Keywords

1 Introduction

Medicinal plants are gaining attention of pharmaceutical industry at global level because of their immense medicinal value and reliability. As a matter of fact, these are the driving factors causing the extensive exploitation and endangering of these treasures. Rauwolfia serpentina (Linn.) Benth. Ex Kurz in Apocynaceae family is one such plant which was exploited indiscriminately due to their unparalleled effects on nervous system. Some other plants belonging to this genus are R. tetraphylla, R. vomitoria, R. hookeri, R. micrantha, and R. verticillate. Rauwolfia serpentina was named in honour of Leonard Rauwolf, a German physician (Goenka 2007). The plant is popular in Sanskrit name Sarpagandha (Snake root) which was first mentioned in historic works of Charak (1000–800 bc). The medicinal properties were also deliberated in other Ayurvedic literatures of Sushruta, Vrindamadha, and Bhavprakasha (Chauhan et al. 2017a).There are a lot of controversies regarding this naming. But the most accepted argument behind the naming is the antivenom property of this plant. Moreover, some assumptions are regarding the resemblance of roots with snake (sarpa). Another unlikely belief is that snakes run away because of the smell of the plant. Some other vernacular names are Chandra, Naakooli, Amalapori, Patalagaruda, etc. The plant is widely observed and cultivated in India, China, Japan, Sri Lanka, and Bangladesh. Appreciable vegetative growth is observed in acidic sandy soil and at an annual rainfall of 200–250 cm. It is found to be indigenous in Himalayas, Punjab, Assam, Meghalaya, Uttarakhand, Sikkim, Andaman, and Western Ghats (Agrawal 2019; Bhanwaria et al. 2021; Dey and De 2010).

This tropical shrub is 6 in. to 2 ft. long and their thick, lengthy roots possess a characteristic lens-shaped structure. These greyish brown clustered roots with uneven cervices generally penetrate deep into the soil. Leaves in whorls are ecliptic or lanceolate. Another unique feature is the clump of pink as well as white flowers. The fleshy, tiny drupe type green fruits become black purple on maturing. The flowering period is usually between November and December (Chauhan et al. 2017a; Agrawal 2019).

The therapeutic significance of plant was dated centuries ago. Advent of use of dried root powder as ‘Pagal ki dawa’ for treatment of mentally violent patients was a milestone. The beginning of twentieth century witnessed several attempts to determine phytochemical constituents of the plant (Chatterjee 1953; Monachino 1954). A revolutionary step in the saga of Rauwolfia serpentina was put forward by Rustom Jal Vakil. This renowned scientist in cardiology carried out tremendous clinical trials for studying the antihypertensive property of dried root powder of the plant. His unparalleled efforts led to the worldwide recognition of the plant as an antihypertensive drug and he was later entitled as ‘Father of Rauwolfia serpentina’ (Goenka 2007). Unfortunately, the overexploitation of R. serpentina had led to a considerable reduction in its overall number and was later categorized as an endangered plant by International Union for the Conservation of Nature and Natural Resources (IUCN) and also in CITES (Convention on International Trade in Endangered Species) (Mehrotra et al. 2015). R. serpentina is a victim of unconstrained human exploitation during the anthropocene epoch, characterized by escalating anthropogenic activities amidst climate anomalies, aggravating climate change impact on the environment, biodiversity, and society as a whole. Tremendous and swift measures are necessary for the safeguarding of these plants.

This chapter emphasizes on the significance of conservation of Rauwolfia serpentina through highlighting renown pharmacological properties and its phytochemicals rendering those activities. Besides, various in vitro propagation and biotechnological approaches utilized in the R. serpentina research have also been discussed.

2 Phytochemicals

Phytochemicals are naturally available chemical compounds which attribute beneficial properties to plants such as defence against pathogen attack and abiotic stresses. Interestingly, these phytochemicals also have therapeutic properties like antimicrobial, anticarcinogenic activity, etc. (Njerua et al. 2013). An array of diverse extraction techniques have been employed for qualitative and quantitative phytochemical profiling of extracts of R. serpentina (Hussain et al. 2015; Kumar et al. 2016b; Namita and Yadav 2016; Das et al. 2019). The concentration of phytochemicals estimated via various detection strategies is depicted in Table 4.1.

Table 4.1 Estimation of important phytochemicals from different parts of R.serpentina and the respective detection methods
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2.1 Alkaloids

Rauwolfia serpentina is rich with monoterpenoid indole alkaloids which are the key factors that attribute the antihypertensive properties (Geissler et al. 2016). For instance, reserpine, serpentine, reserpiline, ajmalicine, ajmaline, ajmalimine, aricine, rescinnamidine, rescinnamine, deserpidine, corynanthine, isoreserpiline, isoreserpine, indobinine, indobine, yohimbine, isorauhimbinic acid, yohimbinic acid, 3 hydroxysarpagine, N(b)-methylisoajmaline, and N(b)-methylajmaline. Alkaloids are basically classified as reserpine-like and yohimbine-like. They considerably vary in their chemical structure (Fig. 4.1). The alkaloid profiling of different tissue extracts of wild plant, cultivated plant, and in vitro regenerated plant has been performed (Panwar et al. 2011). Furthermore, they have been successfully isolated and crude forms were used for various experimental studies (Sagi et al. 2016; Singh et al. 2017b). On the other hand, large bunches of root are necessary for the isolation of minor alkaloids (Falkenhagen et al. 1993). Ammoniacal chloroform was elucidated to be the best solvent for alkaloid extraction (Bindu et al. 2014). The techniques employed for the alkaloid examination were thin layer chromatography (Le Xuan et al. 1980), High-performance liquid chromatography (Wachsmuth and Matusch 2002), High-performance thin layer chromatography (Klyushnichenko et al. 1995; Panwar and Guru 2011), spectrophotometry (Singh et al. 2004), and GC-MS (Hong et al. 2013; Hussain et al. 2015).The advance techniques with more productivity like liquid chromatography with quadrupole time-of-flight mass spectrometry (Kumar et al. 2016a), UHPLC with Hybrid Triple Quadrupole Linear Ion Trap Mass Spectrometry (Kumar et al. 2016b), UPHLC-photo diode array-mass spectrometry (Sagi et al. 2016), and high-performance liquid chromatography coupled with electrospray ionization quadrapole time of flight tandem mass spectrometry (HPLC–ESI-QToF-MS/MS) (Bindu et al. 2014) were also later employed. The pharmacological properties of these alkaloids, particularly remedies on mental discomforts, have been established in the mid-1900s itself (Banerjee and Lewis 1954). Even though the concentration of alkaloids varies with plant parts, the roots are considered to be most therapeutically significant part of the plant (Lucas 1963). Later, studies were conducted to evaluate the dependency of alkaloid content on geographical conditions. Large variations were attained underlining the necessity of genetic breeding programmes for developing high yield variety (Usmani et al. 2015).

