Is there an osmotic regulatory mechanism in algae and higher plants?

Abstract

Under water-stress conditions the amounts of various polyols and also of the imino acid proline are found to increase significantly in different algae and higher plants. These substances have been interpreted until now to act osmotically by increasing the concentration in the cell, thus causing water reflux and balancing osmotic pressure difference from outside the cell to inside. A new concept is described, which proposes that the regulatory function of these accumulating substances is conducted by two mechanisms quite different from osmotic regulation. It is assumed that these regulatory pathways are connected with the hydrophobic groups of biopolymers in the cell cytoplasm. (1) Polyols can replace water molecules by means of their water like OH-groups and thus participate in the hydrophobically enforced water structure. (2) Proline is postulated to associate via its hydrophobic phot with hydrophobic side chains, thereby converting them into hydrophilic groups by exposure the carboxylic and imino group versus water molecules. The advantage is due to the fact that water associated with hydrophilic groups is bound via hydrogen bonding forces, in contrary to hydrophobic groups. In addition, the number of water molecules adjoining hydrophilic groups is far less than those involved with hydrophobic residues. By means of these two alternative mechanisms complete hydration of the biopolymers is maintained, even with a reduced number of available water molecules.