**4.1 Mycorrhiza and auxiliary bacteria**

Garbaye first proposed the concept of MHB by analyzing the isolation, identification, and symbiosis of bacteria in mycorrhiza [24]. MHB acts on mycorrhiza through root sensitivity to fungi, rhizosphere soil improvement, and fungal reproductive germination. At present, in the study of exogenous mycorrhizal, MHB can promote spore germination and mycelial growth by producing growth factors, detoxification antagonistic substances, and inhibiting competitors. The change of mycelium growth represents the adaptability of MHB to fungi and the close relationship between fungi and host plants [29]. MHB can improve the infection rate of exogenous mycorrhizal fungi on plant seedlings, enhance their colonization, and then promote plant growth two categories [30, 31]: the first category is through the production of enzymes that break down the spore wall and other volatile substances such as terpenoids, thereby promoting spore germination and bacterial formation [32]. The second category is by changing the absorption of inorganic salt by mycorrhizal fungi to affect spore germination, the establishment of symbiotic system [33, 34]. The MHB isolated and identified mainly includes *Agrobacterium*, *Burkholderia*, *Pseudomonas*, *Bacillus*, *Paenibacilus*, and *Streptomyces* [35].

#### **4.2 Mycorrhizal and rhizosphere microorganisms**

Mycorrhiza fungi treatment can significantly increase the species of beneficial fungi and reduce the species of pathogenic bacteria. Mycorrhizal fungi and some rhizosphere beneficial microorganisms have synergistic promotion relationship [36], which can significantly improve plant resistance [37–39]. Arbuscular mycorrhizal (AM) fungi have effects on rhizosphere microbial community structure and activity under atrazine stress, but there are few reports on the composition and diversity of rhizosphere bacterial community. Arbuscular mycorrhizal fungi (AMF) increased the phospholipid fatty acid analysis (PLFA) biomass of AM fungi but decreased the phospholipid fatty acid analysis (PLFA) biomass of microbial bacteria and fungi in rhizosphere soil.

Under the same site conditions, the number of fungi in rhizosphere soil of larch mycorrhizal fungi was significantly different from that in non-rhizosphere soil, and the species composition and dominant population were also significantly different. Since the relationship between tree roots, mycorrhizal fungi, and soil is very complex, many studies have been reported. However, the research field of mycorrhizal rhizosphere fungi has just started, and there are still many contents to be further studied. At the same time, mycorrhizal fungi have direct or indirect effects on plant growth and development. Enzymes, hormones, and toxins produced by mycorrhizal fungi affect the mineral nutrition, water absorption, and root development of plant roots in soil environment.
