## 2 Endophytes-Current State of the Art
### Endophytes as Inhabitants of Agricultural Plants
#### 2.2.4 Endophytes as Plant Defense Against Pathogens  
Endophytic microorganisms also play an important role in plant defense against pathogens, which is also critical from an agricultural point of view. They can reduce or neutralize the harmful effects of fungi, viruses, bacteria, insects and nematodes [84]. Many endophytes produce chelators, antimicrobial metabolites, lytic enzymes and hydrogen cyanide, inhibiting the growth of phytopathogens. Furthermore, they compete with pathogenic microorganisms for available nutrients and space, thereby excluding them from the plant's ecological niche. They also have the potential to induce systemic resistance [84]. One of the most powerful and thoroughly studied mechanisms is the production of antibacterial, antifungal and antiviral antibiotics. Many compounds produced by endophytic bacteria have been identified, e.g., phenazine, pyrolnitrin, 2,4-diacetylplnologicinol (DAPG), pyoluteorin, oomycin A produced by _Pseudomonas_ sp., or oligomycin A, canozamine, zwittermycin A and xanthobacin synthesized by bacteria of the genera _Bacillus_, _Streptomyces_ and _Stenotrophomonas_[36]. Hydrolytic enzymes such as chitinases, cellulases, 1,3-glucanases, proteases and lipases cause cell lysis of fungal pathogens, while biosurfactants can act as antimicrobial compounds [78,84]. Bacteria belonging to the genera _Pseudomonas_ and _Bacillus_, were reported to produce hydrogen cyanide, which has a deleterious effect on weeds, thus enabling plants to compete with them [85]. Bacterial endophytes also make low molecular weight bioactive compounds that are active at low concentrations and act antagonistically against a range of plant pathogens [85].  
An indirect antagonism mechanism occurs when bacteria reduce or prevent harmful effects of pathogens on plants by producing inhibitory substances or inducing induced systemic resistance (ISR) [86]. There are many examples of endophytes that reduce the susceptibility of plants to pathogens. Furthermore, endophytic microorganisms are characterized by a faster and more intense induction of defense mechanisms in host plants, against various pathogens, compared to rhizosphere microorganisms [84]. Studies conducted on endophytes isolated from mulberry, among which bacteria from the genera _Pantoea_, _Bacillus_, _Pseudomonas_, _Curtobacterium_ and _Sphingomonas_ predominated, showed that the isolates had high activity against the fungal pathogens _Sclerotinia scleroorum_, _Botrytis cinerea_ and _Colletotrichum glooseporiogle_[87]. [PERSON] et al. [88] demonstrated that _Pseudomonas tremae_ and _Curtobacterium herbarum_ strains were sources of bioactive compounds providing resistance to phytopathogens. A _Nicotiana benthamiana_ plant grown from seed inoculated with the aforementioned bacteria showed several times higher concentrations of endogenous salicylic acid and increased resistance to a disease caused by _Pseudomonas syringae_ pv. _tabaci_. Some fungal species, such as _T. harzianum_ and _T. longbrachidium_, are also known for their beneficial interactions with plants [88]. Various studies also report the extensive capabilities of this beneficial fungus in alleviating plant stress when conditions are unfavorable for plant growth [89]. Also of interest is the genus _Fusarium_, which includes species described as both endophytes and plant pathogens. This is due to its ability to colonize and produce various mycotoxins, in addition non-pathogenic _Fusarium_ species show biocontrol effects against potential pathogens [89].  
The use of a stress-tolerant consortium of endophytic bacterial strains can be used to improve plant growth under abiotic and biotic stress conditions by regulating plant hormones, improving nutrition, producing siderophores and enhancing the antioxidant system [90,91]. It has been suggested that endophytes'sense' physiological changes in plants and adjust gene expression to adapt to the new environment, and can therefore be used as protective agents in agricultural systems under extreme climatic conditions [12].