Vacuolar energization: Pumps, shunts and stress

Abstract

Two H+-pumps may co-reside at the plant vacuolar membrane-the V-PPase and V-ATPase. Elucidation of their transport characteristics by patch clamp electrophysiology has indicated the possibility of distinct physiological roles. The V-ATPase may predominate in vacuolar acidification while the V-PPase may facilitate vacuolar K+ accumulation. Little is known of how pump activity is regulated in vivo, but an ATP4- -regulated vacuolar shunt conductance may help clamp the membrane voltage to permit continued lumenal acidification. Both pumps respond to stress in order to maintain the critical functions of the vacuole. Under chill or hypoxic stress, V-PPase transcript levels and activity can increase to counter the impaired activity of the V-ATPase as ATP levels drop and the latter enzyme dissociates. During salt stress, the subunit composition of the V-ATPase may be modulated and its activity increased to power enhanced vacuolar Na+ sequestration.