Effects of Host-Specific Toxins on Electropotentials of Plant Cells

Gardner, James A.; Scheffer, R. P.; Higinbotham, Noe

doi:10.1104/pp.54.3.246

Cited by 34 publications

(9 citation statements)

References 6 publications

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“…In agreement with inhibitor studies on other plant material [7,13,34,32], rapid depolarizations were observed when cells were treated with CCCP or with cyanide ( Fig. 2A and B).…”

Section: Effects Of Inhibitors On Membrane Potentialsupporting

confidence: 87%

Electrogenic pump activity in red beet: Its relation to ATP levels and to cation influx

Mercier

Poole

1980

J. Membrain Biol.

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Summary. In storage tissue of Beta vulgaris L., carbonyl cyanide m-chlorophenylhydrazone or cyanide+salicylhydroxamic acid reduce cell electropotentials from about -200 to below -100 inV. The relationship between potential and cellular ATP level is examined during treatment with different concentrations of inhibitors. At low ATP levels the potential rises sharply with increases in ATP, but above an ATP level of approximately 50% of the uninhibited level the potential changes very little with ATP concentration. A plot of membrane potential vs. 86Rb+ influx or of potential vs. net K + uptake indicates that as the level of inhibition is decreased, the potential tends to reach a limit while cation influx and net uptake continue to increase. Resistance measurements, although subject to difficulties of interpretation, indicate no change in conductance with potential, ion flux, or ATP level. Thus the membrane potential should directly reflect electrogenic pump activity, attributed to active uncoupled H + efflux. K + uptake can occur against its electrochemical gradient and is attributed to a coupled K + influx/H + efflux pump. The results show that the electrogenic pump activity is independent of the K+/H + exchange rate. Thus electrogenic H + efflux and K+/H + exchange may represent different transport systems, or different modes of operation of a single pump with variable stoichiometry.Electrogenic pumps, i.e., primary transport processes which contribute directly to the membrane potential, are a conspicuous feature of plant cells [26]. In most cases it appears to be the efflux of H § which gives rise to an energy-dependent hyperpolarization. Net * Current address: Department of Biology, University of Calgary, Calgary, Alberta T2N 1N4, Canada.H + efflux is usually associated with the accumulation of K § or other cations, but the link between these ion movements is not yet clear. Slices of storage tissue of red beet (Beta vulgaris L.) appear to be a favorable material for investigation of the relation between these transport processes [25] since a large and prolonged efflux of H+, an equivalent influx of K +, and a conspicuous hyperpolarization are all evoked by raising the external pH from about pH 6.5 to pH7.5. Other ion movements such as active (K + +C1-) influx are not sensitive to pH and are much smaller than K+/H + exchange at high pH. In the present study, membrane potentials in beet cells at high pH are investigated in relation to changes in ATP level brought about by metabolic inhibitors, especially cyanide. In addition, membrane potentials are compared with cation influx over various levels of metabolic inhibition, and the results are interpreted in terms of possible schemes of energy coupling for the electrogenic pump and for the accumulation of cations. Materials and Methods Tissue Preparation

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“…In agreement with inhibitor studies on other plant material [7,13,34,32], rapid depolarizations were observed when cells were treated with CCCP or with cyanide ( Fig. 2A and B).…”

Section: Effects Of Inhibitors On Membrane Potentialsupporting

confidence: 87%

Electrogenic pump activity in red beet: Its relation to ATP levels and to cation influx

Mercier

Poole

1980

J. Membrain Biol.

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“…Toxininduced increases in the reduction of nitrate by susceptible and resistant maize tissues resulted from an increased uptake of nitrate within a few hours of HC toxin treatment (17). HC toxin also causes an increase in negative electropotential across the plasma membrane (2). The inhibition of Chl synthesis in susceptible and resistant maize is the most rapid inhibitory effect of HC toxin observed to date (1 1).…”

Section: Discussionmentioning

confidence: 99%

Isolation and Biological Activities of Four Selective Toxins from Helminthosporium carbonum

Rasmussen

Scheffer²

1988

Plant Physiol.

