## 4 Plant Tissue and Organ Cultures in Bioreactors
### Temporary Immersion System  
TIS (Temporary immersion system) are automated platforms for controlled short-time contact of the plant explants with a liquid medium in an aseptic environment, which could be a particularly interesting option in terms of _Rhizobium_-mediated genetic transformation techniques. This type of reactor allows the culture of various plant tissues and organs by periodically immersing them in the culture medium. This type of reactor allows for a good balance between the access of oxygen and nutrients, thus obtaining good biomass growth. Moreover, optimization of the culture conditions is easily accomplished by manipulating the frequency and time of immersion in a liquid medium. Most importantly, the tissues and organs are not exposed to the destructive effects of shear forces occurring in the other types of reactors. The top vessel is the rearing chamber; it is equipped with a water jacket for precise temperature control and an integrated UV light source, which is mounted on the top cover. The plant material is supported by a stainless-steel screen installed inside the growing chamber. The bottom vessel is the nutrient tank; it is designed with two external ports--one on the top end for air supply and one at the bottom end for media loading and sampling. The above examples are intended as a brief introduction to the most commonly used reactors for the in vitro culture of plant material. It is important to note that each system should be set up individually depending on the cultured cells, tissues, organs, or whole plants. When used on an industrial scale, many approaches are based on combinations of different strategies that are intended to ensure the maximum productivity of secondary metabolites or recombinant proteins [23; 88]. The types of bioreactors described previously are shown in Figure 2.  
Transgenic Plants Manipulated in Metabolic Pathways as a Source of Bioactive Secondary Metabolites Grown in a Bioreactor  
The World Health Organization defines any plant that contains a substance that can be used for therapeutic purposes or that is a precursor to a new, semi-synthetic pharmaceutical as a medicinal plant [89]. Over the course of evolution, plants gradually gained the ability to synthesize various types of secondary metabolites with valuable medicinal properties [90]. Secondary metabolites are useful natural products synthesized by the secondary metabolism of plants. The production of some secondary metabolites is associated with  
Figure 2: The types of bioreactors most often used for the cultivation of plant cultures in vitro. (**A**) Bubble column bioreactor, (**B**) Stirred tank bioreactor, (**C**) Nutrient mist or sprinkle bioreactor, (**D**) Wave-mixed bioreactor, (**E**) Temporary immersion system. the induction of morphological differentiation and maturation in some cells during plant growth [53]. Plant secondary metabolites are most often classified according to their biosynthetic pathways. Individual compounds are synthesized via a variety of enzyme-catalyzed reactions using simple building blocks. There are several major biosynthetic pathways in plants, including the mevalonate pathway (joining three molecules of Acetyl-CoA) and the non-mevalonate pathway--which both produce isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) (terpenes)-- the shikimic acid (phenylpropanoids) pathways, the 2-C-methyl-d-erythnitol-4-phosphate (quinones) pathway, the amino acid pathway (alkaloids), the acetate-malonate pathway (fatty acids, phenols, and quinones), and the complex pathway (flavonoids) [13; 91]. Generally, four groups of large molecules are recognized: phenols, steroids, terpenes, and alkaloids. While phenols are involved in the synthesis of lignin and are characteristic of all higher plants occurring in nature, other bioactive compounds, such as alkaloids, are sparsely distributed in the plant kingdom and are much more specific to a particular genus and species.