Effects of Fusicoccin and Abscisic Acid on Sugar and lon Transport from Plant Glands

Fusicoccin (FC) inhibited net excretion of Cl^– by theglands of the pitchers of the carnivorous plant Nepenthes hookeriana;of Na⁺ and Cl^– by the salt glands of the halophytes Limoniumvulgare and L. pectinatum and of K⁺ in the nectar of Acer platanoidesflowers. It had no effect on K⁺ elimination with nectar of Impatienswalleriana (extrafloral nectaries) and Abutilon striatum. Abscisicacid (ABA) stimulated net excretion of K⁺ and Cl^– in Nepenthesand of Na⁺ and Cl^– in Limonium but had no effects on K⁺in nectar. Thus, FC and ABA had opposing effects on ion excretionby the salt eliminating glands of Limonium and Nepenthes. Bothcompounds, however, had similar effects on sugar secretion ofnectary glands which was either inhibited or unaffected by FCand ABA. It is suggested that the effects of FC and ABA on ion excretionby gland cells could be reconciled with literature showing FC-stimulationand possible ABA-inhibition of proton pumps at the plasmalemmaof plant cells. Nepenthes hookeriana, Limonium vulgare, Limonium pectinatum, Acer platanoides, salt-glands, nectaries, excretion, fusicoccin, abscisic acid, proton pump     

Dynamics of Salt Secretion by Sporobolus spicatus(Vahl) Kunth from Sites of Differing Salinity

Secretion of salts by bicellular salt glands and the water relationsof the grass Sporobolus spicatus were investigated at four sitesalong the coast of the Red Sea in Egypt that differed in theextremity of salinity and drought. Salt eliminated by the leaveswas similar in its composition at all sites. Na⁺and Cl^-werethe dominant ions in the soil, and together comprised about93% of the dry weight of secreted salt. The molar ratio of K⁺:Na⁺inthe plant leaves was more than ten-fold that in the interstitialsoil solution and thirteen-times that in the secreted salts,reflecting the high selectivity of the secretion mechanism forNa⁺. The concentration of Na⁺in the solution transported tothe leaves between 0900 and 1500 h was less than 0.1% of thatin the soil solution. Accumulation of salts by the plant shoots,which increased with increasing soil salinity and drought, wasmaximal during the day when the extent of secretion greatlyreduced. The ionic osmotic potential (     

Inhibition of Sugar and Salt Elimination by Glands with the Amino Acid Analog p-Fluorophenylalanine

The amino acid analog p-fluorophenylalanine (FPA) inhibitedsugar and K⁺ secretion by nectary glands. FPA specifically reducedthe net excretion of Na⁺ and Cl^– by the salt glands ofthe halophyte Limonium vulgare and ³⁶Cl^–excretion by theglands of the pitcher walls of the carnivorous plant Nepenthes.Net uptake and net accumulation of Na⁺ and Cl^– by Limoniumleaf tissue and ³⁶Cl^– accumulation in Nepenthes pitchertissue were much less inhibited than excretion. The resultsare discussed in relation to literature reporting similar specificeffects of FPA on transport of ions from the symplast of barleyroots into the dead xylem elements. Limonium vulgare, Nepenthes hookeriana, salt-glands, excretion, p-fluorophenylalanine     

Gibberellic Acid Controls the Secretion of Acid Phosphatase in Aleurone Layers and Isolated Aleurone Protoplasts of Avena fatua

The role of gibberellic acid (GA₃) in controlling the secretion(across the plasma membrane) and release (through the cell wall)of acid phosphatase (E.C. 3.1.3.2 [EC] .) from Avena aleurone layershas been investigated. Evidence from this comparative studywith intact aleurone layers and isolated aleurone protoplastsreveals that the secretion of acid phosphatase is under GA₃control. The mechanism underlying secretion and release of theenzyme from aleurone cells is discussed. Key words: Avena fatua, Acid phosphatase, Aleurone protoplasts, Gibberellic acid, Secretion     

Anatomy, Ultrastructure and Assimilate Concentrations of Roots of Citrus Genotypes Differing in Ability for Salt Exclusion

