Hydrolytic and pumping activity of H+-ATPase from leaves of sugar beet (Beta vulgaris L.) as affected by salt stress

Cell wall extensibility plays an important role in plant growth. According to the acid-growth theory, lower apoplastic pH allows extension growth by affecting cell wall extensibility. A lowered apoplastic pH is presumed to activate wall-loosening enzymes that control plant growth. Plasma membrane (PM) H+-ATPases play a major role in the apoplastic acidification by H+ transport from cytosol to the apoplast. A salt-induced decrease in H+-pumping activity of plasma membrane H+-ATPases in salt-sensitive maize plants has previously been found. This led us to formulate the hypothesis that salt-resistant plant species such as sugar beet (Beta vulgaris L.) may have a mechanism to eliminate the effect of higher salt concentrations on plasma membrane H+-ATPase activity. In the present study, sugar beet plants were grown in 1 mM NaCl (control) or 150 mM NaCl in hydroponics. H+-ATPase hydrolytic and pumping activities were measured in plasma membrane vesicles isolated from sugar beet shoots. We found that plasma membrane H+-ATPase hydrolytic and pumping activities were not affected by application of 150 mM NaCl. Moreover, apoplastic pH was also not affected under salt stress. However, a decrease in plant growth was observed. We assume that growth reduction was not due to a decrease in PM-H+-ATPase activity, but that other factors may be responsible for growth inhibition of sugar beet plants under salt stress.