Skimmia anquetilia N.P. Taylor and Airy Shaw (Rutaceae): A Critical Appriasal of its Ethnobotanical and Pharmacological Activities

Skimmia anquetilia N.P. Taylor and Airy Shaw (Rutaceae) is a perennial, aromatic, gregarious wild ornamental shrub native to the Western Himalaya. The plant is used in the traditional medicinal system to treat copious health conditions like rheumatism, fever, inflammation, headache, influenza, body-ache, clearing of the nose, diabetes, lowering the body temperature, smallpox, wounds, burns, snake, and scorpion bites. Phytochemical and gas chromatography-mass spectrometer (GC-MS) analysis of S. anquetilia showed the presence of alkanes, alkenes, carboxylic acids, fatty acids, and their esters, simple coumarins, terpenes, phenylpropanoid, and so on. These active principles exhibit a wide array of pharmacological effects, including anti-inflammatory, antioxidant, anti-cancerous, anti-feedant, and antibacterial properties. Most pharmacological studies were based on the essential oil and the crude extracts of the plant and the bioactive compounds responsible for the bioefficacy have not been well-identified. Further investigations are required to transform the experience-based claims on the use of S. anquetilia in traditional medicine practices into evidence-based information. Detailed in-vitro and in-vivo studies on the mechanisms of action of pure bioactive compounds and more elaborate toxicity studies to ensure plant safety for human use should be conducted. This review recapitulates the current status of its use in the ethnobotany, phytochemistry, and pharmacological activities. It also offers a critical assessment of the plant’s existing information which would help to recuperate its potential as a source for drug development of lead molecules.


Introduction
Medicinal plants have achieved broader recognition in recent times since these plants are natural products, they have minimal side effects and better effectiveness than their synthetic equivalents (Batiha et al., 2020). Approximately 80% of people in the world rely upon conventional medicine as a vital source of their basic medical care (Ekor, 2014). Most treatments use medicinal plant extracts and bioactive molecules (Michel et al., 2020). Medicinal plants are important sources of crude drugs that are used to treat various pathological conditions to maintain a status of well-being (Shakya, 2016). Medicinal plants have always been a potential source to treat various ailments, either in the form of traditional preparations or as pure active principles, and perhaps they are often the only source of medicine for the majority of folks in the developing nations (Taylor et al., 2001).
S. anquetilia is a medicinal plant that belongs to the genus Skimmia of the Rutaceae family (Saqib and Sultan, 2005). The plant is endemic to the Western Himalaya and is distributed in the mountain ranges of Afghanistan and Indian sub-continent: India, Pakistan, and Nepal (Nissar et al., 2018). In India, the plant originates in the subalpine region of the Garhwal (Gaur, 1999), Jammu and Kashmir, Uttar Pradesh, and Himachal Pradesh (Walters et al., 1986;Sharma et al., 1993). In the conventional medicinal system, different parts of the plant have been used for the treatment of various ailments in parts of the Western Himalayas of India, Nepal, and Pakistan (Epifano et al., 2015). The purpose of this review is to provide an up-to-date and comprehensive overview of the botany, phytochemistry, traditional uses, and pharmacological activities of S. anquetilia. Furthermore, the present knowledge obtained mainly from experimental studies was critically evaluated to provide evidence and validation for local and traditional uses of S. anquetilia and to suggest future research scenarios and prospective therapeutic uses for this plant.

Searching strategies
An extensive literature search related to the plant species S. anquetilia of the genus Skimmia was conducted to gather all relevant information about the traditional uses, phytochemicals, and pharmacological activities. Publicly accessible databases and primary sources were searched, including PubMed, SciFinder, Web of Science, Science Direct, Google Scholar, and so on (Figure 1). A large number of literature articles published from 1956 to 2022 were reviewed. The extracted data included vernacular plant names, plant descriptions, traditional uses, purified compounds, and pharmacological activities. The species name was validated using The Plant List (2013). 1 All studies and reviews that investigated the ability of S. anquetilia to heal illnesses in a laboratory (in-vitro) and 1 www.theplantlist.org Frontiers in Plant Science 02 frontiersin.org Several databases used to access various phytoconstituents and pharmacological activities of Skimmia anquetilia.
animals (in-vivo) were included as long as the effects were explicitly stated.

