Numerous studies have demonstrated significant variations in the plant growth-promoting effects among different species of arbuscular mycorrhizal fungi (AMF). However, the underlying mechanisms remain incompletely understood, particularly regarding how distinct AMF species regulate the rhizosphere microbiome. Five AMF species (Funneliformis mosseae, Diversispora versiformis, Clariodeoglous etunicatum, Rhizophagus intraradices, and Acaulospora delicate) were inoculated to investigate their effects on tobacco seedling growth and rhizosphere microecological regulation. The results showed that all AMF inoculations significantly increased shoot and root biomass, N/P/K uptake, morphological traits (height, stem diameter, leaf area), chlorophyll content (SPAD), and root architecture (length, surface area, volume, diameter) of tobacco seedlings. Among them, the treatment inoculated with R. intraradices showed the most outstanding growth-promoting effect in all growth indicators. Metagenomic analysis indicated that AMF inoculation significantly altered the diversity and community structure of rhizosphere substrate microorganisms. Among them, R. intraradices inoculation yielded the highest microbial diversity, with an associated network exhibiting enhanced complexity. KEGG functional annotation revealed metabolic pathways (IAA biosynthesis, iron-siderophore transport regulation, exopolysaccharide production, and nutrient cycling) consistently associated with tobacco growth promotion in all AMF inoculations. However, species-specific mechanisms were observed: F. mosseae promotes tobacco seedling growth by enhancing IAA synthesis through the recruitment of beneficial microorganisms such as Nostoc, Flavisolibacter, Frateuria, and Sphingomonas. D. versiformis enhanced carbon fixation via the hydroxypropionate-hydroxybutyrate cycle, driven by the proliferation of Glaciecola, Pedococcus, Phycicoccus, and Hephaestia. C. etunicatum facilitated phosphorus/iron accumulation through organic phosphorus mineralization, phosphate transport, and iron acquisition accompanied by the recruitment of, Hartmannibacter, Lysobacter, Moheibacter, and Pseudolabrys. R. intraradices improved nitrogen assimilation through augmented nitrogen transport and assimilatory nitrate reduction (ANRA), correlated with the recruitment of Azospirillum, Sphingobium, Mesorhizobium, Paracoccus, and Parafilimonas. A. delicate stimulated plant growth via polyphosphate degradation and exopolysaccharide biosynthesis, associated with the enrichment of Segetibacter, Ferruginibacter, Hyphomicrobium and Pseudomonas. Notably, this study revealed that functional divergence in rhizosphere microbiomes associated with the five tested fungal species was primarily reflected in the abundance rather than the composition of functional genes. In summary, AMF inoculation significantly enhanced tobacco seedling biomass and agronomic traits by improving mineral nutrient assimilation efficiency and restructuring the rhizosphere microbial community. Different AMF species exhibited distinct microecological regulation patterns. This study elucidated the growth-promoting mechanisms of AMF from a microbial interaction perspective, providing a theoretical basis for establishing a sustainable tobacco cultivation system.

Root-knot nematodes (RKNs) are among the most destructive pests in protected watermelon production under continuous cropping systems. Although both pepper rotation and arbuscular mycorrhizal fungi (AMF) inoculation have shown potential for suppressing RKNs and promoting plant growth, their combined effects remain unclear. This study conducted greenhouse pot experiments using continuously cropped watermelon soil over two consecutive cycles. In the first cycle, chili pepper (Capsicum annuum) or watermelon (Citrullus lanatus) was planted, followed by watermelon cultivation in the second cycle with inoculation of Funneliformis mosseae or Glomus versiforme. Compared with continuous watermelon cropping, both rotation and AMF inoculation improved root vitality, osmotic regulation, antioxidant enzyme activities, and photosynthetic performance, thereby enhancing watermelon growth and resistance to RKNs. Among all treatments, chili pepper rotation combined with Glomus versiforme showed the best performance, increasing shoot fresh weight by 31% and reducing disease index (DI), gall index (GI), and egg mass index (EI) by 30.8%, 77.0%, and 57.1%, respectively. In addition, the populations of second-stage juveniles (J2) in soil and roots and adult females in roots decreased by 85.4%, 55.5%, and 50.8%, respectively. High-throughput sequencing results showed that the combined treatment enriched several potentially beneficial microbial taxa, including Ochrobactrum, Bacillus, Acinetobacter, and Delftia. In addition, it enriched predicted metabolic pathways that may be associated with plant growth promotion and stress tolerance. Overall, pepper rotation combined with Glomus versiforme inoculation represents a promising, environmentally friendly strategy for the management of watermelon root-knot nematode disease.