Abstract
Elicited soybean (Glycine max (L.) Merrill, Leguminosae) seedlings can produce prenylated isoflavonoids from different subclasses, namely pterocarpans (glyceollins), isoflavones and coumestans. These prenylated isoflavonoids serve as defence compounds and can possess antimicrobial activity. Recently, we showed that priming with reactive oxygen species (ROS) specifically stimulated the production of glyceollins in Rhizopus spp.-elicited soybean seedlings (ROS + R). In this study, we achieved diversification of the inducible subclasses of prenylated isoflavonoids in soybean, by additional stimulation of two prenylated isoflavones and one prenylated coumestan. This was achieved by using a combination of the relatively long-lived ROS representative, H2O2, with AgNO3 prior to microbial elicitation. Microbial elicitation was performed with a live preparation of either a phytopathogenic fungus, Rhizopus spp. or a symbiotic bacterium, Bacillus subtilis. B. subtilis induced 30% more prenylated isoflavones than Rhizopus spp. in (H2O2 + AgNO3)-treated seedlings, without significantly compromising the total levels of glyceollins, compared to (ROS + R)-treated seedlings. The most abundant prenylated isoflavone induced was 6-prenyl daidzein, which constituted 60% of the total isoflavones. The prenylated coumestan, phaseol, was also induced in the (H2O2 + AgNO3)-treated and microbially elicited seedlings. Based on previously developed quantitative structure-activity relationship (QSAR) models, 6-prenyl daidzein and phaseol were predicted to be promising antibacterials. Overall, we show that treatment with H2O2 and AgNO3 prior to microbial elicitation leads to the production of promising antibacterial isoflavonoids from different subclasses. Extracts rich in prenylated isoflavonoids may potentially be applied as natural antimicrobial agents.
Highlights
The increasing demand for novel, natural antimicrobials for food preservation (Hintz et al, 2015) and for combating drug-resistant pathogens (Subramani et al, 2017) has triggered research to find methods for efficient production of structurally diverse, yet chemically-related compounds
The RP-UHPLC-PDA chromatograms of ethanol extracts of (H2O2 + AgNO3)-treated soybean seedlings are shown in Fig. 3 and compared to the recently proposed reactive oxygen species (ROS) + R, i.e. (H2O2 + Fe (II)), which is used as a benchmark in this study
When (H2O2 + AgNO3)-treated seedlings were subsequently elicited with Rhizopus spp. or Bacillus subtilis, the levels of prenylated isoflavones increased significantly
Summary
The increasing demand for novel, natural antimicrobials for food preservation (Hintz et al, 2015) and for combating drug-resistant pathogens (Subramani et al, 2017) has triggered research to find methods for efficient production of structurally diverse, yet chemically-related compounds. Fungal elicitation is the most employed strategy to stimulate production of prenylated isoflavonoids in legumes (Aisyah et al, 2013; Feng et al, 2007; Simons et al, 2011a; Sobolev et al, 2008). Reactive oxygen species (ROS) generated through Fenton’s reaction (H2O2 + Fe (II)), have been exogenously applied alone (Degousee et al, 1994) or as a primer prior to fungus elicitation to stimulate glyceollin biosynthesis (Kalli et al, 2020). A combination of H2O2 and AgNO3 was selected to treat soybean seedlings prior to microbial elicitation to stimulate the pro duction of prenylated isoflavonoids from different subclasses, including the more antibacterial prenylated isoflavones.
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