Inclusion and exclusion criteria
The literature published up to 2022 was used in this review paper to assess the biological efficacy of S. anquetilia concerning six health conditions related to humans. The search was limited to studies published in the English language, independent of the sample size or prospective development period.

Data extraction
The titles, abstracts, and full articles were used to make a preliminary assessment of the publications by the researchers. Manuscripts that met the study's predetermined addition and exemption criteria were selected and included. The papers were then used to collect the necessary data on experimental design (animal model and extraction methods), interventions delivered, and treatment findings.

Collection and identification of plant specimen
The whole plant of S. anquetilia N.P. Taylor

Ethnobotany of S. anquetilia
Ethnobotany is a field of research that applies indigenous plant knowledge to health care. In India S. anquetilia has long been used in Ayurveda and Unani system of medicine, which describes its numerous uses ( Table 1). The paste prepared from the roots of S. anquetilia has been used as an antidote against scorpion and snake bites (Bhattarai, 1992;Ahmed et al., 2004;Qureshi et al., 2009;Gondwal et al., 2012b). The powder of the plant bark has been found effective in curing wounds and burns (Negi et al., 2011;Singh and Rawat, 2011). The cold infusion of fresh leaves has been used to treats smallpox (Namiki, 1990;Saqib and Sultan, 2005;Prakash et al., 2011;Kumar et al., 2012), headache, and fever (Bhattarai, 1992;Gaur et al., 1993). The smoke of dried leaves has been used for freshness (Bhattarai, 1992;Gaur et al., 1993;Kunwar et al., 2010), and air purification (Chopra et al., 1956;Chauhan et al., 2017). Some tribal sections of India use its leaves in the preparation of curries (Anon, 1966;Chauhan et al., 2017), an alcoholic drink "Soor" high in calories (Rana et al., 2004), and as a flavoring agent (Anon, 1966;Chauhan et al., 2017). A paste prepared with the mixture of leaves of S. anquetilia and turmeric has been used to treat inflammation and rheumatism (Negi et al., 2011;Singh and Rawat, 2011). In Nepal, leaf infusion is taken for headache and for freshness, leaves are aromatic and used for headache and general fever (Baral and Kurmi, 2006). Personal communications with locals highlighted that that the leaf extract is used to treat diabetes in the Kupwara district of Kashmir valley. Moreover, its dried leaf powder has been used as an insecticidal and pesticidal agent (Bhattarai, 1992). The whole plant has been used in anesthesia and to treat several other health complications like pneumonia, paralysis, and lung cancer (Wani et al., 2016).

Pharmacology of S. anquetilia
The varied traditional uses of S. anquetilia have contributed to the initiation of several pharmacological studies. Preceding research shows that the S. anquetilia extracts exhibit a wide array of bioactivities, viz: antibacterial (Sharma et al., 2008a), antioxidant, anti-inflammatory, and other activities like antifeedant and anticancer activities (Gondwal et al., 2012a;Kumar et al., 2012;Negi et al., 2012;Wani et al., 2016; Table 3). At the same time, a wide array of in-vivo and in-vitro models has been used to evaluate the pharmacological properties of S. anquetilia. Evidence-based laboratory analysis indicates that petroleum ether, chloroform, ethyl-acetate, methanolic, and aqueous extracts of S. anquetilia possess several promising pharmacological properties.

Antibacterial activity
The methanol leaf extract and isolated active constituents, namely; skimminan and skimmin of S. anquetilia exhibited broad-spectrum antibacterial activity by inhibition of Agrobacterium tumifaciens, Pseudomonas syringae, and Pactobacterium carotovorum (Gram-negative plant pathogens) at a dose of 200 µg/disc using disc diffusion method. Results showed that the methanol extract and skimminan exhibited inhibitory activities against all three pathogens, whereas skimmin was only effective against A. tumifaciens. The highest zone of inhibition (12.6 ± 0.8) was exhibited by methanol extract against A. tumifaciens (Sharma et al., 2008a). Recently, (Nabi et al., 2022a,b) reported the antibacterial activity of methanol leaf extract and n-hexane, ethyl-acetate, and methanol root extracts of S. anquetilia against Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Salmonella typhi, and Staphylococcus aureus at different concentrations (10, 20, 40, 80, and 160 mg/ml) using the agar well diffusion assay. The methanolic leaf extract showed the highest zone of inhibition against E. coli (19.0 ± 0.57), followed by P. aeruginosa (18.0 ± 0.57) and K. Pneumoniae (17.0 ± 0.57) at 160 mg/ml. Among the root extracts, ethyl acetate extract showed the highest zone of inhibition against P. aeruginosa (18.0 ± 1.0), followed by S. aureus (17.0 ± 1.0). Furthermore, the minimum inhibitory concentration (MIC) of methanol leaf extract against P. aeruginosa (2 mg/ml) and ethyl acetate root extract against S. aureus (4 mg/ml) demonstrated therapeutically significant antibacterial activity.

