The effect of arbuscular mycorrhizal fungi on heavy metal elements in the rhizosphere soil, fibrous roots, and rhizomes of Paris polyphylla var. yunnanensis. Paris polyphylla var. yunnanensis belongs to the lily family and is a perennial medicinal plant. Its dry rhizomes have the effects of cooling the liver, calming the nerves, reducing swelling and pain, clearing heat and detoxifying, and have received widespread attention and use for their unique therapeutic effects and low toxicity. With the increasing demand in the pharmaceutical industry year by year, wild resources are becoming increasingly depleted. Therefore, artificial cultivation of Dianzhong lou has become an effective way to achieve sustainable resource utilization. Improving the medicinal quality of Dianzhong lou through biotechnology has become a hot topic in the research of standardized cultivation system for Dianzhong lou in China, and it is also the only way to achieve effective protection and sustainable utilization of wild resources.

With the rapid development of industry and the widespread use of fertilizers and pesticides, the problem of heavy metal pollution in soil is becoming increasingly serious. Heavy metals in soil can only migrate in the environment and cannot be decomposed by soil microorganisms to continuously accumulate. When the accumulation of heavy metals reaches a certain limit, it will have toxic effects on plant systems, and then endanger human life and health through the food chain. For example, excessive cadmium (Cd) can induce diabetes, reproductive toxicity, osteoporosis, kidney function damage, spleen and liver and other organs to cause cancer, excessive mercury (Hg) can affect the central nervous system of human beings, and lead (Pb) has a harmful effect on the hematopoietic system, nervous system, endocrine system and immune system. Therefore, the problem of heavy metal residues in traditional Chinese medicine is one of the primary issues to ensure that traditional Chinese medicine is “safe, controllable and effective”. Arbuscular mycorrhizal fungi (AMF) are the most widely distributed plant symbiotic fungi in nature. Exogenous inoculation of AMF can effectively reduce the absorption of heavy metal elements by host plants. For example, Li et al. used winged pod wood as the host plant, and exogenous inoculation of AMF can form a good symbiotic relationship with winged pod wood, reducing the concentration of Cu and Pd in winged pod wood; Wei et al. found that exogenous inoculation of Glomus mossae (Gm), Scutellospora pellucida (Spe), and Scutellospora calospora (Sca) can significantly reduce the absorption of heavy metal elements (Cd, Cu, Hg, Pb, and As) in the roots of wood fragrance using wood fragrance as the host plant; Li et al. used peanuts as the host plant and exogenous inoculation of AMF reduced the Cd content in the aboveground part of the plant, changing the morphological structure of the root system to adsorb and retain Cd. It can be seen that AMF can reduce the absorption, transport, and accumulation of heavy metals by host plants through direct effects such as chelation of heavy metal ions by extracellular hyphae and filtration protection of roots, as well as indirect effects such as promoting the absorption of nutrients by host plants, changing the bioavailability of heavy metals in rhizosphere soil, and rhizosphere environment. Therefore, exogenous inoculation of AMF can effectively reduce the residual heavy metals in Chinese medicinal materials, ensuring their safety and effectiveness.

At present, research on heavy metal elements in Dianzhong Lou mainly focuses on inoculating individual plants with AMF and different production areas, while there are few reports on the comparative study of AMF mixed microbial agents on heavy metal elements in the rhizosphere soil, fibrous roots, and rhizomes of Dianzhong Lou. Therefore, in this study, 12 excellent individual AMF plants selected by the research group in the early stage were mixed and inoculated into sterile potted Dianzhong Lou for symbiotic cultivation. The study investigated the effects of inoculating different AMF mixed microbial agents on heavy metal residues in the rhizosphere soil, fibrous roots, and rhizomes of Dianzhong Lou, in order to identify AMF mixed microbial agents that can effectively reduce heavy metal residues in the rhizosphere soil, fibrous roots, and rhizomes of Dianzhong Lou. This provides a theoretical basis for the application of different AMF mixed microbial agents in Dianzhong Lou production and technical support for mycorrhizal cultivation of Dianzhong Lou.

 

 

