Ion transport and osmotic adjustment in plants and bacteria.

Plants and bacteria respond to hyperosmotic stress by an increase in intracellular osmolality, adjusting their cell turgor to altered growth conditions. This can be achieved either by increased uptake or de novo synthesis of a variety of organic osmolytes (so-called 'compatible solutes'), or by controlling fluxes of ions across cellular membranes. The relative contributions of each of these mechanisms have been debated in literature for many years and remain unresolved. This paper summarises all the arguments and reopens a discussion on the efficiency and strategies of osmotic adjustment in plants and bacteria. We show that the bulk of osmotic adjustment in both plants and bacteria is achieved by increased accumulation of inorganic osmolytes such as K+, Na+ and Cl-. This is applicable to both halophyte and glycophyte species. At the same time, de novo synthesis of compatible solutes is an energetically expensive and slow option and can be used only for the fine adjustment of the cell osmotic potential. The most likely role the organic osmolytes play in osmotic adjustment is in osmoprotection of key membrane transport proteins and reactive oxygen species (ROS) scavenging. The specific mechanisms by which compatible solutes regulate activity of ion transporters remain elusive and require more thorough investigation. It is concluded that creating transgenic species with increased levels of organic osmolytes by itself is counterproductive due to high yield penalties; all these attempts should be complemented by a concurrent increase in the accumulation of inorganic ions directly used for osmotic adjustment.