Anticancer activity
Among the 35,000 plant species screened against cancer, about 3,000 have demonstrated potential anticancer activities (Surendran et al., 2021). Wani et al. (2016) determined the anticancer activity of the essential oils extracted from the leaf, stem bark, and root bark of S. anquetilia. The study was carried out on four different cell lines viz: MCF-7 (Breast), HeLa (Cervix), PC-3 (Prostate), and Caco-2 (Colon) using sulphorhodamine (SRB) assay. The stem bark essential oil was found to be the most active against all tested human cancer

Anti-inflammatory activity
Although various anti-inflammatory drugs have been discovered and are in clinical use, the inflammation condition is still challenging. Most of the existing drugs are opioids and non-steroidal anti-inflammatory drugs (NSAIDs), but they produce many side effects. Hence, the discovery of novel drugs is necessary. Plants possess various phytoconstituents that have displayed anti-inflammatory properties with few side effects (Virshette et al., 2019). Phytoconstituents, for instance, tannins, saponins, alkaloids, flavonoids, and phytosterols, have shown promising anti-inflammatory activities (Abdul-Nasir-Deen et al., 2020). The anti-inflammatory effect of S. anquetilia has been reported previously. Kumar et al. (2012) evaluated the anti-inflammatory activity of S. anquetilia leaf extract (LESA) by in-vitro and in-vivo methods using the human red blood cell (HRBC) membrane stabilization model and the carrageenan-induced rat paw edema model. The authors indicated that the anti-inflammatory effects of LESA revealed the concentration-dependent activity. For HRBC membrane stabilizing agent, S. anquetilia methanol extract exhibited the highest anti-inflammatory effect compared to the other leaf extracts and showed a result value of 68.50 ± 1.57.
The chloroform, ethyl-acetate, and methanol leaf extract of S. anquetilia at a dose of 400 mg/kg showed 58.22, 60.17, and 67.53% inhibition respectively in albino rats. The methanol extract showed a maximum anti-inflammatory activity of 67.53% at a 400 mg/kg dose against the standard drug Diclofenac (10 mg/kg) . Recently, Verma et al. (2020) reported the anti-inflammatory activity of ethyl-acetate leaf extract (EESA) of S. anquetilia by in-vitro methods using the HRBC membrane stabilization model at doses of 50, 100, 200, and 400 mg/ml. The EESA extract exhibited concentrationdependent inhibition, and the maximum inhibitory effect found was 90.70% at 400 mg/ml in comparison with the standard drug diclofenac sodium which showed 94.88% protection. Prakash et al. (2011) have analyzed 2,2-diphenyl-1 picryalhydrazide (DPPH) radical scavenging activity, reducing power assay, and chelating activity of Fe 2+ of aqueous extracts of seeds and fruit pulp of S. anquetilia using butylated hydroxytoluene (BHT), catechin, and gallic acid as standards. The results of the study revealed that both extracts exhibited moderate in-vitro antioxidant potential. Gondwal et al. (2012b) determined the antioxidant efficiency of essential oils of leaves and flowers of S. anquetilia by reducing power, chelating properties of Fe +2 , and 2 2 -diphenylpicrylhadrazyl (DPPH) radical-scavenging assay. DPPH radical scavenging activity was higher in the leaf essential oil and extract, whereas the maximum Frontiers in Plant Science 07 frontiersin.org chelating activity was observed in the flower's essential oil and aqueous extract and the highest reducing power was shown by flower essential oil and leaf extract. John et al. (2014) have determined the antioxidant activity of crude methanol extract, n-hexane, dichloromethane, ethyl-acetate, n-butanol, and aqueous fractions of S. anquetilia leaves by eight distinct assays viz: 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulpohonic acid) (ABTS) radical cation scavenging activity, the ferric reducing antioxidant power (FRAP), 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, total phenolic contents (TPC), total flavonoid contents (TFC), total antioxidant activity by phosphomolybdenum method, superoxide anion radical scavenging activity, and metal chelating activity. They opined that the ethyl-acetate fraction showed the highest total phenolic content, 2,2 -azinobis-(3-ethylbenzothiazoline-6-sulpohonic acid) (ABTS) radical cation scavenging activity, the ferric reducing antioxidant power (FRAP), and the DPPH radical scavenging activity. Dichloromethane fraction showed the highest antioxidant activity. The highest superoxide anion radical scavenging activity was displayed by the aqueous fraction. The crude methanolic extract exhibited the highest total flavonoid contents.

