The response of filamentous fungus Rhizopus nigricans to flavonoids

Abstract

The saprophytic fungus Rhizopus nigricans constitutes a serious problem when thriving on gathered crops. The identification of any compounds, especially natural ones, that inhibit fungal growth, may therefore be important. During its life cycle, Rhizopus nigricans encounters many compounds, among them the flavonoids, plant secondary metabolites that are involved in plant defense against pathogenic microorganisms. Although not being a plant pathogen, Rhizopus nigricans may interact with these compounds in the same way as plant pathogens--in response to the fungitoxic effect of flavonoids the fungi transform them into less toxic metabolites. We have studied the interaction of R. nigricans with some flavonoids. Inhibition of hyphal spreading (from 3% to 100%) was observed by 300 μM flavones, flavanones and isoflavones, irrespective of their basic structure, oxidized or reduced C-ring, and orientation of the B-ring. However, a hydrophobic A-ring was important for the toxicity. R. nigricans transformed some of the flavonoids into glucosylated products. Recognition of substrates for glucosylating enzyme(s) did not correlate with their fungitoxic effect but depended exclusively on the presence of a free -OH group in the flavonoid A-ring and of a hydrophobic B-ring. Although the fungus produced glucosyltransferase constitutively, an additional amount of the enzyme was induced by the substrate flavonoid. Moreover, effective detoxification was shown to require the presence of glucose.

Bioconversion of Kaempferol and Quercetin Glucosides from Plant Sources Using Rhizopus spp.

“…Therefore, quercetin, which possesses one extra -OH group at the 3 -position, was degraded at a lower rate compared to kaempferol during fungal degradation. This could also be a reason for the faster degradation of kaempferol by R. oryzae after 2 days when its accumulation in fermented medium seems to be sufficient for fungal detoxification as mediated by the fungal enzymes [35].…”

Improved Release and Metabolism of Flavonoids by Steered Fermentation Processes: A Review

“…Similar to quercetin, kaempferol was glycosylated by the filamentous fungus Cunninghamella echinulata [ 67 ]. According to Slana et al [ 82 ], under conditions whereby flavonoids are toxic, e.g., when they are present in high concentrations, microorganisms such as fungi could produce glycosylating enzymes (glycosyltransferase) transforming the phenolic compounds into less toxic metabolites. Not only glycolysltransferases but also other enzymes can be involved in the glycosylation of phenolic compounds, as it was shown e.g., for glucansucrase of Leuconostoc mesenteroides acting on luteolin, quercetin and myricetin [ 65 ], cellulase of Penicillium decumbrens on quercetin [ 69 ], cellulase of Aspergillus niger on catechin [ 64 ], or α-amylase of Bacillus sp.…”

The Role of the Plant Flavonoid Pathway in Adaptation to Elevation

“…Flavones and flavonols can act as defensive compounds within vegetative tissues as protection against herbivores and fungal and bacterial pathogens Slana et al, 2011), but their role in defense against herbivores and pathogens is less clear than their role in protection against UV radiation. Flavonoids are produced constitutively and also induced by infection and browsing, but interactions between specific herbivores and plant flavonoid defenses vary in the literature (reviewed in .…”