Keywords

1 Introduction

The use of medicinal plants as therapeutic agents against various diseases has been notable for thousands of years (Gopal 2001). The World Health Organization (WHO) appraises that more than 80% of global population utilize plants and plant products for their primary health care (Cordell 1995). There is a huge demand for traditional herbal medicine which has increased several folds globally during the recent past. The primary reason behind this boon is thought to be the introduction of traditional knowledge-based drugs like Taxol (anticancer), Artemisinin (antimalarial), Forskolin (antihypertensive) (Ghisalberti 1993), and many others in the Western market.

Fransworth et al. (1985) estimated that at least 119 compounds derived from 90 plant species are considerably significant drugs currently practiced in one or more countries and 77% of these are obtained mostly from botanicals used in traditional medicine. The traditional Chinese medicine (TCM), the Indian system of medicine (ISM), the Japanese Kampo, and African folklore are still practiced by major population of Asia and Africa. The botanical-based treatments have remained a major part of TCM, and even today it comprises approximately 50% of total drugs used in China. Of the 5500 medicinal plants used in TCM, about 300–500 are commonly used in day-to-day prescriptions (Corizier 1974). One such drug that has been used in China for the past hundreds of years is Ephedra sinica (Ma huang), the active principle of which is a potent sympathomimetic amine, ephedrine. This medicine in the form of various salts is now used in Western medicine to treat bronchial asthma.

The Indian system of medicine (ISM), mainly constituting of Ayurveda, Siddha, and Unani, is considered as one of the traditional system of medicine with thoroughly documented plant-based therapeutics. In India and abroad also, Ayurveda is being practiced by a vast population. In ISM, the remedies are primarily based on plants and plant products that usually work on the human system to boost immunity, resistance, and strength in order to cure the diseases (Swami Tirtha 1998). Some of the classical examples are as follows: (a) reserpine, an indole alkaloid as a potent antihypertensive and tranquilizing agent developed from the Indian medicinal plant Rauwolfia serpentina (Warrier et al. 1996), and (b) forskolin, a highly oxygenated labdane diterpenoid from the roots of Indian medicinal plant Coleus forskohlii exhibiting potent antihypertensive, antithrombotic, and positive ionotropic properties (Rastogi and Meharotra 1990). The Kampo system of medicine is practiced by a considerable population in Japan, where about 70% of physicians prescribe one or another Kampo formulations (Ishibashi 2002). Some of the bioactive isolates from the plants used in Japanese Kampo are baicalein (from Scutellaria baicalensis) (Huang 1999), glycyrrhizin (from Glycyrrhiza uralensis) (Tang and Eisenbrand 1992), and saikosaponin I (from Bupleurum falcatum) (Shibata et al. 1923).

In most of the African countries, up to 80% of population depends entirely on botanicals for therapeutics (Suzuki 2002; David 2000), many of which have been documented in the form of an African pharmacopeia issued by a scientific technical research commission of the organization of African Unity in 1984. A few promising drugs developed from African medicinal plants include (a) caffeine from the coffee tree Coffea arabica (from the highlands of southwest Ethiopia), a purine alkaloid exhibiting potent CNS activity and positive inotropic activity and activates lipolysis, and (b) ouabain (γ-strophanthin) isolated from Strophanthus gratus (from tropical West Africa) – a cardiac glycoside used to treat heart problems in acquits cardiac insufficiency (Hostettmann et al. 2000).

Due to the devoted work from the researchers, good proportion of promising drugs, numerous therapeutic leads, and many novel pharmacologically potent drugs have been developed from botanicals (Fig. 28.1) (Phillipson 1999). E. Merck, in the year 1826, manufactured an analgesic morphine, which was derived from opium poppy on commercial level, which was marked as the dawn of commercialization of plant-based drugs (Galbley and Thiericke 1999). Reserpine, the chief constituent from the roots of the Indian medicinal plant R. serpentina, introduced by CIBA (USA) in 1953 is another classical example of plant-derived drug. The following (Fig. 28.1) are the examples of plant-derived drugs which are under commercialization.

