Introduction

The outburst situation caused due to the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) represents a serious public health crisis across the globe. Since, December 2019, the whole world is suffering from the crisis of Corona virus. The virus is supposed to be originated from bats and latter on transmitted to humans. The outbreak of this human pathogen emerged in the city of Wuhan in China, and resulted to human disease, termed as COVID-19 (Arti and Bhatnagar 2020; Burki 2020; Chen et al. 2020; Huang et al. 2020). World Health Organization (WHO) declared Public Health Emergency of International Concern (PHEIC) owing to its fast rate of transmission within the humans (Chan et al. 2020; Chen et al. 2020; Li et al. 2020a, b; Sun et al. 2020). Over 14.57 million cases have been detected in 213 countries till now. The virus possesses crown like spikes on its outermost layer, so it was named as coronavirus (Fig. 1). The SARS-CoV-2 belongs to the beta (β), Coronavirus genus, closely related to the previously identified severe acute respiratory syndrome Coronavirus (SARS-CoV), family Coronaviridae, order Nidovirales and the sub family Orthocoronaviridae (Lu et al. 2020; Shereen et al. 2020; Wu et al. 2020; Zhou et al. 2020). Middle East Respiratory Syndrome (MERS) and severe acute respiratory syndrome (SARS) are commonly caused by viruses belonging to the Coronaviridae. Coronaviridae has two sub families; Torovirinae and Coronavirinae and the members of Coronaviridae family affects the mammals and aves (Zhou et al. 2020). The virus is 65–125 nm in diameter and the genomic content is estimated to be 26–32 kilobases. SARS-CoV-2 is an RNA virus and it contains single-stranded RNA genome. The virus has four subgroups i.e. Alpha (α), Beta (β), Gamma (γ) and Delta (δ). The Alpha (α) and Beta (β) corona virus infect mammals. The Beta (β) corona virus causes respiratory illnesses to humans and the Gamma (γ) and Delta (δ) corona virus affect aves and some selective mammals (Woo et al. 2012). SARS-CoV-2 makes use of its own proteins to safely harbour in the host cells (Fig. 1). Reportedly, a densely glycosylated spike (S) protein, SARS-CoV-2 main protease (Mpro) and RNA-dependent RNA polymerase (RdRp) are defining features paving the path of virus from entry to infection in the host cell (Cui et al. 2019; Lung et al. 2020; Ton et al. 2020; Wrapp et al. 2020). Essentially, the S protein undergoes extensive structural reorganization in order to fuse with the host membranes, thus establishing a physical link between the virus and the host cell (Booth et al. 2003; Li et al. 2020a). Eventually, a stable conformation is established (Wrapp et al. 2020). RdRp catalyzes the replication event, resulting in the synthesis of viral RNA. Remarkably, SARS-CoV and SARS-CoV-2 share similar nucleotide sequences and resulting RNA-dependent RNA polymerase (Liu et al. 2020). The SARS-CoV-2 proteins, metabolites and host cell factors can be targeted to reduce the viral replications within the host cells as for instance; the viral protein Mpro facilitates the synthesis of functional viral proteins out of the precursors (Fig. 2). Mpro, S and RdRp proteins can be targeted for develo** diagnostics, antibodies and vaccines. Human corona virus (HCoV), HCoV OC43, MERS-CoV and SARS-CoV are some of the viruses which causes respiratory problems in human and are being transmitted from one individual to another. This viral infection is transmitted from human to human through the coughed, sneezed droplets or by the contact of infected surface. The solved three dimensional structures of the viral proteins provide an outstanding ground for discovering specific ligands (Liu et al. 2020). In South-east countries like South Korea, China, Japan and India, many traditional medications are used to treat SARS-CoV-2, but the efficiency of the compounds are limiting and molecular mechanisms are also unclear. Plants possess amazing defense competence towards diseases (Panigrahi and Satapathy 2021; Panigrahi et al. 2021). The phytomolecules are also known as the bioactive compounds which have the tendency to modify the cellular physiological processes. Here, we highlight several phytomolecules having the ability to restrict the activity of SARS-CoV-2 and thus future studies may reveal the underlying molecular mechanism(s) and the efficacy of these phytomolecules.

Fig. 1
figure 1

A typical schematic view of SARS-CoV-2. The RNA virus possesses 4 structural proteins known to be spike protein, membrane protein, nucleocapsid protein and envelope protein

Fig. 2
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Possible potential targets to restrict the activity of SARS-CoV-2. There are number of promising targets which can be exploited to inhibit the viral infection. Metabolites including nucleoside analogues and anti-metabolites can be targeted to prevent viral replication. Likewise, host cell proteins such as ACE-2 (Angiotensin Converting Enzyme-2), Importin α1-β1 inhibitors and Signal transducers and activators of transcription 1 (STAT1) can also be targeted. Three categories of proteins; structural, non-structural and accessory can also be targeted to repress the activity of the virus

Covid-19 and its transmission

People are worstly affected by Covid-19 in different ways. The most common symptoms of Covid-19 include fever, dry cough, and tiredness. It also shows symptoms like diarrhoea, headache, loss of taste or smell, aches and pain, rash on skin, discoloration of finger on toes etc. The symptoms of Covid-19 are same as the symptoms of corona virus (CoV) which had appeared in 2003 as SARS. It was named as SARS-CoV-2 by WHO on 11th February 2020 and the disease was termed as CoV disease-19 (Covid-19) (Jiang et al. b, c; Sahoo and Satapathy 2020).

