Phosphate Absorption Rates and Adenosine 5′-Triphosphate Concentrations in Corn Root Tissue

Abstract

The correlations between ATP concentration in corn (Zea mays) root tissue and the rate of phosphate absorption by the tissue have been examined. Experimental variation was secured with 2, 4-dinitrophenol, oligomycin, mersalyl, L-ethionine, 2-deoxyglucose, N2 gassing and inhibition of protein synthesis. It is concluded that ATP could be the energy source for potassium phosphate absorption, but only if the transport mechanism possesses certain properties: oligomycin-sensitivity; creation of a proton gradient susceptible to collapse by uncouplers; phosphate transport via a mersalyl-sensitive Pi--OH-transporter; good activity at energy charge as low as 0. This question is receiving a good deal of attention (9,10,15,16,19,28,37), and it seems to be widely believed that the requirement for aerobic respiration or light can be explained in terms of ATP production, at least for cation uptake. The recent work of Hodges and associates demonstrates that a Mg-requiring, cation-stimulated ATPase is present on plasmalemma from oat roots (20,26,27), and that the correlation between ATPase activity and ion uptake is exceptionally good (12 (2,32) could find no correlation between ATP levels in washed carrot and beet tissue and the rates of K+, Na+ and Cluptake. Experimental variations were obtained with anaerobiosis, uncouplers, inhibitors of ATP formation, and adenosinetrapping agents. In washed beet root tissue with low concentrations of salt, anaerobiosis rapidly depressed salt uptake without affecting ATP concentrations significantly. Conversely, L-ethionine lowered ATP concentrations with little depression of ion uptake. Uncouplers gave inhibition of both ion uptake and ATP concentration, but the inhibition of ion uptake preceded the extensive depletion of ATP. Inhibitors of ATP production, such as oligomycin or Dio-9, had a delayed effect on ATP concentrations and ion uptake. Polya (31) found no evidence directly linking protein synthesis to ion uptake, although indirect effects were suggested. Polya and Atkinson (32) believe their results are consistent with transport mechanisms directly involving the mitochondria and oxygen-terminated electron transport.Cram (8) studied active C1-influx into washed carrot root discs and concluded that uncouplers and oligomycin affected only transport into the vacuole. The Cl-influx pump at the plasmalemma was deduced to be more closely linked to the redox reactions of the respiratory chain than to ATP production, thus supporting Atkinson and Polya.Here we report experiments similar to those of Atkinson and Polya and with the same end in mind, but using washed corn root tissue. The correlation obtained between phosphate absorption rates and ATP concentrations is highly dependent on the treatment used to vary ATP. However, in our opinion, the results are not inconsistent with ATP being the basic energy source for active phosphate transport, provided that the mechanism has certain properties.MATERIALS AND METHODS Two-centimeter segments of primary root of corn seedlings (Zea inays L...

Potassium and Phosphate Uptake in Corn Roots

“…In the present study, washing corn root segments in aerated K-phosphate and CaCl2 solutions increased the rate of K+ (wRb) uptake by 5-fold with little or no lag and the rate leveled off after 4 h. In contrast, washing increased phosphate uptake by only 2-to 3-fold during the same 4-h period with an initial lag of 30 to 40 min. These results agree well with previous studies on the stimulation of ion uptake by washing (5,13,16,19 …”

“…DES concentrations of above 50 I.M strongly inhibited both K+ and Pi uptake but the maximum degree of inhibition was greater for K+ than Pi. A plausible explanation for this differential effect of DES could be that DES at low concentration inhibited only plasmalemma ATPase-mediated K+/H+ exchange (2, 10, 18) without affecting nonplasmalemma ATPase-mediated Pi uptake (8,11,16,17). The inhibitory effect of DES at high concentrations on Pi uptake could be due to the inhibition of ATP production and utilization inside the cells (2).…”

“…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) are used for K+ and Pi uptake, respectively, it should be possible to differentiate the energycoupling requirements for these two ions either by changing the proton gradient across the membrane or by chemically modifying the transport carrier. Accordingly, several chemical probes such as the ATPase stimulator, FC2 (18), the inhibitor, DES (2), the OH-/Pi antiporter inhibitor, mersalyl (8,16) 4 h, or more, are "stable" in their physiological activities (7,20), most of the experiments were conducted using washed tissues.…”

Inhibition of Anion Transport in Corn Root Protoplasts

“…Except for the time course experiments, a 15-min radioactive exposure period was employed. Previously described methods were used to measure the ion influx into 4 h-washed excised root segments at 0.2 mm salt concentration (12) and plasmalemma ATPase activity (14). All experiments were duplicated.…”