Fig. 28.1
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Potent plant-based drugs derived from various traditional medicinal systems of the world

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The widespread skepticism on synthetic medicines and worry of their usage due to adverse side effects such as toxicity, teratogenicity, and carcinogenicity which steered to the noticeable change in the status of herbal drugs as alternative medicine. The recent boon in the demand for herbal drugs is attributed to the introduction of promising plant-based drugs, viz., (1) Taxol (anticancer), (2) artemisinin (anti-malarial), (3) forskolin (antihypertensive), etc. in the Western market. The present annual global market for herbal medicine is 62.0 billion US dollars. However, it is very sad to note the Indian share amounts to a meager 1%.

2 Plant Secondary Metabolites

According to Verpoorte (1999): “Secondary metabolites are compounds which act as a defensive role in the interaction of the organism with its environment for survival in the ecosystem and are restricted to particular taxonomic group”. Three major secondary metabolites present in plants such as triterpenoids, alkaloids and phenolics.

2.1 Terpenoids

Terpenoids or isoprenoids are the largest class of secondary metabolites represented by nearly 27,000 compounds. Triterpenoids are classified on the basis of number of isoprenoid units present in any terpenoidal compound. For example, the monoterpenes are made up of two isoprenoid units (citronellal in lemon and menthol from peppermint), sesquiterpenes are made up of three isoprenoid units (zingiberene and artemisinin), diterpenes are made up of four isoprenoid units (gibberellic acid), triterpenes are made up of six isoprenoid units (ursolic, oleanolic, and betulinic acid), and tetraterpenes are made up of eight isoprenoid units (lanosterol, stigmasterols, diosgenin, lupeol) (Gershenzon and Kreis 1999).

2.2 Alkaloids

The alkaloids contain nitrogen besides carbon, oxygen, and hydrogen, commonly as part of a cyclic system, and are numerous among plants with diverse pharmacological properties depending on the specific alkaloid structure. The well-known examples of potent alkaloids are reserpine, ajmaline, vincristine, colchicines, mescaline, nicotine, cocaine, and morphine. Alkaloid biosynthesis in plants is from amino acids, terpenes, and aromatic compounds (Herbert 2001).

2.3 Plant Phenolics

Phenolic compounds are comprised of carbon, hydrogen, and oxygen atoms arranged as an aromatic ring having a hydroxyl substituent. Among plant polyphenols, the flavonoids form the largest group followed by phenolic quinones, lignans, xanthones, coumarins, and monocyclic phenols (Croteau et al. 2000).

2.4 Transport, Storage, and Turnover of Plant Secondary Metabolites

The biosynthesis of secondary metabolites is restricted in plants to various compartments, and their accumulation, storage, and release occur in highly specialized organs or tissues such as in glands, bark, roots, stem, and flowers (Croteau et al. 2000). In the vacuole the high polar water-soluble compounds are generally accumulated (Wink 1999; Boller and Wiemken 1986), whereas low polar compounds are restricted in other parts of plants such as resin ducts, lactifers, glandular hairs, trichomes, thylakoid membranes, or the cuticle and defend plants from abiotic and biotic stress (Wiermann 1981). In the peppermint monoterpenes, essential oils are accumulated in glandular trichomes during the development of leaves (McGarvey and Croteau 1995).

