Plants heavily rely on chemical defense systems against a variety of stressors. The glucosinolates in the Brassicaceae and some allies are the core molecules of one of the most researched such pathways. These natural products are enzymatically converted into isothiocyanates (ITCs) and occasionally other defensive volatile organic constituents (VOCs) upon fungal challenge or tissue disruption to protect the host against the stressor. The current review provides a comprehensive insight on the effects of the isothiocyanates on fungi, including, but not limited to mycorrhizal fungi and pathogens of Brassicaceae. In the review, our current knowledge on the following topics are summarized: direct antifungal activity and the proposed mechanisms of antifungal action, QSAR (quantitative structure-activity relationships), synergistic activity of ITCs with other agents, effects of ITCs on soil microbial composition and allelopathic activity. A detailed insight into the possible applications is also provided: the literature of biofumigation studies, inhibition of post-harvest pathogenesis and protection of various products including grains and fruits is also reviewed herein.

The glucosinolate pathway, which is present in the order Brassicales, is one of the most researched defensive natural product biosynthesis pathways. Its core molecules, the glucosinolates are broken down upon pathogen challenge or tissue damage to yield an array of natural products that may help plants defend against the stressor. Though the most widely known glucosinolate decomposition products are the antimicrobial isothiocyanates, there is a wide range of other volatile and non-volatile natural products that arise from this biosynthetic pathway. This review summarizes our current knowledge on the interaction of these much less examined, non-isothiocyanate products with fungi. It deals with compounds including (1) glucosinolates and their biosynthesis precursors; (2) glucosinolate-derived nitriles (e.g. derivatives of 1H-indole-3-acetonitrile), thiocyanates, epithionitriles and oxazolidine-2-thiones; (3) putative isothiocyanate downstream products such as raphanusamic acid, 1H-indole-3-methanol (= indole-3-carbinol) and its oligomers, 1H-indol-3-ylmethanamine and ascorbigen; (4) 1H-indole-3-acetonitrile downstream products such as 1H-indole-3-carbaldehyde (indole-3-carboxaldehyde), 1H-indole-3-carboxylic acid and their derivatives; and (5) indole phytoalexins including brassinin, cyclobrassinin and brassilexin. Herein, a literature review on the following aspects is provided: their direct antifungal activity and the proposed mechanisms of antifungal action, increased biosynthesis after fungal challenge, as well as data on their biotransformation/detoxification by fungi, including but not limited to fungal myrosinase activity.

Influenza A and B viruses are a global threat to human health and increasing resistance to the existing antiviral drugs necessitates new concepts to expand the therapeutic options. Glycopeptide derivatives have emerged as a promising new class of antiviral agents. To avoid potential antibiotic resistance, these antiviral glycopeptides are preferably devoid of antibiotic activity. We prepared six vancomycin aglycone hexapeptide derivatives with the aim of obtaining compounds having anti-influenza virus but no antibacterial activity. Two of them exerted strong and selective inhibition of influenza A and B virus replication, while antibacterial activity was successfully eliminated by removing the critical N-terminal moiety. In addition, these two molecules offered protection against several other viruses, such as herpes simplex virus, yellow fever virus, Zika virus, and human coronavirus, classifying these glycopeptides as broad antiviral molecules with a favorable therapeutic index.

The interaction between plant defensive metabolites and different plant-associated fungal species is of high interest to many disciplines. Volatile organic compounds (VOCs) are natural products that are easily evaporated under ambient conditions. They play a very important role in inter-species communication of microbes and their hosts. In this study, the VOCs produced by 43 different fungal isolates of endophytic and soil fungi during growth on horseradish root (Armoracia rusticana) extract or malt extract agar were examined, by using headspace-gas chromatography-mass spectrometry (headspace-GC-MS) and a high relative surface agar film as a medium. The proposed technique enabled sensitive detection of several typical VOCs (acetone, methyl acetate, methyl formate, ethyl acetate, methyl butanol isomers, styrene, beta-phellandrene), along with glucosinolate decomposition products, including allyl cyanide and allyl isothiocyanate and other sulfur-containing compounds—carbon disulfide, dimethyl sulfide. The VOC patterns of fungi belonging to Setophoma, Paraphoma, Plectosphaerella, Pyrenochaeta, Volutella, Cadophora, Notophoma, and Curvularia genera were described for the first time. The VOC pattern was significantly different among the isolates. The pattern was indicative of putative myrosinase activity for many tested isolates. On the other hand, endophytes and soil fungi as groups could not be separated by VOC pattern or intensity.

