determinants based on functionality (Borsani et al., 2003).
Several genetic model systems, apart from the favorite subjects of stress studies, like tomato and tobacco, offer excellent opportunities for research in salt tolerance.
Examples are the common ice plant, Arabidopsis, yeast, recently found halophyte
Thellungiella halophila (Zhu, 2000, 2001) and salt-tolerant green alga Dunaliella salina
(Cowan et al., 1992).
Improved salt tolerance may be achieved by the maintenance, activation, and
enhanced function of physiological systems that are especially sensitive to disruption
by increased levels of salts (Winicov, 1998). On the other hand, an opposite approach
could consider the favoring and improvement of those systems exhibiting distinct
tolerance to salinity.
Plant responses to salt stress are complex, extremely variable, mutually linked,
and include a wide range of effects at the molecular, cellular, tissue and whole-plant
level. It is therefore unlikely in the near future, to have simple answers and solutions
related to the problem of salt tolerance. However, there is hope that many aspects of salt
stress would be resolved, particularly through the knowledge from molecular biology,
biotechnology and bioinformatics. Further, knowledge on salt-inducible genes, genetic
control of salt responses and signaling pathways offers a chance for creating a clearer
picture of plant responses and adaptations to salinity.

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