Rapid, Futile K+ Cycling and Pool-Size Dynamics Define Low-Affinity Potassium Transport in Barley

Abstract

Using the short-lived radiotracer 42K+, we present a comprehensive subcellular flux analysis of low-affinity K+ transport in plants. We overturn the paradigm of cytosolic K+ pool-size homeostasis and demonstrate that low-affinity K+ transport is characterized by futile cycling of K+ at the plasma membrane. Using two methods of compartmental analysis in intact seedlings of barley (Hordeum vulgare L. cv Klondike), we present data for steady-state unidirectional influx, efflux, net flux, cytosolic pool size, and exchange kinetics, and show that, with increasing external [K+] ([K+]ext), both influx and efflux increase dramatically, and that the ratio of efflux to influx exceeds 70% at [K+]ext ≥ 20 mm. Increasing [K+]ext, furthermore, leads to a shortening of the half-time for cytosolic K+ exchange, to values 2 to 3 times lower than are characteristic of high-affinity transport. Cytosolic K+ concentrations are shown to vary between 40 and 200 mm, depending on [K+]ext, on nitrogen treatment (NO3− or NH4+), and on the dominant mode of transport (high- or low-affinity transport), illustrating the dynamic nature of the cytosolic K+ pool, rather than its homeostatic maintenance. Based on measurements of trans-plasma membrane electrical potential, estimates of cytosolic K+ pool size, and the magnitude of unidirectional K+ fluxes, we describe efflux as the most energetically demanding of the cellular K+ fluxes that constitute low-affinity transport.

Tracing Cationic Nutrients from Xylem into Stem Tissue of French Bean by Stable Isotope Tracers and Cryo-Secondary Ion Mass Spectrometry

“…Nevertheless, a symplastic tracer fraction was detectable by 20 min, even though a normally high symplastic concentration (Leigh, 2001) would dilute the tracer. This suggests a fast potassium turnover there, maybe a sign of the transmembrane fluxes associated with the regulation of turgor and concentration (Szczerba et al, 2006).…”

Apoplast Acidification in Growing Barley (Hordeum vulgare L.) Leaves

“…Growth started to increase within minutes ( Having tested the responsiveness of the LVDT setup, treatments were applied. (Kronzucker et al, 2003;Rodriguez-Navarro & Rubio, 2006;Szczerba et al, 2006). The effect of fusicoccin on elongation growth was dependent on the K + concentration in the bathing medium which was in direct contact with the leaf elongation zone ( Fig.…”

Relaciones amonio/nitrato en soluciones nutritivas ácidas y alcalinas para arándano

“…Estos niveles de K + rebasaron por mucho a los señalados por Hanson y Hancock (1996) como óptimos para arándano (0.4-0.65%). Se ha señalado (Szczerba et al, 2006) que la absorción, y por tanto la concentración de K + en hojas y otro tejidos de la planta dependen en gran medida de nivel de NH 4 + , ya que éstos compiten para ingresar a la célula.…”

“…These levels of K + greatly exceeded by those reported by Hanson and Hancock (1996) as optimal for Blueberry (0.4 to 0.65%). It has been noted (Szczerba et al, 2006) that the absorption, and therefore the K + concentration in leaves and other tissues of the plant depend greatly on the level of NH 4 + , as they compete to enter the cell.…”