Fig. 5
figure 5

A variety of plant metabolites involved in regulating plant physiology. The primary and secondary metabolites have different group of compounds. The major compounds that are categorized as primary metabolites and found in all the living systems include proteins, lipids, carbohydrates, nucleic acid and chlorophyll. Likewise, secondary metabolites include flavonoids, phenol acid, alkaloids, volatile oils, terpenes, saponins, tannins, and many more

Bioactive molecules

The modern science is exploring new natural derivatives present in variety of living organisms and subsequently these natural products display highly efficient therapeutic properties for treating various diseases (Dash et al. 2020). These natural products have diverse application and unique properties which are useful for medicine production processes (Kang et al. 2013). From ancient time, the traditional natural medicines are playing vital role in disease control and ancient medicine practitioners were expert in this field with or without knowledge of bioactive molecules. Now-a-days researchers are focusing more on plant-baed bioactive molecules to overcome dreadful disease as these products have negligible side effects on humans (Kang et al. 2013). The therapeutic capacity of medicinal plants is due to their chemical composition which primarily comprises of vitamins, minerals and bioactive molecules (Singh et al. 2020a). Many scientific studies suggest that the synthetic vitamins and minerals cannot give the benefits as much as natural products. The medicinal plants contain many minerals and vitamins that easily get assimilated by human body. Natural products contain more bioactive molecules as it has complementary action between vitamins, enzymes and minerals (Singh et al. 2020b). Synthetic drugs posses more side effects and have more disadvantages towards human body as compared to natural medicines (Singh et al. 2020b). The plant products have phytomolecules containing active ingredients with therapeutic properties (Dias et al. 2012). The secondary metabolites are specific from species to species. Bioactive molecules harbour therapeutic, toxicological and immune stimulation properties and could prove to be an effective alternative against viral diseases (Panigrahi and Satapathy 2020d; Sahoo and Satapathy 2020).

Across the globe, the mankind is facing the deadly effect of Covid-19. Every vaccine is under the trial basis and it has been reported that the drugs that were generally used for the Human Immunodeficiency Virus (HIV) such as lopinavir/ritovir can be used for treating Covid-19 patients (Wang et al. 2020a, b, c). Some other drugs like pitavastatin, nelfinavir, perampanel and praziquantel can also be used against covid-19 (Xu et al. 2020). Plants are repositories of several types of natural bioactive molecules which might play a critical role in addressing the current pandemic. Essentially the various secondary metabolites which are secreted by the plant cells can be utilized for develo** anti-viral drugs using plant biotechnological approaches (Fig. 6). Usually secondary metabolites such as flavonoids, alkanoids, terpenoids and polyphenols possess antiviral properties (Yang et al. 2018; Khaerunnisa et al. 2020; Singh et al. 2020c). Medicinal plants are abundantly rich in phenolic metabolites (Yang et al. 2018). Molecular docking studies revealed that medicinal plants-derived phytomolecules such as quercetin, curcumin, kaempferol, catechin, naringenin, buteolin-7-glucoside, apigenin-7-glucoside, demethox-yeurcumin, obeouropein and epigallao-catechin that have the potential for combating against SARS-CoV-2 (Singh et al. 2020c). These bioactive molecules display similar pharmacophore properties as nelfinavir as revealed in in silico analysis. The bioactive phytomolecules released from a range of plant species bears therapeutic properties against SARS-CoV-2. Many phytomolecules such as rutin, diacetyl curcumin, diosmin, (E)-1-(2-Hydroxy-4-methoxyhenyl)-3 [3-[(E)-3-(2-hydroxy-4-methoxyphenyl)-3-oxoprop-1enyl]phenyl]prop-2-en-1-one, beta′-(4-Methoxy-1,3-phenylene)bis(2′-hydroxy-4′,6′dimethoxyacrylophenone) and apiin are also effective anti-virals (Adem et al. 2020; Singh et al. 2020c; Table 1). Lupane-type triterpenes, R-cadinol, labdane-type and abietane-type diterpenes, lignoids, curcumin and sesquiterpenes are few other identified bioactive compounds which would also play an vital role in defending the host cells against SARS-CoV-2 (Wen et al. 2007; Yang et al. 2020; Gong et al. 2008; Nguyen et al. Full size image

Table 1 Potent phytomolecules against coronavirus