## 2 Endophytes-Current State of the Art
### Possibilities of Using Endophytes in Agriculture-Bioproducts and Strains Used in Biofertilizers
It is also worth emphasizing that several rhizobacteria have the potential to fix atmospheric N using the nitrogenase enzyme-mediated reduction of nitrogen into ammonia that is accessible for plants [167]. This feature resulted in the common application of rhizobacteria (especially _Rhizobium_ and _Bradyrhizobium_) as components of biofertilizers. Some rhizobacteria are also able to produce siderophores and therefore are useful in enhancing plant Fe nutrition [167]. All aforementioned features make rhizobacteria important candidates for improving soil fertility and plant health [167]. To summarize, the use of rhizobacterial biofertilizers reduces the environmental pollution caused by chemical fertilizers and protects plants against many soil-borne pathogens [167; 168]. Another type of biological products are biostimulants, which support plant growth already in small amounts. Their main purpose is to increase crop yields in organic farming as a result of increased solubility of nutrients in the soil [169]. They are also used to stimulate natural processes that increase abiotic stress tolerance and yield quality, the effect of which is not dependent on nutrient content [57]. According to the European Biostimulant Industry Council (EBIC), the major aim of biostimulant application is \"to stimulate natural processes to benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and/or crop quality, independently of its nutrient content\" [170]. It was suggested that biostimulants may be an alternative to the auxin-based rooting enhancers because they contain active substances, which, during application to plants, stimulate their life processes, i.e., photosynthesis [171]. Also, biostimulants can increase the microbial and enzymatic activities of soil and change the solubility of micronutrients [169; 172]. Some of the most common plant biostimulants are humic substances that stimulate root growth and consequently increase nutrient uptake together with soil pH neutralization [169]. Preparations that can provide an alternative to chemical pesticides in many crops are also biopesticides [156]. In Europe, the increased interest in such products is linked to the increased demand for environmentally safe products and the expansion of areas of organic farming [173]. Endophytic microorganisms that are the active ingredients of biopesticides are bacteria from the genera _Pseudomonas_ and _Bacillus_ and fungi from the genera _Trichoderma_, _Coniothyrium_, and _Beauveria_[174]. It is worth clarifying that biopesticides include not only preparations based on living organisms, but also products containing chemicals of natural origin, plant growth regulators, or pheromone traps [161]. Bacterial and fungal vaccines are also frequently used in agricultural practice [52]. Vaccines containing endophytic microorganisms, e.g., nitrogen-fixing bacteria in symbiosis with legume roots, and fungi (e.g., of the genus _Trichoderma_) that protect plants by producing antibiotics and enzymes that degrade the cell walls of pathogens are available on the market [163]. It is worth mentioning that vaccines are approved for marketing after fulfilling the requirements of the registration procedure. Therefore, they are classified as products of proven quality and efficacy [156]. The use of different types of bioproducts mentioned above is associated with many advantages. Treating seeds and plants with preparations based on endophytic microorganisms increases yield quality and promotes plant growth and development [72; 175]. Through their biological activity, endophytes supply the soil with nutrients and improve their availability [96]. The use of bioproducts also has a positive effect on the soil structure. It has been evidenced that the use of biopreparations can increase soil organic carbon and soil porosity [175]. Use of biological products is also characterized by high yields even when applied in small quantities and this in turn affects the lower costs of plant cultivation [12].