**2. Advantages of plant tissue culture techniques for the production of secondary metabolites**

In a context where consumers demand for safe natural products increases, because synthetic chemicals are perceived as potentially toxic, the interest in plant secondary metabolites from research and industry also increases [10]. Few important plant products with simple chemical structures can be produced via chemosynthesis; however, many compounds like alkaloids are difficult to be synthesized or the cost of their synthesis outweighs their commercial availability [11, 12]. Some compounds can be obtained from naturally grown plants, but sometimes there are regional and environmental restrictions, which can limit the commercial production [13]. Also, traditional cultivation of some plant species is difficult or takes several years. In this context, plant cell and tissue culture techniques appear as environmentally friendly alternative methods for the production of secondary metabolites when natural supply is limited and traditional methods are unfeasible. The mass propagation of plants in aseptic and environmental controlled conditions, and the large-scale production of secondary metabolites in a year-round system without seasonal constraints, are some of the advantages of plant tissue culture techniques [3]. Moreover, cultures can be established in any part of the world independently of the plant growth requisites and are free of microbes and insects avoiding the use of pesticides and herbicides [14, 15]. Plant tissue culture techniques provide a reliable and predictable method for isolating the secondary metabolites at a high efficiency within a short time when compared to the extraction from wild plant populations [16]. Also, the simplicity in the extraction of the metabolites from *in vitro*-produced tissues makes the method appellative for commercial application [17].

Apart from the abovementioned advantages, there are metabolites that are generally not found in the intact plant but can be produced by *in vitro* cultures [18]. Biotechnology opens the opportunity to apply traditional or metabolic engineering strategies to promote the accumulation of desired compounds by *in vitro* cultures. Products from *in vitro* cultures can still be used as models of whole plants, and cell cultures can be radiolabeled so that secondary products can be traced metabolically [19].

The massive and indiscriminate collection of plant material from plants producing important bioactive compounds has threatened the survival of some species. *In vitro* propagation through plant tissue culture techniques allows the large-scale multiplication of true-to-type plants within a short span of time and without a negative impact on the natural resources [16]. This method is particularly valuable for plants difficult to propagate by conventional techniques or with slow propagation rates. In this context, in the last years, there has been an increased interest on the use of these methodologies for the propagation and conservation of medicinal plants.