Seedlings of Rangpur lime (Citrus reticulata var. austera hybrid?)and Etrog citron (C. medica) were treated in water culture with0, 25, 50 or 100 mol m^–3 NaCl, and in sand culture with0 or 100 mol m^–3 NaCl. Leaf chloride analyses indicatedthat Etrog citron accumulated the most chloride at all levelsof salinity. The structure, ultrastructure and concentrationsof chloride and reserve assimilates of the primary root up to50 mm back from the tip were compared between genotypes andbetween salt treatments. There were no differences in root anatomy in the absence ofsalt between the two genotypes. The hypodermal cells developedlignified and suberized walls which blocked the plasmodesmataand resulted in degeneration of the cell contents. Frequentthin-walled passage cells in the hypodermis had living contentsand may represent major sites of ion uptake into the symplasm,which was connected with the stele via plasmodesmatal connectionswith and between the cortical cells. The primary endodermalcells had lignified casparian strips and plasmodesmata in othercell wall areas. These connections were blocked by secondarysuberization of all except the endodermal passage cells oppositethe protoxylem arcs. Suberization of the hypodermis and endodermis and the appearanceof granular deposits in the vacuole occurred closer to the roottip of both genotypes after treatment with 100 mol m^–3NaCl. Levels of starch and triglyceride in 10 mm serial segmentswere similar between roots of control and salt-treated plantsand increased with distance back from the root tip. Chlorideconcentrations increased with salt treatment but values (ona tissue water basis) were similar between genotypes and betweenthe apical and proximal ends of the root. Reducing sugar concentrationsdecreased with salt treatment to a similar extent in roots ofboth genotypes. Key words: Citrus, Roots, Salinity     

The Ultrastructure of Ion-Secreting and Non-Secreting Salt Glands of Limonium platyphyllum

The ultrastructure of salt glands in developing leaves of Limoniumplatyphyllum is described prior to exposure to 3% NaCl solution(with non-secreting glands) and after 4.5 and 18 h exposureto the salt solution. It is shown that in most glands, the transitionto active chloride transport was accompanied by the displacementof vacuoles toward the cell periphery and by the establishmentof plasmalemma contact sites with the tonoplast which appearedsimilar to gap junctions in animal epithelial cells. No evidencefor the exocytosis of vacuoles was found. It is suggested thatgland vacuoles may have a primary role in chloride secretionand that the tonoplast may be functionally asymmetrical, sothat the free part facing the hyaloplasm bears ion pumps, whereashighly permeable ion channels are active along the zone of contactwith the plasmalemma. It follows that the active step in chloridetransport in Limonium glands is the influx of ions into thevacuoles. Within the inner cup cells of the gland, vacuolescome into contact with the plasmalemma only at sites where thecell wall is adjacent to secretory and accessory cells. Suchan asymmetry appears to ensure the directed flux of ions intothis cell wall. Wall protuberances in the gland cells are rudimentaryand presumably not involved directly in NaCl secretion. Thenucleolus is activated during secretion and the frequency offree ribosomes is significantly increased, which is suggestiveof their involvement in the synthesis of membrane transportproteins. The ultrastructure of about one-third of the glandsremained unchanged in salt-treated leaves. Key words: Salt glands, ultrastructure, ion fluxes     

Salt Glands in the Zoysieae

Salt glands were found in two species of the genus Zoysia ofthe tribe Zoysieae, sub-family Chloridoideae (Poaceae). Glandsprotrude from and are recumbent to the leaf epidermis, and consistof two cells, a basal cell and upper cap cell. Glands were betterdeveloped on the adaxial surfaces. Those on the abaxial surfaceappear to be non-functional. Zoysia matrella, the more salt-tolerantspecies, had a higher density of larger glands, and secretedmore sodium per unit leaf mass, resulting in much lower leafsap osmolalities than those of the more salt-sensitive Z. japonica.The finding of salt glands in the tribe Zoysieae confirms itsrelation to the four other tribes within the sub-family Chloridoideaein which salt glands have previously been reported. Salt glands, Zoysieae, Poaceae, Japanese lawngrass, Zoysia japonica, Manilagrass, Zoysia matrella, sodium chloride, salt tolerance, secretion     

