Smart Engineering of Genetic Resources for Enhanced Salinity Tolerance in Crop Plants

Salinity is a consistent factor of crop productivity loss in the world and in particular arid and semi-arid areas where the soil salinity and saline water are major problems. Plants employ various mechanisms to cope with salinity stress and activate an array of stress-responsive genes to counteract the salinity-induced osmotic and ionic stresses. Genetic improvement for salinity tolerance is challenging, and thus progress attained over the several decades has been far less than anticipated. The generation of an explosion of knowledge and technology related to genetics and genomics over the last few decades is promising in providing powerful tools for future development of salinity-tolerant cultivars. Despite a major progress in defining the underlying mechanisms of salinity tolerance, there are still major challenges to be overcome in translating and integrating the resultant information at the molecular level into plant-breeding practices. Various approaches have been suggested to improve the efficiency of plant breeding for increasing plant productivity under saline environments. In this context, breeding for salinity tolerance in crops largely depends upon the availability of genetic resources of tolerance, reliable screening techniques, identification of genetic components of tolerance, and successful genetic manipulation of desired genetic backgrounds. The efficiency of selection and breeding in the stressful environments can be improved through marker-assisted selection. To date, this is almost exclusively applied to major genes, but this requires to be extended to quantitative trait loci (QTLs) controlling complex traits such as salinity tolerance to greatly enhance the impact. Moreover, methodologies for high-throughput genotyping, and the development of an array of "functional" markers can be much supportive. The introduction of novel genes or alteration in the expression patterns of the existing genes through the generation of transgenic plants can also be employed to overcome the limits in classical plant breeding. The introgression of wild halophytic attribute genes facilitated by genetic engineering is an alternative approach to bypass interspecific hybridization barriers, which will stimulate breakthrough in the future agriculture. The molecular dissection of salinity-tolerance trait, accompanying the classical quantitative genetics, is a substantial progress in updating tools and methods for the manipulation of plant genomes. Methods of gene discovery such as identification of candidate genes, QTL cloning, linkage and association mapping, and functional genomics such as identification of transcripts and proteins involved in salinity tolerance are necessary to manipulate the molecular mechanisms underlying the complex phenotype of salinity tolerance. Some of the challenges and opportunities have also been addressed in the present review with a particular emphasis on molecular breeding approaches to be employed in combination with other crop improvement strategies to develop salinity-tolerant cultivars.