Anti-feedant activity
Anti-feedants are substances with anti-feedant characteristics that, at low concentrations, act on the pest's extremely specific receptor cells. Anti-feedant sensory-linked neurons either dissuade or inhibit insect feeding (feeding suppressant effect), or limit the functionality of a feeding stimulant receptor of herbivores, or the capacity to attach directly to normal feeding cues like carbohydrates and amino acids (Purrington, 2003). The essential oils from leaves and flowers of S. anquetilia showed suppression in the potential of egg-laying by Caryedon serratus, damaging the beetle for groundnut seeds at a 1.5% concentration. The suppression increased with the increase in oil concentration with no interference with the further development of larvae in adults (Gondwal et al., 2012a). The same effects of S. anquetilia extracts on Lepidoptera (forest pests) have been reported by Negi et al. (2012).

Anti-arthritic activity
The production of autoantigens in certain arthritic diseases may be due to the denaturation of protein and membrane lysis action. Denaturation of proteins causes the production of autoantigens in conditions such as rheumatic arthritis, cancer, and diabetes, which are considered inflammatory conditions. Therefore, by inhibition of protein denaturation, inflammatory activity can be inhibited. The anti-arthritic activity of ethylacetate leaf extract (EESA) of S. anquetilia at concentrations 50, 100, 200, and 400 mg/ml was determined using the protein denaturation assay. The results were compared with standard acetylsalicylic acid (100 mg/ml). The EESA extract showed dose-dependent inhibition of protein denaturation, the maximum inhibition of protein denaturation was found 92.41% at 400 mg/ml in comparison to the standard which showed 96.21% inhibition at 100 mg/ml (Verma et al., 2020).

Future prospects
S. anquetilia is proving to be an unreliable option for the future. Various active compounds viz, alkanes, alkenes, coumarins, carboxylic acids, fatty acids, and esters of fatty acids, terpenes (monoterpenes, diterpenes, sesquiterpenes), etc., have been reported to be the major bioactive compounds in this plant (Bhatt et al., 2021). The varied bioactivities including anti-arthritic (Verma et al., 2020), anticancer (Wani et al., 2016, anti-inflammatory, antibacterial (Nabi et al., 2022a,b), antioxidant, and anti-feedant (Gondwal et al., 2015) activities have been studied with potential findings. Despite the positive outcome, most of the studies are based on the in-vitro models and mechanisms of action are not wellstudied. Various traditional medicinal sources indicate that this plant has been used to treat diabetes, smallpox, burn injuries, etc., but, no pharmacological studies have been conducted to validate these activities. Furthermore, in some areas, there is inadequate information and limited research is available. Therefore, further studies concerning the basic chemical composition of phytoconstituents and the mechanisms involved in traditional uses are needed. The pharmacological activities must be experimented to the next levels for generation of novel drugs. This might prove helpful to use its immense therapeutic efficacy as a potent phytomedicine. Thus, systemic research experiments must be carried out for the development of drugs and medicines for their better economic and therapeutic utilization.

Conclusion
S. anquetilia is probably a possible herbal treatment for various diseases. The plant provides several promising perspectives for both traditional as well as modern medicine. S. anquetilia is a wealthy source of essential oils containing various important bioactive compounds but there is inadequate information concerning the basic chemical composition and their mechanisms involved. Most of the plant parts have been used in traditional medicine including leaves, stem, flower, fruit, and root bark. Therefore, determining research analysis of the bioactive constituents is needed, particularly its pharmacological properties and toxicity in terms of both in-vitro as well as in-vivo test systems to authenticate the safety of such plant-based phytochemicals and to develop standard novel drugs.

Author contributions
MN: conceptualization and writing-review and editing the manuscript. NT and BAG: reviewing and editing the manuscript. All authors contributed to the article and approved the submitted version.