## 6 Secondary Metabolite Production and Elicitation Technique  
Plants produce thousands of organic compounds that are divided into two large groups: primary and secondary metabolites. Primary metabolites are essential for plant growth and development, and the majority appear to be common in all plants. The plant secondary metabolites are often referred to as compounds that have no essential role in the plant's development and growth yet possess other important roles such as plant defense and adaptation against abiotic stresses. Plant secondary metabolites are considered very valuable and unique sources for pharmaceuticals, food additives, flavors and industrially important biochemicals [85]. Many studies agree that the accumulation of particular secondary metabolites is achieved by exposure to abiotic stresses. Secondary metabolites are connected with other chemicals that are involved in salinity and drought stress resistance, including calcium, ABA, SA, JA, ethylene and other signaling molecules [86; 87]. Some of the predominant secondary metabolites in _Camellia sinensis_ are the catechins, which are a type of polyphenolic flavonoids [88]. Studies on catechins have found that these metabolites are able to either generate or scavene reactive oxygen species [89]. In the matter of abiotic stress, as reported above, ethylene is a critical plant hormone in signal transduction and resisting the adverse effect of the stress. A study conducted on _Camellia sinensis_ showed that the treatment of a precursor of ethylene known as 1-aminocyclopropane-1-carboxylic acid (ACC) showed enhanced accumulation of phenolic compounds. Additionally, the antioxidant enzyme activity of SOD, CAT and POX enzymes was decreased, whereas that of the APX enzyme was increased due to the precursor treatment [88]. These findings indicate a presumable implication of induction of the secondary metabolites in regulating the balance of ROS in the plant via ethylene signaling. This information indicates that the secondary metabolites contribute significantly to the plant's resistance to abiotic stresses. Along with their contribution to preventing harmful effects of salinity and drought stresses, they also possess economically important values such as acting as a source of food or traditional medicine. Many medicinal plants are sources of very important drugs such as alkaloids from Catharanthus (for chemotherapy) [90], melatonin from _Tunacatum partheinum_[91] and antioxidant secondary metabolites from many plants. Thus, understanding how the economically significant secondary metabolites are produced is very important. Although plant phytochemicals have powerful beneficial values as plant protectants and human therapeutic values, their production in plants is often at low concentrations in different organs of the plant [92]. Therefore, scientists focus on the development of a variety of enhancement tools for the production of secondary metabolites using cell and molecular genetic approaches. One famous strategy used to enhance the production of phytochemicals in plants is the elicitation technique. An elicitor is globally used for \"any physical or chemical agent capable of inducing or stimulating defense response in plant cells/tissues via production of secondary metabolites\" [93; 94]. These elicitors modulate the regulation of the biosynthetic pathways that are involved in the natural biosynthesis of significant phytochemicals in the plants. An elicitor could result in the induction of transduction pathways that lead to the production of the secondary metabolite. There are many types of factors that could be considered as elicitors for the plant enhancement to produce secondary metabolites. An elicitor could be endogenous or exogenous. An endogenous elicitor is one produced by the plant without foreign participation. An exogenous elicitor originates from outside the plant. In this review, we focused on the elicitation of abiotic factors such as salinity and water deficit stresses [94].