**1. Introduction**

Mycorrhizal fungi can infect plant roots to form mycorrhiza [1, 2]. In recent years, there have been many studies on the symbiotic relationship between mycorrhizal fungi and plants [3, 4]. The dynamic interaction between plants and mycorrhizal fungi can promote the effective absorption of minerals by plants from soil and provide protection for various environmental stresses [5, 6]. However, some studies have found that this interaction is closely related to mycorrhizal helper bacteria (MHB). Mycorrhizal helper microorganism is a kind of special rhizosphere and endophyte that can specifically bind to mycorrhizal fungi, promote mycorrhizal fungi infection to the host, and promote plant growth and development [7, 8]. The interaction of mycorrhizal fungi and auxiliary microorganisms can promote plant growth, reduce the occurrence of soil-borne diseases, and improve the availability of nutrients in soil and plant absorption rate [9–11]. However, there are still insufficient studies on the synergistic effect of mycorrhizal fungi and auxiliary microorganisms on agricultural crops and forests.

The purpose was to elaborate the effects of the synergy mechanism between mycorrhizal fungi and mycorrhizal fungi, plant microflora, rhizosphere microorganisms and mycorrhizal relationships, and the relationship between endophytic bacteria and mycorrhizal fungi.

## **2. Mycorrhiza and mycorrhizal fungi**

Mycorrhiza refers to the reciprocal symbiosis formed by fungi and plant roots, according to different morphological structures. Mycorrhiza is divided into seven types: arbuscular mycorrhiza, ecto mycorrhiza, endo mycorrhiza, orchid mycorrhiza, arbutoid mycorrhiza, monotropoid mycorrhiza, and ericoid mycorrhiza. Among them, arbuscular mycorrhiza, ecto mycorrhiza, orchid mycorrhiza, and ericoid mycorrhiza are four common types [12]. The characteristic is that fungi do not invade the inner cortex and can form a typical structure of hyphal ring in root cortex cells. Aging hyphal rings are eventually digested and absorbed by plant cells. Studies have shown that the biological binding force of orchid mycorrhiza (OM) symbionts to each other is not symmetrical, and plants are the beneficiaries of symbiotic relationships [13]; some OM fungi are not specifically associated with orchids [14]; fungal heterotrophic orchids have more diverse mycorrhizal fungal lineages than autotrophic orchids [15].

Mycorrhizal fungi are fungi that can infect plant roots to form mycorrhiza [16]. Mycorrhizal fungi and terrestrial plants have formed mutually beneficial, mutually conditional physiological wholes and symbionts with different morphological characteristics 500 million years ago. The core of symbiosis is the bidirectional exchange of plant carbon sequestration and nutrients obtained by fungi. Mycorrhizal fungi mycelium can interact directly or indirectly with other beneficial organisms in soil interior and surface and participate in nutrient transformation, absorption, and recycling processes in soil ecosystems [17]; it plays a role in maintaining the balance of atmospheric composition, regulating ecosystems, increasing biodiversity, and stabilizing and maintaining sustainable productivity of ecosystems [18]. It can also improve the absorption of nutrients and water and salt resistance and disease resistance of plants by combining with the roots of host plants, releasing plant hormones and enzymes to promote plant growth [19].

In recent years, there have been many studies on the symbiotic relationship between mycorrhizal fungi and plants. The dynamic interaction between plants and mycorrhizal fungi can promote the effective absorption of minerals by plants from soil and provide protection for various environmental stresses [20]. However, some studies have found that this phenomenon is also closely related to the mycorrhizal helper bacteria (MHB) [21], which is less studied. MHB is a kind of special rhizosphere bacteria that can specifically bind to mycorrhizal fungi, promote the infection of mycorrhizal fungi to the host, and promote the growth and development of plants [22]. Studies have shown that MHB not only promotes mycorrhizal symbiosis by triggering plant growth factors but also promotes spore germination, root colonization, metabolic diversity, and biological control of soil-borne diseases [23]. The interaction of mycorrhizal fungi and MHB can promote plant growth, reduce the occurrence of soil-borne diseases, and improve the availability of nutrients in soil and plant absorption rate [24, 25]. However, there are still insufficient studies on the synergistic effect of mycorrhizal fungi and MHB on crops in China and abroad.
