Silvano Sacco

Effect of Mentha × piperita essential oil and monoterpenes on cucumber root membrane potential

Peppermint (Mentha piperita L.) essential oil and its main components were assessed for their ability to interfere with plant plasma membrane potentials. Tests were conducted on root segments isolated from etiolated seedlings of cucumber (Cucumis sativus L.). Increasing the concentration of peppermint essential oil from 5 to 50 ppm caused a decrease in membrane potential (Vm) hyperpolarization of 10-3 mV, whereas concentrations from 100 up to 900 ppm caused an increasing depolarization of Vm (from 5 to 110 mV). When tested at 300 ppm, (+)-menthyl acetate, (-)-limonene and 1,8-cineole did not exert any significant effect on V(m), whereas (+)-menthofuran (73 mV), (+)-pulegone (85 mV), (+)-neomenthol (96 mV), (-)-menthol (105 mV) and (-)-menthone (111 mV) showed increased ability to depolarize V(m). A plot of log of octanol-water partition coefficient (K(ow)) against their depolarizing effect showed a significant negative correlation, suggesting that among all monoterpenoids increased membrane depolarization depends on lower K(ow). However, among monoterpene ketones, alcohols and furans, increased membrane depolarization is associated with a decline in water solubility. The possible effect of monoterpenoids on membrane ion fluxes is also discussed, since changes in the bioelectric potential of cells imply changes in the flux of ions across the plasma membrane

The effect of isosakuranetin (5,7-dihydroxy 4′-methoxy flavanone) on potassium uptake in wheat root segments

Abstract Isosakuranetin (ISK; 5,7-dihydroxy 4′-methoxy flavanone) is a plant exudate with known cytotoxic and fungicide properties. When tested on wheat root segments at a concentration of 70 μ M it inhibited K + dependent H + extrusion and net uptake of K + , while leaving the membrane potential (PD) unaltered. Fusicoccin (FC) + ISK treatment resulted in a slight membrane depolarization, while ISK alone did not alter O 2 consumption or alternative oxidase activity. ISK did not increase the pyruvate content in incubated root tissues or inhibit Fe 2+ uptake. The observed drop in K + net uptake depended on a decrease in K + influx into the cell, leading to the suggestion that ISK may act on wheat root segments as an inhibitor of K + permeation. The lack of proton leakage and membrane disruption by ISK made this compound a weak candidate as a phytoalexin, and suggesting a major role as an allelopathic molecule.

Stimulation of oxygen uptake by fusicoccin in conditions of inhibited H+ extrusion

InElodea densa leaves, the fusicoccin-induced increase of H+ extrusion and K+ uptake is associated with an increase of respiration by some 20%, interpreted as depending on a decrease of the ATP/AP ratio. In the absence of K+ as a permeant cation, no effect of fusicoccin on either H+ extrusion or on the ATP/ADP ratio is observed, while the hyperpolarization of the transmembrane electrical potential difference (Em) is maximal, and the increase of QO2 is increased from 20 to about 80%. This effect of fusicoccin on oxygen uptake in condition of inhibited operation of the ATP-dependent H+ pump suggests that the interaction between fusicoccin and its receptor at the plasma membrane can involve other systems besides the proton transporting ATPase.RiassuntoLo stimolo dell’estrusione di H+ (elettrogenica ed ATPasi-dipendente) da parte della fusicoccina (FC) richiede la presenza nel mezzo di K+ o altri cationi permeanti. In assenza di questi, l’effetto della fusicoccina sull’perpolarizzazione del potenziale transmembrana (Em) è amplificato, e il consumo di O2 sale a circa l’80%. Tale effetto della tossina in condizione di blocco della pompa protonica suggerisce che l’interazione tra la fusicoccina e il suo recettore possa coinvolgere a livello della membrana plasmatica anche altri sistemi oltre alla H+ -ATPasi.

