Salt Sensitivity and the Activities of the H+‐ATPases in Cotton Seedlings

Abstract

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 hvpocotyl length of seedlings grown in 75 rm M / NaCI 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 PMand V‐ATPases from roots in salt‐sensitive cotton seedlings grown with or without 75 rm M / NaCI. Higher PM‐ATPase activity (42%) was observed in seedlings grown in 75 rm M / NaCI. 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 part, from a lack of effective driving force for compartmentalization of Na + .