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A new and simpler purification procedure was developed for host selective toxins from Helminthosporium carbonum race 1. Four analogs or forms of toxin with the same selectivity as the fungus were isolated from culture fluids; two forms (HC toxins IH and IV) have not been reported by other workers. Crystals of the major form of toxin (HC toxin I) were recovered in high yields (>80 milligrams per liter of culture fluid) without the use of high performance or preparative thin layer liquid chromatography. EDs5 values, based on inhibition of root growth of susceptible seedlings, for HC toxins I, II, III, and IV were 0.2, OA, 2.0, and 20 micrograms per milliliter, respectively. The specific activity of crystalline HC toxin I matched the most active preparation reported previously-, the preparation of HC toxin II was more active than that reported previously. Resistant seedlings tolerated 100-fold higher concentrations of each form of toxin than did susceptible seedlings. Hydrolysis of the epoxide group of HC toxin I to a diol destroyed toxicity to susceptible and resistant seedlings. The data suggest that the same mechanisms are affected in resistant and susceptible plants.Helminthosporium carbonum (Ullstrup) race 1 (anamorph of Cochliobolus carbonum Nelson), the causal agent of a leaf spot disease of maize, produces a host-selective toxin that is required for pathogenicity (12). Leisch et aL (8) identified HC toxin as a cyclic tetrapeptide containing alanine (2 residues), proline (1 residue), and an unusual epoxide-containing amino acid, Aoe3 (1 residue). Subsequent papers confirmed the amino acid composition and cyclic nature of the compound, and indicated the amino acid sequence of the molecule (9, 15) ( Fig. 1, toxin I). The toxin was first identified as cyclo-(2-amino-8-oxo-9,0 O-epoxydecanoyl-prolyl-alanyl-alanyl) by Walton et al. (15). This form of HC toxin has been synthesized (4), confirming the structure. Hydrolysis of the epoxide to a diol produced a compound that is nontoxic (1,16). An analog of HC toxin was discovered in culture fluids of the fungus (5) (Fig. 1, toxin II). The compound differed structurally from the previously known HC toxin by the substitution of glycine for alanine adjacent to Aoe. The substitution did not alter specificity of HC toxin, but reduced its (Fig. 1), the most abundant and potent form of the toxin. At least three analogs of toxin I were separated with the new purification scheme. These include the glycine-containing compound (HC toxin II) (5), the hydroxyproline-containing compound (HC toxin III) (14) and an additional form, not reported previously (HC toxin IV). Chemical characterization of toxin IV is not yet complete. MATERIALS AND METHODSToxin Production and Initial Purification. Procedures for production and initial purification of HC toxins were modified from those used previously (10). Pathogenic, single spore isolates of H. carbonum race 1 were maintained on potato dextrose agar slants at 4°C. For toxin production, the fungus was grown in a modified Fries solutio...

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“…Hence, measurements of membrane potential changes with conventional electrophysiological methods are at least 1000 times more sensitive in indicating changes in membrane properties than continuous K+ concentration measurements. To detect victorin-induced membrane changes as early as possible, this method was introduced in 1974 (3,17). At that time, the toxin was available only as a mixture of several related compounds and only partly purified from the fungal culture filtrate.…”

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Electrical Membrane Properties of Leaves, Roots, and Single Root Cap Cells of Susceptible Avena sativa

Ullrich¹,

Novacký²

1991

Plant Physiol.

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The effect of the purified host-selective toxin victorin C, a cyclized penta peptide, was compared to that of CCCP and vanadate on membrane functions of susceptible leaves, roots, and single root cap cells of Avena sativa with conventional electrophysiology. The plasmalemma depolarized irreversibly by about 80 millivolts and to below the diffusion potential within 1 hour. Concentrations as low as 12.5 picomolar were effective in the susceptible but not the resistant cultivar. Electrical membrane potential difference changes were independent of pH and could not be prevented by fusicoccin or Ca2 . Membranes began to depolarize after a lag phase that never was shorter than 6.5 minutes, even with concentrations as high as 1.25 micromolar. Membrane depolarization was accompanied by a distinct decrease in specific membrane resistance from 4.5 to 1.0 ohm times square meter on average. These changes were followed by K+ and Cl-efflux and extracellular alkalinization. ATP level and 02 uptake did not decrease within 2 hours. It is concluded that the victorin-induced deleterious membrane alterations are not caused by direct interaction with the plasmalemma H+-ATPase, K+ channels, lipid structure, nor energy metabolism, but they seem to be triggered by a cascade of events leading to an unspecific increase in membrane permeability.Victorin C is a host-selective toxin, a cyclized penta peptide (32), produced by the phytopathogenic fungus Cochliobolus victoriae Nelson. It is known to increase the cell membrane permeability of susceptible oat plants (Avena sativa), containing the Victoria (Vb) gene, which also encodes for resistance (Pc-2) against Puccinia coronata (16,31). Changes in membrane permeability were most typically expressed in irreversible K+ efflux from leaves, roots, and leaf protoplasts (2,12,18,24,30). K+ efflux might be triggered by several mechanisms: (a) a direct or indirect (by stalling the energy supply) inhibition of the plasmalemma H+-ATPase and thus depolarization of the membrane and opening of the K+ channels; (b) an interaction with K+ channels; (c) a direct change in channel-unrelated unspecific membrane permeability by alteration 'Supported by the Deutsche Forschungsgemeinschaft (C. U.), by an Alexander von Humboldt Senior U.S. Scientist award, and by the National Science Foundation, DMB-8516038 (A. N.) 675 of the lipid structure or an indirect one via an intracellular sequence of events, starting with binding of the toxin to a specific membrane-located binding protein (31). The molecular mechanism, however, is still unknown.With K+ selective electrodes only flux changes beyond the nmol range can be detected. However, for a measurable change in the electrical membrane potential of plant cells, a charge transfer ofless than one pmol * cm-2 is required. Hence, measurements of membrane potential changes with conventional electrophysiological methods are at least 1000 times more sensitive in indicating changes in membrane properties than continuous K+ concentration measurements. To detect victor...

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Effects of Host-Specific Toxins on Electropotentials of Plant Cells

Cited by 34 publications

References 6 publications

Electrogenic pump activity in red beet: Its relation to ATP levels and to cation influx

Electrogenic pump activity in red beet: Its relation to ATP levels and to cation influx

Isolation and Biological Activities of Four Selective Toxins from Helminthosporium carbonum

Electrical Membrane Properties of Leaves, Roots, and Single Root Cap Cells of Susceptible Avena sativa

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