Effects of 3,4-dihydroxybenzoic acid on root membrane potential in micropropagated tobacco plants

In order to understand the mechanism of action of the phenolic compound 3,4-dihydroxybenzoic acid, we tested its effect on tobacco root membrane potential. Tobacco root segments, excised from micropropagated plants grown in liquid media, were perfused with 0.1–5 mM 3,4-dihydroxybenzoic acid. Activity on the plasma membrane potential was compared with that obtained after perfusion with 0.05 mM indole-3-acetic acid, 0.05 mM kinetin and 0.05 mM gibberellic acid. Possible interactions between 3,4-dihydroxybenzoic acid and plant growth regulators were evaluated by the means of successive applications. When applied to tobacco root segments, 3,4-dihydroxybenzoic acid elicited a transient membrane depolarization. The membrane depolarization induced by 3,4-dihydroxybenzoic acid was followed by a repolarization phase, as for auxin applications. In roots preconditioned with the other growth regulators, the activity of 3,4-dihydroxybenzoic acid on membrane potential was non-specifically affected. In roots preconditioned with 3,4-dihydroxybenzoic acid, indole-3-acetic acid activity on cell membrane was altered, suggesting a specific reciprocal interaction.

The induction of the oxidative burst inElodea densa by sulfhydryl reagents is independent of the inhibiting action of these reagents on photosynthesis

The sulfhydryl group-reagents NEM (N-ethylmaleimide), p-CMBS (parachloromercurybenzenesulfonate), Ag+ and Cu2+ inhibit very rapidly and drastically photosynthetic O2 evolution inElodea densa leaves. This effect has been compared with that of the same reagents in inducing in non illuminated leaves a strong increase of O2 consumption (oxygen burst), presumably due to activation of some plasma membrane redox systems. The results suggest that the structures and mechanisms involved in induction of the oxidative burst on one hand and in the inhibition of photosynthesis on the other are substantially independent.RiassuntoReagenti che modificano i sulfidrili (NEM, p-CMBS, Ag+ e Cu+2) risultano inibire precoeemente e drasticamente la fotosintesi in foglie diElodea densa. Questo effetto è stato confrontato con quello indotto da questi reagenti in foglie non illuminate nell’indurre un forte aumento della respirazione, presumibilmente imputabile all’attivazione di sistemi ossidativi del plasmalemma. I risultati indicano una completa indipendenza nei meccanismi dei due ordini di fenomeni considerati.

Effect of benzoic acid hydroxyl- and methoxy-ring substituents on cucumber (Cucumis sativus L.) root membrane potential

Abstract The allelopathic effect of some benzoic acid (BA) OH- and OCH3-ring substituents was studied on cucumber root transmembrane potential difference (Vm). Most of the methoxy-BAs induced a rapid Vm depolarization, followed by a Vm hyperpolarization, with the only exception for p-anisic acid (pA). On the other hand, salicylic acid (SA) and 3,4-dimethoxybenzoic acid (DHB) strongly depolarized Vm. A positive correlation was found between Vm hyperpolarization and lipophilicity of methoxylated BAs, whereas a positive correlation was found between lipophilicity and Vm depolarization of hydroxylated BAs. The influence of BAs on K+ was studied by means of specific blocking with Cs+ indicating a possible direct interaction of SA, gallic acid (GA), vanillic acid (VA) and 3,4-dimethoxybenzoic acid (DMB). Interference of BAs with the Vm hyperpolarizing effect of root perfusion with the fungal toxin fusicoccin were also observed.

The alkaline tetraphenylborates: a tool for maintaining a low and controlled K+ activity in the high- affinity K+ uptake process

The use of a suspension of a sparingly soluble K-salt may be useful as a K+-buffering system whenever low K+ concentrations are needed but the volume of medium limits the amount of K+ available for the tissue. In this investigation we propose K tetraphenylborate (K-PhB) as a candidate. The anion tetraphenylborate up to a concentration of 0.35 mol m-3 onElodea densa leaves and maize roots segments does not influence the transmembrane electric potential nor enter the cell in detectable amount, and no aspecific electrolite leakage can be detected. Moreover, it does not affect the rate and percentage of germination of lettuce seeds.RiassuntoL’uso di sospensioni di sali potassici poco solubili può essere utilizzato per creare sistemi tamponanti per il K+ stesso, quando le condizioni operative (volume ridotto del mezzo, basso rapporto tra questo e la quantità di tessuto) limitino il K+ disponibile. In questaNota proponiamo l’uso del tetrafenilborato di K+ o di RB+ , il cui anione, qui usato fino alla concentratione di 0,35 moli m-3 non influenza il potenziale cellulare inElodea densa e segmenti subapicali di radici di mais, non entra in quantità apprezzabile nel tessuto, non provoca da questo erflusso aspecifico di elettroliti e non sembra influenzare la germinabilità dei semi (lattuga).