Fig. 4.1
The chemical structures of reserpine, rescinnamine, ajmaline, ajmalicine, yohumbine, reserpiline, and serpentine.

Chemical structure of major indole alkaloids

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2.1.1 Reserpine

Reserpine is considered as the most potent and therapeutically efficient indole alkaloid derived from the root extracts of R. serpentina. Chemically, the compound is 11, 17 α-Dimethoxy-18 β-[(3,4,5-Trimethoxybenzoyl) Oxy]-3 β, 20 α-yohimban-16 β-carboxylic acid methyl ester (C33H40N2O9) (Singh et al. 2017b). It is whitish yellow powder, insoluble in water. The compound was extracted and used by Robert Wallace Wiggins (Lobay 2015). The bioavailability of the compound ranges from 50 to 70%. The ingested reserpine distributed throughout the body even in breast milk. Tissue-specific biosynthesis of reserpine in roots was spotted in many investigations (Panwar et al. 2011). In addition to this, reserpine has been successfully isolated and quantified (Negi et al. 2014). The mass production of reserpine from the plant at industrial scale later began (Zafar et al. 2020a).

Reserpine is now a commercially valuable pharmaceutical drug approved by FDA in 1953 for hypertensive patients (McQueen et al. 1954). Reserpine can act on sympathetic-parasympathetic equilibrium of central nervous system and evokes a sedative sensation. Along with this, some studies pointed out the effect of reserpine on level of glycogen, acetylcholine, and anti-diuretic hormones (Goel et al. 2009). Interestingly, it drastically reduced the blood pressure and exhibited vasodilating effect on comparison with synthetic drugs. The action on blood vessels was found to be persistent (Mcqueen and Blackman 1955; Shamon and Perez 2016). It was also perceived that frequent administration of minimum doses of reserpine induced variations in motor signals in pharmacological models of Parkinson’s disease (Fernandes et al. 2012). Moreover, apoptosis inducing ability of reserpine has been noted in many studies. It inhibited DNA synthesis as well as destabilized the mitochondrial membrane potential. Recently, it had been proven that reserpine is an efficient therapeutic agent in cancer treatment. This alkaloid supressed cell proliferation and DNA repair and even induced apoptosis by regulating TGF-β signalling pathway (Ramu et al. 2020). However, certain assumptions circulated regarding the chances of genetic mutation on regular administration of reserpine. But in early 1990s itself, these controversies were proved to be wrong (von Poser et al. 1990). Various drug formulations with distinct trade names like Regroton, Demi-Regroton, Salutensin, Hydroserpine, and Hydropres-50 are now available in market (Singh et al. 2017b).

2.1.2 Rescinnamine

Rescinnamine (C35H42N2O9), the 3,4,5-trimethoxycinnamic acid ester of methyl reserpate, is another bioactive compound with antihypertensive as well as cytotoxic property (AbdelHafez et al. 2013). It was reported that reserpine could easily dissolve and exert vasodilating effect on blood vessels (Mcqueen and Blackman 1955). The underlying factor behind this result was the ability of rescinnamine to block the conversion of angiotensin I to angiotensin II. Further studies proved that it also reduced aldosterone secretion. The earlier studies evidenced the hypotensive, bradycardic, and depressive nature of this compound (Klohs et al. 1954). Rescinnamine, its similar compounds, and the synthetic production procedure were officially patented in 1975. Commercially, now it is available in the name ‘Tsuruselpi S’ (Singh et al. 2017b).

2.1.3 Ajmaline

Ajmaline is another therapeutically consumed indole alkaloid for antiarrhythmic effect. Ajmaline (C20H26N2O2) is chemically a monoterpenoid alkaloid with ajmalan backbone in which hydroxyl groups are substituted at 17 and 21 positions. This alkaloid was isolated in 1931 and named ajmaline in honour of Hakim Ajmal, a legendary person in Unani medical system (Agrawal 2019).

Ajmaline is a class I antiarrhythmic agent which can alter the electrocardiogram characteristics (Roten et al. 2012). It decreases J wave and extends P-Q interval, Q-T interval, QRS complex, and R wave. It is a potent sodium channel blocker and employed in the diagnosis of Brugada syndrome and its subtypes (Rolf et al. 2003). Moreover, it is also administrated in the treatment of tolerated monomorphic ventricular tachycardias and atrial fibrillation cases in Wolff-Parkinson-White syndrome (Singh et al. 2017b). Cardenolide and bufadienolide are two important synthetic derivatives of ajmaline with antiarrhythmic and cardiotonic property. These compounds as well as the protocol for production was patented in 1975 (Makarevich et al. 1979). Now it is commercially available in brand names like Ajmalinum, Gilurytmal, Ajmalina, etc. (Singh et al. 2017b).

2.1.4 Yohumbine

Yohumbine (C21H26N2O3), the α-adrenergic blocker, is widely used in erectile dysfunction disorders as it could increase blood flow towards penis (Ernst and Pittler 1998). It specifically blocks presynaptic neuron and is experimentally validated in rabbits (Starke et al. 1975). It was reported that administration of yohumbine stimulated the fight or flight responses like anxiety, shivering, palpitations, hot, cold flashes, pupil dilation, etc. in experimental subjects (Charney et al. 1984). Yohumbine is also often considered as a sexual desire-inducing agent. It is also well-known for its hypnotic activity (Mehrotra et al. 2015).

