1. Introduction

There are approximately 450,000 plant species exist on earth, and one third of these plants are under the risk of extinction (Pimm and Joppa 2015). The current estimated total number of plant-produced metabolites within a given plant species are greater than 10,000, however, at present, it has been projected that with currently available metabolome techniques, only less than 20% of these metabolites can be analyzed (Lei et al. 2011; Abdelrahman et al. 2018). During life span, human has frequently used plant derived natural products as traditional medicines for millennia. However, the full potential of these plant derived natural products remains to be exploited, because they are difficult to synthesis in vitro, exist in very low amounts in a given plant species and/or produced by rare plant species and thus cannot be utilized for the large scale production. Generally speaking plants synthesize a diverse array of primary and secondary metabolites, which have different structures and vary greatly in their richness (Arbona et al. 2013; Hong et al. 2016). For instance, primary metabolites are crucial for plant growth and development, whereas secondary metabolites have more explicit functions; and both types of metabolites have major roles for plant responses to a specific stress (Fujii et al. 2015; Abdelrahman et al. 2019).