3 Biotic and Abiotic Factors Influencing Accumulation of Secondary Metabolites

In order to become a key player at the global level and to provide efficacious herbal remedy to the society, it is absolutely necessary to consider certain regulatory aspects. The primary aspect is the identification of high bioactive secondary metabolite-producing plants. The collection of secondary metabolites in a plant species depends principally on (1) soil nature, (2) climatic condition, and (3) altitude. Microbes also play a pivotal role in accumulation of secondary metabolites in herbal plants. Some important microbes such as plant growth-promoting rhizobacteria (PGPR) and mycorrhiza influence the bioactive phytochemical accumulation in herbal plants. Detailed investigation in this aspect will help in identifying the high-yielding plant source as well as will be useful in optimizing the parameters in which the medicinal plants may be cultivated under stringent and unfavorable conditions. Arbuscular mycorrhizal fungi (AM fungi) exist in mutually beneficial symbiotic coexistence with the higher land plants (Smith and Read 1997). Clark and Zeto (2000) and Augé (2001) have indicated that AMF improves plant nutrient assimilation specially phosphorus along with nitrogen, zinc, iron, and copper as well as water relations. Furthermore, AMF ameliorate cultivation condition and abiotic stresses (Jeffries et al. 2003) through high underground root biomass and improved absorption of soil nutrients and secretion of different enzymes by AMF colonized roots and/or hyphae (Marschner 1995; Smith and Read 1997).

3.1 Arbuscular Mycorrhizal Fungi (AMF)

AMF is symbiotic fungi that form a beneficial relation with the roots of a diverse group of terrestrial plants (Fig. 28.1). The AM fungi are classified under the order Glomales under the Zygomycota (Redecker et al. 2000), but currently it is moved to a new phylum, the Glomeromycota (Schüßler et al. 2001). These fungi are widespread soil-borne fungi, whose origin and occurrence have been established more than 450 million years (Redecker et al. 2000). AMF association with higher plants can be seen in temperate, tropical, and arctic regions but was found to be absent in waterlogged conditions (Smith and Read 1997). These fungi have a synergistic effect on plant growth performance in comparison with any microbes by an increase in biomass of the root system of the plant and also by enhancing the root surface area for absorption of minerals and water (Leake et al. 2004). Thus, it can be inferred that arbuscular mycorrhizal (AM) symbiosis with terrestrial plants is of significant utility in forest ecology, land reformation, improved growth, and better yield of plants in low-input systems (Sieverding et al. 1991).

3.2 Agronomic and Ecological Roles of AMF

The economically and medicinally important crops are known to be colonized by AMF (Sieverding et al. 1991). AMF improves the phosphorus nutrition by increasing the root biomass and surface area and by improving the P uptake (Koide 1991). AMF act as phosphate solubilizers by producing organic acids that increased the availability of insoluble mineral phosphates (Lapeyrie 1988) and enhanced the phosphorus uptake by host plants. Moreover, AMF also improves the uptake of macronutrients and micronutrients (Clark and Zeto 2000; Smith et al. 2004). Marschner (1998) and Hodge and Campbell (2001) on their studies on mycorrhiza established that the better plant nutrition is dependent on (i) enhanced root surface via extra-radical hyphae that extend beyond root depletion zone, (ii) decomposition of organic substances, and (iii) enhanced microbial consortium in the rhizosphere zone.

Contemporary studies reveal that AMF is an ideal tool for sustainable agriculture, landscape resurrection, and horticulture via decomposition of organic material, seedling establishment, enhanced pathogenic resistance, herbivore tolerance, soil stability, heavy metal detoxification, water stresses/cold temperature resistance, and reducing desertification (Jeffries et al. 2003; Hart and Trevors 2005). The function of AMF to their hosts in nutrient suppressive soil (less P) environment is high whereas it is less under P-sufficient conditions (Koide and Schreiner 1992), and plant growth rates can be reduced by AM colonization when available P is present (Peng et al. 1993).

3.3 Mycorrhizal Association with Medicinal Plants

Inoculation of crop plants with AMF is known to enhance growth, nutritional, and secondary metabolite contents of many medicinally and industrially significant crops which is attributed to enhanced uptake of nutrients, production of growth-promoting factors, biotic and abiotic stress tolerance, and synergistic interaction with PGPR such as nitrogen-fixing rhizobacteria and PSB (Bagyaraj and Varma 1995; Rajan et al. 2000). Enhanced mineral nutrition, i.e., N, P, Fe, Cu, Zn, and B, helps in the synthesis of chlorophyll thus enhancing photosynthetic rate (Bian et al. 2001; Feng et al. 2002; Toussaint 2007). In addition, AMF inoculation on medicinal plants is responsible for reduction in the intensity of disease caused by bacterial and fungal pathogens by morphological modulations such as thickening of the cell wall, stronger vascular bundles, etc. Moreover AMF inoculation leads to physiological alterations in host such as enhanced levels of P, phenolics, sulfur-containing amino acids, etc. (Boby and Bagyaraj 2003).