2.1.5 Ajmalicine

Ajmalicine (C21H24N2O) is an indoline alkaloid extracted from R. serpentina as well as Catharanthus species (Zenk et al. 1977). The concentration of ajmalicine was reported to be more in stem extract when compared to that of roots (Deshmukh et al. 2012). It was reported that the amount of ajmalicine in roots could be raised up by optimizing growth media conditions like utilizing sucrose, phosphate, and 6.5 pH (Bhagat et al. 2020). The commercial production has also been enhanced by use of CdCl2 (Zafar et al. 2020a) Interestingly, similar to other phytochemicals, ajmalicine maintains cerebral blood pressure and also possesses psychotic effects (Agrawal 2019). Ajmalicine also possesses renal vasodilation properties and it selectively inhibits postganglionic functioning of sympathetic system (Mehrotra et al. 2015).

2.1.6 Other Alkaloids

Reserpiline is a yohimban alkaloid with antipsychotic property (Gupta et al. 2012). Serpentine is an anhydronium alkaloid with sedative effect. However, it also excites heart rate. Interestingly, serpentine also exhibited antihistaminase activity and was testified in guinea pigs (Sachdev et al. 1961). It also inhibits succinate dehydrogenase in liver and brain tissues. Isoajmaline, neoajmaline, and rauwolfinine are some other alkaloids with apparent actions on nervous system (Singh et al. 2017b). 3-hydroxysarpagine, yohimbinic acid, isorauhimbinic acid, Nb-methylajmaline, and Nb-methylisoajmaline are some other indole alkaloids. Remarkably, yohimbinic acid exhibited significant inhibition on topoisomerase which could be further researched for anticancer therapy (Itoh et al. 2005). It was noted that alkaloids like vomilenine and perakine were produced from cell suspension cultures in contrast to wild plant (Nikolaeva and Alterman 1993).

2.2 Other Phytochemicals

R. serpentina is also rich in phenols, flavonoids, and tannins. Phenols are intensely valuable because of its antioxidant, antimicrobial, anticancer, antidiabetic, anti-inflammatory, hepatoprotective, neuroprotective, and cardioprotective nature (Saibabu et al. 2015). Flavonoids are free radical scavengers and widely studied phytochemical in fields of cancer, inflammation, etc. (Panche et al. 2016). The presence of radical scavenging potential of phenols and flavonoids of R. serpentina was assessed and found to be higher (Keshavkant et al. 2008). Later, the use of salicylic acid (elicitor) and AcCN/H2O (solvent) was suggested with the intention of elevating the production of phenolic acids and flavonoid rutin at commercial scale (Nair et al. 2013). Quercetin, a most potent flavonoid antioxidant, was also isolated from the leaves of R. serpentina. It exhibits anti-inflammatory, anticancer properties and is used in the treatment of arthritis, bladder infections, asthma, eczema, and diabetes (Gupta et al. 2015). Similarly, rutin was also detected. Rutin prevents DNA alteration, lipoprotein peroxidation, and thus mutations (Gupta and Gupta 2015). Tannins are peculiar with its antiseptic property (Vieira Pereira et al. 2015). In addition to this, the presence of saponins and fat was identified in n-hexane extract of roots of R. serpentina (Hussain et al. 2015). The presence of carbohydrates, glycosides, starch, coumarins, emodins, and phlobatanins was also later revealed. Anthraquinones were also ascertained in stem extracts of the plant (Deshmukh et al. 2012; Das et al. 2020; Vaishnav and Sahoo 2020).

The plant is also nutritionally copious with vitamins like riboflavin, ascorbic acid, thiamine, and niacin. The macronutrients like calcium, phosphorous, magnesium, and sodium were detected in the plant extract. However, zinc and iron are major microelements. (Singh et al. 2017b). All these elements are essential for the proper growth and development of our body. Unfortunately, presence of some heavy metals like arsenic, chromium, and cadmium was also detected in trace amounts via atomic absorption spectroscopy (Gupta and Gupta 2016). Recently, the presence of titanium, tin, vanadium, molybdenum, chromium, aluminium, silicon, mercury, strontium, nickel, cadmium, lead, and bismuth in limited amounts was identified in leaves and seeds by means of direct current arc optical emission spectroscopy (Bharti et al. 2020). Above all, antioxidant enzyme superoxide dismutase has been identified in the extracts (Kirillova et al. 2001).

3 Pharmacological Properties

Curative properties of R. serpentina have been efficiently utilized by tribal people in different parts of the world for the treatment of various ailments. According to literature surveys, tribal groups in Similipal Biosphere Reserve are using the root extract of R. serpentina for curing malaria (Panda 2014). As reported in Indian Ayurvedic literature, the root extracts of the plant were used in doctoring mental illness, sleeplessness, cardiovascular diseases, erotic violent behaviours, snake bites, diarrhoea, and severe pain (Lobay 2015; Bunkar 2017a). It was also administrated to ease uterine contraction during delivery (Singh and Singh 2009). It is now available in the market in the form of herbal drugs (Rungsung et al. 2014). When compared to the synthetic drug, extracts of R. serpentina have the following advantages like fewer side effects, no digestive issues, increased bioavailability, and reduced chances of drug resistance development (Bunkar 2017a). Figure 4.2 represents the phytochemical properties of R. serpentina.

Fig. 4.2
A radial diagram of R, Serpentina extract. The surrounding text boxes are antioxidant, antidiabetic, hypolipidemic, antimicrobial, antihypertensive, anticancer, and antivenom. A few points are listed in each text box.

Depicts the various pharmacological properties of R. serpentina and the possible reasons or ways behind the action

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3.1 Antioxidant Nature

The formation of oxidative stress in our body leads to various diseases like cancer and diabetes. Medicinal plants are chemically rich in antioxidants. In this context, extracts of the R. serpentina could be efficiently utilized to prevent and treat oxidative damages. Various studies have assessed the antioxidant potential of extracts of R. serpentina via 2, 2′-diphenyl-1-picryl Hydrazyl Radical (DPPH) assay (Fazal et al. 2011). Interestingly, methanolic extracts of roots showed higher free radical scavenging activity than leaves and buds (Dey et al. 2016). In addition to this, further studies exposed that wild varieties showed more activity than cultivated ones (Chauhan et al. 2017b).

