Aromatic plants produce essential oils (EOs) as their secondary metabolites. EOs are produced by glandular trichomes and specialized secretory tissues and are diffused on to the surface of plant organs such as flower and leaves (Iriti et al., 2006; Lange and Turner 2013; Sharifi-Rad et al., 2017). EOs and their components, target cell membrane, cytoplasm, enzymes and protein that can completely change the conformation of the microbial cell (Nazzaro et al., 2013). Essential oils as natural plant protectant may be used as bactericide, viricide, fungicide, antiparasitics and insecticide (Bakkali et al., 2008). The antimicrobial and antifungal activity of EO might be due to terpenes and terpenoids, which are lipophilic in nature capable of disrupting the cell membrane leading to cell death (Nazzaro et al., 2017). EOs may act as prooxidant affecting inner cell membranes and mitochondria. In few cases changes in intracellular redox potential and mitochondrial dysfunction induced by EOs can be associated with their ability to exert antigenotoxic effects (Bakkali et al., 2008). Antifungal agent can inhibit the function of mitochondrial electron transport chain, reducing mitochondrial membrane potential (Nazzaro et al., 2017). The EOs of Oliveria decumbens completely suppressed the mycelial growth of the fungus Alternaria solani (Bahraminejad et al., 2016). The major constituent of its EO are carvacrol, thymol, p-Cymene, ϒ-terpiene and myristicin. The EOs of this species exhibiting highest anti-Alternaria activity can be used as natural fungicide. Thyme essential oil induced apple resistance against Botrytis cinerea leading to accumulation of the pathogenesis-related proteins PR-8 which codes for a class III chitinase (Banani et al., 2018). The terpenes alphapinene destroyed the cellular integrity and modified mitochondrial activity in some microorganisms (Andrews et al., 1980). The antimicrobial activity of EOs is due to their solubility in phospholipid bilayer of cell membrane. The solubility of EOs in water is related to their ability to penetrate the cell walls of bacterium and fungus (Knobloch et al., 1989). Hu et al.(2017) studied the antifungal activity in vitro and anti-aflatoxigenic efficiency in vivo of EOs derived from turmeric (Curcuma longa L.) against Aspergillus flavus. Suppression of fungal contamination in maize exhibits that turmeric EO has an ability of a natural antifungal agent.
Cristani et al.(2007) speculate antimicrobial effect of thymol, carvacrol, p-cymene and gamma-terpiene may result from perturbation of lipid fraction of the plasma membrane of the microorganisms. The effect seems to be dependent on the lipid composition and net surface charge of the microbial membrane. The compound may cross the cell membranes thus penetrating into the interior of the cell and interacting with intracellular sites critical for antibacterial activity (Cristani et al., 2007). EOs from Gaultheria procumbens is composed of methylsalicylate (MeSA) a compound that can be metabolized in plant tissues to salicylic acid inducing plant immunity against phytopathogens (Vergnes et al., 2014). Systemic acquired resistance (SAR) can protect plant against a wide range of pathogens, the molecules that induce this response can represent a strategy to protect plant. For induction of SAR through salicylate pathway natural source of active compounds have to be identified (Vergnes et al., 2014). EOs and hydrolysate are effective against phytobacterial pathogens (Proto et al., 2022). Mafakheri and Mirghazanfari (2018) result demonstrated peppermint and fennel EOs as a promissory natural fungicide which can control plant disease. The inhibitory and fungicidal effect of peppermint EO was two mg/ml against Alternaria species (Mafakheri and Mirghazanfari 2018). Amini et al.(2016) result demonstrated that the application of EOs of Cymbopogon citratus and Ocimum basilicum provides protection against soil-borne oomycete pathogen Phytophthora capsici, P. drechsleri and P. melonis in pepper, melon and cucumber plant. The essential oil from Thymus vulgaris suppressed the mycelial growth of Colletotrichum gloeosporioides, Fusarium oxysporum and Rhizoctonia solani. While Cymbopogon citratus was active to only Fusarium oxysporum (Sun-Og et al., 2007). Terpinenol and α-terpineol the components of EO of Pistacia lentiscus L., completely inhibited the mycelial growth of Aspergillus flavus (Barra et al., 2007).
The fungal cell wall play a role in the growth and viability of fungi. Trans-Anethole a major component of anise oil possess antimicrobial activity. Anethole demonstrated antifungal activity against the filamentous fungus Mucor mucedo IFO 7684exhibiting morphological changes in hyphae such as swollen hyphal tip(Yutani et al., 2011). The morphological changes of M. mucedo can be explained by the fragility of cell walls caused by chitin synthase activity inhibition (Yutani et al., 2011). Effect of EO from Citrus sinensis (L.) Osbeck epicarp on Aspergillus niger resulted in loss of cytoplasm in fungal hyphae and budding of hyphal tip. The hyphal wall and its diameter became thin, distorted and resulted in cell wall disruption (Sharma and Tripathi 2008).EOs could be an option to tackle fungal diseases (Omar and Kordali 2019).
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