Osmotic adjustment and plant adaptation to environmental changes related to drought and salinity

The salinization and water deficit of soil are widespread environmental problems in limiting plant survival, growth, and productivity. However, some plants could adopt some strategies to resist salinity and drought stresses. Among these strategies, the mechanism of osmotic adjustment could help plants and algae to avoid ion toxicity and maintain water uptake in both stresses by accumulating large quantities of osmolytes. Two types of osmolytes, organic solutes and inorganic ions, play a key role in osmotic adjustment. Different osmolytes and their osmotic adjustment actions are different according to their distribution in different plants. Organic solutes, known as compatible solutes, include amino acids, glycerol, sugars, and other low molecular weight metabolites, serve a function in cells to lower or balance the osmotic potential of intracellular and extracellular ions in resistance to osmotic stresses. Inorganic ions for osmotic adjustment are mainly Na+, K+, Ca2+, and Cl-. Inorganic ions make great contribution in osmotic adjustment by ion transport processes with related ion antiporters and ion channels. The aim of this review is to integrate recent data on the mechanisms of osmotic adjustment by osmolytes in plants and algae, and to illustrate the variety of related molecular mechanisms, to introduce new concepts and techniques into this research field. Genetic manipulation including the application of transgenic techniques in plants provides promising strategies to elevate the tolerance capability of plants under osmotic stress conditions.