Multiphasic uptake of phosphate by corn roots

Abstract

The concentration dependence of phosphate uptake was studied using root sections of corn (Zefl mays L. cv. Ganga 5). Detailed and widerange (57 concentrations in the range 1 /uniol m~^-75 mol m"-^), precise (average SEM <2.5%, n = 6) and reproducible (similar patterns in three independent experiments and for 5, 10, 15, 20, 25 and 30 °C) data revealed six (or seven) concentration-dependent phases separated by 'jumps' or sharp breaks. These transitions were independent of temperature and occurred over relatively narrow concentration ranges (0.0001-0.0004, 0.08-0.31, 1.0-3.5, (7.5-10), 18-20 and 57-59 mol m"-^). The intermediate phases obeyed Michaelis-Menten kinetics, whereas sigmoidal kinetics were observed at lower concentrations. Uptake within each of the two highest phases increased more rapidly with increasing external phosphate concentration than predicted from Michaelis-Menten kinetics but also saturated more rapidly. The latter finding is not consistent with free diffusion across the plasmalemma at high external phosphate concentrations. Kinetic models yielding continuous isotherms, e.g. the sum of one or two Michaelis-Menten terms and a diffusion term, cannot account for the data.

pho2, a Phosphate Overaccumulator, Is Caused by a Nonsense Mutation in a MicroRNA399 Target Gene

“…A single-phase or multiphase Pi transport system has been described in many plant species or cultured cells (Ullrich-Eberius et al, 1984;McPharlin and Bieleski, 1987;Nandi et al, 1987;Mimura et al, 1990;Furihata et al, 1992;Shimogawara and Usuda, 1995;Dunlop et al, 1997;Narang et al, 2000). Our data suggest that Arabidopsis possesses a dual Pi uptake system under the conditions we analyzed: a lowaffinity system operating at high Pi concentration (K m 5 120-130 mM) and a high-affinity system operating at low Pi concentration (K m 5 10-11 mM; Fig.…”

Multiphasic Uptake of Potassium by Corn Roots

“…The former model (also termed single + diffusion) has also been shown to be inadequate for many other sets of data for uptake of potassium and other solutes (13,17), whereas the multiphasic model has been extensively tested and validated (13,(15)(16)(17)20 Despite all the evidence against it, the existence ofa separate linear component has recently been widely accepted. Inadequate consideration of available evidence is partly to blame for this.…”

“…The finding of separate uptake and transition sites (25) provides good evidence for such discontinuities, as does detailed and precise data for uptake of phosphate (13) and other solutes (17). Models yielding continuous isotherms cannot satisfactorily account for the kinetics of ion uptake in plants (17,19).…”

“…2). Significance of fit to adjacent phases was determined by an F-test (20).As a measure of goodness of fit, MSD' was used:Despite extensive studies over a quarter of a century, there is still no consensus about the number and nature of uptake mechanisms in plants (13,17,21). Recently, the existence of a 'linear component' has been widely accepted, in part on the basis of work (6-9) purporting to show that uptake of potassium by corn roots can be resolved into a saturable and a linear component.…”

An altered phosphate uptake system in arsenate‐tolerant Holcus lanatus L.

“…The observed departure from single hyperbolic kinetics in the uptake of a range of ions is usually explained in terms of a dual uptake mechanism, consisting of high-and low-affinity uptake systems (Epstein, 1976). Although other uptake mechanisms have been used to explain ion uptake isotherms, the major alternative to the dual uptake is the single, multiphasic uptake mechanism (Nissen, 1973;Nissen & Nissen, 1983;Nandi, Pant & Nissen, 1987), where uptake isotherms are represented as consisting of a number of successive Michaelis-Menten phases, separated by discontinuous transitions. Criticism of this mechanism is based on the statistical tests applied to the data to show the multiplicity of transition points (Borstlap, 1981).…”