Apoplast redox metabolism: Effect of acetovanillone (apocynin) and acetosyringone, on their co-oxidation and redox properties

https://doi.org/10.1016/j.pmpp.2020.101481Get rights and content

Highlights

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    Co-oxidation of apoplast phenolics can increase the redox potential.

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    Apoplast phenolics can act as prooxidants rather than antioxidants.

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    Apoplast phenolics produce relatively stable free radicals.

Abstract

A common response of plant leaves and roots to stress, biotic or abiotic, is the oxidative burst. In leaves, this results in reactive oxygen species being produced within the apoplast, the space surrounding the responding cells. A second response that occurs in the tobacco apoplast during this period is the production and accumulation of phenolic metabolites. The interaction of these redox active metabolites should have major effects on the redox potential of the apoplast and subsequently affect the outcome of the plant tissue. To begin to understand the effects of these responses on the redox potential, two induced phenolics, acetovanillone (AV) and acetosyringone (AS) were studied in vitro with peroxidase and H2O2, all of which are present in the tobacco apoplast. While both phenolics were observed to be substrates for peroxidase, AV was found to react 10x faster than AS. The oxidation of AV resulted in an increase of the redox potential to a maximum of 0.35 V. The oxidation of AS resulted in an increase to a maximum of 0.55 V. Interestingly, the redox potential was not proportional to the oxidation of either phenolic. It was found that when present together, co-oxidation occurs with the faster acting AV as a catalyst to oxidize AS. This has an immediate effect on the redox potential. The rate of AS oxidation is dependent on the AV concentration and thus so is the redox potential reaching 4.5 V. The results suggest that these phenolics could play a role in the regulation of the apoplast redox potential during the early stages of stress.

Introduction

The plant apoplast is often the site at which bacterial pathogens and host first encounter each other, and in this location, many offensive and defense responses are triggered by both participants. A notable characteristic in the early responses is their redox sensitivity. The plant produces apoplast phenolics in response to the invading bacteria and then follows with an oxidative burst, resulting in the generation of reactive oxygen species in the apoplast [1,4,5,9,12,14]. In cell suspensions inoculated with bacteria, phenolics are detected in the extracellular medium within hours [1,9]. In whole plants, increased phenolics have been shown in the apoplast wash fluid of leaves inoculated with pathogens [2,5]. Acetovanillone (AV) and acetosyringone (AS) were two of the major inducible phenolics found in interactions of tobacco with bacteria. The concentration of the phenolics increased and decreased independently during the first hours of interaction. Since this period is also when the oxidative burst occurs with the production of reactive oxygen species in the apoplast, we wanted to examine the redox interactions of these phenols with reactive oxygen species.

Previous work with AS [1,6,7], demonstrated that it had bioactivity in the realm of the tobacco/bacterial pathogen interaction; increased concentrations up to 100 μM reduced the time between inoculation and the oxidative burst response. It was also found to magnify the redox potential of the associated oxidative burst in tobacco cell suspensions. When Pseudomonas syringae pv. syringae was in contact with oxidizing AS, it immediately entered a ‘viable but not culturable’ (VBNC) state [11]. We recently examined the oxidation of AV in detail [3]. AV is also known as ‘apocynin’ in the human disease studies, where it has been known for its inhibitory effects on several major maladies. Most of the beneficial properties of AV were attributed to its reduction chronic oxidative in the tissue.

The redox status of the plant apoplast environment during the invasion of pathogens appears to tumultuous and likely important in determining the outcome of the interaction. The redox potential, like pH, can have multiple effects including regulation of important pathways as well as affecting molecular recognition. Our objective in this study was to gain insight into how two major phenolics, acetovanillone and acetosyringone, would affect each other during an oxidative burst, as well as, affect the redox potential.

Section snippets

Chemicals

All chemicals were obtained from Sigma-Aldrich Chemicals, Inc (St. Louis, MO, USA): Acetosyringone (3′,5′-dimethoxy-4′-hydroxyacetophenone, AS), acetovanillone (3′-methoxy-4′-hydroxyacetophenone, AV), UPLC grade water and methanol, and horseradish peroxidase (P8250). Horseradish peroxidase was assayed with pyrogallol as described by Sigma Chemical Co. One unit of peroxidase will form ~18 μM of purpurogallin from pyrogallol per minute at 25 °C.

Reaction mixtures

Reaction mixtures, 10 ml, were contained in 20 ml

Oxidation of acetosyringone (AS) -constant AS

The kinetics of acetosyringone oxidation was first examined using varying amounts of H2O2, ranging from 25 to 200 μM (Fig. 1A and B). The reaction mixtures also contained 200 μM AS and, routinely, 100 mU/ml of horseradish peroxidase, in 10 ml of 50 mM potassium phosphate buffer, pH 6, 25 C. The concentration of AS was monitored by UPLC-UV and the H2O2 concentration was monitored by the Fox Assay. The initial rates of AS degradation were similar, around 1.5–2 μM AS min−1, in all initial

Discussion

The phenolics that are found to increase in the apoplast soon after inoculation with bacteria, have several important characteristics that affect the redox status of the apoplast. One is their affinity for oxidation by peroxidase; another is their redox potential, which relates to the energy of the molecular orbitals involved. The results of this study demonstrate that two phenolics with very different characteristics can interact and significantly affect the surrounding redox potential. When

CRediT authorship contribution statement

C. Jacyn Baker: Conceptualization, Investigation, Supervision. Jodi Smith: Investigation, Methodology, Resources. Clifford Rice: Investigation, Methodology, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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