Search

Plant roots are exposed to hypoxia in waterlogged soils, and they are further challenged by specific phytotoxins produced by microorganisms in such conditions. One such toxin is hexanoic acid (HxA), which, at toxic levels, causes a strong decline in root O 2 consumption. However, the mechanism underlying this process is still unknown. We treated pea (Pisum sativum L.) roots with 20 mM HxA at pH 5.0 and 6.0 for a short time (1 h) and measured leakage of key electrolytes such as metal cations, malate, citrate and nonstructural carbohydrates (NSC). After treatment, mitochondria were isolated to assess their functionality evaluated as electrical potential and O 2 consumption rate. HxA treatment resulted in root tissue extrusion of K + , malate, citrate and NSC, but only the leakage of the organic acids and NSC increased at pH 5.0, concomitantly with the inhibition of O 2 consumption. The activity of mitochondria isolated from treated roots was almost unaffected, showing just a slight decrease in oxygen consumption after treatment at pH 5.0. Similar results were obtained by treating the pea roots with another organic acid with a short carbon chain, that is, butyric acid. Based on these results, we propose a model in which HxA, in its undissociated form prevalent at acidic pH, stimulates the efflux of citrate, malate and NSC, which would, in turn, cause starvation of mitochondrial respiratory substrates of the Krebs cycle and a consequent decline in O 2 consumption. Cation extrusion would be a compensatory mechanism in order to restore plasma membrane potential., (© 2023 The Authors. Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Pea (Pisum sativum) stem mitochondria, energized by NADH, succinate or malate plus glutamate, underwent a spontaneous low-amplitude permeability transition (PT), which could be monitored by dissipation of the electrical potential (ΔΨ) or swelling. The occurrence of the latter effects was dependent on O2 availability, because O2 shortage anticipated the manifestation of both ΔΨ dissipation and swelling. Spontaneous ΔΨ collapse was also monitored in sucrose-resuspended mitochondria and again O2 deprivation caused an anticipation of the phenomenon. However, in this case ΔΨ dissipation was not accompanied by a parallel mitochondrial swelling. The latter effect was, indeed, evident only if mitochondria were resuspended in KCl (as osmoticum), or other cations with a molecular mass up to 100 Da (choline+). PT was also induced by protonophores (carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) or free fatty acids) or valinomycin (only in KCl). The FCCP-induced dissipation of ΔΨ and swelling were inhibited by ATP and stimulated (anticipated) by cyclosporin A or O2 shortage. The FCCP-induced PT was accompanied by the release of pyridine nucleotides from the matrix and of cytochrome c from the intermembrane space of KCl-resuspended mitochondria. The spontaneous and FCCP-induced low-amplitude PT of plant mitochondria are interpreted as due to the activity of a recently identified KATP+ channel whose open/closed state is dependent on polarization of the inner membrane and on the oxidoreductive state of some sulfhydryl groups. [Copyright &y& Elsevier]

A soluble protein with a molecular mass of 55 kDa has been purified from etiolated pea stem mitochondria. The protein exhibits a Mg2+-requiring PPiase activity, with an optimum at pH 9.0, which is not stimulated by monovalent cations, but inhibited by F−, Ca2+, aminomethylenediphosphate and imidodiphosphate. The protein does not cross-react with polyclonal antibodies raised against vacuolar, mitochondrial or soluble PPiases, respectively. Conversely, it cross-reacts with an antibody for the α/β-subunit of the ATP synthase from beef heart mitochondria. The purified protein has been analyzed by MALDI-TOF mass spectrometry and the results, covering the 30% of assigned sequence, indicate that it corresponds to the β-subunit of the ATP synthase of pea mitochondria. It is suggested that this enzymatic protein may perform a dual function as soluble PPiase or as subunit of the more complex ATP synthase. [Copyright &y& Elsevier]

The H+/PPi stoichiometry of the mitochondrial H+-PPiase from pea (Pisum sativum L.) stem was determined by two kinetic approaches, and compared with the H+/substrate stoichiometries of the mitochondrial H+-ATPase, and the vacuolar H+-PPiase and H+-ATPase. Using sub-mitochondrial particles or preparations enriched in vacuolar membranes, the rates of substrate-dependent H+-transport were evaluated: by a mathematical model, describing the time-course of H+-gradient (ΔpH) formation; or by determining the rate of H+-leakage following H+-pumping inhibition by EDTA at the steady-state ΔpH. When the H+-transport rates were divided by those of PPi or ATP hydrolysis, measured under identical conditions, apparent stoichiometries of ca 2 were determined for the mitochondrial H+-PPiase and H+-ATPase, and for the vacuolar H+-ATPase. The stoichiometry of the vacuolar H+-PPiase was found to be ca 1. From these results, it is suggested that the mitochondrial H+-PPiase may, in theory, function as a primary H+-pump poised towards synthesis of PPi and, therefore, acting in parallel with the main H+-ATPase. [ABSTRACT FROM AUTHOR]

Proton pumping pyrophosphatase (H+-PPiase) of pea stem mitochondria appears to be localized on the inner surface of the inner membrane. Aminohexanediphosphonate and dichloromethylenediphosphonate exert different inhibitory effects on this activity and on that of tonoplast. Antibody raised against membrane-bound mitochondrial H+-PPiase does not react with tonoplast vesicles. Thus, plant mitochondrial H+PPiase seems to have a molecular structure different from that of vacuolar H+-PPiase. [ABSTRACT FROM AUTHOR]