Salt sensitivity and the activities of the H⁺-ATPases in cotton seedlings

Salinity is a major problem confronting agriculture in arid environments. Sensitivity to high levels of salt in plants is associated with an inability to effectively remove Na⁺ ions from the cell cytoplasm. The ability to compartmentalize Na⁺ may result, in part, from stimulation of the H⁺-ATPases on the plasma membrane (PM-ATPase) and vacuolar membrane (V- ATPase). These H⁺-pumping ATPases may provide the driving force for Na⁺ transport via Na⁺-H⁺ exchangers. In a salt-sensitive line of cotton (Gossypium hirsutum L.), greater relative reductions in root length and root fresh weight than in hypocotyl length of seedlings grown in 75 mM NaCl indicated that the root was most affected by salt stress. To determine if the H⁺-ATPases are involved in the response to salt, we compared activities of the PM- and V-ATPases from roots in salt-sensitive cotton seedlings grown with or without 75 mM NaCl. Higher PM-ATPase activity (42%) was observed in seedlings grown in 75 mM NaCl. This stimulation was specific for Na⁺, was not observed when Na⁺ was added to membrane fractions, and was not due to an increase in PM-ATPase protein levels. V-ATPase protein accumulation was unaffected by growth in the presence of Na⁺, and activity was unaffected by Na⁺ in the growth medium or by Na⁺ added to membrane fractions. These studies suggest that although the PM-ATPase responds to increased Na⁺, activity of the transport proteins on the plasma membrane alone may be insufficient to regulate intracellular Na⁺ levels. In addition, the inability of the V-ATPase to respond to increased levels of Na⁺ indicates that salt sensitivity in cotton seedlings may result, in port, from a lack of effective driving force for compartmentalization of Na⁺.