3.2 Antidiabetic Activity

The aqueous extracts of R. serpentina were used as an antidiabetic drug for periods by the local clan in northern Thailand (Manosroi et al. 2011). Ample clinical studies revealed a decrease in blood glucose level on the administration of Rauwolfia therapy. The antidiabetic activity of R. serpentina was primarily investigated via alpha-amylase assay. The hypoglycaemic efficacy of wide and cultivated varieties of the plant was explored and it was observed that the plant is a good candidate drug for diabetic treatments (Chauhan et al. 2017b). Interestingly, the methanolic root extracts showed the antidiabetic effect on alloxan-induced diabetic rats (Qureshi et al. 2009). Furthermore, 10–60 mg/kg was found to be the appropriate dose of an extract with hypoglycaemic property based on trials in Wistar mice (Azmi and Qureshi 2012b). The level of glycosylated haemoglobin in blood was used as a parameter to investigate the effects of methanolic extract of R. serpentina (Azmi and Qureshi 2012a). Later, the glucose suppressing activity of hydromethanolic extracts was also experimentally perceived. In contrast to previous studies, higher doses of the extract were effective and with less side effects (Azmi and Qureshi, 2014). The action of extract over pancreatic cells helped in maintaining blood glucose homeostasis (Azmi et al. 2018). Similarly, investigation in fructose-induced type 2 diabetic mice was also performed. The findings suggested that the hypoglycaemic effect was either because of reduced intake of fructose into the intestine or of low insulin resistance (Azmi and Qureshi 2013). These results were further corroborated by docking studies. The phytochemicals like ajmaline, serpentine, rauwolfine, yohimbine, and sarpagine in R. serpentina were found to be activators of insulin and thus mediated the diabetic effect (Ganugapati et al. 2012). The dry lab studies using aldose reductase as a target led to the identification of potent phytochemicals like indobine in attributing antidiabetic nature (Pathania et al. 2013).

3.3 Hypolipidemic Property and Role in Cardiovascular Diseases

The hypolipidemic effect of methanolic extracts of R. serpentina was experimentally proven in Wistar albino mice. Remarkably, a significant reduction in the level of total cholesterol, triglyceride, and low density lipoprotein was observed underlining the role of the herbal drug in maintaining lipid equilibrium (Azmi and Qureshi 2013). Parallel studies were performed to evaluate the antilipidemic activity of hydromethanolic extract. These actions were endorsed most likely probably because of inhibition of HMG-CoA reductase and AcetlyCoA carboxylase or by promoting lipase (Azmi et al. 2018). More studies were conducted on albino mice and alloxan-induced mice, validating the antihyperlipidemic activity with less side effects (Qureshi et al. 2009; Shah et al. 2020). Additionally, MolDock studies revealed the contribution of phytochemicals like reserpine, ajmalicine, yohimbine, and indobine in inhibiting the 3-hydroxy-3-methyl-glutaryl-CoA reductase and thus the cholesterol biosynthetic pathway (Azmi et al. 2021).

Herbal medicines are good alternatives for the expensive pharmaceuticals for healing cardiovascular diseases (Rastogi et al. 2016). The hypolipidemic property of R. serpentina root extract is the core for its healing role in cardiovascular ailments. Clinical trials using human subjects in the 1950s proved the efficacy of Rauwolfia therapy in treating angina pectoris and coronary artery disease (Lewis et al. 1956). In vivo studies were performed by inducing oxidative stress on left anterior coronary artery ligation in dogs, followed by administration of the herbal formulation of R. serpentina with other medicinal plants. The plant formulation succeeded in maintaining the biochemical, haemodynamic parameters, and thus the cardiac health (Afsheen et al. 2018). The blood pressure moderating ability of the reserpine was also reported (Bunkar 2017a). Increased blood pressure leads to ailments like hypertension. The action of the reserpine on the vaso-motor centre leads to generalized vasodilation and subsequently reduced blood pressure. Several clinical trials have been carried out to appraise the antihypertensive efficacy of root extracts of R. serpentina. In mid of 1900s, Vakil conducted a notable work on 50 hypertensive patients. The uptake of rauwolfia resulted in a fall of blood pressure for around 80% of patients (Vakil 1949). Alseroxylon, a drug derived from Rauwolfia, was administrated to a group of 84 patients with varying intensity of hypertension. Most of them showed positive results with harmless side effects (Livesay and Moyer 1954). Correspondingly, studies on patients with hypertensive disorder for almost 11 months by giving R. serpentina resulted in a decrease in blood pressure (Markvotiz et al. 1955). A series of studies succeeding Vakil’s findings were performed during 1950s. All of them unequivocally pointed out the impact of R. serpentina, specifically reserpine on hypertension. Moderate side effects like sedation and drowsiness were observed in most cases. But it was considered as blessing in disguise for patients (Lobay 2015). A review work was performed to expound the dose-dependent action of reserpine by exploring all the available clinical trials. They reported the effectiveness of reserpine similar to the available drugs, but couldn’t make a conclusion regarding exact dosage of reserpine. However, it was suggested that low dosage of drug is acceptable with least side effects. (Shamon and Perez 2016) Recently, a major fall in diastolic BP, systolic BP, and level of angiotensin converting enzyme was observed as a result of experimental tests using a combination of R. serpentina and Curcuma longa on hypertension-induced dogs (Farieha et al. 2019). Besides, the cardioprotective indices were elevated and the risk of coronary attacks was reduced on the use of methanolic extracts of R. serpentina in alloxan-induced mice (Azmi and Qureshi 2012a).

