High salinity in agricultural field has been a problem since the starting of cultivation practice. While irrigation has made it viable to extend agriculture to semi-arid and arid regions of land and has been partly accountable for the large increases in food production of the last 40 years, it has also resulted in large-scale water logging and salinity. Land degradation due to multiplied salinity presently impacts approximately 20 % of world’s area under irrigation, without taking into consideration arid areas or deserts, which comprise a quarter of the total land of the planet (Yeo, 1999). Most plants are very sensitive to salt, which severely affects yield. The excessive presence of salt also has a very negative effect on the soil structure, affecting porosity and water retention properties and can eventually affect fields and make them unsuitable for agricultural practices. More sustainable use of natural resources, land, and water is therefore essential to reverse the degradation of the environment and to ensure sustained productivity.  The development of crop varieties with increased tolerance to abiotic stresses such as drought and salinity is therefore an important strategy (Yamaguchi and Blumwald, 2005) for crop improvement.

 Salt stress and development of salt-tolerant crops by conventional breeding

Salt stress successfully decreases the provision and availability of water inside the soil to plants and hence there is considerable overlap between plant responses to drought,   salinity and photosynthesis and it cause the manufacturing of toxic reactive oxygen species (ROS). The existence of plants that thrive in soils with high level of salts (halophytes) and the occurrence of version among crop cultivars in salt sensitivity, imply that salt tolerance to a great extent which is below genetic control. Halophytes represent approximately 2 % of plant species, however, they may be found among half of the terrestrial plant families and are very variable and diverse. Although the development of tolerance to salt is believed to have occurred independently, during the evolution of landraces or cultivated crop plants, halophytes seem to have evolved the same basic method for dealing with salinity i.e. storing harmful salt ions in the cell vacuole and accumulating organic solutes which act as most protect ants inside the cell cytoplasm (Glenn et al., 1999). Conventional breeding programs for salinity tolerance consist of the improvement of rice, wheat and Indian mustard varieties, tolerant to salt and to alkali soils by means of the Central Soil Salinity Research Institute in Karnal, India and efforts to incorporate salt tolerance to wheat from wild related species (Colmer et al., 2006). A wide variety of genomic tools, which include molecular markers and gene profiling methods, can substantially improve the efficiency of breeding packages, and should be absolutely exploited for conventional breeding initiatives.

 Engineering salt tolerant crops by genetic modification

Arabidopsis has performed a vital role within the elucidation of the basic methods underlying stress tolerance and the knowledge acquired has been transferred to a certain extent for improvement of crop plants (Zhang et al., 2004). Several features make Arabidopsis a perfect model organism i.e. having a small fully-sequenced genome, small size and a quick life cycle. In addition, genomic resources are available for Arabidopsis and can be used to enhance our information of the identical approaches in crop plants that are less responsive to genetic studies. Many of the genes recognized to be involved in stress tolerance were isolated from Arabidopsis (Shinozaki and Yamaguchi-Shinozaki, 2007). The development of salt-tolerant crops by genetic engineering have focused on the subsequent strategies by increasing the plant’s potential to restriction the uptake of salt ions from the soil, extrusion of salt ions and improving the compartmentalization of salt ions in the cell vacuole where they do not affect cellular functions.

Salt intake is controlled by low and high affinity ion transporters i.e. trans-membrane proteins that move ions across the cell membrane, which are also required for the intake of K+ ions and Ca2+ ions. The efflux of ions (Figure 1) from the plant depends on the activity of the SOS1 gene (for Salt Overly Sensitive1), initially characterized in Arabidopsis but recently identified in rice, and shown to be functionally conserved between dicots and monocots (Martínez - Atienza et al., 2007).  Over-expression of NHX1 genes in Arabidopsis, rice and tomato have been reported to increase the tolerance to salt stress. A scheme of salt tolerance mechanism in plant is represented in Figure 1 which is based on SOS response and involves in entry of Na+ ion to vacuole, sodium entry blocked into cytoplasm and efflux to cytoplasm conferring salinity tolerance in plant.

 Conclusion