## 1 Introduction  
Flavonoids are plant secondary metabolites which have gained importance in the biotechnology industry due to their unique health related properties [1]. They are found in almost all of the growing parts of plants responsible for the pigmentation of fruits, flowers and leaves. Moreover, flavonoids such as anthocyanins, tannins and phytoalexins impart protection to plants against ultraviolet radiation, herbivores and pathogens, respectively [2]. Besides their relevance in plants, flavonoids are also associated with a vast spectrum of health enhancing effects and are an indispensable component in a variety of pharmaceutical, nutraceutical, cosmetical and medical applications [3,4]. Flavonoids possess significant antiallergic, antihepatotoxic, anti-inflammatory, antioxidant, antitumor, antiviral, antibacterial, antiosteoporosis as well as cardio-protective activity. Antioxidant, antihepatotoxic, anti-inflammatory, antiallergic, antitumor, antiosteoporosis, antiviral, antibacterial and cardio-protective potential have been reported in many flavonoids from various plant species [5, 6, 7, 8, 9]. In vivo studies also support the possible use of flavonoid rich plants in treating diseases. Recently, Bergamot juice, which is rich in flavonoid content, was used to treat experimental periodontitis in rats. The results showed that flavonoids present in Bergamot juice reduced the expression of inflammatory markers associated with gingival inflammation, and downregulated NF-kB activation as well as apoptosis [4]. Therefore, when consumed in the human diet, these compounds promote health and prevent diseases [4,5]. Flavonoids also influence the quality and stability of food as they add flavor and color to food and act as a food preservative [10]. These properties make flavonoids an important commercial biomolecule, which has led to attempts for the in vitro production of flavonoids via the plant tissue culturing technique. Plants are capable of synthesizing an overwhelming variety of low molecular weight organic compounds, called plant secondary metabolites or plant specialized metabolites, which help plants to colonize diverse and challenging environments [11]. Currently, about 100,000 such compounds have been isolated from plants classified into major groups, which include terpen, phenolics and alkaloids [12]. Flavonoids form a part of a large group of phenolic compounds and an essential component of plant cells [13]. Phenolic compounds have one or more hydroxyl groups attached directly to an aromatic ring, while their subgroup, flavonoids, is characterized by the presence of 15 carbon atoms in the flavonoids' basic skeleton containing two aromatic rings. The carbon atoms in flavonoids are arranged in C6-C3-C6 form, which corresponds to two aromatic compounds linked together via three carbon atoms [14,15]. The arrangement of the C3 group determines the classification of flavonoids, giving rise to three classes: flavonoids, isoflavuoids and neoflavonoids [16] (Figure 1). The biosynthesis of flavonoids is initiated by the condensation of one \(p\)-counzoyl-CoA molecule with three molecules of malonyl-CoA in the presence of chalcone synthase to form chalcone. Chalcone is isomerized by an enzyme chalcone isomerase, which converts chalcone to flavonenes. From these intermediate compounds, the pathway diverges into several side branches, each giving rise to a different class of flavonoids [17]. Figure 1: Flavonoids’ structures and classification. Recently, medicinal plants have been exploited for flavonoids to treat human maladies naturally [18]. With the rise in industrial demand for bioactive compounds from medicinal plants, there is a shift towards the use of in vitro culture systems for the mass production of secondary metabolites in a shorter time. The in vitro culturing technique avoids the destruction of plants' natural habitat, cutting down plant roots or underground parts and the over-exploitation of wild species [19].