Rapid auxin- and fusicoccin-enhanced Rb(+) uptake and malate synthesis in Avena coleoptile sections

Metabolic regulation of pH in plant cells: role of cytoplasmic pH in defense reaction and secondary metabolism

A new biochemical pH-stat hypothesis that revised the classic hypothesis is presented to understand the metabolic regulation of intracellular pH in plant cells. Alternative pathway glycolysis, alternative pathway respiration and malate-derived lactic and alcoholic fermentation (alternative pathway fermentation), all unique to plants, are integrated into a regulatory mechanism of pH in the cytoplasm. Its uniqueness to plant kingdom is discussed from the evolutionary viewpoint: it is suggested that when the ancestors of extant terrestrial plants expanded their habitat from oceans to freshwater, they abandoned a "sodium system" and adopted a "proton system" for nutrient uptake. Validity of the new hypothesis is examined with available data on a secondary active transport, anoxia and other experimental evidence. The hypothesis predicts that biotic and abiotic stress-induced cytoplasmic acidification triggers synthesis of phytoalexins and other secondary metabolites. Possible roles of cyanide-resistant alternative pathway respiration in the secondary metabolite production, metabolic switching between primary and secondary metabolisms, and defense reactions are proposed.

Mechanisms of fusicoccin action: kinetic modification and inactivation of K(+) channels in guard cells

Fusicoccin commonly is thought to promote secondary solute transport via an increase in electrical driving force which follows the enhancement of primary, "electrogenic" H(+) extrusion by the plant plasma membrane H(+)-ATPase. However, previous electrical studies ofVicia faba L. guard cells in FC (Blatt, 1988, Planta174, 187) demonstrated, in addition to a limited rise in pump current, appreciable declines in membrane conductance near and positive to the free-running membrane potential (V m). Much of the current at these potentials could have been carried by outward-rectifying K(+) channels which were progressively inactivated in FC. We have examined this possibility in electrical studies, using whole-cell currents measured under voltage clamp to quantitate steadystate and kinetic characteristics of the K(+) channels both before and during exposure to FC; channels block in tetraethylammonium chloride was exploited to assess changes in background 'leak' currents. The cells showed little evidence of primary pump activity, a fact which further simplified analyses. Under these conditions, outward-directed K(+) channel current contributed to charge balance maintainingV m, and adding 10 μM FC on average depolarized (positive-going)V m. Steady-state current-voltage relations revealed changes both in K(+) channel and in leak currents underlying the voltage response. Changes in the leak were variable, but on average the leak equilibrium potential was shifted (+)19 mV and leak conductance declined by 21% over 30-40 min in FC. Potassium currents were inactivated irreversibly and with halftimes (current maxima) of 6.2-10.7 min. Inactivation was voltage-dependent, so that the activation ("gating") potential for the current was shifted, positive-going, with time in FC. Channel gating kinetics, inferred from the macroscopic currents, were also affected; current rise at positive potentials accelerated 4.5-fold and more, but in a manner apparently independent of voltage and extracellular potassium concentration. Current decay at negative potentials was quickened, also. These results identify the outward-rectifying K(+) channels as one site of action for FC at a higher plant cell membrane; they complete the link introduced in the preceding paper between K(+) channel current, K(+)((86)Rb(+)) flux and irreversible cation uptake in the toxin. The data also offer some insights toward a kinetic description of channel gating. Finally, they provide a vehicle for interpreting FC-induced changes in K(+) and net H(+) flux, and in membrane potential without the necessity for postulating gross changes in H(+) pumping.

