Genetic basis of ion exclusion in salinity stressed wheat: implications in improving crop yield

Productivity of food crops like wheat, a staple food of major portion of the world, is hampered due to salinity stress, thereby threatening food security. With the advancements in plant physiology and molecular biology based techniques, a number of structural and regulatory genes (transcription factors, miRNA, siRNA etc.) have been identified that contribute to inducing salinity tolerance responses. Location of these salt tolerant genes or genetic loci on specific chromosomes has also been partially characterized through QTL mapping. This information helps in efficient transfer of these genes into other crop cultivars through molecular breeding tools. Although plant salt tolerance mechanisms include osmotic adjustment, ion exclusion, ion inclusion, efficient antioxidant system, hormonal signaling etc., ion exclusion has long been recognized as central to salinity tolerance in wheat. Ion exclusion involves highly coordinated activity of a variety of channels, pumps and antiporters at parenchyma cells in root, stem and leaves associated with xylem. A number of molecular markers have been identified which are associated with ion exclusion or Na(+)exclusion. However, some of them are associated with undesirable traits thereby producing lower crop productivity, e.g.Kna1. Thus, it is imperative to assess novel sources of ion exclusion with subsequent characterization before their introgression into other crop cultivars. In this review, recent advancements in identifying novel sources of ion exclusion in wheat have been discussed at length. In addition, up to what extent marker assisted breeding using these ion exclusion sources will help improve crop salt tolerance in wheat, thereby reducing global food security threat. An effective new paradigm is the targeted identification of specific genetic determinants of stress adaptation that have evolved in nature and their precise introgression into elite varieties.