**1. Introduction**

The total mankind is dependent on plants as a source of carbohydrates, proteins, vitamins, food, and shelter. Plants are studied for their important constituents and the nutritional factors

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for over decades. Along with the essential primary metabolites, higher plants are also capable to produce a number of low molecular weight compounds. A diverse group of organic compounds that are produced by plants to facilitate interaction with the biotic environment and the establishment of a defense mechanism are called as plant secondary metabolites [1– 3]. The production of these metabolites is very low (less than 1% dry weight) and depends greatly on the physiological and developmental stage of the plant [4,5]. Plant natural products have been an important part of medicine throughout human history. In recent years, the use of herbal medicines has steadily increased worldwide [6]. With this increasing demand comes growing concerns about the safety and efficacy of herbal medicines. Although the potential for medicinal plants seems almost limitless, there are a few major obstacles that hinder largescale utilization by the western medical system. Among them is the lack of reproducibility common in testing many plant extracts (up to 40%), which has limited the enthusiasm for developing plant-based pharmaceuticals [7]. Unlike standardized single-entity pharmaceuti‐ cal drugs, herbal medicines consist of complex mixtures with multiple compounds responsible for therapeutic activity, making standardization difficult [8]. Further complicating the issue is the fact that plants, unlike synthetic medicines, are living organisms, with inherent biological variation [9]. Just because plant material originates from the same species, it does not neces‐ sarily mean that the chemical content will be identical. This lack of reproducibility may be due to two main factors, genetic variability and differences in growing conditions.

In addition, plants are a valuable source of a wide range of secondary metabolites, which are used as pharmaceuticals, agrochemicals, flavors, fragrances, colors, biopesticides, and food additives. Plants are producing new compounds and in future new chemical models are drawing for new drugs because the most of the plants chemistry is yet to be explored [10]. The characterization of molecular structures and chemical analysis helped us to pinpoint the activities of plants under controlled conditions. Although all these advancements, we still depend on the secondary metabolites of biological sources including pharmaceuticals [11].

Due to various agro alimentary, perfumes, flavors, colors, and pharmacological effects, the secondary metabolites are having extensive demand and various commercial preparations are available in the market. Besides, the appeal of using natural products for medicinal purposes is increasing, and metabolic engineering can alter the production of pharmaceuticals and help to design new therapies. The evolving commercial importance of secondary metabolites has in recent years resulted in a greater interest in secondary metabolism, particularly in the possibility of altering the production of bioactive plant metabolites [12]. Secondary metabolites are separated into nitrogen compounds (alkaloids, nonprotein amino acids, amines, alcamides, cyanogenic glycosides, and glucosinolates) and nonnitrogen compounds (monoterpenes, diterpenes, triterpenes, tetraterpenes, sesquiterpenes, saponins, flavonoids, steroids, and coumarins).

The plant tissue culture plays an important role in the rapid clonal propagation, regeneration of genetically manipulated superior clones, conservation of germplasm, production of secondary metabolites, and ex vitro conservation of valuable phytodiversity [13,14]. The plant, cell, tissue and organ culture techniques have come up with an escapable tool with the possibilities of acclaiming and supplementing the conventional method in plant breeding, plant improvement, and biosynthetic pathways. This technique has several potential applica‐ tions in crop improvement, and efficient regeneration is a prerequisite in such improvement programs. The biotechnological production of secondary metabolites in plant cell and organ cultures is an attractive alternative to the extraction of the whole plant material [15]. In particular, plant–specific important compounds are obtained by using the plant cell and organ cultures [2]. The faster proliferation rates and shorter biosynthetic cycle of cell and organ cultures leads to have a higher rate of metabolism when compared to field grown plants [16]. Further, plant cell/organ cultures are under controlled conditions proliferates at their optimum growth rates when compared to the cultivated plants, which are facing environmental, ecological, and climatic variations. In recent years, various strategies have been developed for use in biomass accumulation and the synthesis of secondary compounds, such as strain improvement, optimization of medium, and culture environments, elicitation, precursor feeding, metabolic engineering, permeabilization, immobilization, and biotransformation methods, bioreactor cultures, and micropropagation [17]. The focus of the present chapter is the influence of abiotic and biotic elicitors on the secondary metabolite production in the in vitro cultured medicinal plants.

for over decades. Along with the essential primary metabolites, higher plants are also capable to produce a number of low molecular weight compounds. A diverse group of organic compounds that are produced by plants to facilitate interaction with the biotic environment and the establishment of a defense mechanism are called as plant secondary metabolites [1– 3]. The production of these metabolites is very low (less than 1% dry weight) and depends greatly on the physiological and developmental stage of the plant [4,5]. Plant natural products have been an important part of medicine throughout human history. In recent years, the use of herbal medicines has steadily increased worldwide [6]. With this increasing demand comes growing concerns about the safety and efficacy of herbal medicines. Although the potential for medicinal plants seems almost limitless, there are a few major obstacles that hinder largescale utilization by the western medical system. Among them is the lack of reproducibility common in testing many plant extracts (up to 40%), which has limited the enthusiasm for developing plant-based pharmaceuticals [7]. Unlike standardized single-entity pharmaceuti‐ cal drugs, herbal medicines consist of complex mixtures with multiple compounds responsible for therapeutic activity, making standardization difficult [8]. Further complicating the issue is the fact that plants, unlike synthetic medicines, are living organisms, with inherent biological variation [9]. Just because plant material originates from the same species, it does not neces‐ sarily mean that the chemical content will be identical. This lack of reproducibility may be due

248 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

to two main factors, genetic variability and differences in growing conditions.

coumarins).

In addition, plants are a valuable source of a wide range of secondary metabolites, which are used as pharmaceuticals, agrochemicals, flavors, fragrances, colors, biopesticides, and food additives. Plants are producing new compounds and in future new chemical models are drawing for new drugs because the most of the plants chemistry is yet to be explored [10]. The characterization of molecular structures and chemical analysis helped us to pinpoint the activities of plants under controlled conditions. Although all these advancements, we still depend on the secondary metabolites of biological sources including pharmaceuticals [11].

Due to various agro alimentary, perfumes, flavors, colors, and pharmacological effects, the secondary metabolites are having extensive demand and various commercial preparations are available in the market. Besides, the appeal of using natural products for medicinal purposes is increasing, and metabolic engineering can alter the production of pharmaceuticals and help to design new therapies. The evolving commercial importance of secondary metabolites has in recent years resulted in a greater interest in secondary metabolism, particularly in the possibility of altering the production of bioactive plant metabolites [12]. Secondary metabolites are separated into nitrogen compounds (alkaloids, nonprotein amino acids, amines, alcamides, cyanogenic glycosides, and glucosinolates) and nonnitrogen compounds (monoterpenes, diterpenes, triterpenes, tetraterpenes, sesquiterpenes, saponins, flavonoids, steroids, and

The plant tissue culture plays an important role in the rapid clonal propagation, regeneration of genetically manipulated superior clones, conservation of germplasm, production of secondary metabolites, and ex vitro conservation of valuable phytodiversity [13,14]. The plant,
