## 2 Endophytes-Current State of the Art
### Endophytes as Inhabitants of Agricultural Plants
#### 2.2.1 Biotic and Abiotic Stresses Related to Crop Productivity  
The major constraints to crop productivity, food quality and global food security are biotic and abiotic stresses [6]. Stress affects several physiological, biochemical and molecular parameters of plants resulting in loss of microbial diversity and soil fertility and competition for nutrient resources [10]. As endophytes have the ability to improve their host's tolerance to stress factors, researchers highlighted their potential use to alleviate plant stress in a rapidly changing climate where plants will be exposed to frequent flooding, water and nutrient deficiencies, low or high temperatures, excessive salinity or the presence of heavy metals in the soil [60]. Drought is a major environmental stress that has caught the attention of ecologists and agricultural scientists. Excessively high temperatures reduce germination rates, growth and plant productivity. In contrast, water deficiency reduces cell size and cell membrane integrity, impairs photosynthetic activity and enzyme function, and causes changes in plant hormone concentrations [10]. Drought stress-tolerant microorganisms have the ability to improve plant growth and development under water-def \begin{table}
\begin{tabular}{c c c} \hline
**Phylum/Class/Species** & **Host** & **References** \\ \hline _Bacillus_ sp. & wheat & [46] \\ _Staphylococcus_ sp. & tomato & [42] \\ \hline FUNGI & & \\ \hline Class 1 & & \\ \hline _Epiolino_ sp. & tall fescue & [47] \\ _Pestalotiopsis_ sp. 9143 & cypress plants & [48] \\ \hline Class 2 & & \\ \hline _Ploma_ sp. & tropical plants & [49] \\ _Fusarium_ spp. & grasses, legumes & [50] \\ _Apergillus_ spp. & maize, rice & [51] \\ _Neothypodium_ spp. & grasses, Poaceae & [52] \\ \hline Class 3 & & \\ \hline _Ganoderma_ spp. & tree species & [53] \\ _Tricholoma_ spp. & trees and shrubs & [54] \\ \hline Class 4 & & \\ \hline _Mortierella_ spp. & maize & [55] \\ _Rhizopus_ spp. & wheat, potato & [55] \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._microorganisms can form a thick cell wall, enter a dormant state, accumulate osmolytes, produce exopolysaccharide and provide access to nutrients [10]. [PERSON] et al. [61] observed positive effects of endophytic bacteria on metabolic balance and a reduction in negative effects of drought stress in wheat grown under unfavorable irrigation conditions. In contrast, [PERSON] et al. [62] investigated how root colonization by endophytic fungi affects _Chenopodium quinoa_ and its ability to cope with extended periods of drought. The endophytic fungus _Penicillium minioluteum_ was used for this purpose. It was shown that under drought conditions, _Penicillium minioluteum_ had a positive effect on quinoa root growth and improved root formation [62]. On the other hand, low temperatures are also associated with many threats to plants. Acclimatization to low temperatures aims to minimize cell damage and increase photosynthetic activity and the accumulation of metabolites, such as phenolic compounds, proline and starch, associated with cold stress tolerance [63]. Numerous endophytes have been shown to increase plant tolerance to low temperatures. For example, the endophytic _Burkholderia phytofirman's_ strain PsIN modified photosynthetic activity and carbohydrate metabolism involved in cold stress tolerance in grapevine plants [58], and improved growth and strengthened cell walls of _Arabidopsis_ sp. [63]. Another growing problem faced by the agricultural sector is salinity. It lowers microbial activity due to ion toxicity and osmotic stress, leading to reduced plant growth and development [64]. Despite this, endophytes have been successfully used to protect various plants under salt-stress conditions. Plants under salinity stress tend to produce higher levels of ethylene, which can be prevented by the action of the enzyme ACC deaminase, produced by many plant-associated bacteria.