Potassium Transport in Corn Roots

Abstract

Influx isotherms were obtained for MRb+ uptake into 2-cm corn (Zea mays IA632 x (C3640 x Oh43)1 root segments for both low-(0.2 millimolar CaSO4) and high-salt (0.2 millimolar CaSO4 + 5 millimolar KCI) grown roots. Unlike the discontinuous curves usually presented for K+ influx, our isotherms were smooth, nonsaturating curves that approached linearity at K+ (Rb+) concentrations above 1 millimolar. The The effects of NEM and PCMBS on K' efflux were also studied. Short NEM exposures had no effect on cytoplasmic efflux, while inhibiting vacuolar efflux significantly. From these data, it is unclear at which site(s) NEM is acting. A more complex response was obtained with PCMBS, where a monophasic efflux curve was observed. Analysis indicated that the vacuolar efflux was stimulated, while the cytoplasmic component was abolished.The nature of the linear component is discussed, and it is proposed that the mechanism may be more complex than simple facilitated diffusion.It is generally accepted that K+ uptake isotherms for roots of higher plants are complex (1 1, 25). The appearance of discontinuities in influx isotherms led to Epstein's formulation of the now well-known dual-isotherm hypothesis (12), in which it was proposed that two carriers were located in the plasmalemma, each carrier having different Michaelis-Menten parameters. In opposition to this hypothesis, Nissen has argued that such kinetics are due to a single, complex transporter which undergoes transitions in response to changes in external ion concentration (29).It has also been hypothesized that a diffusion limitation of ions exists at low external concentrations, such that their availability for uptake by root cortical cells is significantly reduced (2,10 (4) has also presented a critique of the multiphasic interpretation of uptake. He argues that many of the frequently cited discontinuous uptake isotherms could be explained by models consisting of one or more Michaelis-Menten terms plus a linear term, particularly if experimental error and biological variability are considered. The existence of this linear term has often been ignored, despite the fact that transport kinetics consisting of Michaelis-Menten and linear terms have been well documented in both plant and animal systems (3,5,8,9,13,20,22,27,28,32

Altered Zn Compartmentation in the Root Symplasm and Stimulated Zn Absorption into the Leaf as Mechanisms Involved in Zn Hyperaccumulation in Thlaspi caerulescens

“…We have used this approach previously in several different studies to investigate the root compartmentation of a number of monovalent and divalent cations (Kochian and Lucas, 1982;Hart et al, 1992;DiTomaso et al, 1993;Lasat et al, 1997) and are very familiar with its limitations. This technique was developed to study ion compartmentation in single giant algal cells, in which the ion content of the individual compartments (vacuole, cytoplasm, and cell wall) can be measured directly (MacRobbie, 1971).…”

“…However, in compartmental studies with oat coleoptiles (Pierce and Higinbotham, 1970) and carrot root tissues (Cram, 1968), the authors provided strong arguments and analyses that allowed them to validate this technique for the application of the threecompartment-in-series model to plant organs such as roots and leaves. For a detailed examination of the application of this technique for ion compartmentation in roots, see Kochian and Lucas (1982).…”

“…Unfortunately, no other experimental method is currently available to investigate ion compartmentation in a semiquantitative manner. Moreover, this technique has been used to provide valuable information on the efflux and compartmentation of several ions such as K ϩ (Kochian and Lucas, 1982), Na ϩ (Pierce and Higinbotham, 1970), NH 4 ϩ (Macklon et al, 1990), Cd 2ϩ (Rauser, 1987), Cu 2ϩ (Thornton, 1991), Zn 2ϩ (Santa Maria and Cogliatti, 1988), and Co 2ϩ (Macklon and Sim, 1987).…”

Can K+ Channels Do It All?

“…Consider K+ absorption into low saltgrown maize roots, for which we have previously characterized high-affinity K+ uptake (Kochian and Lucas, 1982;Newman et al, 1987;Kochian et al, 1989). The steady state net K+ influx into the maize root, measured using avibrating extracellular K+ microelectrode, was approximately 7 pmol cm-2 s l , and the E , measured simultaneously on the same root epidermal cell had a value of -110 mV.…”

Membrane depolarization induces K+ efflux from subapical maize root segments

“…Nevertheless, even if this technique allows in vivo measurements in whole organs or plants, it does not resolve net ion fluxes into their unidirectional components. The unidirectional radioactive or nonradioactive tracer technique is commonly used for the in vivo study of the activities of ion channels in intact organs or isolated cells (Kochian & Lucas, 1982;Smith et al , 1987;MacRobbie, 1997). Nevertheless, the in vivo characterization of K + efflux from the cells is limited by the activity of the high-affinity K + uptake system (Maathuis & Sanders, 1996;RodriguezNavarro, 2000), which rapidly and efficiently reabsorbs K + released from cells.…”