Application of the Chemiosmotic Hypothesis to Ion Transport Across the Root

Abstract

The evidence on how ions accumulated in the root symplasm are released to the xylem vessels is exam_ied. It is suggested that Mitchell's chemios-motic hypothesis as appliUed to Ion transport might account for the process. A model based on this hypothesis shows the symplasm as an osmotic unit connecting two isolated solutions, but with no significant difference in proton motive force across the unit. If it is assumed that the resistance to transport by plasmalemma uniports and antiports differs in the cortical and stelar ends of the symplasm, the model will provide for an influx of ions to the xylem. The reported properties of isolated steles suggests that the porters (carriers) do have properties in accord with the model. There is still uncertainty as to how ions actively accumulated by the epidermal and cortical cells of the root are passed to the xylem. The Crafts-Broyer hypothesis (9) proposed cell-to-cell cytoplasmic transport (symplasm transport) through the osmotic barrier of the endodermis into the compact stele, where there is leakage of ions into the xylem due to lower 02 tension and lower metabolic rate. There is now little doubt that the living cells of the root do form a symplasm through interconnecting plasmodesmata much as Crafts and Broyer visualized (1, 8, 19, 26) but support for the latter part of the hypothesis has not been obtained. Microelec-trode determinations of 02 partial pressure in the stele do not indicate concentrations limiting to respiration (4, cf. 13). Roots accumulate ions into the stele at concentrations comparable to those in the cortex (2, 5, 11, 28), and x-ray microanalysis shows the xylem parenchyma to have high concentrations of K+ (19). There is substantial evidence that ion transport into the dead xylem vessels shares critical properties with transport into living root cells: i.e. transport is energy-linked and produces concentration and electrical potential gradients relative to the external solution (1, 5, 8, 11). Furthermore, there is an electrogenic component to the electrical potential (10, 25). The uncertainty lies with what these observations signify about release of ions from symplasm to xylem. Lauchli (19) and Pitman (24) summarized the arguments for an active secretion of ions from the xylem parenchyma, which serves as a sink for ions that move across the symplasm from the epidermis-cortex. Briefly, xylem parenchyma cells show characteristics of an active metabolism , including cytoplasm with abundant mitochondria and ER, and plasmalemma ATPase activity; inhibitor studies clearly show that transport to the xylem can be blocked without blocking ion absorption. Oddly enough, certain inhibitors of protein synthesis, cycloheximide orp-fluorophenylalanine (which produces ineffec-'bana, Illinois 61801 (address for reprint requests). tive proteins), are particularly effective, suggesting rapid protein synthesis as essential to ion secretion. On the other hand there are opinions that the release is passive (2, 5, 18). Bowling (5) concluded "that radial transport is driven by an active step at the outer surface of the root whilst movement from the living cells into the vessels is passive" (i.e. down an electrochemical gradient). Baker (2) expressed a similar opinion. Bowling (5) showed that cell potentials and ion activities appear to be stable across the symplasm but drop significantly in the xylem exudate, although the latter still has high ion activities and a negative electrical potential compared with the external solution. An important experiment in demonstrating the intimate relationship between symplast and xylem was that of Dunlop and Bowling (12) showing rapid and parallel depolarization of epidermal cell and xylem exudate electrical potential upon increasing the KCI concentration of the medium. Baker (2) found that steles in intact corn roots were active in ion accumulation, as reported by Yu and Kramer (28), while freshly isolated steles were not, in agreement with Laties and Budd (18) and Luttge and Laties (20). Uncouplers prevented ion uptake into the stele by preventing uptake into the epidermis and cortex. Leakage of accumulated ions from isolated steles was more rapid than from the cortex (2, 18) and uncouplers accelerated the loss (2). With respect to these opposing hypotheses, there is an aspect of Mitchell's chemiosmotic hypothesis (22, 23) which has not been considered; i.e. the proton motive force (electrochemical gradient of protons) created by respiratory chain "loops" or by ATP hydrolysis can be utilized to drive a net influx or a net efflux of transportable ions. The concept is simple, and elements of it have been around for some years. In this paper I attempt to show how it might be applied to symplasm transport. HYPOTHESIS The phenomenon of influx and efflux salt pumping has been extensively studied in mitochondria, and the evidence from animal (6) and plant (14) mitochondria has been reviewed. Brierley and colleagues (7, 16) have recently shown a dynamic energy-linked influx and efflux of K+ in heart mitochondria at osmotic steady-state. In corn mitochondria a similar dynamic osmotic steady-state has been deduced from the rapid osmotic swelling or shrinkage resulting from inhibition or acceleration of carrier activity (15). Figure 1 is a chemiosmotic model of influx and efflux transport. In general terms the model shows an equivalent of salt influx or efflux driven by an equivalent of electrogenic H+ efflux.3 The 3 This equivalency is correct for the model, but it is not established as fact. It is conceivable, for example, that an equivalent of anion influx might be by symport (or co-transport) with two or more equivalents of H+ thus utilizing part of the energy available in the electrical gradient as well as that in the pH gradient. However, this possibility does not affect the basic question explored here, and the hypothesis is presented in its simplest form. 402