On the possible role of fusicoccin-binding proteins in plant adaptation to stress

Protoplasts alkalizing the cytoplasm when bound to fusicoccin (FC) were isolated from the suspension culture of sugar beet (Beta vulgaris L.) cells. Two kinds of FC-binding sites, which differed in their affinity for FC by two orders of magnitude, were detected on plasma membranes of isolated protoplasts. The balance between FC-binding proteins of different affinity observed at the standard conditions changed in favor of high-affinity proteins under the conditions of cytosol acidification. This shift was reversible and occurred in the narrow physiological range of pH values. It is supposed that the changes in affinity may be accounted for by the conformational changes generated in FC-binding proteins by the shifts in cytosolic pH. A possible scheme for the control of H+-ATPase activity of the plasma membrane by FC-binding proteins in stress-affected plants is proposed.