Effect of Sudden Salt Stress on Ion Fluxes in Intact Wheat Suspension Cells

Although salinity is one of the major problems limiting agriculturalproduction around the world, the underlying mechanisms of highNaCl perception and tolerance are still poorly understood. Theeffects of different bathing solutions and fusicoccin (FC),a known activator of plasma membrane ATPase, on plasma membranepotential (E_m) and net fluxes of Na⁺, K⁺and H⁺were studied inwheat suspension cells (Triticum aestivum) in response to differentNaCl treatments. E_mof cells in Murashige and Skoog (MS) mediumwas less negative than in cells exposed to a medium containing10 mM KCl + 0.1 m M CaCl₂(KSM) and to a basic salt medium (BSM),containing 1 m M KCl and 0.1 m M CaCl₂. Multiphasic Na⁺accumulationin cells was observed, peaking at 13 min after addition of 120m M NaCl to MS medium. This time scale was in good agreementwith net Na⁺flux changes measured non-invasively by moving ion-selectivemicroelectrodes (the MIFE system). When 120 m M NaCl was addedto all media studied, a quick rise of Na⁺influx was reversedwithin the first 20 min. In both 120 and 20 m M NaCl treatmentsin MS medium, net Na⁺efflux was observed, indicating that activeNa⁺transporters function in the plant cell response to saltstress. Lower external K⁺concentrations (KSM and BSM) and FCpre-treatment caused shifts in Na⁺fluxes towards net influxat 120 m M NaCl stress. Copyright 2000 Annals of Botany Company Sodium, potassium, proton, membrane potential, fusicoccin, salt stress, wheat, Triticum aestivum     

Carbon Metabolism in Seagrasses: III. ACTIVITIES OF CARBON-FIXING ENZYMES IN RELATION TO INTERNAL SALT CONCENTRATIONS

The internal salt content and distribution in photosynthetictissues as well as the effect of NaCl on photosynthetic carbonfixation enzymes was investigated in two seagrass species fromthe Red Sea. Concentrations of both Na⁺ and Cl^– were lower in the chloroplast-richepidermis than in underlying cell layers in Halophila stipulacea.In Halodule uninervis, the concentration of Na⁺ was lower inthe epidermis than in the underlying cells, while K⁺ was evenlydistributed between cell layers. The epidermal concentrationsof Na+ were estimated to be 0.17 and 0.10 M for Halophila stipulaceaand Halodule uninervis, respectively, which were about to the average leaf concentrations. Epidermal Cl^– concentrationof Halophila stipulacea was estimated to be 0.08 M, a valueonly about of the overall leaf concentration. Phosphoenolpyruvate carboxylase (PEPcase) extracted from leavesof these seagrasses showed increased activity at 0.05–0.3M NaCl in vitro. Ribulose-l, 5-bisphosphate carboxylase (RuBPcase)activity, on the other hand, was inhibited by NaCl at all testedconcentrations. At epidermal NaCl concentrations, PEPcase activitywas thus stimulated while RuBPcase was inhibited. The reducedRuBPcase activity at such concentrations compared to salt-freeconditions was still sufficient to account for observed photosyntheticrates. We conclude that these seagrasses have adapted to a saline environmentboth by maintaining relatively low ion concentrations in theepidermis where photosynthesis occurs and by having carbon-fixingenzymes capable of functioning in the presence of salt.     

Comparative Studies of Leaf Tissue 4: III. EFFECTS OF WALL-DEGRADING ENZYMES AND OSMOTIC STRESS

Commercially available cell wall-degrading enzymes frequentlyused for protoplast isolation inhibited CO₂ fixation and photosyntheticO₂ evolution, and stimulated dark respiration by leaf tissueand isolated mesophyll protoplasts of Nicotiana tabacum L. andAntirrhinum majus L. They also depolarized the membrane potentialof cells of leaf tissue, inhibited uptake of ⁸⁶Rb by tobaccoleaf tissue and isolated mesophyll protoplasts, and stimulated³⁶CI uptake by tobacco leaf tissue. Where studied, these effectswere found to be reversible. The depolarization effect on Antirrhinumleaf cells occurred even when the enzyme preparations had beendenatured, dialysed, or desalted, and the effect was greatestin those fractions of the enzyme preparation which showed thehighest cellulase activity. Plasmolysis of tobacco leaf tissue inhibited photosyntheticO₂ evolution, CO₂ fixation, and ⁸⁶Rb uptake to levels belowthose exhibited by isolated protoplasts in media of the samecomposition and osmolarity. The implications of these resultsfor work with leaf tissue and isolated protoplasts are discussed.     

