Aspergillus neoalliaceus MR-86 Promotes the Growth of Saposhnikovia divaricata by Regulating the Rhizosphere Microbiome

Plant growth-promoting fungi (PGPF) have shown broad potential to improve soil conditions and enhance root growth and development. However, few studies have examined the effects of exogenous PGPF inoculation on the growth of the medicinal plant Saposhnikovia divaricata and the associated changes in the rhizosphere microbiome. In this study, Aspergillus neoalliaceus MR-86 exhibited phosphate solubilization, growth in nitrogen-free medium, potassium solubilization, IAA production, and siderophore production. PCR assays did not detect the aflatoxin biosynthesis-related genes aflR, aflS, and omtA in strain MR-86. Pot trials demonstrated that inoculation with MR-86 significantly increased the plant height and root dry weight of S. divaricata by 10.32% and 21.05%, respectively (p < 0.05). In the rhizosphere, soil pH decreased, whereas soil alkaline-hydrolyzable nitrogen and available phosphorus levels, as well as the activities of protease, urease, and cellulase, increased significantly. Illumina NovaSeq sequencing revealed that MR-86 inoculation altered the soil microbial community structure and specifically enriched several microbial taxa, including Talaromyces, Subulicystidium, and Aspergillus. Moreover, MR-86 inoculation did not alter the composition of dominant bacterial and fungal phyla, but significantly modified microbial interactions and the topology of microbial networks. Correlation analysis indicated that the specific microbial taxa Subulicystidium, Aspergillus, and Talaromyces were positively associated with soil nutrient indices, enzyme activities, and plant growth parameters. Functional prediction analysis indicated that MR-86 treatment was predicted to be enriched bacterial metabolic pathways, including flavone and flavonol biosynthesis and ether lipid metabolism, and was predicted to increase the relative abundance of functional fungal groups such as ectomycorrhizal and wood-decomposing fungi. In summary, A. neoalliaceus MR-86 may contribute to improved growth of S. divaricata by enhancing nutrient availability and transformation and by modulating the structure and function of the rhizosphere microbiome.