3.4 Antimicrobial Activity

Medicinal plants have been extensively used for microbial infections for centuries ago. Being an important medicinal plant, the antimicrobial potential of R. serpentina was also investigated extensively. The bactericidal activity of the root extract was first validated via MIC and MBC against S. typhi. It was also observed that indole alkaloids like reserpine confer this property (Deshmukh et al. 2012). Later, methanol, chloroform, and aqueous extracts of the plant exhibited inhibition against a number of pathogenic microbes. Among them, methanol extract had shown the highest MIC against S. aureus. The root extract was found to be effective against bacteria like Saccharomyces cerevisiae, Bacillus subtilis, Enterococcus faecalis, Micrococcus luteus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Streptococcus pneumonia and Proteus vulgaris as well as against fungi like Aspergillus niger and Candida albicans (Owk and Lagudu 2016). Interestingly, one of the bactericidal activity-bestowing factors was identified to be stigmasterol in n-hexane fraction of methanolic extract of the plant (Dey et al. 2016). The ethanolic extract of the plants has also showed similar repressing effect on bacteria (Bunkar 2017a, b). Analogously, the methanolic, ethanolic, and acetonic extracts of the plant were considered for investigation for fungi like Trichoderma viride, Fusarium oxysporum, and Penicillium notatum. The antifungal and antibacterial nature of R. serpentina was once again substantiated (Singh et al. 2017a, b).

3.5 Role in Mental Health

R. serpentina is legendary in Ayurvedic literature for its extraordinary effects on the nervous system and subsequent curing of various mental disorders. This antipsychotic herb was widely employed in the treatment of insomnia, tremor, and frenzy conditions. Studies on schizophrenic patients revealed the impact of rauwolfia in reducing aggressiveness. Further studies on neurotic patients induced dreams (Azima et al. 1957). Similarly, raudixin, a drug derived from the roots of R. serpentina, was proven to be a probable dream-inducing agent. But more elaborate studies are required to overcome the objections on the efficiency of the drug as well as to explain the underlying mechanism (Azima 1958). Clinical studies on 118 patients suffering from mental illness revealed the sedative potential of reserpine. Substantial reduction in nervousness and the comforting effect was produced on administration (Glynn 1955). The herbal drug was also found to be effective in reducing overanxiety in psychiatric patients. Unfortunately, no impact was observed in depression (Lowinger 1957). Interestingly, studies on autistic children with low doses showed hopeful changes like reduced hyperactivity, calmness, and better sleep (Lehman et al. 1957). Another study proved that reserpine in the form of drug serpasil had lessen the insane treatments of drug and alcohol addicts (Avol and Vogel 1955). The mitigating effect of phytochemicals like ajmaline and reserpine on Alzheimer’s disease was evaluated via in vitro and in silico studies. Both of the compounds exhibited β-site amyloid cleaving enzyme (BACE-1) suppression, anti-cholinesterase, and monoamine oxidase-B (MAO-B) inhibition. The neuroprotective ability of these bioactive compounds promises a new treatment strategy for alleviating Alzheimer’s disease (Kashyap et al. 2020).

3.6 Anticancer Property

Anticancer potential of reserpine, an important phytochemical extracted from roots of R. serpentina, was an experimental subject among researchers in the nineteenth centuries. But was not prized because of the toxic nature. At the same time, lot of rumours were circulated regarding the incidence of breast cancer on use of R. serpentina. Later, proper clinical studies contradicted these false assumptions (Lobay 2015). Fortunately, exceptional role of reserpine in killing resistant tumour cells was later identified and is now a hope in the anticancer studies. Resazurin reduction assay, doxorubicin uptake assay, was employed to study cytotoxicity and further confirmed by molecular docking studies (Abdelfatah and Efferth 2015). Moreover, it was evidenced that reserpine suppressed cell proliferation and also induced apoptosis in oral cancer cells via TGF-β signalling (Ramu et al. 2020). It was also found that anticancer activity of leaves extract was comparatively higher through studies on HeLa cell lines (Deshmukh et al. 2012). The anticancer property of alstonine, serpentine, was experimentally proven on groups of mice bearing either Ehrlich ascites carcinoma cells or lymphoma cells. These are bioactive compounds present in R. serpentina (Beljanski and Beljanski 1986). More elaborate studies are demanded to explore the anticancer potential of this wonder drug.

3.7 Antivenom Property

The Indian ethnic folks have been reported to use the paste of roots and leaves of R. serpentina as an herbal antidote (Upasani et al. 2017, 2018). It was found to be effective with combination of Tylophora indica paste (Ignacimuthu et al. 2006). This traditional knowledge was later proven to be true by identifying lead compounds in the plant extract against the venom of cobra. The docking studies demonstrated that phytochemicals have a suppressing effect on phospholipase A2 like potential factors in venom (Sreekumar et al. 2014). Stigmasterol is an important phytochemical attributing this antivenom nature. It was also noted that the concentration of stigmasterol in tropical plants varies considerably with the altitudinal location of growth. The factors like wind, rainfall, temperature, and humidity have influential roles in the content of stigmasterol and thus antidote nature (Dey and Pandey 2014).

3.8 Miscellaneous Properties

Investigation on guinea pigs with extracts of R. serpentina unwrapped the antihistaminase nature. Serpentine was the potent phytochemical that hindered histaminase in vivo (Sachdev et al. 1961). Fascinatingly, studies on HIV antigens uncovered the anti-HIV activity of the root extracts of the plant (Sabde et al. 2011). Aqueous and ethanolic extracts were considered for testing the antimalarial activity in mice diseased with malaria. The results indicated substantial suppression in the growth of plasmoids. Topoisomerase II inhibiting the nature of serpentine was the crucial factor that leads to the death of parasites. This promises R. serpentina as a candidate herbal for developing an antimalarial drug (Omoya et al. 2019). The presence of symbiotic actinomycetes has also been discovered within R. serpentina. These actinomycetes are efficient in the production of bioactive chemicals (Gohain et al. 2015). Correspondingly, Cladosporium sp., an antibactericidal fungus, has been isolated from the leaves of R. serpentina. Additionally, potential metabolites like naphthoquinones were also found to be synthesized from this fungi (Khan et al. 2016). A unique strain of rhizobacterium named Delftia tsuruhatensis was identified from R. serpentina with plant promoting properties. Above all, these bacterium also produced a novel antibiotic(amino(5-(4-methoxyphenyl)-2-methyl-2-(thiophen-2-yl)-2,3-dihydrofuran-3-yl)methanol) with profound fungicidal activity (Prasannakumar et al. 2015). Twenty fungal batches belonging to Fusarium sp., Aspergillus sp., Xylaria sp., Phomopsis sp., Cladosporium sp., Alterneria sp., Gleomastix sp., and Colletotrichum sp. have been isolated and four of them showed signs of noticeable antibacterial activity (Singh et al. 2016). Lately, another fusarium sp. with antistaphylococcal action was observed in leaves of R. serpentina. Sixteen more potent antibacterial fungal strains were also identified in this category. The potential of these fungal groups was not only limited to antibacterial activity, but they also mimicked the antioxidant and hypolipidemic effect. These studies also pointed out that C. gloeosporioides is the most colonized fungi within R. serpentina (Nath et al 2013) Another noteworthy property is the action of the plant against the genotoxic impact produced on the exposure of carbofuran in freshwater fishes. The antioxidant nature of R. serpentina attributes to this mechanism of protection (Tiwari and Trivedi 2019). Recently, the hepatoprotective and renoprotective nature of methanolic extracts of R. serpentina was revealed through investigations on albino mice. These studies shed new light on the pharmacological properties of R. serpentina and cancelled the proposed side effects of the use of these drugs (Shah et al. 2020). One more striking property of R. serpentina was corrosion inhibition activity on aluminum alloys. The inhibitory activity was greatly increased by intensifying the concentration of plant extracts and temperature (Chaubey et al. 2017). Likewise, the alkaloid extracts of the plant demonstrated the formation of protective layer over steel supressing the corrosion. Reserpine was also proposed as the possible candidate involved in this biological protection (Raja and Sethuraman 2010).