Fenugreek is used as a spice and a traditional herbal medicine for a variety of purposes, given its antidiabetic and antioxidant effects. Self-emulsifying drug delivery systems (SEDDS) of herbal drugs are targets of extensive research aiming to increase bioavailability and stability. The study’s objective was to formulate SEDDS containing Trigonella foenum-graecum extract to improve the stability of herbal extract and to increase their permeability through a Caco-2 monolayer. A characterized fenugreek dry extract was used for the formulations, while the SEDDS properties were examined by particle size analysis and zeta potential measurements. Permeability assays were carried out on Caco-2 cell monolayers, the integrity of which was monitored by follow-up trans-epithelial electric resistance measurements (TEER). Cytocompatibility was tested by the MTT method, and an indirect dissolution test was performed, using DPPH antioxidant reagent. Two different SEDDS compositions were formulated from a standardized fenugreek dry extract at either the micro- or the nanoemulsion scale with sufficient stability, enhanced bioavailability of the compounds, and sustained release from HPMC capsules. Based on our results, a modern, non-toxic, cytocompatible fenugreek SEDDS formulation with high antioxidant capacity was developed in order to improve the permeability and bioavailability of all components.

The glucosinolates of Brassicaceae plants are converted into bioactive isothiocyanates and other volatiles during a challenge by pathogens and other biotic stressors. However, the role of alternative downstream products with weaker potency (e.g., nitriles) is far from being fully understood. This study tested the possible synergistic antifungal interaction between various glucosinolate-derived nitriles and 2-phenylethyl isothiocyanate (PEITC) on 45 fungal strains, including endophytes from horseradish roots (Brassicaceae) and soil fungi, using an airtight system enabling the accurate study of extremely volatile antifungal agents. The median minimal inhibitory concentrations (MICs) were 1.28, 6.10, 27.00 and 49.72 mM for 1H-indole-3-acetonitrile (IAN), 3-phenylpropanenitrile (PPN), 4-(methylsulfanyl)-butanenitrile (MSBN) and 3-butenenitrile (BN, = allyl cyanide), respectively. Thus, nitriles were considerably weaker antifungal agents compared to PEITC with a median MIC of 0.04 mM. For the same nitriles, the median fractional inhibitory concentration indices (FICIs) of the combinations were 0.562, 0.531, 0.562 and 0.625, respectively. Altogether, 47.7%, 56.8%, 50.0% and 27.3% of tested fungal strains showed a synergistic antifungal activity (FICI ≤ 0.5) for the nitrile–isothiocyanate combinations, respectively. Hypocreales strains showed the least sensitivity towards the GSL decomposition products and their combinations. The mean MIC values for PEITC showed 0.0679 ± 0.0358, 0.0400 ± 0.0214, 0.0319 ± 0.0087 and 0.0178 ± 0.0171 mM for Hypocreales, Eurotiales, Glomerellales and Pleosporales, respectively. In addition, nitriles, especially IAN, also showed significant differences. For the same fungi, the median FICI values fell in the ranges of 0.61–0.67, 0.52–0.61, 0.40–0.50 and 0.48–0.67, respectively, depending on the nitrile. Our results suggest that glucosinolate-derived nitriles may enhance isothiocyanate antifungal activity and that they may play an active role in shaping the plant microbiome and contribute to the filtering of microbes by plants.