Effect of Intracellular ATP Levels on the Light-Induced H+ Efflux from Intact Cells of Cyanidium caldarium

The light-induced H⁺ efflux observed at acidic pH in Cyanidiumcells was shown to be an active H⁺ transport depending on theintracellular ATP produced by cyclic photo-phosphorylation.Triton X-100 was found to act as an effective uncoupler in intactCyanidium cells without collapsing the pH gradient across theplasma membrane. Triton X-100 at 0.015% significantly reducedthe intracellular ATP levels, stimulated the p-BQ, Hill reactionand completely inhibited the light-induced H⁺ efflux. Inhibitionof the H⁺ efflux by Triton X-100 correlated well with the depressionof the apparent rale of light-induced ATP synthesis as wellas the decrease in the intracellular ATP level in light. The light-induced H⁺ efflux was completely inhibited by diethylstilbestrol,a specific inhibitor of plasma membrane ATPase, without anychanges in the intracellular ATP level, thereby suggesting theparticipation of the plasma membrane ATPase in the light-inducedH⁺ efflux. ¹The data in this paper are included in the Ph. D. dissertationsubmitted by M. Kura-Hotta to Tokyo Metropolitan University. (Received February 3, 1984; Accepted June 14, 1984)     

Tin protoporphyrin induces intestinal chloride secretion by inducing light oxidation processes

Heme induces Cl^– secretion in intestinal epithelial cells, most likely via carbon monoxide (CO) generation. The major source of endogenous CO comes from the degradation of heme via heme oxygenase (HO). We hypothesized that an inhibitor of HO activity, tin protoporphyrin (SnPP), may inhibit the stimulatory effect of heme on Cl^– secretion. To test this hypothesis, we treated an intestinal epithelial cell line (Caco-2 cells) with SnPP. In contrast to our expectations, Caco-2 cells treated with SnPP had an increase in their short-circuit currents (I_sc) in Ussing chambers. This effect was observed only when the system was exposed to ambient light. SnPP-induced I_sc was caused by Cl^– secretion because it was inhibited in Cl^–-free medium, with ouabain or 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). The Cl^– secretion was not via activation of the CFTR, because a specific inhibitor had no effect. Likewise, inhibitors of adenylate cyclase and guanylate cyclase had no effect on the enhanced I_sc. SnPP-induced I_sc was inhibited by the antioxidant vitamins, -tocopherol and ascorbic acid. Electron paramagnetic resonance experiments confirmed that oxidative reactions were initiated with light in cells loaded with SnPP. These data suggest that SnPP-induced effects may not be entirely due to the inhibition of HO activity but rather to light-induced oxidative processes. These novel effects of SnPP-photosensitized oxidation may also lead to a new understanding of how intestinal Cl^– secretion can be regulated by the redox environment of the cell. heme oxygenase; electrolyte transport; carbon monoxide; cGMP; reactive oxygen species     

Effect of acidification on the location of H+-ATPase in cultured inner medullary collecting duct cells

In previousstudies, our laboratory has utilized a cell line derived from the ratinner medullary collecting duct (IMCD) as a model system for mammalianrenal epithelial cell acid secretion. We have provided evidence, from aphysiological perspective, that acute cellular acidification stimulatesapical exocytosis and elicits a rapid increase in proton secretion thatis mediated by an H⁺-ATPase. Thepurpose of these experiments was to examine the effect of acutecellular acidification on the distribution of the vacuolar H⁺-ATPase in IMCD cells in vitro.We utilized the 31-kDa subunit of theH⁺-ATPase as a marker of thecomplete enzyme. The distribution of this subunit of theH⁺-ATPase was evaluated byimmunohistochemical techniques (confocal and electron microscopy), andwe found that there is a redistribution of these pumps from vesicles tothe apical membrane. Immunoblot evaluation of isolated apical membranerevealed a 237 ± 34% (P < 0.05, n = 9) increase in the 31-kDa subunitpresent in the membrane fraction 20 min after the induction of cellularacidification. Thus our results demonstrate the presence of this pumpsubunit in the IMCD cell line in vitro and that cell acidificationregulates the shuttling of cytosolic vesicles containing the 31-kDasubunit into the apical membrane.     

Production and Secretion of Sucrose by Sugar-cane Leaf Tissue

Leaf discs, cut from mature plants of Saccharum officinarumwere found to accumulate large quantities of starch and sugar,amounting to some 25 per cent of their initial dry weight, during3 days of constant illumination. Secretion into the externalmedium of about one-third of the accumulated sucrose was effectedby 10^–4 M sodium iodoacetate. Concomitantly the inhibitorincreased apparent photosynthesis. Various factors, includingother respiratory inhibitors, failed to induce sucrose secretion,indicating a selective action of iodoacetate on the cell membranes.     