4 Growth and Conventional Propagation Methods

Generally, the plant grows in wide geographical area with an annual rainfall of 1500–4000 mm and a temperature range of 10–38 °C (Agrawal 2019). It grows well in humus-rich loamy soil. Seeds are usually used for propagation. Seeds are collected around the beginning of June and can be maintained for 6 months in proper storage conditions. Tap roots with filiform secondary roots are chosen sometimes to propagate. Soft stem cuttings could also be employed. Sproutings are seen within a month. Comparatively, low germination rates were observed on the use of these plant parts. Moreover, generated plants are susceptible to pathogen attacks and climate fluctuations. Most importantly, each generation may show genetic variability and which will subsequently affect the alkaloid content within the plant (Mukherjee et al. 2019).

5 Biotechnological Approaches

The various disadvantages of conventional propagation strategies, increased economic value of phytochemicals like reserpine, and the categorization of R. serpentina as an endangered plant attracted the biotechnologists more towards the vast realm of this medicinal plant.

5.1 In Vitro Propagational Strategies

Growing cells, tissues, and organs of plants under artificial synthetic media and appropriate environmental conditions is termed as tissue culture (Van Eck and Smith 1997). Tissue culture is a better option for the mass propagation of endangered plants like R. serpentina. Shoot tips, hairy root, leaf, callus, axillary meristems, nodal and internodal segments, and seeds could be used as explant to develop the plant. Even inflorescence was efficaciously exercised to develop callus (Kaur 2018). Young leaves have been reported as an assuring explant in many studies (Nurcahyani 2008). Nodal, internodal segments, and leaves were compared to develop callus and it was observed that leaf is a better choice for callus induction and plant regeneration (Panwar et al. 2011). Apposite sterilization is a major step ensuring the success of tissue culture. A proper sterilization pattern is followed for R. serpentina cultures; Bavistin (0.1–0.2%) followed by Tween-20 (10%) for 5 min, Sodium hypochlorite for 5 min, cetrimide solution (1%) for 5 min, ethanol for 30 s, and HgCl2 (0.1%)for 5 min (Mukherjee et al. 2019). Several other variations in terms of concentration and exposure of chemicals have been practiced (Bahuguna et al. 2011; Mallick et al. 2012; Pan et al. 2013). Murashige and Skoog (MS) medium is generally used for organogenesis, embryogenesis, and callus induction. Half concentration of MS media (pH—5.8) was employed for root induction as well as for maintaining shoot and leaves (Benjamin et al. 1993; Goel et al. 2009). MS medium with BAP (0.5–1.5 mg/L) with KIN (0.5–1.0 mg/L) or NAA (0.5–1.0 mg/L) was reported to be best medium for callus induction. The use of other media like Gambrog’s B5 medium (Pan et al. 2013), Linsmaier Skoog medium (Yamamoto and Yamada 1986), and Woody plant medium (Alatar 2015) were also successfully established. (Yamamoto and Yamada 1986) They used LS media for 13 years and successfully alleviated the reserpine production via persuading stress conditions by amending hormone concentration. One of the studies proved that woody plant medium supplemented with 5.0 μM BA and1.0 μM NAA was found to be the best choice to generate shoot artificially (Abdurahman 2012). Sucrose (3%) is the major carbon source for nourishment of growing tissues. However, MS media with1.5% sucrose was also to be effective in inducing root (Bhadra et al. 2009). Additionally, fruitful use of MS with 4% sucrose for producing roots was also reported (Pandey et al. 2007). Optimum temperature and light intensity were reported to be 20–25 ± 1–2 °C and 3000 lux, respectively. Moreover, 50–70% of relative humidity and 16 h exposure to light were enough for the growth of callus in synthetic media. Auxins, cytokinins, and gibberellic acid were the common growth regulators used by most investigators and variations in the concentration and combination ratios were also attuned to favour root and shoot initiation (Mukherjee et al. 2019). The impact of concentrations of growth regulators is depicted in Table 4.2. Various other addictives like coconut milk (Aryal and Joshi 2009), mixture of proteins, ascorbic acid, citric acid (Kataria and Shekhawat 2005), copper sulfate (Ahmad et al. 2001), and antibiotics (Benjamin et al. 1993) have also been utilized to induce and enhance the growth of in-vitro plants. Use of thidiazuron in media caused profused shooting in explants (Pandey et al. 2007). Moreover, prior treatment of thidiazuron before culturing explants in liquid MS media resulted in amplified number of shoots (Alatar 2015). Total alkaloid quantity was also elevated by adequate supply of nitrogen (Akram and Ilahi 1986). Interestingly, the shoot tips were treated with colchicine to induce autotetraploidy with the intention of ameliorating alkaloid content (Mathur et al. 1987). Direct and indirect regeneration systems were compared to study the augmentation in reserpine production. In vitro regeneration was proved to be more effective (Mukherjee et al. 2020). These direct and indirect organogeneses have been employed in many studies to explore the ways of micropropagation (Bahuguna et al. 2011). Somatic embryos were also successfully established from roots of R. serpentina in liquid MS media. Additionally, flow cytometry studies evidenced the stability of regenerated plants from these embryos (Zafar et al. 2019). Proper acclimatization must be followed after all these for the generation of healthy plants. Commonly used are soil, sand, and leaf manure or farmyard wastes (Mukherjee et al. 2019). Different combinations of vermiculite, soil, and sand were other good alternatives to generate plantlets (Kad et al. 2017).

