Evidence is presented that K+ uptake in corn root segments is coupled to an electrogenic H+/K+-exchanging plasmalemma ATPase while phosphate uptake is coupled to an OH-/Pi antiporter. Previous studies with excised corn root segments have shown that washing immediately increased K+ uptake activity (5, 19) and membrane potential (15) with uptake increasing by 3-to 7-fold and the membrane potential increasing to a maximum of 25 mv within 4 h. In contrast to K+, Pi uptake (13, 16) increases only 2-3-fold after a 30-min lag period. Based on these washing response data and the study of the effect of dithioerythritol on membrane potential, resistance, and proton fluxes (17), Hanson and Lin (7,17) suggested that active salt transport and its regulation arise from the functioning of a proton-extruding, cation-carrying ATPase in the membrane, balanced by H+/cation and OH-/anion antiporters. If the main driving forces, A I and ApH generated by electrogenic proton pumps (10)
MATERIALS AND METHODSCorn seeds (Zea mays L. B73 x Missouri 17) were germinated and 2-cm (0.5-2.5 cm from the tip) root segments from 3-day-old etiolated seedlings were collected and washed as described by Leonard and Hanson (13). The term "washed tissue" in this paper refers to root tissue washed for 4 h at 30 C in a standard medium of0.2 mM K-phosphate (pH 6.0) plus 0.2 mm CaCl2. Pretreatment with inhibitors was carried out in the same medium for an additional 30 min. Uptake rates were measured by incubating 20 cheesecloth-bound root segments (about 0.3 g of fresh weight) for 30 min (except those in the kinetic studies as noted in figures) in 100 ml of either washing solution labeled with 10,000 cpm/ml of carrier-free 3P in standard medium or wRb in 0.2 mM KCI and CaCl2 solution at pH 6.0. The incubation temperature for uptake was kept at 30 C. Washing, pretreatment, and absorption solutions were held in a water bath with constant shaking (50 cycles/min) and forced aeration. Since tissues that have been washed for 4 h, or more, are "stable" in their physiological activities (7,20), most of the experiments were conducted using washed tissues.In the pH experiments 1 mm Hepes was added to the absorption solution and the pH was adjusted with HCI or NaOH. For proton flux measurements, 80 root segments (about 1.2 g fresh weight) were placed directly in 200 ml of aerated standard medium at 30 C and the pH of the solution was monitored continuously. Net proton flux rate at steady-state was calculated as described by Lin and Hanson (15). Respiration of root tissue was determined with an 02 electrode at 30 C with 10 1-cm root sections in 10 ml of airsaturated standard medium as previously described (17).FC was a generous gift from Dr. E. Marre, and DES, mersalyl, and PCMBS were purchased from Sigma Chemical Co. All other chemicals were ACS reagent grade.
RESULTS AND DISCUSSIONAging of plant tissue by washing enhances many physiological activities such as ion uptake, respiration, and cell membrane potential (7,20). In the present study, washing corn ro...