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Soil Syst. 2018, 2(2), 26; https://doi.org/10.3390/soilsystems2020026

Formation of the Azodication (ABTS2+) from ABTS [2,2′-Azinobis-(3-ethylbenzothiazoline-6-sulphonate)] in Sterile Plant Cultures: Root–Exuded Oxidoreductases Contribute to Rhizosphere Priming

Institute of Earth Sciences, Friedrich-Schiller-University, Burgweg 11, D-07749 Jena, Germany
Received: 2 March 2018 / Revised: 26 April 2018 / Accepted: 27 April 2018 / Published: 1 May 2018
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Abstract

Rhizosphere priming by terrestrial plants comprises increased or repressed efflux of CO2 and N from soil organic matter (SOM), decaying under the impact of temperature, moisture, and the composition of rhizodeposits. Contemporarily, increases in water solubility vs. losses in molecular size, aromaticity, and the content in phenolic OH groups denote the degradation of SOM in planted soil. Root peroxidases (POs) and ‘polyphenoloxidases’ are surmised to contribute to these effects, however, final evidence for this is lacking. Therefore, seedlings of white mustard, alfalfa, and oilseed rape with wide spans in PO release were grown in hydroponic cultures at variable levels of Cu/Fe/Mn as Fenton metals, but also under P and Fe starvation to stimulate the release of carboxylic acids that form catalytic Mn3+ chelants from Mn2+ and MnO2. The shortage in active oxygen as a cosubstrate of POs delayed the immediate oxidation of 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) supplements to the green ABTS•+ by PO/H2O2, the possible formation of Mn3+ via PO catalyzed aryloxy radicals from root–released phenolics, and of HO by metal cations in H2O2 dependent Fenton–like reactions. Enhanced by exuded and external malate, O2 independent MnO2 supplements in some treatments formed ABTS•+ spontaneously. The culture fluids then turned red in all treatments within 24–60 h by the formation of azodication (ABTS2+) derivatives in a second plant initiated oxidation step that is known to be catalyzed by substrate radicals. It is concluded that plants initiate oxidative activities that contribute to rhizosphere priming in an environment of oxidoreductase and carboxylate exudates, the indicated presence of mediating substrate radicals, and the cations and (hydr)oxides of transition metals. Pathways of H2O2 production upon the degradation of carboxylates and by the POs themselves are indicated. View Full-Text
Keywords: rhizosphere priming; humic substances; plant peroxidase; carboxylates; active oxygen; transition metals; Fenton–like reactions; ABTS cation radical; azodication derivatives rhizosphere priming; humic substances; plant peroxidase; carboxylates; active oxygen; transition metals; Fenton–like reactions; ABTS cation radical; azodication derivatives
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Gramss, G. Formation of the Azodication (ABTS2+) from ABTS [2,2′-Azinobis-(3-ethylbenzothiazoline-6-sulphonate)] in Sterile Plant Cultures: Root–Exuded Oxidoreductases Contribute to Rhizosphere Priming. Soil Syst. 2018, 2, 26.

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