Characterization of the Vacuolar Proton Pumps of the Cortex, Stele and Tip of Maize Roots: Effects of Salinity

Abstract

AbstractTonoplast and Golgi ATP‐driven proton pumping was measured in membrane vesicles isolated from the cortex, stele and tip of maize roots. Two nitrate‐sensitive peaks of tonoplast proton pumping were detected on sucrose gradients, a light peak at 16% sucrose, believed to represent small vacuoles, and a denser peak at 25% sucrose, possibly derived from large vacuoles. The nitrate‐insensitive Golgi peak occurred at 31 % sucrose. Membrane preparations of the stele and tip were active in H+‐transport activity and were rich in mitochondria. In contrast, the membranes of the cortex had low levels of both ATP‐driven proton pumping and mitochondrial activities. Salinity caused a doubling of the tonoplast H+‐transport activity of the tip and cortex, particularly in the 25 % sucrose peak. The enhancement of the 25 % peak by salinity was correlated with an increase in vacuolation of the tip. The stelar proton pumps were only slightly stimulated by salinity. The results indicate that the cortex of maize roots is deficient in the metabolic machinery needed for vacuolar solute accumulation, but that it shows some ability to adapt to saline conditions. The physiological implications of these findings are discussed.