Table 4.2 Depicts the influence of various growth regulators on the induction of explants
Full size table

5.1.1 Use of Elicitors

The massive production of phytochemicals like reserpine could be achieved by use of elicitors. Elicitors can be either biotic or abiotic (David Paul Raj et al. 2020). Use of elicitors like copper and zinc imparted the increase in number of shoots generated (Nurcahyani 2008; Ahmad et al. 2014). Use of other chemicals like melaphene exhibited solid impact on alkaloid content (Kozlova et al. 2005). Studies conducted on different concentrations of methyl jasmonate showed that higher concentrations caused a 7.3 times rise in total reserpine content (Harisaranraj et al. 2009). Similarly, acetyl salicylic acid (ASA), salicylic acid (SA), and methylsalicylic acid were applied directly onto the plants in form of sprays. This caused tremendous increase in the phenolic acid-derived metabolites like cichoric acid (Nair et al. 2013). The eliciting property of salicylic acid (SA), dimethyl sulphoxide (DMSO), and abscisic acid (ABA) was assessed. A positive effect was imparted by all except DMSO. An association between the action of SA and trypamine (precursor) was also uncovered. About 57.64 mg g−1 of reserpine was produced on 36 days of elicitation (Panwar and Guru 2013). Similarly, the use of CdCl2 caused considerable increase in reserpine and ajmaline content by inducing stress (Zafar et al. 2020b). An ascent in reserpine synthesis was also reported on the use of AlCl3 (0.15 mM). The antioxidant assays evidenced the stress induced by AlCl3. However, it became the source of a positive effect (Zafar et al. 2017). Outstandingly, the 2.9-fold of rise in amount of ajmaline was recorded on use of mannan from Saccharomyces cerevisiae (100 mg/L). Meanwhile, use of sodium chloride elevated the production of ajmalicine (Srivastava et al. 2016). All the reports underline the fact that induction of stress via chemicals caused a considerable advance in phytochemical production.

5.1.2 Synthetic Seed Development

The artificial seed technology was a breakthrough in the field of in vitro propagation. It can be defined as encapsulated somatic embryo, any meristematic tissue like shoot tips that can be cultivated as seeds and which are capable to regenerate the whole plant (Chandrasekhara 2012). Meristematic nodal segments, shoot tips are most often used for producing synthetic seeds. The explants were encapsulated using 3% sodium alginate and calcium chloride. The efficacy of different media in generating artificial seeds was also considered in the above study. Woody plant medium supplemented with plant hormones like NAA and BA showed sproutings within 2 weeks of growth (Faisal et al. 2012). Later, studies testified the optimum concentration of Sodium alginate as 3% and of Calcium chloride as 75 mM. In contrast to the above study, comparatively higher germination rates were observed in ½ MS liquid and semisolid media (Gantait et al. 2017). Storage at 4 °C was evaluated as a means for maintenance of alginated shoot ends up to 18 weeks (Ray and Bhattacharya 2008). It was found that proper storage at 8 °C will retain the reserpine content and viability of seeds (Gantait and Kundu 2017).

5.2 Use of Genetic Markers

Variation in genetic stability was a common consequence of in vitro propagation studies. Therefore, fidelity of the clone produced must be assessed via various gene markers. RAPD (Random Amplified Polymorphic DNA) and ISSR (Inter Simple Sequence Repeats) markers were used to study the genetic stability of artificial seeds. Genomic DNA was extracted from young leaves using CTAB method followed by PCR. Out of 20 RAPD primers, 19 generated distinct, consistent bands. In case of ISSR, all the seven primers produced bands. All the clones produced bands monomorphic to mother, endorsing the efficacy of in vitro propagation methodology (Faisal et al. 2012). Similarly, it was reported that 10 RAPD primers produced 97 reproducible bands, while ISSR produced 68 consistent maps. This study once again cemented the reliability of synthetic seed approach for micropropagation (Gantait et al. 2017). Finger printing bands generated via RAPD markers confirmed the uniformity between mother and clones produced by direct organogenesis (Goel et al. 2009; Senapati et al. 2014). The ISSR markers were fruitfully employed to investigate the genetic diversity within R. serpentina collected from different geographical locations of Southern Western Ghats. Gene flow (Nm), Nei’s gene diversity (h), Degree of genetic differentiation (Gst), and Shannon index of genetic diversity (I) were also considered. The high polymorphism observed signified the need of proper conservation strategies for this vital plant. Moreover, it also strengthened the reliability of ISSR markers in genetic studies (Pillai et al. 2012). They have been used to evaluate the clonality of indirect organogenesis-regenerated plants. Out of 18 primers used, one that was reported to be not monomorphic band was further characterized (Saravanan et al. 2011). By the same token, genetic diversity was assessed via RAPD markers in R. serpentina variants of Western ghats. Large variation in the quantitative traits within R. serpentina varieties was observed. Along with RAPD, researchers also analysed the horticultural and chemical profiling. Implementation of proper conservation strategies localizing each small area is mandatory for protecting this natural treasure (Nair et al. 2014). In contrast to above all, DNA barcoding of different tissues and time periods of R. serpentina was performed. They detected species unique indels in the rps16 intron for R. serpentina (Eurlings et al. 2013).

