## 2 Endophytes-Current State of the Art
### The Basic Characteristic of Endophytes
Opposingly, Acidobacteria, Planctomycetes and Verrucomicrobia occur at a somewhat lower frequency, most often as associated microbiota [24; 26]. Within Proteobacteria, \(\alpha\)-, \(\beta\)-, \(\gamma\)- and \(\delta\)- classes can be distinguished, with the \(\gamma\)-Proteobacteria being the most diverse and containing the highest number of bacterial endophytes [28]. Most endophytic \(\gamma\)-Proteobacteria belong to the following genera: _Acineobacter_, _Enterobacter_, _Pantoea_ and _Pseudomonas_[6]. \(\alpha\)-Proteobacteria is also characterized by  
Figure 1: The abundance of endophytic microorganisms belonging to different phyla, according to [PERSON] et al. [27]. high abundances of the genera _Rhizobium_, _Methylobacterium_, _Sphingomonas_ and _Bradyrhizobium_[29]. Numerous studies have shown that the most commonly known endophytic genera of bacteria include _Bacillus_, _Burkholderia_, _Microbacterium_, _Micrococcus_, _Pantoea_, _Pseudomonas_ and _Stenrotrophomonas_[28], _Achromobacter_, _Azarcus_, _Collimonas_, _Curbobacterium_, _Enterobacter_, _Flavobacterium_, _Gluconoacetobacter_, _Herbspirillum_, _Klebsiella_, _Microbispora_, _Micromonospora_, _Nocardioides_, _Planomonaspora_, _Serratia_, _Streptomyces_ and _Thermonomospora_[6; 29]. Endophytic fungi are classified into three main ecological groups: mycorrhizal, bal-ansicaeous or pasture fungal endophytes and non-pasture fungal endophytes [30]. Endophytic fungi can also be divided into the two groups: non-_claicipitaceous_ endophytes (NC-endophytes) and _claicipitaceous_ endophytes (C-endophytes) [31]. C-endophytes are grouped into Class 1 and are commonly found in plant shoots, where they cause systemic intracellular infections [32]. There are three categories of C-endophytes: symptomatic and pathogenic species (Type I), mixed-interaction and asymptomatic endophytes, and asymptomatic endophytes (Types II and III, respectively). NC-endophytes are divided into the three subclasses: 2, 3 and 4 [32]. Class 2 fungi, also known as Ascomycetes, are a significant group of fungi that often serve as endophytes in various plants. Class 3 is exceptionally diverse, with its members belonging to the Basidiomycota and the fungal group Dikaryomycota [32]. Class 4 are facultative biotrophic fungi that penetrate plant roots and are distinguished by melanized dark septate hyphae [32]. The endophytic fungi of Class 4 are classified mainly in the phylum Zygomycetes [3; 32]. The largest group in Class 4 are the Dark Septate Endophytes (DSE). They can be found in probably all land plants worldwide [33]. For an endophytic fungus to be classified as a DSE, it should create the specialized structures in the roots of the host and occur in plant roots as asexual, both melanised and septate hyphae [32]. Genera belonging to Class 4 but not to the DSE are, for example, _Ilyonectria_, _Fusarium_, _Cylindrocarpon_, _Gibberella_, Neonectria and the Sebaciales [32]. The diversity of endophytic microorganisms described for many plant species are summarized in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Phylum/Class/Species** & **Host** & **References** \\ \hline BACTERIA & & \\ \(\alpha\)-Proteobacteria & & \\ \hline _Rhizobium wenzxiniae_ sp. & maize & [34] \\ _Rhizobium_ sp. MP1 & chickpea & [35] \\ _Aectobacter diazotropicus_ & maize, wheat, rice & [36] \\ _Azospirillum_ sp. & maize & \\ \hline \(\beta\)-Proteobacteria & & \\ \hline _Burkholderia_ sp. GE 17-7 & ginseng & [37] \\ _Achromobacter insolitus_ & wheat & [38] \\ _Herbspirillum seropedicate_ & rice & [39] \\ \hline \(\gamma\)-Proteobacteria & & \\ \hline _Acinetobacter xylos oxidans_ & pink cataranth & [40] \\ _Citrobacter_ sp. RPT & fern & [41] \\ _Enterobacter ludwigii_ & tomato & [42] \\ \hline Actinobacteria & & \\ \hline _Corynebacterium_ sp. & maize & [43] \\ _Microbacterium_ sp. LKL04 & rod millet & [44] \\ _Micrococcus_ sp. & tomato & [45] \\ \hline Firmicutes & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of identified endophytic microorganisms from different plants.