The effect of arbuscular mycorrhizal fungi on the residual heavy metal content in the rhizosphere soil of Dianzhonglou. Mycorrhizal fungi can promote the growth and development of medicinal plants and improve their adaptability through bidirectional nutrient cycling. Mycorrhizal plants can transport a large amount of organic matter synthesized in their leaves to their roots through AMF. At the same time, mineral elements (heavy metals) in the rhizosphere soil can also be input into plant tissues through AMF to regulate the content of mineral elements (heavy metals) in plant tissues. Therefore, exogenous inoculation of AMF is one of the main effective strategies for microbial remediation to reduce heavy metal residues in Chinese medicinal materials. This study found that inoculation with AMF mixed microbial agents can reduce the residual amount of heavy metals in rhizosphere soil to varying degrees. Among them, S4 treatment had the best regulatory effect on Pb residue in soil, S1 treatment had the best regulatory effect on Cu residue in soil, and S6 treatment had the best regulatory effect on As, Hg, and Cr residue in soil. This indicates that AMF mycelia have different adsorption potentials for heavy metals, and different fungal mycelia have different adsorption abilities for various metal ions. The main reason for this is that different AMF secrete different organic compounds such as proteins, amino acids, and organic acids, which can chelate with different heavy metal ions, thereby reducing the bioavailability or mobility of heavy metals in rhizosphere soil. Reduce the toxicity of heavy metals. Furthermore, from the perspective of heavy metal residues, the residual levels of Pb, Hg, and Cr in the rhizosphere soil of Dianzhong Lou exceed the background values of soil elements in China. This indicates that the AMF mixed microbial agent has differential absorption and strong biological adsorption potential for heavy metal elements in the rhizosphere soil of Dianzhong Lou, leading to varying degrees of accumulation and enrichment of Pb, Hg, and Cr elements in the rhizosphere soil of Dianzhong Lou after bacterial colonization.
The effect of arbuscular mycorrhizal fungi on heavy metal residues in the fibrous roots and rhizomes of Dianthus polyphylla. The accumulation and tolerance of heavy metals in plants is a complex process, and different plants often use different adaptive mechanisms to accumulate or reject heavy metals, thereby maintaining their own growth and development. It is an undeniable fact that exogenous inoculation of AMF can effectively reduce or avoid the residue of heavy metals in traditional Chinese medicine. This study found that exogenous inoculation of AMF mixed microbial agents can promote the increase of heavy metal residues in the fibrous roots of Dianzhong Lou to varying degrees. This is because the fibrous roots can first establish a good symbiotic relationship with AMF, forming mycorrhizal fungi that increase the direct contact area between the roots and rhizosphere soil, accelerate the binding of phosphates and heavy metals in the hyphae, and promote the increase of heavy metal residues in the fibrous roots. This study found that exogenous inoculation of AMF can significantly reduce the residual levels of heavy metals in the new and old rhizomes of Dianthus chinensis, which is consistent with the research results of Zhang et al. As the main medicinal part of Dianthus chinensis, the heavy metals in the rhizomes mainly come from two sources: one is the transfer from the roots, and the other is that some of the heavy metals absorbed by the fibrous roots growing on the surface of the rhizomes are deposited on the surface of the rhizomes. However, in the process of artificial cultivation, the main harvesting target of medicinal farmers is the main root, resulting in a large number of fibrous roots being left in the soil; Furthermore, the overall metabolic activity of the new and old rhizomes is slower, and AMF can directly chelate heavy metals to accumulate in fungi, resulting in lower levels of heavy metal residues in the old and new rhizomes of Dianthus polyphylla compared to the control. From the perspective of heavy metal residues in the roots of Dianzhong Lou, the order of heavy metal residues in the fibrous roots of Dianzhong Lou is As>Pb>Hg>Cu>Cr. The order of heavy metal residues in the new and old rhizomes of Dianzhong Lou is Cr>As>Pb>Cu>Hg. It can be seen that different organs of the same plant have different absorption capacities for heavy metals. According to the limit standards of the Chinese Pharmacopoeia and the Green Industry Standard for Import and Export of Medicinal Plants and Preparations, all the As and Hg heavy metal residues in the treated old roots in this study did not meet the limit standards. Except for the Cr residue in the old roots treated with T3 and S5, which met the limit standards, the Cr residue in the other treatments did not meet the limit standards. This may be related to the differences between AMF species and the synergistic effects between strains. Different strains or mixed microbial agents have different sensitivities to heavy metals. Inoculating AMF promoted the absorption of a certain heavy metal in Dianzhong Lou, but there is no consistent pattern. The specific reasons need further in-depth research.
The effect of arbuscular mycorrhizal fungi on the enrichment coefficient of heavy metals in the rhizomes of Dianzhong. The heavy metal enrichment coefficient is an important indicator that describes the accumulation trend of heavy metals in organisms, indicating the enrichment of heavy metals in plants. Wild chrysanthemums have a strong enrichment effect on Cd, with an enrichment coefficient of 2.72. Therefore, during the cultivation of wild chrysanthemums, attention should be paid to the residual amount of Cd in the soil and environment to avoid excessive enrichment of Cd by wild chrysanthemums, which can lead to Cd exceeding the standard in medicinal materials. This study found that the BCF of Dianzhong Lou for As and Hg were both greater than 3.0, indicating that Dianzhong Lou has a strong enrichment effect on As and Hg. During the artificial cultivation of Dianzhong Lou, the rhizosphere soil is prone to accumulation and enrichment of Hg and Cr, resulting in excessive residual Hg and Cr in some Dianzhong Lou roots and stems. Therefore, in the construction of the Good Agricultural Practice (GAP) base for the production of Chinese medicinal materials in Dianzhong Lou, the soil properties of the cultivation should be strictly controlled, and suitable AMF strains should be selected for mixing. This provides a new technical approach for building an efficient and high-quality Dianzhong Lou planting system.
In summary, the mixed microbial agent (S6) composed of six single plant AMF strains, namely Aspergillus oryzae, Aspergillus oryzae, Fusarium graminearum, Fusarium graminearum, Fusarium graminearum, and Fusarium graminearum, showed the best regulatory effect on heavy metal residues in the rhizosphere soil, fibrous roots, and rhizomes of Dianzhong Lou. The adsorption of heavy metals by AMF is often influenced by various factors, such as the type of host plant, soil fertility, soil pH, etc. In addition, different fungi also have differences in hyphal growth, infection ability, and phosphorus transport efficiency. Therefore, when applying AMF in production practice, these aspects should be considered.