5.3 Rhizogenes-Induced Hairy Roots

It is a declared fact that roots are the major source of alkaloids in R. serpentina. Transformation strategies were mainly focussed to produce bulk amount of roots via Agrobacterium rhizogenes. Ri-induced cultures are a better conservational strategy to guard and propagate this endangered plant (Srivastava and Misra 2017). The first attempted was reported in 1983 where shoot cultures were infected with AT 15834. Remarkably, callus with hairy roots was produced and later fully developed into plants. Unfortunately, much variation in alkaloid content was not reported. However, the advent of transformation techniques gave an economic preface to the studies of R. serpentina (Benjamin et al. 1993). Relatedly, the strain A4 was used to produce roots in wounded leaf explants. This study also led to the discovery of a new alkaloid called 12-hydroxyajmaline (Falkenhagen et al. 1993). Analogous studies were reported in the other species of Rauwolfia (Sudha et al. 2003). Another, Agrobacterium strain LBA 9402, was used to transform shoots and leaves of R. serpentina. The presence of rolA and rolB genes was detected by PCR to mark the transformation. Variations in the alkaloid content were observed in transformed root lines. Physiological condition of resultant cell, copy number of T-DNA, insertion site, and expression of rol genes were most possible reasons (Ray et al. 2014a). Genetic variability in root lines was also reported on studies using strains of LBA 9402 and A4. Various cytogenic studies showed the difference in TL and TR DNA integration pattern and in ajmalicine content too. It was recommended to choose the variant with more alkaloid content for scale-up studies (Ray et al. 2014b). Relatively, transformation efficiency of A4 was reported to be more. Plants generated from Ri-induced roots were found to be normal and produced higher amount of secondary metabolites compared to cultivated plants. It unfolds the possibility of using Ri-induced plants for reserpine production commercially (Mehrotra et al. 2013a). Successful in vitro studies were later upgraded to industrial level by means of Erlenmeyer flasks and bioreactors. The use of 5 L bioreactor with provisions for air supply and all other necessities was reported for the scale up of root cultures of R. serpentina. The growth rate of cultures was considerably high in bioreactor than flasks (Mehrotra et al. 2015). A standardized protocol by the above-mentioned authors for mass production of root cultures via AT 5 strain in B 5 medium in a 5 L bench top bioreactor was later published (Mehrotra et al. 2016). Interestingly, a ANN-based combinatorial model was predicted by them standardizing the culture conditions to obtain maximum metabolite yield from root cultures of R. serpentina (Mehrotra et al. 2013b). In between, the idea of using table sugar instead of sucrose was auspicious which will reduce the heavy expenses of industrial manufacturing. They also reported the upturn in the ajmaline, yohimbine, and reserpine contents on a long-term culture of 6 years and also suggested that this longevity elevated the genetic stability of strain (Pandey et al. 2014). Since the inoculants are highly susceptible to contamination, a better cryopreservation storage strategy was established. Root tips were first encapsulated using sodium alginate and were priorly cultured in sucrose. Gel beads were stored in cryovials and kept in liquid N2 for storage. It was successfully revived later (Mehrotra et al. 2015).

5.4 Metabolic Engineering

Increased demand for bioactives like reserpine for pharmacological needs caused a considerable upsurge in metabolic engineering approaches. Metabolic engineering is defined as the manipulation of metabolic pathway in a selected organism with the intention of producing economically valuable metabolites. The decade of 1900s witnessed early attempts of researchers to alleviate the alkaloid content by incorporating microbes as bioreactors. Interestingly, strictosidine synthase, a key catalytic agent in alkaloid biosynthesis, was isolated from R. serpentina and was expressed in E. coli. According to the report, this is the first enzyme from plant secondary metabolism to be expressed in an microorganism (Kutchan 1989). The enzyme was structurally and chemically similar to the plant-derived enzyme. Later, microsomes were artificially synthesized using cell cultures of R. serpentina and guinea pig liver. Vomeline, an intermediate in ajmaline biosynthetic pathway, was successfully produced via vinorine hydroxylase (Cytochrome P 450-Dependent) at 40 °C and pH 8.3 (Falkenhagen and Stöckigt 1995). Interestingly, phytoene desaturase (PDS) was silenced via biolistic-mediated VIGS in R. serpentina. The construct (pTRV2-RsPDS) was introduced in eight plantlets and bleaching of leaves was perceived indicating the transformation (Corbin et al. 2017). In addition to this, Catharanthus tryptophan decarboxylase (tdc) was overexpressed in roots of R. serpentina via AT 4 strain. tdc is elicitor responsive gene in TIA pathway. An intensification in reserpine and ajmalicine biosynthesis was observed (Mehrotra et al. 2013c). Sadly, very few metabolic engineering works are reported. Elaborate researches are recommended to study the possible ways to ameloriate the bioactives in R. serpentina.

5.5 Novel Approaches

Nanotechnology has been efficaciously employed in the research of R. serpentina. Green synthesized Copper oxide nanoparticles (CuO Nps) were subjected to antibacterial study. These highly porous, spongy nanoparticles were found to be active against both gram positive and gram negative bacteria. Interestingly, these particles decayed trypan blue dye on exposure to ultraviolet rays (Lingaraju et al. 2015). Ample studies resulted in the development of transcriptomic assembilies for R. serpentina(Góngora-Castillo et al. 2012). In silico approaches carried out in R. serpentina resulted in the identification of functional MicroRNAs. MicroRNAs are short non-coding RNAs with specific functional roles. Remarkably, 15 conserved miRNAs were detected and most of them are proposed to be involved in developmental metabolic roles of the plant. These studies can be considered as benchmark in recognizing the possibilties of genetic pathways involved in metabolite production. Interestingly, the investigators also performed the phylognetic study of the evoluntionary relationships between rse-miRNA precursors and stem-loop sequence found in some other plants (Prakash et al. 2016).

5.6 Conclusion

R. serpentina plants are well-known for their medicinal properties and were found easily in many regions of India. However, the colossal exploitation of these valuable medicinal plants during the Anthropocene epoch has led to the considerable reduction in their number. Rampant effective strategies are mandatory to sustain a considerable population needed by society. Effectual in vitro propagation and biotechnological approaches should be utilized to curtail the endangering of plant in the wild. Unfortunately, a very few metabolic engineering approaches have only been reported. More intricate and effective studies are indispensable to get insights of the unexplored possibilities of this plant. Ample computational studies will be revoluntionary in determining the genes involved in secondary metabolite production and subsequent engineering. Besides, stringent regulations are required for the efficacious protection of plants without exploiting them.