On the Microbe-Essential Oil Interplay: Processes, Mechanisms, Impacts and Applications

Dear Colleagues,

With more than six thousand years of continuous use, essential oils are amongst the most ancient plant-derived, non-food products in the service of humankind. Artifacts, inscriptions, and manuscripts from Egypt, Mesopotamia, China, India, and Greece show that they have played an important role in the life of people since early human civilization, serving their spiritual and physical needs: to honor their gods, to embalm the dead, to cure and perfume themselves, etc. Their use continues uninterrupted to this day. Along with scent, antimicrobial activity has been their most valued property. A large number of studies show this activity to be expressed against bacteria, fungi, viruses, and many invertebrates which are invisible to the naked eye—microbes sensu lato (e.g., mites), along with other activities like antifeedant, insecticidal, or allelopathic. As expected, such inhibitory effects are not universal. We know insects are attracted by flowers’ scent, fungi sporulate intensely in the presence of essential oils, bacteria proliferate next to the glandular hairs of the leaves of Lamiaceae representatives, and weeds grow in abundance in cultivations of aromatic plants. Many other activities related to our physical and mental health are reported, adding more uses to the traditional ones. At present, essential oils find applications in the sanitary, cosmetic, pharmaceutical, food, and agricultural industries, with research underway aiming at new targets and innovative ways to use them. This includes their use as feed additives in ruminants to reduce methane emanating from food digestion, or as green pesticides in the form of nano-capsules or nano-emulsions.

The hugely diverse biological activity of essential (or volatile) oils is due to their composition. They are mixtures of low-molecular-weight compounds, belonging primarily (but not exclusively) to the chemical family of terpenoids. More than a hundred such compounds may be present in an essential oil, with concentrations varying from trace to higher than 70%. These compounds may behave in additive, synergistic, or antagonistic ways, producing unique effects depending on the target organisms. Research so far with microbes shows that essential oils interact with cell membranes. Round-shaped and Gram-negative bacteria are generally less sensitive to essential oils than rod-shaped and Gram-positive ones, with the hydrophilic or lipophilic properties of essential oil constituents playing a role in the produced effect, and with oxygenated compounds being more active than hydrocarbons as growth inhibitors.

Because of their volatile nature, essential oils emanate in the environment, playing their role in chemical communication and offering resistance against both biotic and abiotic stresses. In the atmosphere, they participate as VOCs (volatile organic compounds) in the formation of air pollutants. It is estimated that chemical degradation is by far the largest sink of VOCs, with deposition having only a minor contribution. Nevertheless, soil and soil dwellers are primary recipients of these secondary metabolites, as plants do not economize in them once they are produced; plants’ dead parts contain large amounts of them. Once in soil, essential oil constituents influence soil communities and soil processes, as they are or after they are transformed chemically or biologically. Succession, litter decomposition, nutrient cycling, and enzymatic activity are among the processes that can be affected, as well as degrading microbes and other communities of the detritus food chain and mycorrhizal fungi. Given our very limited knowledge on soil responses in the presence of essential oils and their impacts on ecosystem functioning, contributions in this area are valuable—particularly if they provide some insight into responses and impacts at different scales and under different environmental regimes.

Aromatic plants are abundant in the fragrant Mediterranean environment, but essential-oil-producing plants also occur in cold climates, so that an understanding of how these secondary metabolites influence processes and communities is of global interest. This interest is further enhanced given the current re-evaluation of the methods and practices of agriculture. The paradigm shift towards sustainable farming asks for inputs like eco-friendly, target-specific, and biodegradable pesticides that do not undermine the ecosystem services. The naturally derived and fully biodegradable essential oils which influence a large spectrum of organisms in various ways are serious candidates as green pesticides, and a great deal of research is currently underway in this area.

Given the ever-increasing interest in aromatic plants and essential oils, as well as the importance of these secondary metabolites, a Special Issue is being prepared in Plants dedicated to their interplay with microbes. We welcome contributions on all sorts of interactions between essential-oil-producing plants and microbes of any kind, on interactions between aromatic plants and other organisms that are mediated by microbes, on effects involving microbes that may be attributed to essential oils or their constituents, on related processes and mechanisms, on impacts that may be direct or indirect, and on applications that they may find, focusing on the natural and agricultural environments. Original contributions will be given priority over review articles. The deadline for manuscript submission is 28 February 2022.

Prof. Dr. Despina Vokou
Guest Editor

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• aromatic plants
• biological activity
• essential oils
• decomposition
• green pesticides
• phyllosphere
• plant–microbe interactions
• soil metabolism
• microbial communities
• terpenoids