K+ channel KVLQT1 located in the basolateral membrane of distal colonic epithelium is not essential for activating Cl- secretion

The cellular mechanism for Cl^– and K⁺ secretion in the colonic epithelium requires K⁺ channels in the basolateral and apical membranes. Colonic mucosa from guinea pig and rat were fixed, sectioned, and then probed with antibodies to the K⁺ channel proteins K_VLQT1 (Kcnq1) and minK-related peptide 2 (MiRP2, Kcne3). Immunofluorescence labeling for Kcnq1 was most prominent in the lateral membrane of crypt cells in rat colon. The guinea pig distal colon had distinct lateral membrane immunoreactivity for Kcnq1 in crypt and surface cells. In addition, Kcne3, an auxiliary subunit for Kcnq1, was detected in the lateral membrane of crypt and surface cells in guinea pig distal colon. Transepithelial short-circuit current (I_sc) and transepithelial conductance (G_t) were measured for colonic mucosa during secretory activation by epinephrine (EPI), prostaglandin E₂ (PGE₂), and carbachol (CCh). HMR1556 (10 µM), an inhibitor of Kcnq1 channels (Gerlach U, Brendel J, Lang HJ, Paulus EF, Weidmann K, Brüggemann A, Busch A, Suessbrich H, Bleich M, and Greger R. J Med Chem 44: 3831–3837, 2001), partially (50%) inhibited Cl^– secretory I_sc and G_t activated by PGE₂ and CCh in rat colon with an IC₅₀ of 55 nM, but in guinea pig distal colon Cl^– secretory I_sc and G_t were unaltered. EPI-activated K⁺-secretory I_sc and G_t also were essentially unaltered by HMR1556 in both rat and guinea pig colon. Although immunofluorescence labeling with a Kcnq1 antibody supported the basolateral membrane presence in colonic epithelium of the guinea pig as well as the rat, the Kcnq1 K⁺ channel is not an essential component for producing Cl^– secretion. Other K⁺ channels present in the basolateral membrane presumably must also contribute directly to the K⁺ conductance necessary for K⁺ exit during activation of Cl^– secretion in the colonic mucosa. HMR1556; K⁺ secretion; epinephrine; prostaglandin E₂; cholinergic     

Membrane Depolarization by Hexacyanoferrate (III), Hexabromoiridate (IV) and Hexachloroiridate (IV)

Three artificial electron acceptors of different E_o and charge,hexacyanoferrate (III) (K₃Fe(CN)₆), hexachloroiridate (IV) (K₂IrCl₆),and hexabromoiridate (IV) (K₂IrBr₆), were compared with respectto their rate of reduction by roots of Zea mays L., the concomitantproton secretion, and to the effect on plasmalemma depolarization. It has been shown that these plasma membrane impermeable electronacceptors were reduced by a plasmalemma reductase activity.At low concentrations proton secretion was slightly inhibited,at higher concentrations, however, the rate of proton secretionwas stimulated. The root cell plasmalemma showed a transientdepolarization after addition of all three electron acceptors.The depolarization was concentration-dependent for the iridatecomplexes but not for hexacyanoferrate (III). For both iridatecomplexes maximum depolarization was reached at 50 µmoldm^–3. A hypothetical model as an explanation of the redox dependentproton secretion will be given. Key words: Hexachloroiridate (IV), hexabromoiridate (IV), hexacyanoferrate (III), plasmalemma redox, membrane potential, Zea mays     