AM fungi that have a symbiotic relationship with medicinal plants belong to several families of angiosperms and gymnosperms. AM fungi are associated with Adhatoda vasica and Datura somnifera (Wei and Wang 1989). Several authors reported symbiotic relation of AMF with plants that belong to Araliaceae such as Panax ginseng (Zhang et al. 1990; Xing et al. 2000, 2003; Li 2003a; Cho et al. 2009; Ren et al. 2007; Zhang et al. 2011). There were many plants that belong to Labiatae family having association with AMF such as Salvia miltiorrhiza (He et al. 2009a, b; Wang and He 2009; Ma et al. 2009; He et al. 2010; Meng and He 2011), Schizonepeta tenuifolia (Wei and Wang 1991), Bupleurum scorzonerifolium (Teng and He 2005), Pogostemon cablin (Arpana et al. 2008), Coleus forskohlii (Sailo and Bagyaraj 2005), Salvia officinalis (Nell et al. 2009), Mentha arvensis (Gupta et al. 2002; Karagiannidis et al. 2011), Ocimum basilicum (Copetta et al. 2006; Toussaint et al. 2007; Lee and Scagel 2009; Prasad et al. 2011; Rasouli-Sadaghianil et al. 2010), and Origanum sp. (Khaosaad et al. 2006; Morone Fortunato and Avato 2008; Karagiannidis et al. 2011). Mycorrhization was also observed in plants that belong to Umbelliferae such as Anethum graveolens, Trachyspermum ammi (Kapoor et al. 2002a), Coriandrum sativum (Kapoor et al. 2002b; Farahani et al. 2008), Foeniculum vulgare (Kapoor et al. 2004), and Angelica dahurica (Cao and Zhao 2007; Zhao et al. 2009; Zhao and He 2011). AMF was reported in several plants that belong to Liliaceae such as Gloriosa superba L. (Yadav et al. 2013; Pandey et al. 2014), Aloe barbadensis (Gong et al. 2002; Mamta et al. 2012; Pandey and Banik 2009; Burni et al. 2013), Ophiopogon japonicus (Pan et al. 2008), Paris polyphylla var. (Zhou et al. 2009, 2010), and Allium sativum (Borde et al. 2009). Several authors reported mycorrhizal plants belonging to Leguminosae such as Pueraria lobata, Astragalus membranaceus, Glycyrrhiza inflata, Castanospermum australe, and Prosopis laevigata (Wang et al. 2006; Liu and He 2008, 2009; He et al. 2009b; Liu et al. 2007; Abu-Zeyad et al. 1999; Rojas-Andrade et al. 2003). AMF reported in plants belong to the Asteraceae family (Binet et al. 2011; Asrar and Elhindi 2010; Jurkiewicz et al. 2010; Araim et al. 2009; Kapoor et al. 2007; Rapparini et al. 2008; Chaudhary et al. 2008; Awasthi et al. 2011; Huang et al. 2011; Guo et al. 2006; Zhang et al. 2010, 2011; Asrar and Elhindi 2010). AMF is also reported in Forsythia suspense, Taxus chinensis var., Pinellia ternata, Catharanthus roseus, Valeriana officinalis, Atractylodes macrocephala, Camptotheca acuminate, Coix lacryma-jobi var., Ginkgo biloba, and Gentiana manshurica (Wang et al. 1998, 2010; Qi et al. 2002, 2003; Zhang et al. 2004; Wu and Wei 2008; Li 2003b; Huang et al. 2003; Zhao et al. 2006, 2007; Yu et al. 2010; Lu and He 2005, 2008; Lu et al. 2008a, b, 2011; Ren et al. 2008; Chen et al. 2009a, b, 2010; Guo et al. 2010; Shen et al. 2011; Zubek et al. 2012; Rosa-Mera et al. 2011; Nell et al. 2010; Geneva et al. 2010) (Fig. 28.2 and Table 28.1).

