ABSTRACT
Crop yield and global food security are under the constant threat of environmental vagaries. The development of stress-tolerant crop varieties by conventional breeding, molecular marker-assisted breeding or transgenic technology is pertinent to ensuring the food security of the burgeoning human and livestock population. In this chapter, the role of osmolyte accumulation in salinity tolerance of crops by combating ion toxicity and oxidative stress-induced damages is detailed. Changes in osmolyte accumulation are a potent strategy employed by halophytes and tolerant varieties of glycophytes. Osmolyte accumulation is primarily implicated in improving cellular water potential, detoxifying reactive oxygen species (ROS) and maintaining the ionic balance of salinity-stressed plants. Sugars, polyols, quaternary amines and amino acids are the four major groups of compatible solutes; in most compatible solutes, the cellular concentration is often far too low for a conventional osmotic function. This indirectly influences cytosolic osmotic adjustment through regulatory or osmoprotective operations. Exogenously supplied proline and glycine betaine rapidly ameliorate NaCl-induced potassium ion (K+) efflux from Arabidopsis and barley roots, while 21 out of the 26 amino acids prevented K+ efflux from the root epidermis of barley when applied in physiologically relevant concentrations. Transgenic plants overproducing glycine betaine or trehalose were efficient in ROS homeostasis and K+ retention. To elucidate the correlation between osmolyte accumulation and ion homeostasis, and salt tolerance, a thorough understanding of osmolyte-related genes and the upstream and downstream signalling pathways is required. The emerging role of different plant hormones and other stress proteins functioning as compatible osmolytes opens a new avenue for improving salt tolerance through genetic engineering and molecular breeding.