Dual effects of n-alcohols on fluid secretion from guinea pig pancreatic ducts

Ethanol strongly augments secretin-stimulated, but not acetylcholine (ACh)-stimulated, fluid secretion from pancreatic duct cells. To understand its mechanism of action, we examined the effect of short-chain n-alcohols on fluid secretion and intracellular Ca²⁺ concentration ([Ca²⁺]_i) in guinea pig pancreatic ducts. Fluid secretion was measured by monitoring the luminal volume of isolated interlobular ducts. [Ca²⁺]_i was estimated using fura-2 microfluorometry. Methanol and ethanol at 0.3–10 mM concentrations significantly augmented fluid secretion and induced a transient elevation of [Ca²⁺]_i in secretin- or dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-stimulated ducts. However, they failed to affect fluid secretion and [Ca²⁺]_i in unstimulated and ACh-stimulated ducts. In contrast, propanol and butanol at 0.3–10 mM concentrations significantly reduced fluid secretion and decreased [Ca²⁺]_i in unstimulated ducts and in ducts stimulated with secretin, DBcAMP, or ACh. Both stimulatory and inhibitory effects of n-alcohols completely disappeared after their removal from the perfusate. Propanol and butanol inhibited the plateau phase, but not the initial peak, of [Ca²⁺]_i response to ACh as well as the [Ca²⁺]_i elevation induced by thapsigargin, suggesting that they inhibit Ca²⁺ influx. Removal of extracellular Ca²⁺ reduced [Ca²⁺]_i in duct cells and completely abolished secretin-stimulated fluid secretion. In conclusion, there is a distinct cutoff point between ethanol (C2) and propanol (C3) in their effects on fluid secretion and [Ca²⁺]_i in duct cells. Short-chain n-alcohols appear to affect pancreatic ductal fluid secretion by activating or inhibiting the plasma membrane Ca²⁺ channel. intracellular calcium; acetylcholine     

Ion Exchange Fluxes of the Cell Walls of Enteromorpha intestinalis (L. ) Link (Ulvales, Chlorophyta)

The ²²Na⁺ and ³⁶CI^– exchange properties of the cell wallsof Enteromorpha intestinalis (L. ) Link in simple monovalentsalt systems have been shown to be similar to a ‘leaky’cation exchange membrane rather than a homogeneous membrane.The ion exchange properties of the cation and anion cell wallcontents are what would be expected of a cation exchange membranei. e. anion exchange is strongly dependent on the bathing electrolyteconcentration and becomes very slow in dilute salt. This wouldlead to the cell wall becoming a barrier to anions in dilutesalt. However, measurements of the anion flux across cell wallsin living and dead tissues show that anion exchange across cellwalls is facilitated by pores. The exchange kinetics of thebulk of the cell wall anions does not limit the anion flux acrosscell walls of this plant. It is concluded that the cell wallis not a critical limitation to plasmalemma fluxes of the livingplant and that unstirred layers are more important than cellwalls in the measurement of anion flux rates.     

Three distinct mechanisms of HCO3- secretion in rat distal colon

HCO₃^– secretion has long been recognized in the mammalian colon, but it has not been well characterized. Although most studies of colonic HCO₃^– secretion have revealed evidence of lumen Cl^– dependence, suggesting a role for apical membrane Cl^–/HCO₃^– exchange, direct examination of HCO₃^– secretion in isolated crypt from rat distal colon did not identify Cl^–-dependent HCO₃^– secretion but did reveal cAMP-induced, Cl^–-independent HCO₃^– secretion. Studies were therefore initiated to determine the characteristics of HCO₃^– secretion in isolated colonic mucosa to identify HCO₃^– secretion in both surface and crypt cells. HCO₃^– secretion was measured in rat distal colonic mucosa stripped of muscular and serosal layers by using a pH stat technique. Basal HCO₃^– secretion (5.6 ± 0.03 µeq·h^–1·cm^–2) was abolished by removal of either lumen Cl^– or bath HCO₃^–; this Cl^–-dependent HCO₃^– secretion was also inhibited by 100 µM DIDS (0.5 ± 0.03 µeq·h^–1·cm^–2) but not by 5-nitro-3-(3-phenylpropyl-amino)benzoic acid (NPPB), a Cl^– channel blocker. 8-Bromo-cAMP induced Cl^–-independent HCO₃^– secretion (and also inhibited Cl^–-dependent HCO₃^– secretion), which was inhibited by NPPB and by glibenclamide, a CFTR blocker, but not by DIDS. Isobutyrate, a poorly metabolized short-chain fatty acid (SCFA), also induced a Cl^–-independent, DIDS-insensitive, saturable HCO₃^– secretion that was not inhibited by NPPB. Three distinct HCO₃^– secretory mechanisms were identified: 1) Cl^–-dependent secretion associated with apical membrane Cl^–/HCO₃^– exchange, 2) cAMP-induced secretion that was a result of an apical membrane anion channel, and 3) SCFA-dependent secretion associated with an apical membrane SCFA/HCO₃^– exchange. chloride/bicarbonate exchange; short-chain fatty acid/bicarbonate exchange; anion channel; pH stat     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.