Fig. 28.2
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Symbiotic association of AMF with plants

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Table 28.1 Medicinal plants that have been investigated for AM effects
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3.4 Synergistic Effects of AMF on Secondary Metabolites Accumulation in Medicinal Plants

There is alteration in the secondary metabolite accumulation due to chemical and biological events taking place during the AMF–host interaction (Akiyama and Hayashi 2002; Allen et al. 1982; Barrios 2007; Cai et al. 2008). AMF is known to enhance contents of secondary metabolites such as terpenoids, alkaloids, and phenolics in many economically and industrially significant crops such as production of flavonoids, cyclohexanone derivatives and apocarotenoids, phytoalexins, phenolic compounds, triterpenoids, and glucosinolates in herbal, and medicinal important plants colonized by AMF has been reported (Gianinazzi et al. 2010; Hadwiger et al. 1986; Harris et al. 2001; Harrison, 1999; Heet al. 2009c; Huang et al. 2004; Janardhan and Abdul-Khaliq 1995, Jie et al. 2007; Loomis and Corteau 1972; Maier et al. 1999; Paterson and Simmonds 2003; Shah et al. 1980; Silva et al. 2008; Singh et al. 2013; Smith and Read 2008; Szakiel and Paczkoski 2011a, b; Toussaint et al. 2004; Volpin et al. 1994; Wink 1997; Wu et al. 2010; Xiao et al. 2011; Yang et al. 2008; Zeng et al. 2007; Zubek et al. 2010, 2013).

3.4.1 Plant Terpenoid Accumulation Associated with AMF

The influence of mycorrhization on the terpenoid production in different plants has been presented in Table 28.2. The enhancements in triterpenoids are both qualitative and quantitative due to modifications in morphology of various aromatic plants due to (atractylol, anethole, thymol, artemisinin, stevioside, rebaudioside A, β-caryophyllene, p-cymene, geraniol, patchoulol, valerenic acid, and glycyrrhizic acid) in AMF-colonized plants (Kapoor et al. 2007; Mandal et al. 2013, 2015; Morone Fortunato and Avato 2008; Khaosaad et al. 2006; Lu et al. 2011; Farahani et al. 2008; Wei and Wang 1991; Arpana et al. 2008; Geneva et al. 2010; Liu et al. 2007; Jurkiewicz et al. 2010). Furthermore many researchers reported that increase in terpenoids (thymol, geraniol, anethol, forskolin) in aromatic and medicinal plants is due to P availability (Torelli et al. 2000; Kapoor et al. 2002a; Strack et al. 2003; Krishna et al. 2005; Sailo and Bagyaraj 2005; Bagheri et al. 2014; Guo et al. 2006; Zhang et al. 2010, 2011) and transcription of gene responsible for terpenoid biosynthetic pathways (Floß et al. 2008; Mandal et al. 2014, 2015). Adams et al. (2004) investigated that composition of essential oil (EO) levels in Vetiver roots is altered in the presence of unidentified bacteria PGPR, and AMF were suggested to be involved in the modulation of essential oil (EO) accumulation. Copetta et al. (2006) and Freitas et al. (2004) reported that AM fungal root colonization enhances the essential oil content (linalool and geraniol, menthol, menthone, carvone, and pulegone) in Ocimum basilicum and Mentha arvensis, respectively. Moreover, many authors reported increase in terpenoids such as menthol, menthone, carvone, pulegone, carvacrol, and thymol in aromatic plants (Gupta et al. 2002; Karagiannidis et al. 2011; Khaosaad et al. 2006; Morone Fortunato and Avato 2008; Rasouli Sadaghianil et al. 2010; Chaudhary et al. 2008). Moreover, studies on anatomical and pharmacognosy suggested that enhanced production of essential oils was due to improvement in the number of glandular hairs in leaves in AM-inoculated plants (Guerrieri et al. 2004). Furthermore, plant growth regulator levels are also known to be amended in both arbuscule-containing cells and whole tissues of mycorrhizal plants (Allen et al. 1980).

