**2. Fungal endophyte-host plant association**

The association between fungal endophytes and their host plant is due to their unique adaptations which enable the endophytes to harmonize their growth with their host plant [12]. The origin of endophytes is not clear due to complex association between the endophyte and its host plant and the multiplicity of the host's living environment. Exogenous and endogenous are the two hypotheses explaining the origin of endophytes. According to endogenous hypothesis, endophytes are gaged from the mitochondria and chloroplast of the plant, and so it has comparable genetic backgrounds to the host [13], whereas exogenous hypothesis believes that endophytes arrive from outside of the plant and got inserted into the host from root wound, induced channels, or surface [14]. During the long period of coexistence and evolutionary processes, different relationships have been established between endophytic fungi and their host plants ranging from (i) a continuum of mutualism, (ii) antagonism, and (iii) neutralism. As once inside the tissues of a host plant, the endophytic fungi assumed a quiescent (latent) state, either for the whole lifetime of the host plant (neutralism) or for an extended period of time (mutualism or antagonism) until environmental conditions are favorable for endophytic fungi [15]. Endophytes due to its cryptic existence also have its role of decomposers in ecosystem, as they are among the primary colonizers of dead plant tissues [16, 17].

#### **2.1 Fungal endophytes**
