## 2 Endophytes-Current State of the Art  
### The Basic Characteristic of Endophytes  
[PERSON] et al. [12] mentioned that endophytic organisms have been found in virtually every plant. In this context, plants are hosts of various endophytes and form a range of relationships with them including symbiotic, mutualistic, commensalistic and trophbiotic [13]. Plant-endophyte interactions are thought to date back over 400 million years, and they have proven to be so successful that plants still interact with and even require endophytes to live in demanding environmental conditions [3]. It is worth remembering that plants have the potential to \"select\" endophytes of microorganisms from their above-ground or below-ground plant-associated microbial communities that provide them with some benefit [3]. Plant endophytes are usually represented by various microbial taxa, including both bacteria and fungi, and also archaea, algae and viruses [3,14]. Their main sources are microorganisms associated with the rhizosphere, phyllosphere and seeds [2]. Bacteria from the plant root zone (rhizosphere) enter the root interior through lateral root formation sites, damaged tissues and micropores. They then migrate inside the plant through the apoplast and conductive tissues [15]. In this way, they can colonize the intercellular spaces, the xylem, and the cell interior of organs such as roots, stems, leaves, flowers, fruits and seeds [16]. Some studies demonstrated that many endophytic bacteria can infiltrate roots and give rise to new endophyte populations in other plant organs [17,18]. As far as endophytic fungi are concerned, in many plants they are the most diverse and commonly occurring microorganisms. Fungal endophytes are found in the roots and above-ground parts of plants, with considerable diversity [19]. As in the case of bacteria, fungi can colonize plants systemically up until the flowering organs [19]. In addition, endophytes can be transmitted by seeds [19]. Many fungal endophytes, such as Epichloae, are seed-transmitted because they are completely reliant on the host's reproductive strength for their survival and dissemination [19]. It is worth mentioning that studies have shown that endophytic fungi never colonize very early embryos and that infection by endophytes mainly takes place at a later stage, during seed formation [19]. It is also important to note that the first stages of growth are generally critical for the overall microbiota, and plants should utilize the microbes present in seeds to ensure proper ecological functions in later developmental stages [20]. [PERSON] et al. [20] evidenced greater biodiversity of seed-borne endophytes in the seed endosperm than in the embryos and concluded that the seed-born microbiome is not statistically significantly dependent on the wheat cultivars; however, it cannot be claimed that every wheat seed is the same [20]. It should be pointed out that the endophytic microbiome of a plant is a function of factors such as plant species, growth stage, plant tissue, soil physicochemical factors (especially pH and moisture), physiological and nutritional status of the plant, the growing season and environmental factors [6]. The root tissues were found to contain the largest numbers of endophytic microorganisms, meanwhile the aerial parts of the plant are characterized by lower abundances of endophytes [12], whereas the generative organs of plants are the least rich in endophytic microorganisms [21]. Through the use of next-generation sequencing (NGS) techniques, it has been established that the endophytic structure of most plants is dominated by six phyla of bacteria (Figure 1): Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes, Acidobacteria and Deinococcus-Thermus [22; 23]. The most common phylum of bacteria isolated from plants is Proteobacteria. Actinobacteria, Bacteroidetes and Firmicutes also belong to phyla frequently encountered as endophytes [24; 25].