**5. Relationship between endophytic bacteria and mycorrhizal fungi in plants**

Endophytic bacteria, as a unique form of arbuscular mycorrhizal fungi (AMF), exist in the host AMF. At present, studies have shown that endophytic bacteria can enhance the ecological adaptability of the host AMF and improve its environmental stress resistance [40]. At present, it can only be preliminarily proved that the growth promotion effect and defense ability are related to endophytic bacteria. However, AMF without endophytic bacteria symbiosis, such as *R. intraradices*, can still produce the effect of disease resistance and injury stress on host plants without endophytic bacteria symbiosis. The interaction mechanism and disease resistance of this kind of AMF with host plants are different from those of the above endophytic bacteria. There are few studies on the interaction mechanism between symbiotic endophytic bacteria and host AMF. Strengthening the research in this direction has important innovative value and scientific significance for further exploration and evaluation of AMF fungal resources.

The unique arbuscular mycorrhizal structure of AMF and its diverse ecological functions formed by its interaction with host plants play a positive role and influence in the restoration of ecological environment vegetation, the restoration of ecological areas with moderate to strong alkaline effects [41], the restoration of wasteland containing metal mines [42], the phytoremediation of heavy metals in sewage-contaminated soil [43], and the rapid restoration of soil vegetation after interference [40]. The interaction between improving crop growth and mineral nutrition [44], plant productivity, and nutrient absorption is still an important research direction at present and in the future [45].

#### *Plant Microbiome and Mycorrhizal Fungi DOI: http://dx.doi.org/10.5772/intechopen.107373*

Ectomycorrhizal fungi (ECMF) usually selects bacterial communities from the surrounding soil according to symbiotic function or habitat requirements, and the short-term contact between soil bacteria and their fruiting bodies can also directly or indirectly promote the endobacteria (EB) community composition of fruiting bodies [46, 47]. According to the ecological function or habitat requirements, ECMF selects specific bacteria to colonize its body, which lays the foundation for the establishment and maintenance of symbiotic relationship. However, at present, it is still necessary to further explore the extent to which these specific species affect ECMF microbial communities and how they interact. Bacteria colonize selectively in ECMFs suitable for their own growth and form a new symbiotic relationship under severe environmental stresses [48].

It has been proved that whether ECMF selects specific bacterial groups according to symbiotic function or habitat requirements, or bacteria colonize in ECMF due to their own nutrient needs or niche expansion, the symbiotic mode formed between the two is more excellent in adapting to environmental changes.

In the symbiotic interaction between bacteria and ECMF, bacteria can loosely interact with the mycelium surface; it can also colonize mycelia and fruiting bodies, or show some symbiotic species specificity, as well as some potential metabolic complementation and ecological functions, thereby promoting mycelial growth, biomass increase, fruiting body formation of host fungi, and providing nitrogen sources for host fungi through nitrogen fixation [40, 46, 49]. At the same time, the mycelia and fruiting bodies of fungi create a suitable habitat for EB by providing different carbon sources for EB and protecting it from environmental stresses [50].
