**4. Antiviral activity mechanisms and their applications**

The application of plant EOs and active components as direct or indirect effects of antiviral or virucidal activity together with the insect pest control and management [40] is an interesting operation. Many research studies have been focused on medicinal pathogenic human and animal viruses. This knowledge can be further database documented, developed, and applied to plant pathogenic viruses and insect vectors for data-driven agriculture and management.

The plant EOs and their components have been effective in increasing physical/ chemical/biological stabilities and their antiviral effectiveness. Several research studies were reported the potential plant EOs for antiviral activity, for instance, showed that the plant EO isolated from star anise (*Illicium verum* Hook.f., Illiciaceae) and fennel (*Foeniculum vulgare* Mill., Apiacae) had potentially inhibited *Potato virus X* (PVX: *Potexvirus*, *Flexiviridae*), *Tobacco ringspot virus* (TRSV: *Nepovirus*, *Secoviridae*), and *Tobacco mosaic virus* (TMV: *Tobamovirus*, *Virgaviridae*). Similarly, Bishop [41] found that the local lesions of TMV on tobacco (*Nicotiana glutinosa* L., Solanaceae) decreased after being tested by the tea tree oil [*Melaleuca alternifolia* (Maiden & Betche) Cheel., Myrtaceae]. In relation, Iftikhar et al. [42] tested the EO of clove [*S. aromaticum* (L.) Merr. & L.M.Perry, Myrtaceae] caused maximum inhibition of *Potato leaf roll virus* (PLRV: *Polerovirus*, *Luteoviridae*). Lu et al. [43] reported that TMV transmission was inhibited by the EO of artemisia (*Artemisia vulgaris* L., Asteraceae), ginger (*Zingiber officinale* Roscoe, Zingiberaceae), and lemongrass [*Cymbopogon citratus* (Dc. Ex Nees) Stapf, Gramineae]. Moreover, Dikova et al. [44] found that lavender oil (*Lavandula* 

*angustifolia* Mill., Lamiaceae) could control *Tomato spotted wilt virus* (TSWV: *Tospovirus*, *Bunyaviridae*). The EOs extracted from billygoat-weed (*Ageratum conyzoides* L., Asteraceae), bottle brush [*Callistemon citrinus* (Curtis) Skeels, Myrtaceae], ajwain (*Carum copticum* L., Apiaceae), holy basil (*Ocimum sanctum* L., Lamiaceae), and pepper elder [*Peperomia pellucida* (L.) Kunth, Piperaceae] have potentially inhibited *Cowpea mosaic virus* (CPMV: *Comovirus*, *Comoviridae*), *Bean common mosaic virus* (BCMV: *Potyvirus*, *Potyviridae*), and *Southern bean mosaic virus* (SBMV: *Sobemovirus*, *Solemoviridae*) [45]. In another study, Helal [46] reported that the plant EOs of thyme (*T. vulgaris* L., Lamiaceae) and peppermint (*M. piperita* L., Lamiaceae) had inhibition effects of *Tobacco necrosis virus* (TNV: *Necrovirus*, *Tombusviridae*) and *Cucumber mosaic virus* (CMV: *Cucumovirus*, *Bromoviridae*).

According to recent studies, Na Phatthalung and Tangkananond [47] applied dot-immunobinding assay (DIBA) for evaluating the potential of plant EO for transmission inhibitory effects on *Rice ragged stunt virus* (RRSV: *Oryzavirus*, *Reoviridae*) by the brown planthopper (BPH: *Nilaparvata lugens* Stål) (Homoptera: Delphacidae). These studies were demonstrated that all the tested plant EO had potential transmission inhibitory in efficiency ranges from 0.002 to 0.1% from the infected rice plants to non-viruliferous BPH status. In addition, viruliferous BPH status was communicated with similar success to viral-free rice plants. These include black pepper (*Piper nigrum* L., Piperaceae), lemongrass, star anise, kaffir lime (*Citrus hystrix* DC, Rutaceae), and kaempfer [*Boesenbergia rotunda* (L.) Mansf., Zingiberaceae] highly effected 10–70% inhibition and lime [*C. aurantifolia* (Christm.) Swingle, Rutaceae], galangal [*Alpinia galangal* (L.) Sw., Zingiberaceae], holy basil, sweet basil (*O. basilicum* L., Lamiaceae), and betelvine (*P. betle* L., Piperaceae) slightly effected 10–30% inhibition, respectively (**Figure 2**). Furthermore, the plant EOs of star anise and lemongrass were selected for assessing the toxicity and physiological effects on the BPH vector. These results showed that the plant EO in the range from 3 to 5% showed malformed structures and completely destroyed within 3–5 days after treatment (DAT) (**Figure 3**). Therefore, the plant EOs paved the possibility and potential candidates for further prototype development as commercial antiviral agents for plant protection and sustainable agricultural management in agriculture 4.0.

#### **Figure 2.**

*The potential of plant EO for transmission inhibitory effects on RRSV by the BPH vector and detection method by DIBA (figure was modified from reference number [47]).*

*Perspective Chapter: Perspectives on Pathogenic Plant Virus Control with Essential Oils… DOI: http://dx.doi.org/10.5772/intechopen.104639*

#### **Figure 3.**

*The morphological effects of plant EOs on the BPH. (figure was modified from reference number [47]).*

It is possible to hypothesize the antiviral mechanisms from the literature reviews about the viral infection cycle in host cell-culture-based systems (*in vitro*) and viral host models (*in vivo*) as well as molecular docking (*in silico*) [48–50]. The summary concept of antiviral mechanisms by plant EO can be divided into direct and indirect actions. Several modes of direct antiviral actions affected the enveloped and nonenveloped (naked) viral progenies by substance and enzyme blocking in different steps of the viral infection cycle (**Figure 4**) [51, 52]. The various plant EOs and active components have potential inactivation viral activities, transmissibility, stability, and infectivity on enveloped viruses more than on the naked viruses [51].

Several modes of indirect antiviral actions affect host properties, viral transmission modes, and infection efficiency. Generally, plant EO has important features

#### **Figure 4.**

*The mechanism of antiviral actions as possible targets for plant EO. (figure was modified from reference number [51]).*

of hydrophobic properties including surface tension, contact angle value, droplet volumes, and lubricating with varied viscosities that affect the external surface area structure properties of viral hosts [53]. Insect vectors or plants that were sprayed with plant EO may be modified the physiology and disturb the metabolism of the inoculated cells [54]. External surface areas of the viral particles and hosts were coated, which affect the infectivity and transmissibility, were inhibited. Developmental and survival periods of insect vectors are significant for viral transmission and nymph stages are most important for viral transmission. Adult stages are important for population increase, migration, and viral spread [55]. However, several plant EOs tended to be more effective on the soft-bodied insects than the hard-bodied insects. They affected host plant manipulation by the induced systemic resistance (ISR) and insecticidal properties [56]. The active plant EO can manage the insect vector damaging effects on crops and also reduce their plant viral transmission ability.

The plant EO has optimal properties for covering with the general surface structure of probing stylet or body-cuticle (extracellular layer) of insect vectors and has optimal activities for viral transmission inhibition. Therefore, the inhibition of virus transmission by plant EO occurs at the virus-vector or virus-vector-plant relationships (tri-partite relationships). All of these significantly play an important role for knowledge applying in future crop protection and successful pest management under the Agriculture 4.0 policy.