Mechanisms of fusicoccin action: evidence for concerted modulations of secondary K(+) transport in a higher plant cell

Fusicoccin (FC) has long been known to promote K(+) uptake in higher plant cells, including stomatal guard cells, yet the precise mechanism behind this enhancement remains uncertain. Membrane hyperpolarization, thought to arise from primary H(+) pumping stimulated in FC, could help drive K(+) uptake, but the extent to which FC stimulates influx and uptake frequently exceeds any reasonable estimates from Constant Field Theory based on changes in the free-running membrane potential (V m) alone; furthermore, unidirectional flux analyses have shown that in the toxin K(+) ((86)Rb(+)) exchange plummets to 10% of the control (G.M. Clint and E.A.C. MacRobbie 1984, J. Exp. Bot.35 180-192). Thus, the activities of specific pathways for K(+) movement across the membrane could be modified in FC. We have explored a role for K(+) channels in mediating these fluxes in guard cells ofVicia faba L. The correspondence between FC-induced changes in chemical ((86)Rb(+)) flux and in electrical current under voltage clamp was followed, using the K(+) channel blocker tetraethylammonium chloride (TEA) to probe tracer and charge movement through K(+)-selective channels. Parallel flux and electrical measurements were carried out when cells showed little evidence of primary pump activity, thus simplifying analyses. Under these conditions, outward-directed K(+) channel current contributed appreciably to charge balance maintainingV m, and adding 10 mM TEA to block the current depolarized (positive-going)V m; TEA also reduced(86)Rb(+) efflux by 68-80%. Following treatments with 10 μM FC, both K(+) channel current and(86)Rb(+) efflux decayed, irreversbly and without apparent lag, to 10%-15% of the controls and with equivalent half-times (approx. 4 min). Fusicoccin also enhanced(86)Rb(+) influx by 13.9-fold, but the influx proved largely insensitive to TEA. Overall, FC promotednet cation uptake in 0.1 mM K(+) (Rb(+)), despite membrane potentials which were 30-60 mVpositive of the K(+) equilibrium potential. These results tentatively link (chemical) cation efflux to charge movement through the K(+) channels. They offer evidence of an energy-coupled mechanism for K(+) uptake in guard cells. Finally, the data reaffirm early suspicions that FC alters profoundly the K(+) transport capacity of the cells, independent of any changes in membrane potential.

Mechanisms of fusicoccin action: A dominant role for secondary transport in a higher-plant cell

Fusicoccin (FC) is commonly thought to promote "electrogenic" H(+) extrusion through its action on the H(+)-ATPase of the plant plasma membrane. Nonetheless, essential support from rigorous electrophysiological analysis has remained largely absent. The present investigation surveys the effects of FC on the charge transport properties at the membrane of a higher-plant cell - stomatal guard cells of Vicia faba L. - for which the electrical geometry is defined, and from which the voltage-dependent kinetic characteristic for the pump has been identified. Current-voltage (I-V) relations of the guard cells were determined before and during treatments with FC, and during brief exposures to NaCN plus salicylhydroxamic acid. Responses of the pump and of the ensemble of secondary transport processes were identified in the whole-membrane conductance-voltage relations and in the difference-current-voltage (dI-V) characteristic for the pump. In 0.1 mM K(+), exposure to 10 μM FC shifted guard-cell potentials negative by 29-61 mV. Current-and conductance-voltage profiles indicated limited changes in the pump I-V characteristic, an observation which was confirmed through explicit kinetic analysis of pump dI-V relations. However, the voltage response was accompanied by a 1.5-to 2.6-fold fall in membrane conductance. These results challenge conventional views of fusicoccin action by ascribing the electrical responses to reduced current passage through secondary transport pathways as well as to enhanced electrogenic ion pumping.

THE SAP OF PLANT CELLS

Recent advances in the understanding of the functional relevance of sap of mature plant cells are reviewed. The emphasis is placed on roles of vacuoles played in the temporary storage of saccharides and organic acids, in the accumulation of water soluble products of secondary metabolism and in the intracellular digestion of protein. Contents Summary 1 I. Introduction 1 II. Functions of vacuoles 2 III. Vacuoles as pools of saccharides 3 IV. Organic acids 7 V. (Potentially) toxic cell saps 9 VI. Pools of protein 14 VII. Digestive cell saps 15 VIII. Tonoplast, cell sap and cytoplasm 18 IX. Cellular homeostasis 19 Acknowledgement 20 References 20.