Table 28.2 Effects of AM symbiosis on secondary metabolism of medicinal plants under the conditions of artificial inoculation
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3.4.2 Plant Alkaloid Accumulation Associated with AMF

The effect of arbuscular mycorrhizae (AM) on accumulation and biosynthesis of alkaloids has been attributed to mycorrhization in various medicinal plants such as Castanospermum australe (Abu-Zeyad et al. 1999), Catharanthus roseus (Rosa-Mera et al. 2011; Andrade et al. 2013), Datura stramonium (Wei and Wang (1989), Phellodendron amurense (Fan et al. 2006), and Camptotheca acuminate (Zhao et al. 2006; Yu et al. 2010). Liu et al. (2007) investigated that enhanced production of alkaloid in medicinal plants are linked with the higher biomass production in AM-inoculated plants which are usually connected to the nutritional benefits of mycorrhization. AM fungi directly controlled the expression of different genes associated with the plant defense system, with consequences for whole-plant fitness and its response to biotic stresses (Pozo and Azco’n-Aguilar 2007; Fan et al. 2006; Zhou and Fan 2007). It was reported in many medicinal plants that mycorrhization enhances alkaloid contents such as ephedrine, camptothecin, and alliin (Guo et al. 2010; Zhao et al. 2006; Yu et al. 2010; Borde et al. 2009). Studies performed by El-Sayed and Verpoorte (2007) and Roepke et al. (2010) reported the improved production of several pharmacologically important monoterpene indole alkaloids (MIAs), such as vinblastine, vincristine, ajmalicine, vindoline, catharanthine, and serpentine in Catharanthus roseus and pyridine alkaloids (PAs) in Nicotiana species.

3.4.3 Plant Phenolic Accumulation Associated with AMF

The effect of AM on the production of phenolics in various plants has been summarized in Table 28.2. The enhancements of phenolics such as anthraquinone glycosides and flavanoids are both qualitative and quantitative (Araim et al. 2009; Silva et al. 2014; Rosa-Mera et al. 2011; Bagheri et al. 2014; De Sousa et al. 2013; He et al. 2009a, b, c; Meng and He 2011; Zhao et al. 2009; Zhao and He 2011; Nell et al. 2009). It was observed by many researchers that the enhancement of phenolics in medicinal plants was due to increase in the biomass of plants due to better nutrition and expression of genes related to the defense system of plants. Jurkiewicz et al. (2010) reported increase in the caffeic and chlorogenic acid in Arnica Montana. Furthermore there was improved production of phenolics in Viola tricolor (Zubek et al. 2015), Salvia spp. (Yang et al. 2017), and Aloe vera (Pandey and Banik 2009; Mamta et al. 2012). There were significant increases in the content of rosmarinic and caffeic acids (Jugran et al. 2015; Zubek et al. 2015; Jurkiewicz et al. 2010; Toussaint et al. 2007; Lee and Scagel 2009) and diobulbinone (Lu et al. 2015).

Structure of Secondary Metabolites Affected by AM

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A. Triterpene

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B. Alkaloids

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C. Flavanoids and Phenolic Compounds

4 Conclusion

Now it is well-established that symbiotic association between medicinal plants and AM fungi can be exploited for the enhanced production of plant secondary metabolites. These secondary metabolites not only play an important role in uplifting the defense system of plants but also can be used for curing human ailments. In production of herb-based materials, there is a need to implement mycorrhizal technology to explore the role of AM fungi in the cultivation of medicinal plants. Thus future research priorities should emphasize on (1) isolation, selection, and screening of promising and effective AM fungal strains which can be adapted to natural habitat where medicinal and aromatic plants have to be grown, (2) strategies for the production of seedlings of medicinal and aromatic plants with mycorrhiza, (3) intensive cultivation of medicinal plants and the production of medicinally important phytochemicals as well as plant parts rich in medicinally important compounds, and (4) strategies by which AM fungi modulate the contents of bioactive principles in medicinal plants.